Distribution and seasonal abundance of Biomphalaria snails and their infection status with Schistosoma mansoni in and around Lake Tana, northwest Ethiopia

Abstract

Biomphalaria snails, namely B. pfeifferi and B. sudanica, are the principal intermediate hosts for Schistosoma mansoni infection in Ethiopia. Epidemiological studies of Biomphalaria snails and their infection status with S. mansoni is vital for public health planning. This study aimed to assess the spatial and seasonal abundance of Biomphalaria snails as well as their infection status with S. mansoni around Lake Tana, northwest Ethiopia. Malacological survey was conducted from January 2021 to December 2021 in ten different collection sites in and around Lake Tana. Snail collection was performed for 20 min from each collection site seasonally (four times in a year) using a standard scoop and handpicking from aquatic vegetation. All collected snails were carefully examined based on their morphological features and all live Biomphalaria snails were subjected to cercariae shedding experiment. Descriptive statistics were used to determine the prevalence of S. mansoni infection and its relationship with snail collection sites and seasons. A total of 3886 freshwater snails were collected from ten collection sites around Lake Tana. Out of the total snails collected, 1606 (41.3%; 95% CI 39.77-42.89%) were Biomphalaria spp. The highest (374) and the lowest numbers (98) of Biomphalaria snails were collected from Shinne River and Qunzela Lakeshore, respectively. Out of the 1375 live Biomphalaria snails, 14.4% (95% CI 12.59-16.37%) snails shed cercariae, but only 4.87% (95% CI 3.79-6.15%) were cercariae of S. mansoni. The infection prevalence of S. mansoni ranged from 10.59% at the Cherechera site to 1.49% at Gumara River. Biomphalaria snail infections with S. mansoni cercariae were observed throughout the season, the highest and the lowest infection rates being in the spring and summer seasons. Significant differences in the prevalence of S. mansoni infection in Biomphalaria snails were observed across study sites and seasons (p < 0.05). Biomphalaria snails were the most abundant freshwater snails found in nearly all of snail collection sites throughout the year. It was revealed that nearly five percent of Biomphalaria snails were infected with S. mansoni cercariae. This study highlights the importance of appropriate snail control strategies to support the ongoing prevention and control of schistosomiasis around Lake Tana.

Figure 1

Map of the study areas around Lake Tana. The map was prepared using ArcGIS online software.

Figure 2

Image taken from some of the snail collection sites around Lake Tana; (a) Enferanz river (b) Cherechera site (c) Garno river (d) Qunzela port (e) Gorgora port (f) Shinne river (g) Dek Island (h) Robit river. All the images were taken by the corresponding author (TH).

Figure 3

Freshwater snail genera collected from Lake Tana; (a) Biomphalaria snails (right and left side view), (b) Bulinus snails (apertural and abapertural view), (c) Lymnaea snails (ventral and dorsal view), (d) Bellamya snails (ventral and dorsal view) and (e) Melanoides snails (ventral and dorsal view). All the images were taken by the corresponding author (TH).

Figure 4

Relative abundance of Biomphalaria snails at different collection sites.

Figure 5

Relative abundance of Biomphalaria snails on a seasonal basis.

Figure 6

Relative abundance of Biomphalaria snails at study habitat.

Figure 7

Trematode cercaria shed by Biomphalaria snails 100 × magnification; (a) Echinostome cercaria, (b) Amphistome cercaria, (c) Brevifurcate-apharyngeate diastome cercaria, (d, e) Schistosome cercaria, (f) Unidentified cercaria. Echinostome and Schistosome cercaria observed from all study sites while Brevifurcate diastome cercariae were observed in Cherechera, Enferanze, Robit and Dek. Unidentified cercaria was observed from Dek Island.

Figure 8

Seasonal variations of S. mansoni and other trematodes infection.