Transmissible mink encephalopathy (TME) in Chinese hamsters: identification of two strains of TME and comparisons with scrapie.

TME from a single source was transmitted by intracerebral injection to Chinese hamsters, producing clinical disease in all seven animals after incubation periods of over 600 days. The brain from each of the primary cases was used to establish separate intracerebral passage-lines of TME and this led to the isolation of two different strains of agent, designated 333K and 333W. These strains were easily distinguished by the incubation periods they produced (about 130 and 230 days, respectively) under standard conditions of infection, and by the characteristic profiles of vacuolation seen in different regions of the brain. Comparisons were made with a strain of scrapie passaged in Chinese hamsters, designated 34W, which could be distinguished from both strains of TME. Nevertheless the properties of the scrapie and TME strains overlapped, with one of the TME strains (333K) resembling the 34W strain of scrapie in Chinese hamsters more closely than the other TME strain (333W). These similarities strengthen the view that TME and scrapie are caused by a similar type of infectious agent. The very large 'species barrier effect' on transmitting TME to Chinese hamsters was in marked contrast to the minimal effect seen with scrapie and an explanation for this is suggested. Two interesting pathological features of the study were (a) the severe loss of pyramidal cells produced in the hippocampus by the 34W strain of scrapie, and (b) the focal, symmetrical vacuolation of the thalamus caused by 333K TME.