As one approach to analysing the genetic barriers between species, we studied the numbers and types of parasitic worms in two species of house mice (Mus musculus and M. domesticus) and in their natural hybrids. Where the ranges of these two species meet in southern Germany, there is a zone of hybridization less than 20 kilometres across, in which about 98% of the mice have backcross genotypes. Fourteen of the 46 mice tested from within the zone have over 500 pinworms per gut, a number far exceeding the mean of 40 per gut for other mice inside and outside the zone. Other nematodes have a similar, non-random distribution. The number of mice bearing 9 or more tapeworms per gut is also excessive in the hybrid zone. These extraordinarily wormy mice may be unusually susceptible to parasitism; the different species may have different genes for resistance, and recombinant backcross animals may lose both. Our findings support the view that the hybrid populations may have reduced fitness and thereby act as a genetic sink, interfering with the flow of genes between the two species. The possibility that environmental or ecological peculiarites in the zone of hybridization make the mice more liable to infection is not supported.
Infective larvae of the rat lungworm Angiostrongylus cantonensis, presumed cause of human eosinophilic meningoencephalitis, are shed in mucus exuded by naturally infected Malayan slugs (Microparmarion malayanus). Larvae passed by slug hosts were recovered from lettuce and produced normal infection in white rats. Lettuce sold in the local public market also yielded small numbers of infective larvae. Experimental evidence from rats suggests that the local human population, exposed to repeated low-level infections, may become immunized against the rare massive exposure and against clinical disease that might otherwise result after ingestion of heavily infected raw mollusks.
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