Nematode parasites and scrapie: experiments in sheep and mice.

To demonstrate the possible role of nematode parasites in the modification of host susceptibility to scrapie, experiments were conducted using sheep naturally exposed to scrapie, chosen by their genotype at the PrP gene, and infected with Teladorsagia circumcincta. Two 4-year duration experiments demonstrated that the nematode infection shortened the development of scrapie with a significant regression between the level of infection and age at first scrapie symptoms (P < 0.006). Investigations by ELISA tests in different species of nematode parasites of the digestive tract collected from scrapie infected ewes did not reveal the presence of PrPSc. In scrapie-infected C57BL mice, infected or not with Heligmosoides polygyrus at various times, parasitized animals showed a slight but significantly longer survival period. Assays on transmission by the larvae hatching from eggs collected from scrapie-infected mice were unsuccessful. We concluded that nematodes modify host susceptibility to scrapie, but their role in the horizontal transmission of the disease was not demonstrated.