Gastrointestinal parasitism is one of the most important constraints for
the growth and developments of poultry industry in world including India. These
parasites cause heavy economic losses to farmers by affecting growth, production
and high mortality and mortality amongst young animals. In gastrointestinal
parasitism coccidian infection plays an important role, various species reported
from the chicken are Eimeria acervulina, E. brunette, E. necatrix, E. tenella,
E. maxima, E. mitis, E. mivati and E. hagani etc. These species have predilection
site in the different part of gastrointestinal tract. E. acervulina occurs
in the epithelial cells of the anterior portion of the small intestine mainly
in duodenum. E. brunette occurs in the mucosa of the lower portion of
the small intestine, caecum, rectum and cloaca. E. tenella is present
in caecum. E. necatrix occurs in the jejunum, mid gut, caecum and other
parts of the large intestine. They produce severe damage to the site of their
predilection in various mammals including human being. In general, there are
three types of coccidiosis occur depending on the site of predilection of causative
||Intestinal coccidiosis due to E. necatrix, E. maxima, E.
mivati, E. acervulina
||Caecal coccidiosis primarily due to E. tenella and to some extent
||Rectal coccidiosis due to E. brunetti. E. tenella is most pathogenic
coccidia and causes severe disease of poultry rated next to that caused
by Salmonella pullorum
Life cycle of avian coccidian consist of 2 asexual cycles which collectively
require one or more days. The life cycle consist of various stages which involve
oocyst-sporozites-trophozoites-schizonts (completion of 1st asexual cycle)-merozoites-trophozoites-schizonts
(completion of 2nd asexual cycle)-merozoites-formation of male and female gametes
(sexual cycle)-sporulating oocyst (outside the host). Whole life cycle requires
7 days for completion. Short life cycle and large number of sporulating oocyst
of parasites helps in increasing the chance of contamination to a large population.
Coccidiosis is important from an economic point of view, where the birds are
raised under intensive conditions. The severity of the disease which occurs
depends both on the infecting species and the density of parasite burden. As
prevention is greater than treatment, so proper control measures should be followed.
Control measures include good husbandry, as a prime requirement and the use
of anticoccidial drugs for both prophylaxis and treatment.
Anticoccidial drugs: The agents used for the prevention and control of coccidia infections are termed as anticoccidial drugs. The agents who destroy the coccidial population are termed as coccidiocidal and agents who prevent the replication and growth of coccidial population are known as coccidiostatic. To make the anticoccidials effective, it should be used prophylactically instead of therapeutically. Some anticoccidials show strong activity during the sexual cycle i.e., day 5 and 6 and during these days signs of anorexia and hemorrhage appear so initiating anticoccidials treatments during these days will provide more benefit. Anticoccidials are used usually in starter rations for meat type birds raised under floor-pen management.
Most of the anticoccidials show their greatest efficacy against the 1st and
2nd asexual cycle, some inhibits sexual stages of the life cycle. Only few anticoccidials
disturb the chemical metabolic pathway by which the drug block the specific
stage of the parasite1. Selection of an anticoccidial
is based on the ability of the drug to improve weight and feed conversion and
to suppress the development of lesions2.
Presence of drug residues in eggs and milk is also a point of concern associated
with the anticoccidials, so there is need for specified withdrawal periods before
slaughter. The emergence of drug resistant strains of coccidia presents a major
problem. To avoid the development of drug resistance there is method used which
include switching around different classes of drugs and the shuttle programme.
Many therapeutic regimens are to maximize the efficiency of treatment and to
minimize the possibility of resistance e.g. anticoccidial drugs given in sub
therapeutic doses to encourage the development of immunity and the use of compound
anticoccidial preparations are common. The speed of emergence of resistant strains
of coccidia in the field is given by Reid (1975) as follows: (1) Glycomide-very
rapid, (2) Quinolones-rapid, (3) Clopidol-less rapid, (4) Sulphonamides, nitrofurons,
robenidine-moderate, (5) Amprolium-slow, (6) Nicarbazine-very slow and (7) Monensin-absent
or very slow. Resistance is more likely to develop in birds reared under intensive
conditions than in farm animals3. Present
overview discuss briefly about various anticoccidials used for the prevention
and control of coccidial infection by affecting different stages of life cycle
Ionophores: Ionophores are the fermentation products of Streptomyces and other fungi species, extensively used as anticoccidials. Monensin, Lasalocid and Salinomycin are the Ionophores which are used commercially and monensin is choice of product for broiler chickens mainly because of its broad spectrum activity against majority of pathogenic species of coccidian and lack of development of drug resistance.
Mechanism of action: Ionophores facilitate transport of Na+ ion in cells and elevates the intracellular concentration of Na+ ion. This increased concentration of Na+ ion inhibits the certain mitochondrial functions such as substrate oxidation and ATP hydrolysis. Intracellular Na+ ion exchanges for extracellular Ca++ and increases intracellular concentration of calcium ions lead to cytotoxicity. In addition some drugs directly facilitates Ca++ transport in cells and increased intracellular concentration of Ca++ in cardiac and skeletal muscle cells are responsible for its toxic effects in cells.
Monensin: It is a fermentation product of Streptomyces cinnamonensis
and the 1st antibiotic used as an anticoccidials. Due to its broad spectrum
activity, it acts on trophozoites and 1st generation schizonts. Its activity
is generally within 1st 2 days of life cycle of coccidian. It gives protection
against all species at 0.01-0.121% concentration in the feed. It also increases
the weight gain and feed conversion and in some cases causes suppression of
necrotic enteritis2. It is superior over
amprolium, clopidol and zoalene in control of coccidiosis. In the USA, there
is a three day premarketing withdrawal requirement for this compound. This drug
has ability to form complexes with sodium and potassium ions in the host and
developing parasite. This monensin-cation complex renders membrane permeability
to sodium and potassium ions.
Lasalocid: It is another fermentation product and has a high degree
of anticoccidials activity. It is effective at 0.005-0.0075% concentration.
It also increases weight gain, feed conversion and reduces the lesion in severe
coccidiosis2. It has different ionic affinities
and accepts divalent cations as well as monovalent ions.
Salinomycin: It was isolated from a culture of Streptomyces albus.
It is more closely related to monensin than lasalocid. It has anticoccidials
activity at 0.01% in the feed and it was as effective as 0.0121% monensin in
controlling coccidiosis4. The ionic affinity
is similar to that of monensin i.e. sodium and potassium ions.
Maduramicin: It is most potent among the polyether Ionophores. It is
given at 5-6 ppm in feed and activity is similar to that of other Ionophores5.
Problem of these Ionophores is that they may cause severe cardiovascular defects
in animal cells.
chloride hydrochloride]: it is quarternized derivative of pyrimidine which is
a thiamine antagonist. It is most active against E. tenella, E. necatrix
and E. acervulina and to lesser extent E. maxima. Combination
of amprolium with ethopabate, sulphaquinoxaline or even pyrimethamine extended
and strengthened the spectrum of activity. It could be fed at several times
the recommended dose with no ill effects and probably, one of the safest antimicrobial
drugs to be used extensively. It is effective against 1st generation of trophozoites
and schizonts and shows peak activity early in day 3 of cycle. It also suppresses
the sexual stages, gametogony and sporulation of oocyst. Continuous use of Amprolium
is resulting into the development of drug resistance which is a major problem
and limiting its use. It is rarely used alone because E. maxima and other
species are resistant and therefore given in combination with other drugs. Amprolium
is available as a premix and is given prophylactically to birds in a final concentration
of 0.0125 percent. In combination with 2 other drugs, it is given at a level
of 0.006% of each in the food with better effectiveness6.
A combination of amprolium and sulphaquinoxaline at levels of 0.006% of each
in the food is more effective against poultry coccidiosis than either of the
two drugs used alone7. There is no premarketing
withdrawal requirement for this compound. Amprolium is compatible with vitamins,
antibiotics, minerals and other ingredients commonly used in poultry ration
but it should not be mixed in concentrates containing high levels of choline
because of tendency for it to break down into picric acid.
Mechanism of action: It is thiamine antagonist and due to its close structural similarity it blocks the thiamine receptors. This blockage of receptors prevents coccidia from utilizing thiamine and as a result thiamine is unavailable to coccidian. This vitamin (thiamine pyrophosphate) is a cofactor of several decarboxylase enzymes which play role in cofactor synthesis. It is only agent which can be used in laying birds both for prevention and treatment of outbreaks. At higher doses, thiamine deficiency can occur in host but it can be prevented by addition of thiamine.
Nicarbazine: This rug is an equimolecular complex of p, p-dinitrocarbanilide
(DNC) and 2-hydroxy-4, 6-dimethylpyrimidine (HDP). DNC is absorbed more rapidly
from the chicken digestive tract but disappear more slowly from the tissues
than HDP. Both are necessary for anticoccidial activity. This compound is used
principally as a prophylactic and therapeutic dose lies near the toxic dose.
Nicarbazine is having broad spectrum activity and effective against all Eimeria
spp. This compound has coccidiocidal activity, mainly against the schizonts
which appear after the 1st generation. Marked inhibitory effect on the second
generation schizonts and moderate action on the sexual stages have been reported
by McLoughlin and Wehr8. It is available
as a 22.5% premix and it is incorporated into feed to bring a final concentration
of 0.0125%. It is coccidiocidal principally because its molecules can enter
the cells of the coccidia and paralyze the intracellular energy-supplying ATP
which leads to the interruption of cellular energy supply and the cease of function
of sodium-potassium ion pump which results in the abundant influx of sodium
ions and with them the influx of abundant water which causes the intracellular
imbalance of ions in the cells of the coccidia or the rupture of the cells and
the death of coccidia occurs. Some strains of coccidia which have become resistant
to other drugs remain sensitive to nicarbazin. The drug is suitable for administration
to broiler flocks and it is usually given for the first 12 weeks of the chickens
life. It reduces both egg production and the proportion of fertile eggs that
hatch. It also causes depigmentation of eggs, mottled egg yolk and poor hatchability,
so it should not be used for laying hens. In broilers, a 4 day withdrawal of
nicarbazin is required before marketing. Losses from heat stress may occur in
broilers if they are medicated with nicarbazin.
Sulphonamides: They have longest history of use as anticoccidial drugs. The common drugs of this group which are used as anticoccidials are sulphadimidine, sulphaquinoxaline, sulphadimethoxine, sulphanitran and sulphaguanidine. Sulphonamides have broad spectrum of activity against eimerian species and have coccidiostatic action. They are used for prevention and treatment of coccidia and in outbreaks. They are more effective against intestinal than caecal forms of coccidia. They stop the onset of the disease by acting against the second generation schizonts of E. tenella and E. necatrix. They can act upon first generation schizonts and possibly against sexual stages but much higher doses are required. Use of these drugs does not impair immunity development.
Mechanism of action: Wood and Fildes proposed mechanism of action of
sulphonamides, coccidian is synthesizing their own folic acid utilizing PABA
(p-amino-benzoic acid) from growing medium because folic acid is required for
growth/replication of DNA9. Sulfonamides
are structural analogues (PABA and Sulfonamide is similar in nature) of PABA
inhibit bacterial folate synthetase resulting into folic acid is not formed
and a number of essential metabolic reactions suffer. Animal cells also require
folic acid but they utilize performed folic acid supplied in diet and are unaffected
by sulfonamides. Therefore they prevent proper development of schizonts. Diaminopyrimidines
inhibits the conversion of folic acid to tetrahydrofolic acid and are used in
combination with Sulphonamides to potentiate their anticoccidial action.
Sulphadimidine: This compound is still used as a curative drug in certain
parts of the world, but its use has largely been discontinued in Western Europe
and North America where it has been replaced by other compounds10.
It is given @ 0.4% in feed or in drinking water as 0.2% solution of the sodium
salt. It is active against E. tenella, E. necatrix and other species
of coccidia. It has been used in the control of clinical outbreaks of coccidiosis.
The problem of this drug is that it interferes with vitamin K synthesis in the
intestine and resulting into prolongation of blood coagulation time. At higher
doses it causes loss of egg production in laying hens and hyperplasia of the
somniferous tubules of testicles of male birds.
Sulphaquinoxaline: It is an important, effective and commonly used coccidiostat
throughout the world. For therapeutic purposes a dose of 0.5% in the feed is
given. In drinking water, a dose of 0.043% is given for two durations each for
2 days with 3-5 days intervals, is satisfactory. Doses ranging from 0.025 to
0.033% over fairly long periods may be used as preventive medication. It is
also active against E. acervulina in addition to E. necatrix and
E. tenella. It exerts marked inhibitory effects on schizogony. Drug at
a level of 0.1% in the ration inhibited invasion by the sporozoites11.
When used at higher dose for long duration it produces toxic effects which include
multiple hemorrhages in many organs accompanied by necrotic lesions in the spleen,
hypoplasia of bone marrow and agranulocytosis12.
This toxicity is associated with an interference with vitamin K metabolism.
This compound has 6 days withdrawal premarketing requirement and eggs from treated
birds should not be used for human consumption.
Ethopabate: It is an arylamide containing one phenyl ring, belonging to monocyclic aromatics, is a very safe drug. It has anticoccidial activity especially against intestinal forms and lacks activity against E. tenella of caecal worms. This drug is a competitor of PABA for absorption by the parasite and interferes with folate synthesis. It has good activity against E. acervulina and some strains of E. maxima and E. brunette. It has been used only in combination with Amprolium first at 4 ppm and later at 40 ppm. This drug has peak activity on 4th day of cycle.
Clopidol: It is the only member of its class i.e. pyridinols having useful anticoccidial properties. It is also called metichlorpindol or clopindol. It has broad spectrum activity. It is almost completely coccidiostatic in action and effects the sporozoites or trophozoites. It is most active against the sporozoite stage of Eimeria. Thus to produce full anticoccidial potential, it should be in the feed of chickens on the day of exposure to coccidial oocyst. Day one of the coccidia cycle is designated as day of peak activity for clopidol. Drug is not active if given after day of exposure, so should be given on day first. Its coccidiostatic activity holds the sporozoites undeveloped in an epithelial or host macrophages cells for as long as 60 days. Latent coccidiosis may appear if drug is withdrawn during the static state, as the parasites resume development. This drug is generally administered at 125 ppm in the feed. It may be used in last 1-3 weeks of the broiler grow out. There is no premarketing withdrawal requirement.
Quinolones: There are hundreds of Quinolones which have been synthesized and a number of them have showed activity against various groups of parasites. Buquinolate, decoquinate and nequinate are the examples of Quinolones which have shown great efficacy against all species of poultry coccidia. Quinolones have limited absorption because they are virtually insoluble in water. Tissue residues of Quinolones are very low and the liver is the main organ which has greatest concentration. They act on the sporozoite stage of the life cycle of coccidian. The sporozoite is evidently able to penetrate the host intestinal cell but its further development is prevented. Thus on day 1st of life cycle, these compounds show maximum activity. So, these drugs must be in feed on day one of exposure to coccidia to give maximum advantages.
Mechanism of action: Anticoccidial activity of these compounds depends on disruption of electron transport in cytochrome system of mitochondria in coccidia while decoquinate inhibits DNA synthesis by inhibiting DNA gyrase and not effective in treatment of clinical coccidiosis. Quinolones is a class of anticoccidials which is not able to give complete control of oocyst production. The compounds of this class are not able to completely eliminate the oocyst which enhances the potential for the development of drug resistant strains of coccidia. Thus their use in chicken as anticoccidials is now limited.
Buquinolate: It has broad spectrum of activity against all chicken coccidia.
It arrests sporozoite development but does not kill these forms. The inhibited
stages may recommence development if it is withdrawn too early. It is given
at the level of 0.00825% in the feed. It favourably increases the feed conversion
rates. This drug has low toxicity and elimination rate is fast from the tissues
following withdrawal of medicated feed13.
Decoquinate: It also has broad spectrum coccidiostat activity and inhibits sporozoite development. It is used at a concentration of 0.003% in the feed. This compound has no premarketing withdrawal requirements.
Robenidine: It is a guanidine derivative and not available in the USA but is used to some extent in Europe and South America. It is a broad spectrum coccidiostatic and coccidiocidal drug, used for the prophylaxis of coccidiosis. It inhibits oxidative phosphorylation in late first generation and second stage schizonts. It may also have an effect on the gametocytes. It is most effective against the maturing first generation schizonts. It is effective as 0.0066% mixture in the feed. It is not used in laying hens and has 5 day withdrawal period for the slaughter of poultry. It imparts unpleasant taste to the flesh of broiler birds if not withdrawn for 5 days before slaughter.
Halofuginone: Halofuginone is a quinazolinone derivative. It is an alkaloid originally isolated from the plant Dichroa febrifuga and tested for antimalarial activity in China many years ago. This drug has potent broad spectrum coccidiocidal and coccidiostatic activity against 1st and 2nd generation schizonts. The mechanism of action against Eimeria spp. is unknown at present. It is used for the prevention of coccidiosis and should only be given to young birds (up to 12 weeks of age for poultry). The drug is effective against pathogenic eimerian species in chicken at a feed concentration of 3 ppm. The drug is not given to the egg laying birds and has 5 days withdrawal period for the slaughter.
It would not have been feasible to develop the modern chicken and turkey
industries without the discovery and use of anticoccidials. These anticoccidials
are extremely effective and enable the animal to achieve optimum performance
by remaining free of the debilitating coccidiosis disease, when used in a structured
and monitored programme.