Abstract:
Background: The epidemiology of human urinary schistosomiasis caused by Schistosoma
haematobium can be complicated by the presence of ruminant schistosomiasis caused, primarily
by S. bovis. The two schistosome species may be transmitted by the same Bulinus species, they
may occur sympatrically in the same habitat, and their cercariae are very similar in morphology
and therefore, difficult to tell them apart. Screening of snails collected from freshwater habitats for
schistosome infections is often used to identify transmission sites or to evaluate success or failure
of interventions. However, pin-pointing sites involved in S. haematobium transmission can be
complicated by the presence of other mammalian schistosomes such as the bovine schistosome,
which is a fairly common parasite. A PCR-RFLP method targeting a unique segment of the second
internal transcribed spacer (ITS2) region of the ribosomal DNA (rDNA) in the schistosomes was
used to identify mammalian schistosome cercariae shed by bulinid snails collected from endemic
freshwater habitats located within Machakos county in south-eastern Kenya, with the aim to
identify the transmission sites and assess the distribution each of the parasite species in the study
area.
Results: A total of 5,034 bulinid snails were collected from 41 different sites and screened for
schistosome infections, and out of these, 43 (<1%) were found to be shedding mammalian
schistosome cercariae. On analysis using the Polymerase chain reaction- Restriction Fragment
Length Polymorphisms (PCR-RFLP) assay, cercariae from 32 snails were identified as S.
haematobium while cercariae from 11 snails turned out to be S. bovis. Only two sites out of 40
namely Kisukioni and Katiwa, were active transmission sites. Both sites were active transmission
sites for both S. haematobium and S. bovis. The assay reliably identified and distinguished between
S. haematobium and S. bovis cercariae, even when only a few cercariae (5-10) were present in the
sample, or when the parasite DNA concentrations were as low as five pico grammes (5pg). The
FTA® paper offered a more reliable way of collecting, transporting and storing DNA material,
and the samples.
Conclusion: The PCR-based assay can potentially be used to support schistosomiasis control
efforts, in epidemiological studies of urinary schistosomiasis, or in transmission ecology studies
of S. haematobium and S. bovis.