As part of a community-based soil transmitted helminth study, stool surveys from ~8600 consenting participants were being conducted in 45 tribal villages in Thiruvanamalai district of Tamil Nadu, India in September 2016. Suspected schistosome eggs were seen in direct wet mounts (2-3 ova/slide) from the faeces of two healthy tribal women aged 50 and 35 years from the same household. The eggs measured between 110-120 µm in length, were suboval and flatter on one side. The eggs displayed a prominent asymmetrical terminal spine with a bluntly rounded opposite end. Miracidia were clearly visible within the eggs. The eggs were morphologically consistent with prior descriptions and illustrations of Schistosoma incognitum eggs.
Schistosoma incognitum is a natural parasite of pigs, dogs, sheep, goats and rodents reported from India, Indonesia and Thailand. Successful experimental infection of rhesus monkeys (Macaca mulatta) with S. incognitum has been undertaken in the past, demonstrating its potential infectivity for some primate species.1 The two cases reported here represent the first reports of S. incognitum from humans since the initial description of the species as a parasite of two humans in India in 1926.2
It could not be definitively determined if these two cases represent transient passage or true human infections with S. incognitum. However, they do serve to add to the growing body of literature describing human schistosome infections on the Indian subcontinent.3,4,5,6 There are too many convincing reports of autochthonous human schistosomiasis from India and Nepal, with credibly absent travel histories, to ignore. We recommend further investigation of the identity, geographic foci, sources, and epidemiology, of the many reports of schistosomiasis in humans from India and Nepal, including surveillance for possible human infections with S. incognitum.