Dev Biol Stand. Basel, Karger, 1993; vol 80, pp 111-118

Department of Animal Health and Biomedical Sciences, University of Wisconsin-Madison, Madison, WI 53706, USA

EPIDEMIOLOGIC AND EXPERIMENTAL STUDIES ON TRANSMISSIBLE MINK ENCEPHALOPATHY

R.F. Marsh and R.A. Bessen

Abstract

Transmissible mink encephalopathy (TME) is a rare foodborne disease of ranch-raised mink produced by an as yet unidentified contaminated feed ingredient. Because of the clinicopathologic similarities to scrapie and the indistinguishable physicochemical properties of their transmissible agents, it was initially assumed that TME was caused by feeding mink scrapie-infected sheep. However, subsequent studies testing the oral susceptibility of mink to scrapie were unsuccessful.

Epidemiologic investigations of individual incidents of TME have not identified an association between the occurrence of disease and the feeding of any particular ingredient. However, there are two incidents in which the rancher was confident that sheep were not fed. The most recent of these was in Stetsonville, Wisconsin in 1985 where the meat portion of the diet was composed almost exclusively of downer dairy cows. To examine the possibility that cattle may have been the source of infection on the Stetsonville ranch, mink brain was experimentally inoculated intracerebrally into two Holstein steers. Both of these animals developed fatal spongiform encephalopathies 18 and 19 months after inoculation. These findings are compatible with the Stetsonville incident of TME being caused by feeding mink infected cattle tissue and they suggest the presence of an unrecognized BSE-like disease in the United States.

Further experimental studies on the Stetsonville source of TME have identified two distinct strains of transmissible agent in Syrian hamsters. These strains vary in length of incubation period, clinical signs, endstage brain infectivity titre, and pathogenicity for mink. Examination of the disease-specific prion proteins (PrP) produced by each strain also show that they differ in sedimentation, protease sensitivity, molecular weight, and immune reactivity with PrP antipeptide sera. This is the first report of biochemical and physical differences in PrP produced by two distinct strains in a single outbred species. These changes cannot be due to differences in primary amino acid sequence as seen in inherited human spongiform encephalopathies. They also emphasize the role of the transmissible agent in determining the outcome of an infection and that this role can be independent of the host's genetic background.