Abstract
Snail borne trematode (Trematoda: Digenea) diseases of farm animals greatly hamper profitable livestock
production in many countries of the world, especially in the humid tropics and sub-tropics, including
Bangladesh. Fasciola, Schistosoma and amphistomes are the most important snail borne trematode (SBT)
parasites affecting cattle, buffalo, sheep and goats. In this study the eco-epidemiology of the SBT diseases
and their putative snail intermediate hosts was investigated from April/2013 to December/2016. Faecal
sample examination of 6596 farm animals (3897 cattle, 925 buffaloes, 570 sheep and 1204 goats) from
different topographic zones of Bangladesh by simple sedimentation technique revealed overall 54.58%
prevalence of the SBT diseases. The highest prevalence of the SBT diseases was detected in buffaloes (80.97)
followed by cattle (59.28%), goats (32.39%) and the lowest in sheep (26.49%). Fascioliasis caused by Fasciola
gigantica was recorded 9.85%, 26.59%, 3.74% and 3.86% in cattle, buffaloes, goats and sheep, respectively.
Amphistomiasis was found in 49.96% cattle, 70.49% buffaloes, 26.5% goats and 23.16% sheep.
Schistosomiasis caused by Schistosoma indicum and Schistosoma spindale was detected in 6.8% cattle, 6.38%
buffaloes, 3.07% goats and 0.35% sheep. The highest prevalence of the diseases was found in the Coastal
areas (including the St. Martin's Island) (69.77%) and the lowest in the Madhupur Tract (24.11%). More than
half of the farm animal population was found to suffer from SBT diseases in Tista Silt Basin (66.18%), Barind
Tract (61.56%), Brahmaputra Alluvium (55.91%) and in the Hilly Areas (55.24%). In Gangetic Alluvium,
48.11% farm animals were found to harbour SBTs. Amphistomiasis was the most prevalent SBT diseases
which was recorded 58.35%, 48.09%, 42.22%, 19.17%, 53.31%, 47.01% and 50.56% in farm animals of the Tista
Silt Basin, Brahmaputra Alluvium, Gangetic Alluvium, Madhupur Tract, Coastal areas, Barind Tract and
eastern Hilly areas, respectively. The highest prevalence of fascioliasis was recorded in the eastern Hilly
areas (14.87%) followed by Tista Silt Basin (12.63%), Brahmaputra Alluvium (11.50%), Coastal areas
(10.36%), Gangetic Alluvium (9.29%), Barind Tracts (7.27%) and the lowest in the Madhupur Tract (6.52%).
Schistosomiasis was found to its highest magnitude in the Coastal areas (14.32%) followed by the Barind
Tract (11.95%), Brahmaputra Alluvium (5.19%), Gangetic Alluvium (3.73%), Tista Silt Basin (3.19%), and
Hilly areas (2.28%). No case of schistosomiasis could be detected in animals from the Madhupur Tract. Postmortem examination of 600 slaughtered cattle revealed 44.5% fascioliasis, 93.0% amphistomiasis and 22.0%
schistosomiasis. The sex, age, physiological condition and health status of the hosts were important risk
factors where higher infections were recorded in the females, adults, pregnant/lactating cows and poor
conditioned animals. Grazing of animals in low lying pasture and/or supplying grasses from these fields
and also water supply from canals, ponds etc. resulted in significantly higher SBT infections. No variation in
prevalence of SBT infections in cattle was detected in the monsoon (68.95%) and the summer (68.25%),
however, significantly (p=0.0004) lower prevalence was recorded in the winter (54.62%). The freshwater
gastropod snails, Lymnaea auricularia and Lymnaea luteola were detected as the vector of F. gigantica,
amphistomes and the Schistosoma spp. Indoplanorbis exustus was detected as the vector of amphistomes and
Schistosoma spp. Thiara tuberculata, Gyraulus convexiusculus and Bithynia sp. were found as the vector of
amphistomes. I. exustus were prevalent throughout the country but not the lymnaeid snails. L. auricularia
and L. luteola were found in the water bodies containing clear transparent, slow flowing water with
vegetation but L. luteola were found more in the stagnant water bodies with a little bit turbid water. І.
exustus were found in the habitats containing clear as well as turbid water with abundant fresh or decaying
vegetations. For the lymnaeid snails and I. exustus, irrigated rice fields were found to be the most suitable
habitats. The Brotia sp. and T. tuberculata were found attached with stones oror lying in the bottom of the
ponds or river. Bithynia sp. was found in the rice fields with clear water and vegetation. The
convexiusculus was found attached with vegetations in clear water bodies like irrigation channels, irrigated
G.
or
in
rice fields and ponds. L. auricularia and L. luteola were found more in the clay soil, also in loamy clay
sandy clay soil. The vector snails were found in the habitats with pH range from 6.3 to 7.7 and 6.1 to 7.8 of
water and soil, respectively. Two seasonal peaks of cercarial release from the snails, one at the beginning of
summer and the other in the mid monsoon were observed. No cercarial infection in snails was detected
the winter. There was a positive correlation between cercarial incidence and the surface water temperature,
rainfall and evaporation rate while cercarial incidence was negatively correlated with the relative humidity.
Taken together, the findings clearly indicate that the SBT diseases are widely prevalent in Bangladesh; from
the hilly areas to the offshore islands and also that, the eco-epidemiological scenario of the SBT diseases is
very complex in Bangladesh. Development of SBT disease risk models and extensive studies on genetic analyses of the SBTs and the vector snails are essentially needed to combat the SBT diseases in Bangladesh.