Background

Urbanization and demographic growth increasingly observed in developing countries, especially in Cameroon, have contributed to an increasing and urgent demand in proteins of animal origin [1]. In Cameroon, bivalve mollusks (i.e., clams) are exploited by a restricted local community fishing groups in the District of Mouanko (Littoral region) along the Sanaga Delta for their flesh and shell, with an estimated annual production of more than 800 000 tons of clams [2]. Many Cameroonian authors expressed their fears regarding declining production of these palatable mollusk species in actual this context of increased demand of animal proteins. Clams are greatly appreciated by Cameroonian populations for the quality of their white flesh and secondary services produced by their shells, notably as a source of calcium in poultry feed and in lime manufacturing [2,3]. The exploitation of clams is an income-making activity as it generates enormous profits for local communities through their employment along the Cross-River waters in Nigeria, Volta in Ghana, and Sanaga in Cameroon, and positively impacts the economy of these three counties [4,5]. However, a set of biotic and abiotic factors including parasitism have been contributing to reduce drastically clam populations, and therefore a decline in food production and financial resources over the course of the fishing season [2]. Parasitism is important in the process of regulating aquatic ecosystem invertebrate populations [6-8]. Unfortunately, parasitism has received little attention despite its impact in clam mortality and economic losses in the exploitation of edible species such as bivalves [8,9]. Egeria radiata (Lamarck, 1804) is one of the main bivalves exploited in Cameroon by local population. Previous studies in Ghana and Nigeria outlined that this clam species was highly vulnerable to contamination by diverse pathogens including coliform bacteria [5], and parasites (trematodes, cestodes, nematodes and protozoans) [10]. There is a lack of parasitological studies on E. radiata in Cameroon, especially in the Lower Sanaga River. The implementation of such study is a crucial prerequisite in an attempt to domesticate successfully this clam species in the country. We therefore conducted a study to determine the diversity and intensity of parasite fauna, and its impact on characteristics of E. radiata populations from the Lower Sanaga River.

Methods

Study area

The study took place from December 2017 to July 2018 in two sites of Mouanko area, namely Bolounga and Maldjedou. Mouanko is a district of the Division of Sanaga Maritime, Littoral region, Cameroon, located on the banks of the Sanaga River (Figure 1) [2]. Its average geographical coordinates range from 3-38' north latitude, to 9-47' east longitude. The district of Mouanko has about 17,000 inhabitants. The majority of local populations live mainly from fishing and their activities are restricted to the vicinity of streams and alluvial forests. River fishing based mainly on Cichlidae, catfish and clams.

Figure 1: Map showing the Egeria radiata clam sampling location between sites in lower Sanaga River

Study design

The E. radiata clams were collected at the study sites of interest with help of local fishermen. Anglers informed the objectives of study, and only those who gave their approval were included in the study for captures. The measurements (length, height, width and bulge) on each clam individual were made on spot. Thereafter, the valves of each clam were opened, and different body parts were collected for identification of parasite diversity and intensity (Figure 2).

Figure 2: Flow diagram of the parasitological study on E. radiata in the Sanaga Rivers

Collection and measurements of E. radiata populations

The clams were collected with the help of local fishermen along the study sites of the Sanaga river, and morphometric parameters (i.e., length, height, and bulge) were made on spot using a sliding foot to the nearest millimeter. The clams were then placed in individual plastic bags, brought to the Animal Biology Laboratory, Faculty of Sciences, The University of Douala, for further identification of parasitic organisms (Figure 3).

Figure 3: Organ harvesting of Egeria radiata and parasitological screening

Identification of the parasites

The identification of different parasite classes was made using published taxonomic identification keys [11-14]. Different body parts (gills, foot, siphons, visceral mass and mantle) and soft body part of each E. radiata individual were analyzed to identify different development stages of main parasite classes (trematodes, cestodes, nematodes and protozoans). Briefly, metazoans were identified by microscopic observation of the crushed sections of the above mentioned body parts. Biopsies of the E. radiata’s soft body were made for the microscopic identification of endoparasites.

Condition, diversity and ecological indexes

The analysis of parasite composition was made using several condition, parasitological and ecological indexes [15, 16]. Parasite communities were studied at the infrapopulation level (i.e., all members of a parasite species found in/on the host), and the different indexes were determined as follows [17-19] :

1. Elongation index (Ei) is the ratio between height and length of the clam. In practice, Ei is low when environmental conditions are favorable while high Ei values reflect unfavorable environmental conditions ;
2. Condition index (K) is calculated using the following formula K = 10⁴× (W/L³) where W and L are weight and length of clam, respectively ;
3. Parasite prevalence (P) was defined as the ratio between the number of parasitized host individuals and the total number of host individuals;
4. Parasite abundance (AM) was computed by dividing the number of parasite individuals found to the number of clams examined;
5. Mean intensity of the parasitofauna (IM) was defined as the total number of a given parasite species divided by the total number of clams infected with this parasite species. IM was categorized as very low (<10), low (10 ≤ IM ≤ 50); moderate (50 < IM ≤ 100), and high (IM >100) as described.
6. Parasite specific richness (S) is the number of species found in each clam examined.

Statistical analysis

The data were keyed into an Excel spreadsheet, and then exported to the statistical package for social sciences v17 (SPSS, IBM, Inc., Chicago, USA) and GraphPad v5.03 (GraphPad PRISM, Inc., San Diego, USA) for statistical analysis. Qualitative variables were presented as percentages while quantitative variables were presented as mean ± standard deviation (SD). Non-parametric tests of Kruskal-Wallis and Mann-Whitney, and parametric unpaired sample Student tests were used to compare mean values. Pearson’s independence chi-square and Fisher’s exact tests were used to compare percentage values. Statistical significance was set at p-value less than 0.05.

Results

Characteristics of the clams collected in the study sites

A total of 110 clams were collected during the study, 50 in Bolounga and 60 in Maldjedou. The comparative analysis of morphometric characteristics of clams between the study sites revealed that: mean values of total shell weight, length and height of clams collected in Maldjedou were significantly higher than those of their counterparts collected in Bolounga. Thus respectively 30.0 ± 17.1 vs 38.9 ± 22.9, p = 0.02 (total shell weight, g), 4.5 ± 1.2 vs 5.2 ± 1.4, p = 0.005 (length, cm), and 3.5 ± 0.8vs 4.1 ± 0.8, p = 0.001 (height, cm) (Table 1).

Data are presented as mean ± SD

Unpaired Student test was used to compare mean values

*Statistically significant at p< 0.05

Table 1: Morphometric characteristics of E. radiata clams by study sites

Prevalence of infection

Of the total number of clams collected, 82 carried at least one parasite species, giving an overall infection prevalence of 74.5 % (95% CI 65.7-81.8 %). The infection prevalence was 56% (95% CI 42.3-68.4 %) in Bolounga and 90% (95% CI 79.9-95.3 %) in Maldjedou, and the difference was statistically significant (p = 0.0001).

Prevalence of the different parasites classes by study sites

The analysis of clams revealed different parasite groups including protozoans, cestodes, trematodes and nematodes. Protozoans were found only in Bolounga (10 %). The prevalence of cestodes, trematodes and nematodes was significantly higher in Maldjedou compared to Bolounga (e.g., 12.0 % vs 31.7 %, p = 0.01 for nematodes). It should be noted that nematodes were the main parasite classes found in both the study sites (Figure 4).

Data are presented as percentages

Pearson’s independence chi-square test was used to compare percentages

*Statistically significant at p< 0.05

Figure 4: Prevalence of different parasite classes with respect to the study site

Elongation and condition indexes

The elongation and condition indexes were higher in Bolounga compared to Maldjedou, but statistical difference was found only for condition index (p = 0.0067) (Figure 5).

Bars represent as mean ± SD

Unpaired Student test was used to compare mean values

*Statistically significant at p< 0.05

Figure 5: Variation elongation (a) and condition (b) indexes with respect to study area

Parasitological and ecological indexes

The parasite abundance was significantly lower in Bolounga as compared to Maldjedou (p = 0.0001) as depicted in Table 2. The same pattern was observed for specific richness, where a maximum of three parasite species was found in Bolounga. Parasite intensity rates were similar between the two sites (p = 0.841).

Data are presented as percentages, and mean ± SD

Pearson’s independence chi-square and unpaired Student’s tests were used to make comparisons

*Statistically significant at p< 0.05

Table 2: Comparative analysis of indexes between the study sites

Impact of parasitism on the characteristics of clams

No statistically significant difference was found for morphometric parameters between clams infected and uninfected with protozoans and trematodes. In contrast, the bulge of clams was significantly higher in those infected with cestodes and nematodes compared to their uninfected counterparts (Table 3a). All morphometric parameters, with the exception of bulge, were significantly lower in clams infected with parasites of not specified class (Table 3b). We compared the morphometric characteristics of the E. radiata populations with respect to the specific richness (Table 4). Only width was significantly lower in clams infected with two parasite species compared to clams infected with one parasite species.

Table 3a: Variation of morphometric characteristics with respect to infection by different parasite groups

Data are presented as mean ± SD

Mann-Whitney test was used to compare mean values

*Statistically significant at p< 0.05
Table 3b: Variation of morphometric characteristics with respect to infection by different parasite groups (continued and end)

Data are presented as mean ± SD

Kruskal-Wallis test was used to compare mean values

*Statistically significant at p< 0.05

Table 4: Impact of parasite species richness on E. radiata morphometric characteristics

Discussion

Clams are an important source of economic incomes and animal protein source for population of sub-Saharan Africa countries, especially in Cameroon. Unfortunately, parasitism is a serious threat to the economy and health risk to these populations. The present study aimed at determine parasite diversity and intensity as well as the impact of this parasitism on E. radiata populations in the Lower Sanaga river, Cameroon.

Parasite groups found in E. radiata populations

All main groups of parasites (i.e., protozoans, trematodes, cestodes and nematodes) were found in this study, and this is consistent with works of Adeyemo et al.[10] who reported the same groups of parasites in the Nun River in Nigeria. The natural capacity of this bivalve species to burrow and filter water expose it to infection with these above mentioned groups of parasites which also live in the aquatic environment of E. radiata clam. In general, clams are victims of a large number of parasites that cause a mass mortality and significant economic losses [7-9]. Marine bivalves are often served as a host of metazoan parasites or symbiosis such as trematode, cestode, nematode and copepode. Such metazoans have been identified from various marine bivalves including oysters, mussels, cockles and clams [20, 21].Several authors have reported on the occurrence of trematode and cestode in clams from Jeju in South Korea, and Ariake Bay in Japan [22,23]. 

The parasitofauna is mainly dominated by nematodes while protozoans were found with the lowest prevalence. This pattern is in line with that of Adeyemo et al.[10]. In addition, a significant difference was found in prevalence of the groups of parasites between the two study sites. This discrepancy is likely the fact of a geographic area-specific distribution of the groups of parasites. The environment conditions are well known shape the distribution of numerous living organisms including parasites. High number of trematodes may affect reproductive processes and possibly retarding gonadal development [24]. The authors found that mite and trematode abundance were greater in smaller freshwater mussels [25].

Values of parasitological clues among E. radiata clams

Depending on the area, there was a significantly higher average specific wealth in Maldjedou than in Bolounga. That parasite indexes were higher for larger clams and could be explained by higher filtration rates. Furthermore, this could also be explained by longer exposure to infective particles [26]. In the present study, although clam shell length was 5.2 ± 1.4 cm on average at Maldjedou compare to 4.5 ± 1.2 cm at Bolounga. However, the maximum value of 3 was obtained for the entire population, as well as in Bolounga. This could be explained by the fact that in Bolounga the parasitic species here would have been more diverse than in Maldjedou, and thus being a filterer more bioaccumulated of parasitic species. This result is lower than that of Adeyemo et al. [10] having found a value of 4 in the same host Galatea paradoxa (Born, 1778). The specific richness in relation to the morphometric parameters depending on the zones, revealed that the Bulging of clams in Bolounga would be the only parameter related to the specific parasitic richness. This could be explained by the fact that according to authors [10], molluscs are bioaccumulators that concentrate many parasitic species present in the water all around it. Acquisition of parasite infective stages can be increased for larger bivalves with high filtration capacities.

Parasite intensity and abundance

The overall prevalence of infection was 74.5%. This value is higher than the 50% threshold used to identify the presence of dominant species according to authors [17]. Similar prevalence values were reported in Nigeria [10]. This high value of infection burden could be due to anthropic activity as riverside residents used to dump wastes of varied nature, and parasitized feces, and thus contribute to the contamination of the aquatic milieu of E. radiata. As above stated, clams are important filterer organisms, and as a consequence are exposed to infection when filtering parasitized water. Parasite indexes were below 10 in the study sites, thereby implying that majority of parasite species have a very low mean intensity of infection [19]. A similar result was reported in Nigeria, where the authors found abundance values of 1 and 1.09 in their study sites [10]. Also, parasite prevalence and infection intensities in Bolounga were lower than those observed in Maldjedou, suggesting that overall health condition of Bolounga E. radiata clams was graded be good. 

Existing relationship between some biotic parameters of bivalve and E. radiata parasitofauna

Regarding the influence of parasitofauna on metric parameters, only the Bulge was influenced for clams parasitized at Cestodes and parasitized to nematodes. This can be explained by the fact that helminths in general and especially these two classes contain endoparasites and mesoparasites that would have a direct influence on the weight of the soft body of the bivalve. Therefore, an effect of parasitism on the metric parameters of the clam [27] to the example of the E. radiata Bulge. Parasite prevalence was higher in adults than in juveniles and could be explained by higher filtration rates and also by longer exposure to infective particles [26]. Condition index in the Bolounga area show that despite low infection intensity an overall infection, Bolounga have better condition with E. radiata bivalves. The low infection intensity in the two study areas could be associated with a low density of clam. Intensity index is positively correlated with the density of host organisms and environmental factors such substrate of area as bed [9,28].

Conclusion

The present study outlines that main parasite groups are found in E. radiata clams, with a predominance of Nematodes in the two study sites. The prevalence and diversity of parasitism was higher in the Maldjedou site, though parasite abundance and intensity were lower than those found in the Bolounga site. Bulge measurements were significantly associated with parasitism with cestodes and nematodes. The domestication of these bivalves should take into account these results of the influence of parasitological diversity.

Declarations

Ethics approval and consent to participate

Not applicable Ethics approval and consent to participate. The data and other materials that support the findings of this study are available from the corresponding author upon reasonable request.

Consent for publication

Authors give consent for publication after each contribution. All authors read and approved the final manuscript freely.

Availability of data and material

The availability of data and material remains guaranteed by the corresponding author for any future solicitation.

Competing interests

Authors have not declared any conflict of interests during each step of reading and may have influenced the study.

Funding

We extend our sincere gratitude to the Wildlife Conservation Society (CWCS) and staff in Mouanko based office that facilitated logistics and the measurements of the various parameters on the field.

We also thank Department of Animals Biology, University of Douala, Cameroon and all our free key informant and guides.

The population and fishermen gave us their freely assistance and warmth hospitality during the study especially his Bilvalve Collection Group.

Authors’ contributions

AD designed the study, conducted the experiments, and wrote the manuscript. GA conducted the experiments, did statistical analyses. KFLP helped in write-up, statistical analysis and critically revised the manuscript for intellectual content. MTE designed the study. All authors read and approved the final manuscript.

Acknowledgement

We extend our sincere gratitude to the Wildlife Conservation Society (CWCS) and staff in Mouanko based office. We also thank the population of Mouanko and the Fishermen during the study especially Mr Samuel Engolo and his Bilvalve Collection Group.

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