| Summary: | <p>Abstract</p> <p>Background</p> <p>To clarify the extent and putative transmission zone of bovine fasciolosis on the slopes of Mount Elgon, Uganda, conjoint parasitological and malacological surveys, inclusive of inspection of animals at slaughter, were undertaken at increasing altitudes.</p> <p>Results</p> <p>A total of 239 cattle were sampled across eight locations ranging in elevation from 1112-2072 m. Faecal material was examined for presence of <it>Fasciola</it> eggs and sera were tested by ELISA for antibodies against <it>Fasciola</it> antigens. Bolstering this, 38 cattle at slaughter from 2 abattoir sites at 1150 m and 1947 m were inspected; in addition, wild buffalo stool (n = 10) opportunistically picked within Mount Elgon National Park (MENP) at 3640 m was examined. By faecal egg detection, prevalence of <it>Fasciola gigantica</it> at low (<1500 m) and high (>1500 m) altitude sites was 43.7% (95% CI 35.4-52.2) and 1.1% (95% CI 0.0-6.0), respectively, while by ELISA was much higher, low altitude - 77.9% (95% CI 69.7-85.4) and high altitude - 64.5% (95% CI 51.3-76.3). The decline in prevalence with increasing altitude was corroborated by abattoir sampling. Thirty seven aquatic habitats, ranging from 1139-3937 m in altitude were inspected for freshwater snails, 12 of which were within MENP. At lower altitudes, <it>Lymnaea (Radix) natalensis</it> was common, and often abundant, but at higher altitudes became much rarer ceasing to be found above 1800 m. On the other hand, <it>Lymnaea (Galba) truncatula</it> was found only at altitudes above 3000 m and within MENP alone. The snail identifications were confirmed by DNA analysis of the ribosomal 18S gene.</p> <p>Conclusions</p> <p>Active infections of <it>F. gigantica</it> in cattle are common in lower altitude settings but appear to diminish with increasing elevation. This is likely due to a growing paucity of intermediate hosts, specifically populations of <it>L. natalensis</it> for which a natural boundary of 1800 m appeared. Although <it>F. hepatica</it> was not encountered, the presence of several populations of <it>L. truncatula</it> at elevations over 3000 m point towards a potential transmission zone within MENP should this parasite be introduced.</p>
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