Types of trematodes infecting freshwater snails found in irrigation canals in the East Nile locality, Khartoum, Sudan

Abstract

Background: The planorbid freshwater snails of the two genera, Biomphalaria and Bulinus -have been vigorously studied due to the role they play as intermediate hosts of schistosomiasis. In Sudan specifically, most studies have focused on the chemical and ecological control of the two genera, but few studies have looked at their biological control. This study explored the coexistence of other species of freshwater snails and the two genera along with their trematode infections in relation to a number of environmental factors in the East Nile locality, Khartoum state, Sudan.

Methods: Freshwater snails from irrigation canals (abueshreens) were sampled monthly from January 2004 to December 2005. The snails were examined for trematode infections by cercarial emergence immediately after collection and then weekly for an additional four weeks to allow for the maturation of prepatent infections. Vegetation cover in the study sites as well as the physicochemical characteristics of the water, including temperature, were also recorded.

Results: A total of 10,493 snails, representing seven species, were collected. The most abundant species was Biomphalaria pfeifferi, representing 48.6 % of the sample. Overall, 14.1 % of the snails were found to be shedding some type of cercariae. Five species were found to have infections; among these the Bulinus truncatus species was found to be the most heavily infected, with an overall prevalence of 46.2 %. Double infections were recorded in only two B. truncatus snails and one Cleopatra bulimoides snail. Twenty different morphotypes of cercariae were recorded, seven of which appeared not to conform to previously described cercariae from Africa. Xiphidiocercariae type 1 was the most common type of cercariae recovered, accounting for 44.3 % of all infections. The density of snails tended to be lower during the summer months than the winter months, except for M. tuberculata snails, which were not affected by seasonal changes.

Conclusion: The findings of this study indicate that besides schistosomes, other larval trematodes are found, and some use the same intermediate hosts as the schistosomes. Further studies should be conducted to determine whether some of these trematodes could be manipulated for the biological control of schistosomiasis.

Fig. 1

Map showing the study area in the East Nile locality, as well as other localities of Khartoum state, Sudan

Fig. 2

Density of snails (number of snails per standard search), infected snails, and mean water and air temperature, by month, from January 2004 to December 2005, in the East Nile locality, Khartoum, Sudan. a Biomphalaria pfeifferi infected with any trematodes, (b), Bulinus truncatus infected with xiphidiocercariae, (c) and Melanoides tuberculata infected with parapleurolophocercous cercariae

Fig. 3

Morphotypes of cercariae recorded from five snail species in the East Nile locality, Khartoum, Sudan. The types are:- a) Schistosoma haematobium (or related species); b) S. mansoni; c) amphistome; d) echinostome; e) parapleurolophocercous; f) lophocercous-apharyngeate; g) avian schistosome, i.e. brevifurcate-apharyngeate distome cercariae; h) Type 1; i –l) four types of xiphidiocercariae: i) Type 1, j) Type 2, k) ornatae, l) armatae; m) LPM; n) Type 5; o) Type 2; p) Type 3; q) Type 4; r) Type 6; s) Type 7; t) longifurcate-pharyngeate distome cercaria (Strigea). The photos are not to the same scale