What we know about the geographic range of a parasite largely depends on historical records, established knowledge and infection prevalence surveys. Sometimes, new data arise forcing us to re-evaluate what we know about where parasites are found. A recently published report (1) seems to suggest that the parasitic worm Schistosoma haematobium, known to occur only in Africa and some parts of the Middle East, may also be present in Nepal.
An unexpected finding
The worm we are talking about is one of the parasites causing schistosomiasis, an important neglected tropical disease (NTD). The report by Sah et al describes a nepalese fisherman (which never reported travelling abroad, with the exception of sporadic trips across the border to India) showing clinical signs of urogenital schistosomiasis. Furthermore, in his urine eggs were found showing the characteristic morphology of S. haematobium.
Although the precise identity of the eggs was not determined, the report mentions previous publications describing very similar eggs found in people from two villages of India in 1952 and then 1969.
The occurrence of this parasite in these areas may have important public health implications, but are these findings the results of accidental (self-limiting) introductions or a sign of ongoing and sustained transmission?
How can we confirm the presence of ongoing parasite transmission?
In general, to answer this question we need to:
- Confirm the parasite identity: sometimes morphology can be misleading, so the identity (species) of the parasite must be determined with genetic analysis and DNA sequencing. Is it actually the parasite we think it is, or could it be a novel species?
- Incriminate vectors or potential reservoir hosts: if the parasite is known to require a vector/intermediate host (as in the case of schistosomes, which develop inside freshwater snails before infecting people), then the occurrence of such hosts and their infection status should be verified. Without these hosts, the parasite cannot sustain itself in an area even if it has been accidentally introduced. Furthermore, some schistosomes are known to mate with animal parasites forming hybrids which are human-infective (2): animal hosts acting as potential reservoirs for infection should then be considered.
- Confirm local human transmission: how common is the infection in the locals can be a good indication of sustained transmission. Also, ruling out travel to countries or areas known to be affected is important when examining individual sporadic cases (to exclude travel-related infections).
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- Sah R, Utzinger J, Neumayr A. Urogenital Schistosomiasis in Fisherman, Nepal, 2019. Emerg Infect Dis. 2020;26(7):1607-1609. doi:10.3201/eid2607.191828
- Webster BL, Alharbi MH, Kayuni S, et al. Schistosome Interactions within the Schistosoma haematobium Group, Malawi. Emerg Infect Dis. 2019;25(6):1245-1247. doi:10.3201/eid2506.190020