Occurrence of helminth parasites and their developmental stages in snails from different regions of Bangladesh

Abstract

Helminth infection study of freshwater molluscs in and around Dhaka, Khulna and Kishoreganj region was conducted at twenty three locations which include eleven from Dhaka region, six from Khulna region and another six from Kishoreganj region from different ponds during July 2011 to June 2013 on a once in a month basis. A total of three species of freshwater molluscs belonging to two families that comprises of Viviparidae, Ampullariidae and three genera (Bellamya, Pila, Brotia) were found in the pond eco-systems. Physico-chemical parameters like pH, temperature, dissolved oxygen, electrical conductivity and its variation at all the collection sites were analyzed. As the molluscs are intermediatory hosts between the parasites and animals, an overview study was also carried out on the known molluscs’ various parasites and their diseases. Bangladesh has been virtually unexplored with respect to its trematode fauna, the sampled freshwater snails from temporary and permanent ponds were collected. Helminth infections of 3 snails species, viz., Bellamya bengalensis, Brotia costula and Pila globosa. Samplings were done primarily on monthly basis. 31,717 snails (B. bengalensis, 10,915; B. costula, 9,961; P. globosa, 10,841) were autopsied, of which 4,099 were infected by helminthes (12.92%). The infection was eight types of cercariae (belonging seventeen species), one nematode adult morpho-type, two Cestodes, one Pentastomida and one Turbellaria. Among the hosts, prevalence of trematode infection was highest in B.bengalensis (14.7%), followed by P. globosa (12.95%) and B. costula (10.95%). Contrary to the general knowledge, infection of snails by nematode parasites, though rather poor in diversity, was higher (1.24-4.55%) than that by the DST (12.92%). Seasonally, DST was more prevalent in the rainy season, whereas nematodes were during the summer. With respect to parasites harboured, Pila globosa was the most diversified refusing all the 3 infector groups. Altogether screened 31,717 individuals of three freshwater snail species (Bellamya bengalensis, Brotia costula and Pila globosa) for shedding cercariae. Collectively, eight morphologically distinguishable types of trematode cercariae were found: ii Parapleurophocercous cercariae; Pleurophocercous cercariae; Xiphidiocercariae cercariae; Furcocercous cercariae; Echinostome cercariae; Amphistome cercariae; Renicolid cercariae and Cotylomicrocercous cercariae. Out of 23 locations the parasitics collected from Pila globosa other than trematode and nematode were two Cestodes, one Turballaria and one Pentastomida. Overall prevalence was found in August to September (1.03-2.26%) in 2011-2013. In the present study monthly fluctuations in the cercarial prevalence were observed due to the variations in the temperature, rainfall and dissolve oxygen of each month. The occurrences of cercariae are maximum in August to September (18.18% - 28.57%) and minimum if February (3.45-8.33%) which is dealt with elevated temperature, rainfall and DO level in the Dhaka region. Whereas, In Khulna and Kishoreganj region the maximum prevalence was in August to Semtember (14.29% - 37.5%) and no infection was found in some months. It is made clear from the results that more number of species related with low DO content. It can be inferred from the above points that DO content plays a vital role in affecting the population dynamics of a particular species than the other physico-chemical factors like pH, temperature and EC. Moreover the molluscan population is abundant in a place where the aquatic plants are available in diverse forms. Since there is not much variation in pH, temperature, EC and salinity in the collection sites, they do not have much influence in the availability of specimens in this present study. Through this analysis, it is inferred that the possibility of variation in pH, temperature, EC can play a vital role in the life cycle of any plant or animal group and when it is moderate or in low, it cannot have much influence on the population dynamics. In 2011-12, occurrence of trematode parasite in Bellamya was significantly higher than that of Brotia and Pila (p<0.05). Moreover, in 2012-13 occurrence of trematode parasite in Bellamya was significantly higher than that of Brotia and Pila. On the other hand, occurrence of this parasite in Bellamya found in 2012-13 was significantly higher than that of 2011-12. No significant different was found in Brotia and Pila in this two years (p>0.05). The prevalence of trematode parasites in Bellamya, Brotia and Pila in 2011-2012 and 2012-2013 sampled from 23 different regions were determined. Each year was divided into three different seasons (Rainy, winter and summer) where the incidence of the parasites in rainy season in most of the cases was significantly higher than that of other seasons. However, the prevalence in summer (in most of the regions) was significantly higher than that of winter. iii The overall natural environmental parameters varied within overlapping ranges among the ponds. Water temperature ranged between 4.7°C and 38°C depending on the day and time of collection. The hydrogen ion concentration was within the range providing protection for the aquatic life (7.2 to 9.8). The concentration of dissolved oxygen showed generally low in some locations due to pollution load (Dhaka region) and higher (> 7.0 mg/l) levels and exhibited a strong negative association with water conductivity (rs = -0.665, p < 0.05). The data identified the correlation between temperature, pH, EC, DO, rainfall and prevalence of parasites in snails of three different regions in 2011-12 and 2012-13 academic years. Sometimes positive correlation with significant level (p>0.05) and without significant level were observed. Significantly positive correlation was found between temperature and prevalence of parasites in snails of Dhaka regions in 2011-12 (p<0.01).Positive correlation was also found between % occurrence and pH, EC, DO and rainfall, but was not significant (p>0.05). Positive correlation was found between temperature and prevalence of parasites and rainfall and prevalence of parasites in snails of Dhaka regions in 2012-13 (p<0.01).Positive correlation was also found between % occurrence and pH, EC, and DO, but was not significant (p>0.05). Also the significant positive correlation was found between temperature, EC, rainfall and prevalence of parasites in snails of Khulna regions in 2011-12 (p<0.01).Positive correlation was also found between % occurrence and pH, but was not significant (p>0.05). Also the significant positive correlation was found between temperature, EC, rainfall and prevalence of parasites in snails of Khulna regions in 2012-13 (p<0.01).Positive correlation was also found between % occurrence and pH, but was not significant (p>0.05). Positive correlation was found between temperature, EC (p<0.05), rainfall and prevalence of parasitesin snails of Kishoreganj regions in 2012-13 (p<0.01).Positive correlation was also found between % occurrence and pH, but was not significant (p>0.05). Also the positive correlation was found between temperature, EC (p<0.05), rainfall and prevalence of parasites in snails of Kishoreganj regions in 2012-13 (p<0.01).Positive correlation was also found between % occurrence and pH, but was not significant (p>0.05).