Review Article
Diseases of Nile Tilapia with Special Emphasis on Water Pollution
Division of Veterinary Research, Department of Hydrobiology, National Research Centre, El-Bohoth Street,11444 Giza, Egypt
LiveDNA: 20.11233
Tilapia is the most popular fish all over the world due to its mild flavor and taste, versatility in preparation and competitive prices. Tilapia has the lowest feed costs of any farmed fishes. Moreover, it has low fat and high quality protein contents1.
Nile tilapia, O. niloticus is the major fish species that populate Nile River and it is one of the most common, cheapest and available fish for Egyptians. Oreochromis niloticus can survive in poor environmental conditions because of its powerful disease resistance and slight respiratory demands so that they can tolerate low oxygen and high ammonia levels2.
Aquaculture problems can be summarized in 2 main factors, fish diseases and water pollution that are complementary to each other. Really, fish diseases are the ultimate result of an interaction between 3 major variables in the aquatic environments; Fish (host; considering its resistance, age and predisposed), pathogen (considering its virulence and environmental requirements) and water environment (temperature, pH, O2 level)3. Fish -microorganism interactions are usually harmless if the fish’s immune system is not compromised by a stressor. However, fish diseases frequently occur after fish are subjected to stressful conditions4.
Fish diseases are arise from either infectious or non-infectious causes. Infectious causes of fish diseases include viruses, bacteria, fungi and parasites. Viruses give rise to clinical or subclinical problems that having negative impacts on the economy of fish production. It is clear that the research on the viral fish diseases in Egypt is insufficient due to the shortage of the cell lines which are needed for viral isolation and identification5. However, few viruses have been incriminated in fish mass mortalities across the Egyptian state6. Bacteria are very important dangerous pathogens for both cultured and wild fish responsible for mass losses in fish production. The most common bacterial diseases with great impact on Nile tilapia (O. niloticus) under Egyptian aquaculture conditions are streptococcosis, pseudomonas septicemia and motile aeromonas septicemia7. Fungal diseases are also considered as the main diseases causing severe economic losses to fish farms especially saprolegniosis (winter kills) and branchiomycosis8. Mycotoxins cause harmful effects in fish farms via reducing feed efficiency, increasing mortality rate and high costs of treatment9. Finally, parasites are the corner stone of all fish outbreaks that accompanied with secondary infection with bacteria and/or fungi resulting in severe economic losses appeared as high morbidity and mortality rates. Parasitic diseases constitute about 80% of fish diseases in Egypt. This may be due to long periods of warm weather and resultant abundance of natural food and the availability of the intermediate hosts, mollusks and water insects10. Myxobolus infections are among the most common parasitic diseases affecting African cichlids including Nile tilapia11,12. Furthermore, cymothoid isopods are emerging pathogens in Egypt that cause severe economic losses among fish species including Tilapia in Qaroun Lake13.
Non-infectious causes of fish diseases include, water pollution, nutritional and climatic factors. Water pollution is a worldwide problem that has a large concern today due to the industrial and agriculture progress and many anthropogenic activities. It should be noted that most of the river water is acceptably healthy and free of toxins but only in near major drains “black zones” the water becomes unhealthy. A lot of these pollutants are persistent organic pollutants that can travel for long distance; continue for long period, bioaccumulate in fish tissues and having lipophilic ability. Also, biomagnifications of these pollutants usually happen through the food chain14. On the other hand, inorganic heavy metals can negatively affect fish quality and quantity, impair most of the physiological functions, impair immunity and even cause fish death15.
One of the most detrimental factors in aquaculture is the nutrition, nutritional deficiencies in Nile tilapia occur mainly in cultured fish and rarely in wild ones. Deficiency signs of cultured tilapia may happen when fish are fed nutrient deficient diets or raised in a low nutrient-input culture system7.Moreover, the global temperature elevations have received much attention in the past few years, due to their worldwide impact on ecosystems. Wild fish or those reared in cages suspended in rivers are sensitive to extreme weather events, such as floods or droughts, seasonal differences in water temperature, flow speeds and water quality16.
Highetening the biosecurity of fish facility through application of hygienic and preventive measures of the environment, such as fish health management practices; sanitation and disease control procedures are critical factors to prevent fish diseases. Also vaccination has become an increasingly important aspect of aquaculture17.
In recent years there has been considerable increase in the probiotic using in aquaculture. The use of probiotics or beneficial bacteria, which control pathogens through a variety of mechanisms viewed as an alternative to chemotherapeutic treatment18. Different medicinal plants are known to have many properties such as anti-stress, appetizers, growth promoters, tonic and immunostimulants. Furthermore, many of these plants remedies have potent anti-viral, anti-bacterial and anti-fungal properties19.
The current review article aims to report Nile tilapia diseases; its causes, prevalence and control. Also it provides the different types of water pollution and its relation to fish diseases.
NILE TILAPIA DISEASES
Infectious diseases of Nile tilapia
Viral diseases of Nile tilapia: There have been few recent records of emerging viruses that have drastically impacted the tilapia culture in the past few years. Tilapia lake virus is the most critical one among the short list of viruses targeting cichlids. There were 3 records of the presence of infectious pancreatic necrosis (IPN virus) and spring viremia virus (SVV) in Egyptian freshwater fish20,21. Soliman et al.22 recorded the first isolation and identification of spring viremia of carp virus (SVCV) from O. niloticus collected from fish farms in El-Behera, Alexandria and Kafr El-Sheikh governorates, Egypt. Oreochromis niloticus is more resistant to the SVCV than the susceptible hosts (Family Cyprinidae), but due to the several stress conditions leading to immunosuppression, O. niloticus can be infected with the virus, with little or no specific clinical signs or post mortem lesions and serve as a carrier to the virus (Fig. 1).
Tilapia specific viruses (Infect tilapia only): Viruses have been recently implicated in several large disease outbreaks with severe mortalities in both farmed and wild tilapia23,24. These outbreaks featured infection by betanodavirus and herpes-like viruses, causing alterations in central nervous system (neuropil vacuolation and meningoencephalitis, respectively) and a novel enveloped RNA-virus causes ocular, cutaneous and meningeal pathology.
Tilapia Lake virus: Tilapia lake virus (TiLV) is a segmented negative sense RNA new virus has been implicated in recent mass tilapia deaths in Israel and Ecuador24,25. The virus seems to manifest itself in Israel as a problem of the brain26, while it attacks the fish liver in Ecuador24,25 and Colombia27. This virus appears to be a real menace to global tilapia aquaculture.
Nicholson et al.6 studied recent massive summer mortalities reported from Egyptian tilapia aquaculture. In eight commercial farms at different regions of the Nile delta with 43-100% morbidities fish showed detached scales, haemorrhagic patches, open wounds, fin rot and dark discoloration. They isolated TiLV from 50% of the investigated farms. The high detection rate of TiLV was due to the application of a newly published nested reverse transcription-PCR27. They also found that the Israeli isolate of the virus is closer to the Ecuadorian isolate than to the Egyptian one.
Tilapia Syncytial hepatitis virus: A novel cause of tilapia mortality suspected of having a viral etiology is syncytial hepatitis of tilapia (SHT), which has been described in Ecuador. This disease appeared grossly with ascites and histologically with hepatocellular lipoprotein accumulation, necrosis and syncytia formation with necrosis of gastrointestinal mucosa24. Also, Del-Pozo et al.25 suggest that the virus associated with SHT is similar ultrastructurally to an orthomyxovirus and that it presents with partial genetic homology with TiLV (190 nt). This homology has been confirmed in a complementary study by Bacharach et al.26.
Viral encephalitis of tilapia larvae (Novel herpes-like virus): Shlapobersky et al.28 reported an outbreak of a novel disease marked by a whirling syndrome and severe mortalities in laboratory-reared tilapia larvae. They designated the disease as viral encephalitis of tilapia larvae. Based on morphological features, molecular and biophysical characteristics of the pathogen they suggested the causative agent is a herpes-like virus.
Common host viruses (Infect tilapia and other fish species): There were few reports of viral diseases of tilapia, but in Taiwan Tung et al.29 isolated a virus from tilapia similar to infectious pancreatic necrosis virus (IPNV), which is known to occur in eels. From studies involved tilapia that was experimentally infected with IPNV, Mangunwiryo and Agius30 deduced that tilapia could act as a reservoir of this virus, which could then infect other fish species that are more susceptible.
Fig. 1: | Diseases of Nile tilapia with special emphasis on water pollution |
Also, Iridovirus reported as one of important pathogens destructive tilapia within intensively produced systems in most Asian countries31.Moreover, Dong et al.32 described the isolation and characterization of the potential pathogens concurrently infected in natural disease outbreaks of Nile tilapia (O. niloticus) and their pathogenicity in the red tilapia fingerling model. Co-infection of bacteria and Iridovirus was found in two affected farms.
Viral nervous necrosis (VNN): Betanodavirus is the cause of viral nervous necrosis (VNN) disease and has been recorded in many cultured marine fish species worldwide and lately for freshwater species causing high mortalities especially at the larval and juvenile fish stages33. The VNN can infect tilapia, Oreochromis spp. and was identified by indirect fluorescent antibody test in striped snakehead (SSN1)cell line and further characterized by sequencing of a PCR product23,34. Awareness must be taken toward this disease where, The Office International des Epizooties (OIE) list Tilapia as one of the species that may be infected by and there were a possibility of tilapia species to be the carrier or reservoir of this virus35.
Bacterial diseases of Nile tilapia: Bacteria are important pathogens for both cultivated and wild fish and are responsible for mass losses in fish production which is an important economic limiting factor in intensive aquaculture. The most important bacterial diseases in Nile tilapia are classified into:
• | Septicaemic bacterial diseases |
• | Localized cutaneous diseases |
• | Chronic granulomatous diseases |
Septicaemic bacterial diseases
Motile aeromonas septicemia (MAS disease): Motile aeromonas septicemia is an acute or chronic septicaemic disease (Table 1) most common in warm water fish in freshwater environments. Nielson et al.36 demonstrated that three species, Aeromonas hydrophila, A. sobria and A. cavieae were included within the motile aeromonas and described as aeromonas complex37.
Motile aeromonas septicemia is more common in the spring when slightly immunosuppressed fish emerge from stressful winter conditions. Infections in small fish are more acute; while in older fish tend to be chronic. Aeromonas hydrophila is the most important bacteria causing motile aeromonas septicemia and is the causative agent of haemorrhagic septicaemia of cultured tilapia3. Motile aeromonas infections are typically associated with some predisposing stress on the fish population such as overcrowding, poor nutrition, temperature shock, low dissolved oxygen concentration, excessive handling and elevated ammonia and nitrite concentrations. A condition known as post-stocking syndrome in which fish begin to die approximately 3 days after transport and stocking into a new body of water is usually the result of MAS38.
In Egypt, Nile tilapia (O. niloticus) in some fish farms at Dakahlia and Sharkia Governorates, recorded high mortality and morbidity rates due to the presence of A. hydrophila in most of examined fish39,40. Moreover, Elgendy et al.41 isolated A. hydrophila from Nile tilapia, O. niloticus during huge mortalities approached 98% at Barsiq farm, northern Egypt after exposure to extreme cold water (5.2°C).
Pseudomonas septicaemia: Pseudomonas fluorescens and P. putida (Table 1) are considered as causal pathogens responsible for bacterial septicemia in freshwater fish especially under culture conditions. Pseudomonades are opportunistic Gram-negative pathogens, naturally occurring in aquatic environment and as a part of normal gut flora of healthy fish, which cannot be distinguished from aeromonas-induced septicemia in fish stocks exposed to different stresses42.
Pseudomonas fluorescens is widely distributed in aquacultures and considered as one of the main cause of bacterial hemorrhagic septicemia in fish associated with stress conditions such as overcrowding, low temperature and injuries, as well secondary invader of injured fish tissue and in chronic virus infection. Natural mortalities were highest when water temperature at 15-20°C43.
In Egypt, Pseudomonas spp. were isolated through an outbreak of cultured O. niloticus in Qaroun and El Rayan lakes. The organisms were found in 30.83% of examined fish and some fish, especially during the episode of mass mortality demonstrate typical signs of pseudomonas septicemia44.
Vibriosis: Vibrio spp. are Gram-negative motile rods causing fish haemorrhagic septicaemia. The main 2 types were Vibrio anguillarum and V. vulnificus (Table 1). Vibrio infections commonly occur in fish from marine and estuarine environments and sometimes are recorded in freshwater fish causing significant mortalities in aquaculture. It can spread rapidly when fish are closed in heavily stocked, commercial systems and the morbidity may reach 100% 45.Vibrio vulnificus is a pathogenic bacterial species that inhabits brackish waters in warm and tropical ecosystems. It is highly heterogeneous and has been subdivided into 3 biotypes.
Table 1: | Gram-negative Bacterial septicemic diseases in Nile tilapia |
Biotype 1 is the most abundant, distributed worldwide and causes sporadic cases of human vibriosis. Biotype 2 is also distributed worldwide and the only one that harbors the genetic information to infect both fish and humans. Finally, biotype 3 is geographically restricted to Israel, causing outbreaks of human vibriosis after fish handling46.
In Egypt, V. anguillarum, as an economically damaging infectious disease, was isolated from 62% of clinically affected Nile tilapia. The percentages of isolation from skin lesions, muscles, spleen, liver and kidney tissues were 35, 22, 43, 48 and 60%, respectively47. Also, Elgendy et al.41 isolated V. anguillarum from Nile tilapia, O. niloticus during huge mortalities approached 98% at Barsiq farm, northern Egypt after exposure to extreme cold water (5.2°C).
Edwardsiellosis: Edwardsiella tarda is the causative agent for edwardsiellosis (Table 1) in freshwater and marine fish species all over the world leading to mass mortalities and causes gastro intestinal infections in humans48. Moreover, Edwardsiella tarda infections with economically important losses have been recorded in Nile tilapia, O. niloticus in different geographical areas43. The enormous loss caused by this pathogen is being felt in the USA, Japan, Europe and Asian countries. Prevalence of the disease in ponds is seldom above 5%, however, it can reach 50% when fish are confined to tanks49.
In Egypt, E. tarda were isolated from Nile tilapia aquaculture in Behera, Kafr El-Sheikh and Alexandria Governorates50. Also, Noor El Deen et al.51 concluded that E. tarda infection in O. niloticus leads to high morbidity and mortality rate resulting in great economic losses in private fish farms at Kafr El Sheikh Governorate, Egypt.
Yersiniosis (ERM): Yersinia ruckeri is a Gram-negative rod of the family Enterobacteriaceae (Table 1). It is the etiological agent of enteric red mouth disease (ERM) which is a serious disease causing significant economic losses in the salmonid farming industry and causing haemorrhagic septicaemia. Over the last years, the disease spread rapidly in European countries, Australia, South Africa and South America37. Some of the infected fish become asymptomatic carriers which may infect other fish after having been moved to another farm. In this way, the pathogen can easily spread from one farm to another. Birds and other animals can also carry Y. ruckeri, but it has not been verified whether they can function as vectors for the disease52.
In Egypt, Y. ruckeri was isolated for first time from both apparently healthy and moribund Nile tilapia from the Nile River in Giza Province53. Also, it was isolated from both apparently healthy and diseased cultured O. niloticus (8.3 and 12.4%, respectively) and from Nile tilapia and monosex tilapia from Behera and Kafr El-Sheikh Governorates54. Moreover, Eissa et al.55 recorded Y. ruckeri infection among O. niloticus, from an earthen pond at a semi-intensive fish farm in Sharkiya Province, Egypt during the late winter-spring transition period with a prevalence of 66.6%.
Streptococcosis: Streptococcosis is a septicemic disease (Table 2) affects freshwater and marine fish in both farmed and wild populations caused by Streptococcus spp., which are Gram-positive cocci isolated from Tilapia nilotica56. The problem of streptococcal disease is worldwide and results in $150 million losses annually. Few studies have examined the epidemiology of streptococcal infections in tilapia (O. niloticus). Certain environmental factors have been suggested to influence streptococcal disease mortality in tilapia57. Moreover, Anshary et al.58 reported that S. iniae, S. agalactiae and L. garvieae is the cause of significant mortalities in Nile tilapia (O. niloticus) cultured in net cages in Lake Sentani, Papua, Indonesia.
In Egypt, Radwan59, isolated S. iniae with an incidence of 86.7% from diseased environmentally stressed Nile tilapia cultured in brackish water in Fayoum Governorate that showed a mortality rate of 73.3%, compared with a mortality rate of 46.6% in non-environmentally stressed fish.
Staphylococcosis: Two species of staphylococcus have been reported to cause staphylococcosis in fish, S. aureus and S. epidermidis (Table 2). Staphylococci may be present in fish throughout the year, but the disease is induced by a sudden rise in water temperatures or other stress factors in the aquatic environment60.
In Egypt, Moustafa et al.61 isolated S. aureus from Tilapia zillii native to Qarun Lake at El-Fayoum governorate throughout the different year seasons. Also, Soliman et al.62 isolated methicillin-resistant S. aureus (MRSA) from kidney and brain of Nile tilapia (O. niloticus) that were collected during an outbreak in August, 2013 at fish farm in Kafr-El-Sheikh governorate, Egypt.
Localized cutaneous diseases
Columnaris disease (Cotton wool disease): Flavobacterium columnare (Table 3) is a Gram-negative, filamentous bacterium of worldwide importance in freshwater fish causing cotton-wool disease. Outbreaks of columnaris disease are rarely spontaneous, but are influenced by a combination of environmental (temperature more than 15◦C) and other stress factors such as high stocking density, high levels of ammonia and organic load63.
Table 2: | Gram-positive bacterial diseases in Nile tilapia |
Table 3: | Localized flavobacteria diseases in Nile tilapia |
In Egypt, El-Hady and El-Khatib64 recorded 4 bacterial isolates from infected gas bladder of O. niloticus including F. columnare, P. fluorescens, V. harveyi and S. aureus. Moreover, F. columnaris were isolated from different sites of skin ulcerations in the Nile tilapia collected from an earthen pond located at an aquaculture station in Sharkiya Province, Lower Egypt during an acute episode of mass kills during12 summer of 2009.
Bacteria gill disease: Bacteria gill disease (Table 3) may result from exposure of the fish to irritants which damage the gill membrane, creating a pathway for opportunistic bacteria to invade the tissue. Ammonia is one of those irritants predisposing fish to bacteria gill disease. Most bacterial organisms associated with this condition are flexibacter, aeromonas and pseudomonas. However, a single species or strains of bacterial organisms are often involved in the disease65.
In Egypt, Abou El-Atta and El-Tantawy66 investigated some bacterial causes of skin ulcer and gills affection in naturally infected Nile tilapia. Bacteriological examination revealed isolation of A. hydrophila by 43.77%, P. fluorescence by 29.63% and S. faecium by 17.51%.
Chronic granulomatous diseases
Mycobacteriosis: Mycobacteriosis (fish tuberculosis) is a sub-acute to chronic granulomatous disease (Table 2) occurs in aquarium and culture food fish, those reared under intensive conditions. It results from infection by several species of Mycobacterium that includes acid-fast, aerobic, Gram-positive, pleomorphic bacilli and members of family Mycobacteriaceae67. There are several mycobacteriosis case reports from farmedO. niloticus in Kenya68. Piscine mycobacteriosis is caused by M. marinum, M. fortuitum, M. chelonae. It is transmitted through the consumption of contaminated feed, through cannibalism of infected fish or aquatic detritus, as the bacterial cells remain viable for 2 years or more69.Intensively cultured warm water fish species including the cichlid Oreochromis spp. are involved67,70.
In Egypt, Nadia et al.71 recorded that the prevalence of infection with Ichthyophoniosis and mycobacteriosis in Nile tilapia was 32%. The prevalence was higher in cultured (40%) than for wild (24%) fish.
Mycotic diseases of Nile Tilapia: The most important mycotic diseases of Nile tilapia fish can be classified according to the site of infection into:
• | Dermal mycosis |
• | Gill mycosis (Branchiomycosis infections) |
• | Systemic mycosis |
Dermal mycosis
Saprolegniasis: It is a disease of freshwater and brackish water fish caused by Saprolegnia ubiquitous in soil and freshwater. Saprolegniasis is seen as cottony white, gray, brown, red, or greenish masses87 on skin or gills of fish which may be localized, however, the fungi can spread rapidly and cover most of the body. Massive mortalities occurred due to saprolegnial dermal mycoses of pond reared tilapia and of cichlids stocked in artificial lakes, during winter months in non-tropical parts of Africa, in the Near East (Israel) and in introduced tilapia in USA as water temperatures decline below 15°C88,89. Economic losses to tilapia farming is considerable, particularly in the colder winters when 50-80% of the overwintering stock become infected and die, including market sized (300-500 g) fish. Eggs are badly damaged by Saprolegnia when infected during artificial incubation. Invasion is advanced by existing necrotic substances such as unfertilized and damaged eggs90.
In Sharkia Province, Egypt Saprolegnia diclina infection was recorded in Nile tilapia hatcheries during winter. Mixed bacteria (54%), parasites (6%) and single infection by Saprolegnia diclina (40%) were observed91. Moreover, Zahran et al.92 isolated highly pathogenic Saprolegnia parasitica from Nile tilapia that caused 95.6% mortalities in an Egyptian fish farm.
Gill mycosis (Branchiomycosis infections): Branchiomycosis is a fungal disease of fish characterized by necrosis of the gill tissue (gill rot). Brachiomyces sauguins and Brachiomyces demigrans (Table 4) are the 2 fungal organisms associated with that disease. Branchiomycosis occurs in ponds with a high load of organic matter and when water temperatures are above 20ºC (hot seasons) infection may spread to all fishes in the pond leading to severe mortalities90.
In Egypt, Khalil et al.93 determined Branchiomyces spp. in fingerlings of Nile tilapia (O. niloticus) which obtained from private farms at Damietta, Port-said, El-Behera and Kafr-El Sheikh provinces during the summer of 2014. Fish mortalities ranged from a few hundred to several thousand/pond, the causative agent was Branchiomyces demigrans. It was found that ammonia, nitrite and organic matter were over the permissible levels in the examined localities.
Table 4: | |
Systemic mycosis
Ichthyophoniosis (Sand paper disease): Ichthyophoniosis is caused by Ichtyophonus hofferi, (Table 4) which is characterized by granulomatous lesions on the skin of the infected fish (sand paper). Transmission is through contact with fecal materials of infected host or by eating of infected carcass. Internally, kidney, liver, spleen, spinal cord, heart and the brain may be swollen, with existence of white or gray necrotic foci. Dorsoventral or lateral curvature of the vertebral column (lordosis or scoliosis) may occur due to damage of brain and spinal cord90.In Egypt, Ichthyophonus hoferi isolated from the eye of cultured O. niloticus showing pope eye and was higher during winter with 68.1% infection rate94. Also, the prevalence of infection with Ichthyophoniosis in Nile tilapia was 32% which was higher in cultured fish (40%) than for wild (24%)71. Moreover, Icthyophonus hoferi was isolated from both cultured and wild Nile tilapia collected from Alexandria, El-Behera and Kafr El-Sheikh Governorates, Egypt during the period from June, 2006 to May, 2008. The infected fish were suffering from deformities of vertebral column, mouth and caudal peduncle. The infection rate was 27.2% in cultured fish and 20% in wild fish95.
Aspergillomycosis: It is a systemic mycotic disease caused by Aspergillus spp. (Table 4). The original route of infection occur via feeds that have been stored in warm (greater than 27◦C), humid (more than 62%) conditions that promote the growth of mycotoxin producing fungi, mainly if moisture content in the feed is more than 14%. Aflatoxins (mycotoxins) produced by Aspergillus flavus and A. parasiticus are the most toxic and common contaminants of fish feed. AFB1 is considered one of the most potent carcinogens. Aflatoxins degenerate vitamins A, C and B196. Systemic infection with Aspergillus flavus is associated with eye affections and others in cultured fishes. Nile tilapia, O. niloticus fed on Aspergillus flavus contaminated diet became inactive, dark, oedematous, exophalmia with corneal changes was common97.
Aly5 concluded that, the most important diseases affecting Egyptian fish farms and leading to severe economic losses were saprolegniasis, aflatoxicosis and Ichthyophonus infection. Furthermore, Atef et al.98 observed that the incidence of Aspergillus spp. was 90% in tilapia fish collected from ponds in El-Wadi -El-Gadid and El-Fayoum governorates. The most identified Aspergillus spp. were A. flavus (40%), A. niger, A. ochraceous (20% for each) and A. terrus (10%).
Parasitic diseases of Nile tilapia
Protozoa diseases of Nile tilapia:
Tissue protozoa (ciliates and flagellates)
Ciliates
Ichthyophthiriasis (white spot disease): It is one of the important diseases affecting mainly cultured and aquarium Nile tilapia caused by Ichthyophthirius multifiliis (Table 5) its morbidity rate may be up to 100% causing severe losses in fish farms. It has been known as Ichthyophthiriasis, sand grain, gravel or Ich disease10,100 (Fig. 2).
Trichodiniosis: It is caused by Trichodina spp. (Table 5) and considered one of the most dangerous protozoal affections in cultured and wild Nile tilapia. It is mainly found as ectozoic but it may appear as endozoic form10,100.
Chilodonellosis: It is caused by Chilodonella pisicola (syn. C cyprini ) and C. hexasticha. It is common in cultured Nile tilapia causing severe losses and few reports recorded it in wild fish10,100.
Flagellates
Ichthyobodoosis: It is one of the most dangerous external protozoal diseases in hatcheries of Nile tilapia fish occur under unfavorable conditions (Table 5). The disease was previously named Costiasis and is caused by Ichthyobodo necator (syn. Costia necatrix)10.
Internal protozoa
Hexamitosis: It is caused by the intestinal protozoan, Hexamita intestinalis, H. Africans (Table 5) and was recorded in both wild and cultured Nile tilapia in Egypt. This disease affects the intestine, gall bladder and may be found in blood10.
Fig. 2: | Parasitic diseases of Nile tilapia |
Table 5: | |
Piscine coccidiosis: Piscine coccidia are intracellular organisms of the epithelium of the gut, gall bladder, swim bladder, kidney tubules and liver caused by Eimeria spp.10.
Diseases caused by myxosporidia: Myxosoma tilapiae is a myxosporean protozoon affect African cichlids including O. niloticus inducing number of external lesions such as corneal opacity due to presence of the plasmodial spores of the protozoon in cornea of the eye, head cysts and frontal skin ulcer which may progress to formation of the so called “hole in the head like lesions”. This protozoan parasite has a life cycle of 2 hosts including fish and aquatic ring worms and it is isolated from 80% of earthen pond reared Nile tilapias in the farm11,12. Moreover, Shawer et al.95 recorded spinal nomalies due to natural infection of Nile tilapia with Myxosoma cerebralis.
Trypanosomiasis (protozoan blood parasites): It is a disease infects both wild and cultured Nile tilapia caused by protozoan blood parasite (Table 5) named Trypanosoma tilapiae. It occurred mostly in summer and spring in fish weaken by overwintering and also in summer in intensely stocked ponds. Young fishes are more liable for the infection than old ones. The disease transmits to other fish through blood sucking invertebrates as aquatic leeches and some crustaceans. The haemorrhagic areas induced by these intermediate hosts become ulcers that may open the gate for secondary infection10. Negm-Eldin101 reported that Trypanosoma mukasi was concurrently transmitted from O. niloticus to Clarias gariepinus using the leech Batracobdelloides tricarinata as a vector. Transmission was successful in immature Cl. gariepinus.
Helminth parasites
Trematodes
Monogenetic trematodes: Monogenesis is caused by monogenetic trematodes (Table 6, 7) that include Dactylogyridae (Gill Flukes), Cichlidogyrides (Gill Flukes of Tilapia only) and Gyrodactylides (Skin Flukes). These flukes feed on the epithelial cells and blood so they cause massive damage to fish skin and gills7,10.
Digenetic trematodes: Digenetic trematodes (Table 8) are found in Nile tilapia fish either in the form of larvae (most common, where the fish act as 2nd intermediate host) or as a mature parasite (the fish act as final host). In case of larvae they are often found as encysted metacercaria in the subcutaneous tissue, gills, eye or internal organs10.
Table 6: | |
Table 7: | |
Table 8: | |
It is not easy to eradicate the metacercariae or the adult trematodes from fish while the control of snails and aquatic birds (1st intermediate and final hosts) may improve the case. Many species of digenetic trematodes can infect human who eat fish infected with the encysted metacercaria either in a raw state, improperly cooked or partially salted. Amer and El-Ashram102 isolated Prohemistomatidae encysted metacercariae (EMC) from the muscles of cultured Nile tilapia in Abbassa fish farm, Egypt. Moreover, Mahdy and Shaheed103 recorded that the infestation rate of Heterophyid EMC in Nile tilapia from three localities in Egypt were 61.3%. They were Heterophyes heterophyes, Pygidiopsis genata, Haplorchis pumilio, H. wellsi, H. taichui and Phagicola mollienesicola.
Cestodes (tape worms)
Diphyllobothriosis: It is internal parasitic disease in which the fish act as intermediate host. The cause of this disease is Diphyllobothrium latum and D. dendriticum (Table 9). The parasitic larvae found in the dorsal musculature and in the internal organs of fish. Death of fish may be occurring due to larvae penetration to the heart. In young fish, the parasites cause severe damage and even mortality10. This disease of public health importance where transmission to man occurs after eating raw, improper cooked or lightly salted infested fishes harboring the plerocercoid larvae of the parasite in the musculature or the gonads. The patients suffer from signs of mild intestinal disorders with megaloblastic anemia.
Nematodes (round worms)
Heart worm disease: It is an internal parasitic disease has a great economic importance affect the heart sinus venosus and the abdominal cavity of cichlids including Nile tilapia (Table 9). The cause of this disease is nematode parasites belonged to genus Amplicaecum10. The rate of infestation increases with increase of the size and age of fish. The 3rd stage larva (L3) of Amplicaecum species act as the infective stage leaved the first intermediate host (copepods) and then harboured in the 2nd intermediate host (fish).
Acanthocephala (thorny headed worms)
Cichlid acanthocephaliasis: In Egypt this disease is recorded in the intestines of mainly wild tilapia species caused by a parasite related to genus Acanthosentis, Acanthosentis tilapiae (Table 9). The disease prevails in summer months but only in wild cichlid fishes, especially O. niloticus. In heavy infections, fish may not show signs of disease but worms may perforate the intestine causing abdominal peritonitis and death10,104.
Parasitic crustacea: Generally, crustacean males are smaller than females and the mature females are the only parasitic in most of the parasitic copepods while the immature developing females are free living (Table 10). Under overstocking, crustacean parasites quickly build up and cause severe mortality in the stocked fish mainly in high water temperature as their life cycles become faster10.
Table 9: | |
Table 10: | |
Copepoda (lower crustacea)
Ergasilosis: It is a crustacean disease caused in Nile tilapia by Ergasilus spp. high levels of infection occur mainly in low salinity and high water temperatures. Ergasilus species are considered as gill parasites so it is called gill maggots. Mature females are characterized by paired multiseriate white egg sacs so Ergasilus species accepted the famed common name “gill maggot”. This crustacean may transmit some bacterial, fungal, viral and hemoflagellates. Also, the lesions induced by them become a mean for secondary infection with branchiomyces and saprolegnia7,10.
Lernaeosis: This disease affects the entire external body surface of Nile tilapia and caused by crustacean parasite named Lernaea spp. (Lernaea elegans in Egypt) and characterized by high morbidity rates10. The clinical signs differ according to the site of infection (ecto or mesoparasites).
Branchiura
Argulosis (fish lice): Argulus spp. are obligate ectoparasites affecting the external body surface of fish and known as fish lice. The lesions made by fish lice provide a site for secondary attacking by bacteria and fungi7,10. Also, they may transmit some trypanosomes through blood sucking. Heavy infestations of Argulus sp. can lead to mass mortalities mainly in young fish.
Isopoda (higher crustacea)
Nerocila disease: This disease is caused by crustacean belonged to order Isopoda, family Cymothoidae. Nerocila orbignyi cymothid isopod was isolated from Tilapia fish, in Egypt. They are blood sucking and feeding on fluids exuded by the walls of the pouch they inhabit10. They occupy oral and branchial cavities and may cause mouth deformation. In Egypt, Elgendy et al.105 reported that Lake Qarun is contaminated with genotoxic substances to Tilapia zillii fish as well as predispose them to numerous opportunistic pathogens as parasitic Isopoda sp. that were noticed attached to the ventral body surface and in the gill chamber of fish.
Usually there are mixed parasitic infestation and it is rarely to find only one parasitic isolate in fish. In Aswan Governorate, Ahmed and Shoreit106 isolated Ichthyophthirius multifiliis and Dactylogyrus spp. that causing high mortalities (60-100%) in O. niloticus hatcheries in Aswan Governorate during 2001. Other study of parasitic infestation in Oreochromis niloticus fingerlings was performed in private fish farms in Giza Governorate where mortality rate was 15% among examined fish. They were examined for external protozoa, monogenetic and digenetic trematodes. The prevalence of isolated protozoa showed high infestation rates with 71.3% Trichodina mutabilis and 60% Chilodonella hexasticha. Monogenetic flukes (Gyrodactylus rysavyi ) and digenetic larvae (Heterophyid EMC) had infestation rate of 40 and 66.6%, respectively107.
Also, Mahmoud et al.108 studied mass mortalities among Nile tilapia at Maryotia stream which is subjected to many sources of pollution through disposing of sewage materials in addition to improperly treated inorganic and organic chemical wastes. The examined fish were heavily infected with different types of parasites (100% infestation rate). Ali et al.104 reported that O. niloticus collected from Nile River at El-Sail drain revealed parasitic infection of monogeneans and crustaceans, Clinostomum sp., Diplostomum sp. EMC, nematodes larvae and Acanthocephala sp. Muscles were infected with mixed parasites; Mxyoboulus tilapiae, Ichthyophthirius multifilis, encysted metacercariae; Diplostomatidae, Prohemistomatidae and Heterphyidae were also recorded in O. niloticus and M. cephalus collected from 3 different private fish farms, that received untreated sewage pollution at El-Faiyum, Port Said and El-Dakahlia governorates109.
Non infectious (environmental) causes of Nile tilapia diseases: Nile tilapia diseases may occur as a result of many environmental causes:
• | Pollution |
• | Climatic causes |
• | Nutritional causes |
Pollution (chemical and physical): Tilapia feeds at phytoplankton and zooplankton (lower level food chain) where the pollutants at a very low levels. Moreover, there were a little amount of fat in its musculature tissue wherever the dirty dozen, the persistent organic pollutants (POPs) bioaccumulate. Besides, in case of cultured Nile tilapia the exposure time for pollutants is short because they take a short period to reach the marketable size, so tilapia is a miracle fish14.
Chemical pollution: Includes organic and inorganic pollution
Organic pollution
Oxygen demanding wastes: They include biodegradable organic compounds coming from domestic and municipal wastewater and many industrial waste waters like food processing, canning, slaughter houses, tanneries and paper and pulp mills. Biodegradation of such organic wastes by aquatic bacterial activity is consuming the dissolved oxygen through an aerobic oxidation reaction110. So, decrease in the dissolved oxygen (DO) will happen and represent a serious problem if falls below 4 mg L1 and adversely affect the aquatic life, mainly fish health111.
Synthetic organic compounds: They include pesticides, detergents, food additives, pharmaceuticals, insecticides, paints, plastics, solvents and polychlorinated biphenyls (PCBs). Majority of such organic compounds are non-degradable and persistent in the environment, persistent organic pollutants (POPs); like aldrin, heptachlor, hexachlorobenzene, DDT, dioxins and toxaphene. The POPs are highly lipophilic compounds, they bio-accumulate in the organisms and their concentrations increase at higher trophic levels (bio-magnification)112,113. They are highly persistence in the environment, resistant to microbial degradation and are toxic to aquatic organisms. Abd El-Gawad et al.14 recorded that Nile tilapia exposed to pesticides showing a variety of reproductive problems and low survivability as a result of endocrine disruption of fish. Some pesticides like Diazinon enters water bodies and remains stable for more than 6 months and accumulate in non-target tissues causing much oxidative damage in protein structure of O. niloticus fish114.
Oil pollution: Oil enters water body through oil spills, leak from oil pipes and wastewater from production and refineries. Oil is lighter than water, it spreads over the surface of water, decreases the air contact with water, resulting in reduction of dissolved oxygen. Also, it reduces the light penetration and the phyto-production in the water. Polycyclic aromatic hydrocarbons (PAH) are belonged to oil pollutants, they are found in both marine and fresh water and some of them are known to be carcinogenic115. Furthermore, pollution occurred by crude oil and its products are more harmful to Nile tilapia survival over a short period of pollution than others over time116.
Inorganic pollution: They include phosphates, nitrates, cyanides, sulphates, heavy metals and its complexes. Phosphates and nitrates compounds increased in water through fertilizers, detergents and animal wastes. They may stimulate the algae growth, leads to eutrophication and low DO in the water117.
Heavy metals (Cd, Cr, Cu, Mn, Hg, Fe, Zn and Pb) represent one of the most hazard groups of water pollutants. They can enter aquatic environments through industrial or domestic sewage, leaching, storm runoff and atmospheric deposits118. They are non-biodegradable and persist in the environment; bio-accumulate in living organisms causing adverse effects on aquatic flora and fauna and may constitute a public health problem where contaminated organisms are used for food. Fish are used as bio-indicators, playing an important role in monitoring heavy metals pollution119.
There are many diseases can affect fish as a result of inorganic pollution of water as environmental gill disease, brown blood disease and environmental hypoxia.
Environmental gill disease and brown blood disease: Excess of non ionized toxic ammonia in water causes the brown blood disease and other gill diseases. Such ammonia increase is a result of accidental releases of ammonia-rich fertilizer during transport, household use of ammonia-containing cleaning products, burning of municipal waste, internal-combustion engines and chemical synthesis (nitric acid, synthetic monomers and plastics, waste gas treatment, sewage treatment plants, ammonium nitrate explosive production). The affected fish were characterized by chocolate brown blood, signs of asphyxia and anemia. Subsequent bacterial problems, such as Aeromonas and Columnaris infections, often occur few weeks after brown blood disease120.
Environmental hypoxia: It affects mainly cultured fishes as a result of low dissolved oxygen in water, characterized clinically by signs of asphyxia associated with acute mortality. The acceptable levels of DO concentration are greater or equal to 5 ppm. Most of tilapias perform well at 4 ppm.
Low dissolved oxygen in water which usually occurs in case of high water temperature during summer seasons, excess green plants and algae in water resulted in signs of asphyxia among fishes especially at the early morning121.
Physical pollution: Any change in the color, odor, taste, turbidity or temperature of the water is a physical pollution; the most effective of them on aquatic organisms is the thermal pollution.
Thermal shock: It includes either cold or hot water. Cold water is released below dams and harming native fish. Hot water originates from discharge of hot water from thermal power plants, nuclear power plants and many industries, increases the water temperature, reduces DO content and adversely affects the aquatic life. Hot water kills billions of fish eggs and larva. Also, elevated water temperature may represent an important factor in increasing the chemical pollution toxicity and some infectious diseases122.
Gas bubble disease: It is a disease of cultured fish occurs due to gas super-saturation as a result of direct use of deep underground water, heavy algal blooms and sudden change in the water temperature and/or mixing of water of different temperatures. The escape of gases that present in ground water into the fish blood leads to formation of gas emboli in skin and different body organs. In acute form the fish is suffering from severe mortalities up to 100% especially in young fishes, floating of fish on the water surface as a result of swim bladder over inflation, exophthalmia, skin blisters due to subcutaneous emphysema, while in chronic form there were variable sized blisters filled with gases present in skin, gills and different internal organs, rupture of the gas blisters is usually associated with secondary bacterial, fungal and parasitic infections7.
Strong water current and mechanical trauma: In intensive farming, fish are usually transported, vaccinated and subjected to stressful handling procedures that may lead to local inflammation, affecting the integrity of the spine leading to spinal deformities123.
Table 11: | |
Climatic causes of diseases in Nile tilapia: Climate as an environmental factor is strongly associated to aquaculture productivity. Being cold blooded animal, fish is influenced by the surrounding water temperature which affects the body temperature, food consumption, feed conversion, growth rate and other body functions124. During the past few years, global temperatures elevation has received much attention due to their worldwide impact on ecosystems. The climate models mentioned by IPCC125 portended that global temperatures are prospectively to increase by 1.4-6.4°C in this century. Wild Nile tilapia or those reared in cages suspended in rivers are sensitive to extreme weather events, such as floods or droughts, seasonal differences in water temperature, flow speeds and water quality16,126. Moreover, Sex ratios change among fish is an important effect of global warming127. Furthermore, different forms of deformities including skeletal deformities may be induced as a result of temperature variation during early fish development128.
Nutritional causes of Nile tilapia diseases: Nutritional diseases of Nile tilapia mainly occur in cultured fish either due to over feeding or deficiency. Overfeeding of cultured fish causes food accumulation, degradation and fermentation of uneaten food leading to increase ammonia level in the water.
Nutritional deficiencies in Nile tilapia: Essential amino and fatty acids deficiency in Nile tilapia causes loss of appetite, poor feed utilization efficiency and retarded growth. Mineral deficiencies are difficult to occur in tilapia as most trace elements are obtained from the dietary ingredients and the culture water129. Calcium deficiency in Nile tilapia is manifested by reduced growth, poor feed conversion and bone mineralization, while magnesium deficiency is characterized with whole-body hypercalcinosis; concerning manganese deficiency is accompanied with reduced growth and skeletal abnormalities129,130. In Nile tilapia, excess magnesium in a low-protein diet produced severe growth retardation and a significant decrease in blood parameters, while magnesium deficiency in a high-protein diet caused whole-body hypercalcinosis131. Adding antibiotics to the feed reduces the vitamin synthesizing capacity of fish. For example, vitamin B12is produced by Nile tilapia in normal conditions however, should be added to the diet when antibiotic treatments were received130,132,133 (Table 11).
RELATION BETWEEN POLLUTION AND NILE TILAPIA DISEASES
Now-a-days, the interrelationship between fish diseases and water pollution has a great interest. This relation is a synergistic and represents a double edged phenomenon, in which the parasite infestation may increase the fish susceptibility to pollution. On the other hand, pollutants affecting the fish immune system causes increase (or in some cases decrease) in the parasite incidence. Adverse environmental conditions decrease the immunological response of fish and increase susceptibility to different diseases. Exposure to sub-lethal doses of zinc for a long time decreases the total proteins, globulins and lysozymes concentrations in fish plasma134.
Thermal pollution leads to faster development of microorganisms and affect both the aquatic organisms and humans health122.Also, it increased the effect of chemical pollution on living organisms. A synergetic relationship between cadmium toxicity in O. niloticus and water temperature was assessed by Abdel-Tawwab and Wafeek135 who reported the maximum Cd toxicity at 32°C. On the other hand, decrease in water temperature may also increase some fish infectious diseases. Francisella sp. was the causative agent for tilapia mortalities in Hawaii, United States, Taiwan and Japan. This bacterium is responsible for outbreaks in cultured and feral tilapia species since 1994, especially in the cooler months136. Furthermore, Elgendy et al.41 reported severe mortalities approached 98% in earthen-pond cultured Nile tilapia, northern Egypt during winter where water temperature was 5.2°C. The recorded values for dissolved oxygen, No2, No3 and NH3, were exceeding the optimum recommended levels. The magnitude of stress formula was extremed by presence of elevated levels of some heavy metals (Co, Cu, Fe, Mn, Pb, Ni, Zn, Cr and Cd) higher than the recommended permissible limits. Furthermore, A. hydrophila and V. anguillarum were isolated from succumbed fishes. Moreover, Eissa et al.12 concluded that the 2 different causative agents (F. columnare and M. tilapiae) under the effect of the acute change in the water quality measures (rise in the water temperature, pH, ammonia, abrupt decrease in dissolved oxygen) have synergized together to induce Nile tilapia mass mortalities.
It was suggested that skeletal deformities in fish is a good indicator for water pollution. Industrial facilities emit high amounts of pollution into the atmosphere that accumulate in upper atmospheric layers by condensation, then forming acid clouds and acid rain. Fish exposed to acid rain may be susceptible to numerous adverse effects including poor respiration, reduced growth, reproductive failure and notable skeletal deformities128. Also, Cd may cause spinal deformities by inducing osteomalacia and neuromuscular damage137. Moreover, cadmium toxicity is increased by high temperature that needs to be taken into account for the prediction and assessment of spinal deformities in fish induced by cadmium pollution138. Regarding lead it is also implicated in causing disturbance in bone ossification and may lead to mutation-like tumor in O. niloticus (Nile tilapia) fish139. Egyptian agriculture drainage water mostly contains organochlorine and organophosphate pesticides. Also, organophosphate pesticides are regularly used for external parasites treatment of fish such as monogeneans and crustaceans (e.g., Argulus and Lernea). These chemicals may induce skeletal deformities in fish by altering biological processes needed for maintaining biochemical integrity of neuromuscular tissue or bones128. Also, Mahmoud et al.108 studied mass mortalities among Nile tilapia fish (O. niloticus) at Maryotia stream, Egypt which is being subjected to many sources of pollution through disposing of sewage materials in addition to improperly treated inorganic and organic chemical wastes. Parasites (100% infestation rate): Copepod, monogenea, protozoa, trematodes larvae and Acanthocephala. There were marked abnormal water parameters (high levels of unionized ammonia, PAHCs, phenol and heavy metals, severe reduction of DO) and environmental pollution incriminated as a primary stress factor promoted the invasion of parasites.
El Sail drain (Kima drain) is one of the major sources of pollution of the River Nile at Aswan governorate, Egypt. It is used for the disposal of household solid waste, both treated and non-treated sewage wastewater, as well as industrial wastewater. Ali et al.104 concluded that consuming fish caught from the studied sites around El-sail drain disposal point represents serious hazard on human health. The study recorded high values of pH, EC, BOD, COD, NO2, NO2 and NH3. Heavy metals concentrations in water especially Ni, Pb and Cd exceeded the permissible limits. Total bacterial count, total coliform, Shigella sp. Salmonella sp. and E. coli were detected in higher numbers in water samples from sites of study. Moreover, Nile tilapia fish (O. niloticus) collected from that site revealed severe bacterial (Enterobacteriaceae spp., E. coli and Salmonella spp.) and parasitic infection (monogeneans and crustaceans in gills, Clinostomum sp. The EMC in gills and branchial cavity, Diplostomum sp. The EMC in muscles, nematodes larvae and Acanthocephala sp. in the intestine).
Fish branchiomycosis (fungal gill rot) occurs most frequently in the warm climatic regions. It depends on water temperature, with elevated load of organic matter, fertilized ponds by organic manure and elevated levels of unionized ammonia in the water raise the incidence of fungal gill rot due to epithelial cell hyperplasia of gills. Also, Branchiomyces spp. was determined in fingerlings of O. niloticus with severe mortalities. It was found that ammonia, nitrite and organic matter were over the permissible levels in the examined localities93. In addition, Atef et al.98 isolated Aspergillus spp. from Tilapia fish and water samples from ponds in El-Wadi-El-Gadid and El-Faiyum governorates at the rate of (90 and 40%, respectively). Also they isolated bacteria; Pseudomonas spp., E. coli, Proteus spp., Klebsiella spp., Citrobacter spp. and Salmonella spp. from fish and water samples. The incidence rate of aflatoxin in Tilapia fish was line(25%). There were extremely high concentrations of iron and ammonia that negatively influenced the pond water quality which caused fish poisoning. There was also hepatic damage manifested by significant elevation in levels of liver function tests in polluted fish due to accumulation of mycotoxins.
Contamination of fish flesh with some bacterial species, Streptococcus sp., Staphylococcus sp., Salmonella, Morganella morganii, Pseudomonas cepacia and Enterococcos durans and pararasites, Mxyoboulus tilapiae, Ichthyophthirius multifilis, encysted metacercariae; Diplostomatidae, Prohemistomatidae and Heterphyidae were recorded in Oreochromis niloticus and Mugil cephalus collected from 3 different private fish farms at El-Faiyum, Port Said and El-Dakahlia governorates in Egypt that received untreated sewage pollution109. Chronic exposure to heavy metals in living habitat may cause persistent metal bioaccumulation in tissue with genotoxic impacts on fish where they have long been considered as one of the chief group of elements inducing DNA damage140. Investigation of DNA alterations in aquatic organisms is highly suitable biomarker for evaluating the genotoxic effects of such contaminants since it can detect exposure to low levels of pollutants141. Also Elgendy et al.105 indicated that Lake Qarun is contaminated with some heavy metals, Zn, Ni and Cu that act as genotoxic substances to Tilapia zilli fish and predispose them to numerous opportunistic pathogens. Such heavy metals showed high values in water exceeding the permissible limits and also accumulated in fish muscles. Genotoxicity (DNA damage) was confirmed in fish liver cells by Comet assay. Bacterial infections, V. alginolyticus, A. hydrophila and Photobacterium damsella subsp. piscicida were detected in fish specimens. Parasitic Isopoda sp. was also noticed attached to the body surface and gill chamber of fish.
PREVENTION AND CONTROL OF NILE TILAPIA DISEASES
Prevention is the basis of any program for health protection and can be as challenging and complex as the existing diseases' actual control.
In wild environment:
• | Enforcement and environmental law implementation |
• | Improving municipal sewage system across stations |
• | Raising public awareness about importance of natural water resources protection from pollution |
• | Removal and movement of industrial complexes from Nile river costs to urban/desert areas |
• | Minimizing environmental pollution through applying 3 treatments system (Mechanical, chemical and biological) at level of factories/drainage stations |
• | For the protection of aquatic systems and fish from heavy metals pollution, new methods of eliminating pollutants from wastewater must be applied142 |
In aquaculture: The control of fish diseases in aquaculture includes both preventive and treatment measures as follows5.
Prevention of fish disease:
• | Preventing exposure to physical, chemical and biological disease agents |
• | Controlling environmental conditions affect fish by farm site selection, water supply, fish handling, transport systems and waste removal |
• | Diet selection, quantities fed and feeding frequency |
• | Application of vaccination programs. Licensed vaccines are available against vibriosis, furunculosis and enteric red mouth diseases and developing vaccines against local pathogens |
• | Application of sanitation programs and egg disinfection for prevention of vertical and horizontal transmission of pathogens |
Disease control methods: Control measures for infectious diseases to reduce or eliminate the root of infection, cut the connection between the root of infection and fish and reduce the fish susceptibility to infection.
Reduction or elimination of the root of infection is done through:
• | Accurate diagnostic techniques and determination of methods of disease spread and prevention of their spreading |
• | Elimination of carriers from the water supply |
• | Quarantine measures to reduce diseases introduction to the aquaculture |
• | Farmers should be familiar with general signs of fish diseases |
Cutting the connection between the source of infection and fish: This can be done through treatment or elimination of diseased fish, using equipment of water sterilization, feed and feed ingredients pasteurization and disinfection of rearing facilities.
Reducing the susceptibility of fish to disease: Adjusting environmental conditions to reduce adverse effects as; regulate water temperatures, alter oxygen and other dissolved gas levels, reduce levels of ammonia and nitrite, reduce population densities, improve methods of handling and using of immunostimulants to improve disease resistance.
Immunostimulants: They are natural and synthetic compounds promoting the non-specific immune response and antibody production. Included among these compounds are vitamins, trace elements, yeasts, glucans and others.
Disease treatment methods
Chemotherapy: Chemicals used for infectious disease treatment.
Antibiotics: Sensitivity test should be done for determining the proper antibiotic. Before using antibiotics, sources of stress should be deleted or reduced. Treatment must be conducted for the required time period. Too high dose or too long treatment times will increase a danger of toxicity to the fish. Too low dose or too short treatment time of antibiotic increase the risk to develop antibiotic resistance. Two week withdrawal period is recommended for all chemotherapeutic treatments prior the intended release or harvest date.
Biotherapeutics: These are biological agents through which the control of destructive organisms occurs by the use of other beneficial organisms.
Probiotics: A probiotic is a living microbial adjuvant which has beneficial effects on host. They improve the feed utilization and increasing its nutritional value, enhancing the host response towards diseases and improving the quality of its ambient environment.
Bioremediators: It is a type of probiotics that improve water quality (the treatment of pollutants or waste by the use of microorganisms that break down the undesirable substances).
Prebiotics: Prebiotics are non-digestible food ingredients that beneficially affect the host by stimulating the growth and/or activity of one or a limited number of bacteria in the intestine and thus improve host health. The main advantage of prebiotics over the probiotics is that they are natural feed ingredient, their incorporation in the diet does not require particular precaution, on their authorization as feed additives may be more easily obtained, in spite of some concerns about their safety and efficacy.
Now-a-days there are preparations combining both agents (probiotics and prebiotics) called symbiotic e.g., Enterococcus faecalis with Mannan-oligosaccharide (increase the immune response toward V. anguillarum).
Medicinal plants: Different herbal plants are known to have many properties such as anti-stress, appetizers, growth promoters, tonic and immuno-stimulants. Moreover, these substances also have other valuable properties, they are non-biodegradable and biocompatible. Many of herbal plants remedies have potent anti-viral, anti-bacterial and anti-fungal properties19.
In conclusion, fish health can be affected with two main factors, fish diseases and environmental pollution both of them are interrelated to each other. Fish diseases are due to either infectious or non-infectious causes. Infectious diseases are viral, bacterial, mycotic and parasitic diseases that usually accompanied with high mortality and morbidity rates. Non-infectious diseases mainly caused by environmental problems and nutritional deficiencies. Water pollution has a large concern due to the industrial and agriculture progress and many anthropogenic activities. These pollutants may be chemical or physical, chemical pollutants either organic (pesticides, detergents, insecticides, paints, plastics, etc) or inorganic (phosphates, nitrates, cyanides, sulphates and heavy metals); most of them have the ability to bioaccumulate in fish tissues. Application of hygienic and preventive measures of the environment heightening the biosecurity of aquaculture.
This study discovered the relationship between fish diseases and environmental pollution. Fish diseases are due to either infectious or non-infectious causes. Water pollution occurring due to the industrial and agriculture progress and many anthropogenic activities. These pollutants may be chemical or physical; most of them have the ability to bioaccumulate in fish tissues. The application of hygienic and preventive measures of the environment competently tightens the biosecurity of fish facility. Vaccination and the use of probiotics and medicinal plants as an alternative to chemotherapeutic treatment have become an important aspect of aquaculture. This study will help the researchers to uncover the critical areas of fish diseases and environmental pollution that many researchers were not able to explore. Thus a new theory on the correlation between fish diseases and environmental pollution may be arrived at.