Prion diseases in wild animals
I read an article about Chronic Wasting Disease in the York Daily Record this past weekend, detailing the imminent spread of CWD into Pennsylvania. Chronic wasting disease is an emerging infectious disease that is becoming epidemic in many North American deer herds, and animals afflicted with this disease have been found within 10 miles of the Pennsylvania border in Allegany County, MD.
CWD has been observed in North American deer since the late 1960’s, and the numbers of afflicted animals is gradually increasing. Animals infected with the etiologic agent show increasing neurological defects, including uncoordinated movement, weight loss, poor posture, excessive thirst, and drooling. The disease is fatal for infected deer. The etiologic agent however is not a virus, or a bacterium or other cellular agent, but instead is a non-cellular pathogenic agent known as a prion.
Prions are a class of cellular proteins that have been found in all mammalian species, and has been proposed to play a role in the development of long term memory in mice, although the actual role remains controversial. The healthy, normal form of the prion protein can be induced to form a pathogenic form of the protein, which leads to runaway neurological damage as the “bad” prion causes the normal, “good” prion to fold into a altered form of the protein. Prion based diseases have been described in many mammalian species, and diseases in humans include: Creutzfeld-Jacob disease, Fatal Familial Insomnia, and Kuru. The first two examples are not communicable diseases, and cannot be transmitted from individual to individual, however Kuru is a transmissible disease found in the Fore tribe of Papau New Guinea that was passed on between individuals during ritualistic cannibalism.
A mechanism analogous to Kuru is also behind (unintentionally) the transmission to humans of Bovine Spongiform Encephalopathy, or Mad Cow Disease. Meat from cows afflicted with BSE enters into the food supply, and is consumed. Pathogenic bovine prions taken up in the diet cause normal cellular prions to shift to the “bad” form, inducing progressive neurological defects in the person who consumed the meat. The transmission of prion diseases across species like this (bovine to human) is not very effective, and although the disease is quite horrific, the actual risk is real, but not very great.
Chronic wasting disease presents another possible avenue for infection of humans with prion based agents, and understanding of how the disease is progressing in deer herds is essential for preventing further spread. A recent research article by Almberg and co-workers sheds some light on how prion-based disease spreads in deer. Prions in their altered form are remarkable for their durability, and are resistant to autoclaving. The “bad” form of prions can remain infective in the environment outside of a host for at least 2 years. Since deer are not renowned for their cannibalistic tendencies, this suggests that the actual avenue for infection in wild deer is due to consumption of prion particles during forage. Almberg and co-workers conclude from the distribution of CWD afflicted deer and the persistence of pathogenic prions in the environment that disease dynamics are going to continue to shift and that the epidemic will continue to expand. Furthermore, as indirect methods of transmission (acquired from environmental pools of prions) become more and more significant, current strategies for managing the epidemic including culling herds and vaccination will become ineffective.
Posted on July 5, 2011, in Danger danger danger!, Kill the wabbit!, Microbes in the News and tagged Chronic wasting disease, Creutzfeldt–Jakob disease, Prion, Transmissible spongiform encephalopathy. Bookmark the permalink. Comments Off on Prion diseases in wild animals.