The Monogeneans are ectoparasites, mostly of fishes; living hung
by their haptor on the skin, fins and gills of their hosts. Nevertheless,
some species live in more hidden habitats such as the rectal cavity, nostrils,
the urethras, stomach and so on. Their development cycle is direct (no
host intermediary) and has a larva with cilia often swimming (oncomiridia),
which is the only free stage (Pariselle and Euzet, 1998). In natural environment,
Monogeneans generally expand in limited number and are in apparent equilibrium
with the host fish. In the conditions of fish culture where the fishes
are more concentrated than in natural environment, we sometimes notice
a large amount of parasites, leading to epizootics (Buchmann and Lindenstrom,
2002), resulting in high mortality of fishes and, therefore, to a loss
of productivity in basins (Aloo, 2002). In addition, severe secondary
infections, favored by lesions caused by the hooks of haptor and the despoiling
action of the parasites, can contribute to this mortality (Mouton et
al., 2001). Therefore, a pre-requisite for the success of fish breeding
is to prevent epidemics and the development of potential infections (Obiekezie
et al., 1992). This requires a good taxonomic and biological knowledge
of pathogen agents.
Monogeneans parasites of continental fishes are mostly known in their
taxonomical aspect. Indeed, only the species has caught the attention
of most of the researchers. They were interested in the taxonomy, the
specificity and biological cycle of Monogeneans parasites (Tombi and Bilong
Bilong, 2004). Quantitative data on population dynamics of Monogeneans
parasites remain scarce, particularly in tropical Africa (Mouton et
al., 2001). So far, only a few study such as Mbahinzireki (1980),
Khidr (1990), Obiekezie et al. (1992), Aloo (2002) and Bilong Bilong
and Tombi (2005) indicate that almost the prevalence of parasites of fishes
of African continental have been identified.
The results presented here are part of a comprehensive study of parasites
of freshwater fishes from Burkina Faso and concern the dynamic of populations
of five gills Monogeneans of Oreochromis niloticus in the dam of
Loumbila (Burkina Faso). Are considered here the rates of infestation
of different stages of maturity of parasites in function of time, sex
and size of the host. The objective of this research is to study the demographic
profiles of Monogeneans parasites of the fish model, in a semi-confined
area close to that of the basins of pisciculture, in order to gather the
first data relating to parasitism of Burkina Faso`s fishes.
MATERIALS AND METHODS
The study site:Loumbila`s dam is located at 20 km from Ouagadougou in the department
that bears the same name and in the province of Oubritenga. Its geographic
coordinates are the following: 12°29`34" north latitude and 01°24`05"
west longitude. This water retention has a capacity of 42 million m3
(ONEA and CASST, 2004). This dam is inhabited by ichthyology fauna that
allow the development of a traditional fishing. Three families of fishes
coexist: Cichlid especially those represented by O. niloticus
and very little by Sarotherodon galilaeus are numerically greater
than that of Claridae with Clarias sp. and that of the Centroponidae
with Lates niloticus.
The host:The host used in this study is Oreochromis niloticus (L.).
It is a fish that has great economic interest in Burkina Faso (Baijot
et al., 1994). Its use is therefore widespread in development projects
of the fish culture in that country. This study was carried from February
2004 in January 2005. During this period, we harvested and autopsied 176
individuals from Oreochromis niloticus (L.), or an average of 15
fish per month.
The sex ratio for this sub-population is 0.83 for males, who were 96
individuals against 80 females. Fish, whose Standard Length (LS) ranged
from 70 to 194 mm, have been placed in 08 classes sizes, which are: Class
A (LS <85 mm), with 9.09% of the specimens; Class B (85 = LS <95
mm), with 5.11% of specimens; Class C (95 = LS <105 mm) with 6.81%,
the D (105 = LS <115 mm) with 15.90% of specimens; class E (115 = LS
<125 mm) with 14.20% of specimens; class F (125 = LS <135 mm) with
31.81% of specimens; class G (135 = LS <145) with 7.38% of specimens
and class H (LS (145 mm) with 9.65% of specimens.
Fishing and fixing of the biological material:The fish examined were caught by fishermen using fishnets. Once
caught, the fish were immediately sacrificed; their gill arches were isolated
from the bucco-pharyngeal cavity by dorsal and ventral sections and then
frozen in liquid nitrogen.
Research, coloration and identification of the parasites:In the
laboratory, after thawing, the parasites were detached from the gills
using strong water current and transferred individually with a needle
directly into a drop of ammonium picrate-glycerine mixture (Malmberg,
1957). The preparation was then covered with a cover slip and sealed with
Glyceel (Gurr, BDH Chemicals).
The identification of species and determining the degree of maturity
of individuals` parasites were made under a microscope. Two stages of
maturity have been identified: adult or mature (testis, ovary and sclerotised
pieces of the observable apparatuses copulator) and the young or immature
(presence of only male genitalia or total absence of genitalia apparatuses).
Epidemiological approach:We have studied the parasitism of this host by Monogeneans in function
of time, size and sex of the hosts. As a result, the abundance or infestation
rate (expressed as a percentage) is the total number of parasites in a
sample of hosts divided by the total number of hosts examined (Margolis
et al., 1982). We used the software STATISTICA version 6 to analyze
the data and the comparison of the rates of infestation was made by the
means of Chi-square tests (χ2). The level of significance
was fixed to 0.05.
Five species of Monogeneans were collected on the gills of the fish:
Cichlidogyrus tilapiae, Cichlidogyrus halli, Cichlidogyrus
thurstonae, Cichlidogyrus rognoni and Scutogyrus longicornis.
Temporal variation of the occurrence of the parasites
Cichlidogyrus thurstonae:C. thurstonae was present in O. niloticus throughout the
study period. The lowest rate of infestation was observed in July 2004
(21.42%). The highest values were observed during the month of June 2004
with a rate of 68.75% and in the month of September 2004 with a rate of
infection of 73% (Table 1). The Chi-square test (χ2)
applied to the temporal variations of the occurrence of C. Thurstonae
shows that the values taken in a comprehensive manner are not statistically
significant at 5% (χ2 = 13.51, df = 11).
Cichlidogyrus halli:This species of Monogenean was present in O. niloticus during
all the study period. Although the values of the rates of infestation
are overall a little low compared to the first parasite described, we
observed that the variation of the infestation rate were serrated. The
lowest rate of infestation was observed in April 2004 with a value of
20%. The highest value in the rate of infestation was observed in September
2004 (60%). The Chi-square test (χ2) applied to the temporal
variations of the occurrence of C. halli shows that these values
||Temporal variation of different species
taken in a comprehensive manner are not statistically significant at
5% (χ2 = 7.19; df = 11), like that was show by the Table
Cichlidogyrus tilapiae:The rate of infestation varies from 26.66% in February with a peak
of 53.33% in September. There was a lack of parasites in July 2004. The
Chi-square test (χ2) applied to the temporal variations
of the occurrence of C. tilapiae shows that these values taken
in a comprehensive manner are not statistically significant at 5% (χ2
= 13.65, df = 11).
Cichlidogyrus rognoni:We observed that the parasite was present in March, April, October
and December 2004 with low values respectively 11.76, 6.66, 12.5, 21.42
and 7.69%. The Chi-square test (χ2) applied to the temporal
variations of the occurrence of C. rognoni shows that these values
taken in a comprehensive manner are not statistically significant at 5%
(χ2 = 16.72, df = 11).
Scutogyrus longicornis:We note that this species of Monogeneans was present in O. niloticus
throughout the study period. Although the values of infestation rates
are low in all, we observed a variation in the rates of infestation was
serrated. The lowest rate of infestation was observed in May 2004 with
a value of 14.28% (Table 1). The highest rate of infestation
was observed during the month of September 2004 (53.33%) and in December
2004 (53.84%). The Chi-square test (χ2) applied to the
temporal variations of the occurrence of S. longicornis shows that
these values taken in a comprehensive manner are not statistically significant
at 5% (χ2 = 14.59, df = 11).
Occurrence of the parasites according to the sex of the host
Cichlidogyrus thurstonae:The rate of infestation of O. niloticus by C. thurstonae
was 48.95% for male fish and 56.25% for female fish. These values were
not statistically significant at 5% (χ2 = 0.92, df = 1).
Cichlidogyrus halli:The rate of infestation of O. niloticus by C. halli
was 33.33% for male fish and 50% for female fish. We note that the females
were much more parasitized than males. However, the Chi-square test (χ2)
applied at the occurrence of Cichlidogyrus halli depending on the
sex of its host, shows that these values are statistically significant
(χ2 = 5.01, df = 1).
Cichlidogyrus tilapiae:The rate of infestation of O. niloticus by C. Tilapiae
was 21.87% (21 male fish parasites on the 96 examined) and 22.50%
(18 female fish parasitized on the 80 examined). These values were not
statistically significant at 5% (χ2 = 0.01, df = 1).
Cichlidogyrus rognoni:The rate of infestation of O. niloticus by C. tilapiae
was 4.16% for male fish (4 fish parasites males on the 96 examined) and
7.50% for female fish (6 female fish parasitized on the 80 examined).
These values were not statistically significant at 5% (χ2
= 0.90, df = 1).
Scutogyrus longicornis:The rate of infestation of O. niloticus by S. longicornis
was 31.25% for male fish and 38.75% for female fish. These values were
not statistically significant at 5% (χ2 = 1.08, df = 1).
Occurrence of the parasites according to the size of the host
Cichlidogyrus thurstonae:The lowest rate of infection was observed in fish belonging to the size
class H (41.17%). The highest value was observed in the fish of the Class
B (66.66%). The Chi-square test (χ2) applied at the occurrence
of C. thurstonae depending on the size of O. niloticus shows
that these values are not statistically significant at 5% (χ2
= 2.20; df = 7). So, C. thurstonae may parasitize O. niloticus
at any age (Table 2).
Cichlidogyrus halli:The highest rate of infestation was encountered in classes A, E,
F and G respectively with 56.25, 48, 42.85 and 53.84%. The Chi-square
test (χ2) applied at the occurrence of C. halli
depending on the size of O. niloticus shows that these values
are not statistically significant at 5% (χ2 = 4.93, df
= 7). So, C. halli may parasitize O. niloticus at any age.
Cichlidogyrus tilapiae:All classes of fish harvested and studied had hosted C. tilapiae.
Classes C, D and H present rate of infestation less important than the
other classes, respectively with 33.33, 28.57 and 29.41.
|| Occurrence of species depending to the size of the
The Chi-square test (χ2) applied at the occurrence of
C. tilapiae depending on the size of O. niloticus shows
that these values are not statistically significant at 5% (χ2
= 2.81, df = 7). So, C. tilapiae may parasitize O. niloticus
at any age.
Cichlidogyrus rognoni:We observed that the fish of size A, B and C had not been infected
by C. rognoni. Fish belonging to other classes are parasitized
all except the size class G. The small sample size may explain the absence
of parasites here. However, the Chi-square test (χ2) applied
at the occurrence of C. rognoni depending on the size of O.
niloticus shows that these values are not statistically significant
at 5% (χ2 = 5.28, df = 7). So, C. rognoni may parasitize
O. niloticus at any age.
Scutogyrus longicornis:We note that all classes of O. niloticus collected and studied
had harbored S. longicornis. The lowest rate of infection
is observed in fish belonging to the size class C (16.66%). The highest
value was observed in the fish of the Class B (55.55%). The Chi-square
test (χ2) applied at the occurrence of S. longicornis
depending on the size of O. niloticus shows that these values are
not statistically significant at 5% (χ2 = 7.45, df = 7).
So, Scutogyrus longicornis may parasitize O. niloticus at
any age (Table 2).
During present study on Monogeneans gill parasites of Oreochromis
niloticus (L.), we found five species belonging to two genera: the
genus Cichlidogyrus with four species and the genus Scutogyrus
with one specie.
The exploitation of a host fish by several genera of Monogeneans has
been already reported by several authors such as Birgi (1988b) with the
species of Clarias jaensis and Clarias pachynema parasitized
by the genera Quadriacanthus and Claridectes, Obiekezie
et al. (1992), which reported the presence in Clarias gariepinus,
the genus Gyrodactylus and several specie of Ancylodiscoides; Koskivaara
and Valtonen (1992), who found genera Datylogyrus, Paradiplozoon
and Gyrodactylus in Rutilus rutilus; Bilong Bilong (1995)
shows that Hemichromis fasciatus was parasited by the genera
Cichlidogyrus and Onchobdella; Douellou and Chishawa (1995)
who collected the species of genus Quadriacanthus and Macrogyrodactylus
in Clarias gariepinus and Tombi and Bilong Bilong (2004) who made
an inventory of the genera Dactylogyrus, Dogelius, a larva
of Gyrodactylus and of Polystomatidae in Barbus martorelli.
Moreover, the exploitation of hosts by several congeneric species was
also reported by Buckmann and Lindenstrom (2002), Tombi (2005) and Simkova
et al. (2006). As thought by the authors mentioned above, our observations
can be explained by the fact that in natural environment, the parasitic
densities are generally weak and therefore, the niches are always available
on the gill biotope (Gutiérrez and Martorelli, 1999; Buckmann and
Lindenstrom, 2002; Simkova et al., 2006), facilitating the simultaneous
colonization of the same host by several species of Monogeneans. In Oreochromis
niloticus (L.), we have not observed any significant difference in
the rate of the parasitism according to the classes of size. Our results
are similar to those of Koskivaara and Valtonen (1992) who did not observe
any significant difference of P. Homoion between different classes
of sizes R. rutilus or of Pseudodactylogyrus bini and P.
anguillae in Anguilla anguilla. However, our observations on
the parasitism following the different classes are contrary to those of
Bakke et al. (2002) who observed a negative correlation between
the growth of the population of Gyrodactylus salaris and age of
salmonidae. They explain this by the immune answer that increase with
the age of the fish. Present results are also contrary to those of Morand
et al. (2002) and Tombi and Bilong Bilong (2004), who found out
a positive correlation between the parasitic abundances or intensities
with the size of the fish. These authors explained their observation by
the fact that as soon as the fish grows up the gill surface enlarges.
Gutiérrez and Martorelli (1999) and Simkova et al. (2006)
also think that since in big fish the volume of water passing through
the gills are higher; the number of oncomiridia might also be high. We
think that considering the mode of infestation of Monopisthocotylea the
volume of water passing through the gills should not influence the prevalence
or intensity parasitic on those biotopes: Indeed, the infesting larva
here settle first on the skin before scrawling and settling on the gills
(Combes and Jourdane, 2003).
On one side, we have not observed any significant difference between
the rates of parasitic infestation in virtually all of the males and females
of Oreochromis niloticus, what confirms the observations of Silan
et al. (1996) and Tombi and Bilong Bilong (2004). On the other
side, following the occurrence of C. halli according to males and
females of O. niloticus (L.), the statistics data give a significant
difference. The females fish are more attacked by the parasites than males.
Present results are similar to those of Tombi (2005), who found that the
species D. bopeleti and D. insolitus are more frequently
isolated in the female Barbus. These results are also similar to
those of Silan et al. (1996), which explained that the females
more-sedentary during the period of reproduction are more infested by
the Diplectanidae than the males of Dicentracnus labrax.
Concerning the temporal variation of the occurrence of parasites, all
species of Monogeneans identified have been present the whole year in
the host fish (O. niloticus), except C. rognoni which was
absent during in July, August and September (Table 1).
Thus, the host fish is vulnerable at any period. Under the tropics, analogous
remarks have been reported. Indeed, in Cameroon, Bilong Bilong and Njiné
(1998) considered eutrophic basin in urban areas and showed that in this
lentic environment the Monogeneans gill parasites of the fish Hemichromis
fasciatus appear the whole year. Later, Bilong Bilong and Tombi (2005)
studied the temporal structure of six species gill parasites of Barbus
martorelli of the Mfoulou stream (under-affluent of the Sanaga river)
in the surrounding areas of Yaounde (Cameroon). This study, conducted
in oligotrophic water showed that, except for the species of Monogeneans
Dactylogyrus maillardi absent during the month of July, all the
species of xeno community are present on that Cyprinid during the whole
year. The same, in Nigeria, Obiekezie et al. (1992) observe that
Macrogyrodactylus clarii appears in a river the whole year. In
Zambia, an artificial lake the Monogeneans of the genus Cichlidogyrus
in three species of Cichlid are met equally the whole year (Batra, 1984).
Bilong Bilong and Tombi (2005) sums up the importance of fish parasites
as follows: they may reduce population numbers by causing mortalities
and sometimes mass mortalities, they may affect the reproductive organs
and reduce the number of offsprings and thus the population size and they
may reduce the weight of fish. Combes and Jourdane (2003) also stressed
that the pathogenic effect is rarely caused by one parasite species. It
was hereby shown that O. niloticus Monogenean community
was made up of cinq species of Dactylogyridae; taking into account all
parasite species, infracommunity sizes could be considerable and the sum
of pathogenic effects so important that it could cause cases of morbidity
or mortality even in a natural environment.
In the prospected site, the addition of the pathogenic effects of the
five parasitic species can become important and provoke cases of morbidity
(or even of mortality), particularly for fish accommodating greater infra
communities. Such mortality can be reinforced by secondary infections
due to lesions caused by the hooks of parasites and the spoliatory action
of those organisms. It can seem insignificant in a pond of the size of
the one prospected, but would certainly expand in ponds in small-scale
diversion (250-40 m2, etc.) where the density of fish increase
fast enough and where the transmission rate of parasites will therefore
be increase. In such a situation and for same species of host, one can
fear serious general pathology strongly influenced by the prevalence,
abundance and distribution (aggregated or not) of the parasites (Paperna
and Smirnova, 1997).
At the dam of Loumbila, the fluctuations of the parasitic abundance
are neither seasonal nor cyclic. Oreochromis niloticus is infested
the entire year; indeed, the recruitment of those organisms, though relatively
weak, is continuous. The result is logically their accumulation in that
The struggle against direct cycle parasites like the Monogeneans can
be done either by the treatment of hosts or by the breaking off of the
development cycle in eliminating the infesting larva from the living environment
of the fish. To reduce the parasitic pressure on the host and to prevent
the potential damage of those Monogeneans, one can disinfect the genitors
before the putting them in the basins. However, if the sowing is done
with young fish, which can be less resistant to the effect of pesticides,
one can wait until the emptying of water. One will then treat the re-population
stock in the basins. After the emptying during which one can spill pesticides
into the basins, a lack of parasites should be observed. Even the use
of any pesticide require the opinion of a specialist who must respect
the local policy (regulating the use of pesticides) and take into account:
(a) problems of preserving the environment (b) cases of resistance of
the parasites to some molecules as those reported in Nigeria (Opara, 2002).
The choice of any substance must therefore be preceded by a test.