Human prion disease and relative risk associated with chronic wasting disease in Colorado deer and elk.
Here is a summary of an article involving prion diseases written by a collaborative group of Colorado researchers. As one of the authors, I have borrowed liberally from the original article (MaWhinney et al., Emerg Infect Dis. 2006, 12 (10). 1527-35). The article (which contains complete references and a discussion of limitations) can be accessed at http://www.cdc.gov/ncidod/EID/vol12no10/06-0019.htm . For background on prion diseases (which elicit no immune response) see the CDC and Wikipedia sites: http://www.cdc.gov/ncidod/dvrd/prions/ and http://en.wikipedia.org/wiki/Prion .
An emerging wildlife epizootic of chronic wasting disease (CWD), a contagious prion disease among deer and elk, has potential public health implications. CWD is related to other mammalian transmissible spongiform encephalopathies (TSEs) or prion diseases such as Creutzfeldt-Jakob disease (CJD) in humans, bovine spongiform encephalopathy (BSE or mad cow disease) in cattle, and scrapie in sheep. In prion diseases a normally produced brain protein accumulates in an abnormal misfolded and aggregated form, resulting in neuron destruction and a universally fatal outcome after a prolonged and variable incubation period.
The highest reported CWD prevalence is in a contiguous region spanning parts of Colorado, Wyoming and Nebraska with estimated prevalence of 5% in mule deer, 2% in white-tailed deer, and 0.5% in elk. In Colorado, CWD was first noted in the 1960s in captive deer and in a wild elk in 1981. There are not clear epidemiological connections between original and more recent cases, suggesting unidentified risk factors contribute to the relatively wide and unpredictable geographic CWD distribution.
Humans and animals can acquire TSEs by consuming prion contaminated food. Cannibalistic outbreaks of prion disease include an epidemic of kuru among the New Guinea Fore tribe and epizootic BSE in the United Kingdom (UK) caused by feeding protein supplements derived from prion infected cattle offal to cattle. Food-based prion transmission between species also occurs, although a phenomenon known as the “species barrier” decreases transmission efficiency. In vitro studies suggest this natural barrier reduces human susceptibility to animal prion diseases. Human prion disease has not yet been linked with CWD.
The otherwise reassuring molecular evidence of species barriers is clouded by disparate experiences with scrapie and BSE as food-borne human pathogens. Scrapie exposure has not been demonstrated to increase CJD risk despite extensive human exposure. Conversely, in Britain, consumption of BSE infected cattle led to an epidemic of variant CJD (vCJD) starting in the mid-1990s. As of 10/2/07, only 166 cases (161 deaths) of vCJD have been identified in the UK despite the dietary exposure of millions. Recent studies suggest little chance of large numbers of future vCJD cases.
Data that define human CWD exposure from consumption of infected game do not exist. However in seven Colorado counties considered endemic areas for CWD, 75% of hunting licenses are issued locally; suggesting county residents consume the majority of regionally harvested game. We used Colorado death certificate data to evaluate CJD death rates. Risk of CJD has not increased for residents of counties where CWD is endemic (adjusted relative risk (RR) = 0.81, 95% CI, 0.40-1.63). [RR >1.0 would indicate an increased CJD risk given CWD County residence.] Nor has the rate of CJD increased over time (five year RR = 0.92, 95% CI, 0.73-1.16).
In Colorado, human prion disease resulting from CWD exposure is either rare or non-existent. However, given uncertainties in the incubation period, exposure, and clinical presentation, the possibility that the CWD agent might cause human disease cannot be eliminated.
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9 comments:
This is a fascinating entry -especially since my son is elk hunting at this moment. In the mid-nineties there was a bit of an uproar about Rocky Mountain National Park that resulted from the publication of a provocative book "Rocky Times in Rocky Mountain National Park" which called into question the game management plan of that national park with regard to elk. At that time it was common knowledge that the elk were hugely overpopulated given the available graze and habitat size. It was common to see positively geriatric elk wondering about. There was also an increase in density-dependent diseases to which ungulates are prone and under native conditions serves to thin herds. Instead, the elk were "treated" through placement of medicated salt licks and winter bundles. The game management techniques have been modified - rangers no longer "protect" the elk from mountain lions and the natural ebb and flow of population numbers.
Is it reasonable to assume that the endemic CWD is a density-dependent disease that was given a food-hold and persists due to the survival of elk, deer etc that might otherwise have perished?
How does this prion disease get spread in the wild? Is it through eating of feces? Cannibalism in elk (guessing not)? Eating grass next to a dead elk? Just curious.
Terrio, Population density certainly contributes to the spread of the disease. I'm not sure that this is as much an issue in the northeast corner of the state and contiguous regions in Wyoming and Nebraska.
Stephenb, CWD spreads more like Scrapie (a sheep TSE) rather than BSE. I believe that saliva is a suspected culprit. According to the CWD link on the CDC website (link above): "CWD can be highly transmissible within deer and elk populations. The mode of transmission is not fully understood, but evidence supports the possibility that the disease is spread through direct animal-to-animal contact or as a result of indirect exposure to prions in the environment (e.g., in contaminated feed and water sources)."
Further to samm's comments, when sheep scrapie agent is injected into the brains of elk (USDA, 2003) it produces a disease indistinguishable from CWD, and prions that are indistinguishable from the elk version, too. This has led some to speculate that the elk may have acquired the agent from old, abandoned sheep enclosures; maybe so old that they are no longer recognizable as such. The agent is almost indestructible in the environment.
Sam,
Very interesting post. The variable latency issue with Prion diseases has led to a bit of controversy regarding transplantation of cadaver parts (think things related to the eyes, etc.) and the possibility for screening. The argument goes something like, more people probably die with the "infection" that die from the symptoms resulting from the infection. Given a possibility of a 30+ years latency for the symptom, a infected person might transmit vCJD or another prion through transplantation. This might necessitate screen for prions. An overview of the issue may be found below.
1. Warwick RM, Eglin R. Cell Tissue Bank. 2005;6(4):263-70
Scotts,
You are exactly right. The screening should apply to all human TSEs. It is not surprising that your article is from the UK where a link between BSE and vCJD has been established.
Along the lines of transmitting infectious agents via transplant: in yesterday's (11/13/07) news, 4 transplant recipients have been infected with HIV and Hep-C. Apparently, the donor had recently acquired the infections and had not yet developed an antibody response. In addition to antibody screens, we may need to consider screening donors for plamsa HIV RNA, Hep-C, etc?
http://www.nytimes.com/2007/11/13/health/13cnd-organ.html?_r=1&8br&oref=slogin
I had an interesting update from Terrio. Her son's elk tested positive for CWD...
Wow that is very interesting blog and very strange about terrio's son's deer. I what are the symptoms of CWD? I think it is very important for people that hunt deer and elk to test these them for CWD before cutting them up and eating them for dinner!
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