Schistosoma malayensis

''Schistosoma malayensis'' is a schistosome parasite. It was first described in 1988 in Peninsular Malaysia and appears to be a zooenotic infection. The species is named after the country of Malaysia. The natural vertebrate host is van Müller's rat ('' Rattus muelleri''). The intermediate hosts are aquatic snails, ''Robertsiella kaporenisis''. Among ''Robertsiella kaporenisis'' are two other ''Roberstiella'' species.

Among humans exposed to this parasite the lifetime incidence appears to be 5–10%.Sagin D.D., Ismail G., Fui J.N., Jok J.J. (2001) ''Schistosomiasis malayensis''-like infection among the Penan and other interior tribes (Orang Ulu) in upper Rejang River Basin Sarawak Malaysia. Southeast Asian J. Trop. Med. Public Health 32(1):27–32 The symptoms are unclear, because the symptoms have never been reported. The disease has been little studied and it is currently considered to be a relatively minor public health problem.

Taxonomy

''Schistosoma malayensis'' is a member of the ''japonicum'' species complex along with ''Schistosoma japonicum'' and ''Schistosoma mekongi''. While the three species are grouped together because of their similarity, ''Schistosoma malayensis'' is more closely related to ''Schistosoma mekongi''.

Morphology

Size is the only morphological difference compared to its natural host possibly due to host-induced variation. The adult ''S. malayensis'' is typically smaller than ''S. mekongi'' and ''S. japonicum''.

''S. malayensis'' eggs have been found in liver granulomas, embedded within dense, fibrous tissue. The eggs of ''S. malayensis'' have a thin-walled, yellowish shell. The eggs contain miracidia and are approximately 50 μm long × 28 μm wide. The ova is not operculated and has no bipolar plugs and the thin covering was not striated.

Epidemiology

Serologic surveys for schistosomiasis due to ''S. malayensis'' indicate of 3.9% prevalence in rural populations. It was unsuccessful at adapting to human host. Infected rodents are often found near snail habitats. Infections in humans are unlikely and are considered rare. Humans are most likely to become infected while fishing or canoeing on small streams.

Intermediate host

Freshwater snails ('' Robertsiella'' sp.) act as an intermediate host for ''S. malayensis'', that can infect humans and other mammals when cercaria are released from the snail and eventually get in contact with the definitive host. ''Robertsiella'' species are Caenogastropoda snails of the family Pomatiopsidae. This species is known to be located in limestone areas in the foothills of the mountain chains of Kedah and Perak States in West Malaysia.

Definitive host

The final or definitive mammalian hosts include ''Rattus muelleri'' and ''R. tiomanicus''. ''R. muelleri'' has been found in lowland secondary, and disturbed primary forests in wet conditions in Western Malaysia. ''R. muelleri'' is often found near river banks due to the species prioritization of crustaceans and mollusks over other food sources, such as insects. The predation of mollusks by ''R. muelleri'' allows ''S. malayensis'' a route into its definitive host. It is possible infected rodents defecate in bodies of water they hunt for prey in, maintaining a constant contamination with ''S. malayensis'' eggs and continuing the life cycle.

References

{{Taxonbar, from=Q7431530

Diplostomida
Parasites of rodents
Animals described in 1988