TRANSMISSIBLE SPONGIFORM ENCEPHALOPATHIES: THE ROLE OF GENETICS, STRAIN VARIATION, AND ENVIRONMENTAL CONTAMINATION IN DISEASE CONTROL
Location: Animal Diseases Research
Title: Disease-associated prion protein in neural and lymphoid tissues of mink (Mustela vison) inoculated with transmissible mink encephalopathy
Submitted to: Journal of Comparative Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 29, 2012
Publication Date: November 7, 2012
Citation: Schneider, D.A., Harrington, R., Zhuang, D., Yan, H., Truscott, T.C., Dassanayake, R., Orourke, K.I. 2012. Disease-associated prion protein in neural and lymphoid tissues of mink (Mustela vison) inoculated with transmissible mink encephalopathy. Journal of Comparative Pathology. 147:508-521.
Interpretive Summary: Transmissible mink encephalopathy is a rare, naturally occurring disorder of farmed raised mink. The epidemiology of the disorder suggests that transmission occurred from oral exposure to food contaminated by the the disease-inducing agent, the prion, from domestic livestock. Because of the epidemic nature of TME outbreaks, transmission among individuals could not be assessed. Transmission among animals in a group appears to be correlated with distribution of prions in the tissues of the infected animal. Sheep, goats, deer, and elk accumulate prions in the lymph nodes and may shed the agent through blood, urine, feces, saliva or placentas. Cattle, on the other hand, have detectable prions limited to the brain and central nervous system, and there is little evidence for transmission among herdmates. Although transmissible mink encephalopathy was first described in 1965, scant data on the distribution of prions outside of the brain of the mink are available. In this study, mink were infected by the oral route, presumably the natural route of infection, and by direct inoculation into the brain, and sacrificed at intervals to determine the early and late distribution patterns of the prion protein. Abnormal prion proteins were found in the lymphoid tissues of infected mink early in disease, well before clinical signs appear. The disease in mink is therefore more similar to that observed in small or wild ruminants and less similar to the disease in cattle. The role of scavenging carnivores in dissemination of prions under field conditions is not known. The findings of this study provide some evidence that small carnivores, should they become hosts for prion diseases, may shed the agent into the environment.
Transmissible mink encephalopathy (TME) is a prion disorder of farmed raised mink. As with the other transmissible spongiform encephalopathies, the disorder is associated with accumulation of the misfolded prion protein in the brain and an invariably fatal outcome. TME outbreaks have been rare but typically affect large numbers of animals in a facility, suggesting a point infection from feeding of prion contaminated feed, possibly through feeding of carcasses of infected domestic livestock. The potential for transmission among mink is not known. Horizontal transmission of the prion diseases of ruminant livestock is limited to the species and genetic subgroups in which abnormal prion proteins accumulate in the lymphoid tissue. Cattle, with no evidence of lymphoid prion, do not appear to transmit the disease to progeny or herdmates. In flocks of sheep with scrapie, horizontal transmission is associated with shedding of the agent at parturition. However, the subgroup of sheep with a prion genotype in which prions are restricted to the central nervous system show no evidence of placental or blood borne prions. The distribution of prions in the extraneural tissues of mink is not well delineated. In this study, mink were inoculated by the oral and the intracerebral route, and the appearance of prion proteins in lymphoid tissues examined at various time points in the 5 to 12 month incubation time. Abnormal prions were detected in the retropharyngeal and mesenteric lymph nodes and to varying degrees in the spleen. These findings suggest that abnormal prions are disseminated through the blood and might be shed at parurition, by bite wounds, or through saliva, urine or feces. Mink are a useful model for examining the role of small carnivores in prion persistence in wild populations and the finding of peripheral prion proteins suggests that they may transmit the disease under certain husbandry conditions.