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Chronic Wasting Disease Does Not Spread to Cattle and Humans

After reading the recent post on the always fatal prion disease known as "chronic wasting disease" spreading in wild deer and elk in the U.S., a reader asked whether chronic wasting disease (CWD) is also found in cattle. Or whether it could cross over to cattle, and so wind up in the beef we eat. Excellent question.

The studies and medical discussions that I read all agreed that: chronic wasting disease is only found in cervids - deer, elk, reindeer, and moose. Medical opinion is that there is a species barrier, a "thin molecular barrier" preventing crossing over of the prion disease to cattle and humans. Several studies that specifically looked at this issue back this up. Whew, what a relief!

A 2017 study by Canadian researchers reviewed 23 studies looking at "transmissibility" of CWD to humans. They did not find any evidence of transmission of CWD prions to humans in the U.S or Canada, and no evidence supporting the possibility of transmission of CWD prions to humans. However, they did not rule out the possibility that a CWD prion more readily transmissible to humans could emerge over time given sufficiently extensive human exposure, or if a variation in CWD prion strains develops.

A good study by Univ. of Wyoming researchers actually had deer sick with CWD and healthy cattle living side by side for 10 years - sharing food, water, paddocks, and with constant interaction. Another group of 12 calves were given orally (by mouth) 45 g of prion tissue from CWD infected deer and kept indoors (2 per room) in an isolation building after that point. (Note that eating only 1 g of such prion tissue makes deer sick with CWD). There was also a healthy control group not exposed to CWD in any way. All brains were examined after death - and all cattle brains were normal.

Interestingly, this 2018 study did mention that if CWD prion material is injected into cattle brains (which was done in some other studies), then they do go on to develop CWD - but that is not how cattle would be exposed to it naturally. Which is why they did the study trying to mimic natural conditions in which any potential transmission could occur - and found no transmission of CWD.

But...scientists are concerned with possible transmission of CWD prions to humans occurring at some point, for example if variation in CWD prion strains develops. Keep in mind that CWD is spreading year by year throughout the US (which should be of special concern to hunters).  There are many, many questions at this point.

Excerpts from Elizabeth S. Williams et al. (2018) in the Journal of Wildlife Diseases: CATTLE (BOS TAURUS) RESIST CHRONIC WASTING DISEASE FOLLOWING ORAL INOCULATION CHALLENGE OR TEN YEARS' NATURAL EXPOSURE IN CONTAMINATED ENVIRONMENTS

Abstract: We conducted a 10-yr study to establish whether chronic wasting disease (CWD) was readily transmissible to domestic cattle (Bos taurus) following oral inoculation or by cohousing cattle with captive cervids in outdoor research facilities where CWD was enzootic. 

Calves (n=12) were challenged orally on one occasion using brain homogenate derived from CWD-infected mule deer (Odocoileus hemionus). Five uninoculated cattle served as unchallenged controls. Two other groups of cattle (n=10–11/group) were housed outdoors for 10 yr in captive cervid research facilities. The environmentally challenged cattle were exposed to CWD-associated prions through common paddocks, feed, and water and via direct daily contact with known and potentially infected mule deer or wapiti (Cervus canadensis) throughout the decade-long study period.

None of the exposed cattle developed neurologic disease during the study. We euthanized cattle surviving to 10 yr postchallenge and examined all for lesions or disease-associated prion protein (PrPd) by histopathology, immunohistochemistry, and western immunoblot analysis of central nervous system and lymphoid tissue. None had evidence of PrPd accumulation. We conclude that the risks of CWD transmission to cattle following oral inoculation or after prolonged exposure to contaminated environments are low.

[Excerpts from the article] Cervid carcasses have not been rendered or used in meat and bone meal intended for ruminant feed since the late 1990s. However, cattle may be exposed to the CWD agent by grazing on shared pastures or ranges and by consuming hay or other crops harvested from fields where CWD-infected cervids forage and could be exposed to infected carcass remains. Such exposure likely would be a byproduct of the horizontal transmission through direct or environmental exposure—including contaminated feed and water sources—that appears to be the primary driver of CWD epizootics among susceptible cervid species (Miller et al. 2000Miller and Williams 2003Williams 2005).

We used two approaches to assess natural susceptibility of cattle to CWD under experimental conditions. The first involved exposing healthy, weanling, beef breed calves to a large, single, oral dose of the CWD agent and maintaining them in isolation for 10 yr. The second involved confining calves in paddocks with CWD-infected mule deer or wapiti for 10 yr and monitoring them for clinical prion disease. In addition to observing exposed cattle for clinical disease for a decade, we collected tissues postmortem and analyzed them for presence of disease-associated prion protein (PrPd). We chose the 10-yr assessment period largely for logistical reasons while anticipating a protracted incubation period for cross-species transmission (e.g., 6 yr for cattle intracerebrally inoculated with CWD; Hamir et al. 2005).

Despite ample opportunity for exposure to infectivity with a decade or more for infections to develop, we observed no evidence of prion disease in any of the 31 exposed subject cattle surviving to the study's end. Lesions typical of transmissible spongiform encephalopathy were absent, including those induced by IC inoculation of the CWD agent. The observed resistance of our study cattle to CWD after exposure by natural routes and the limited susceptibility to IC challenge reported previously are consistent with a substantial species barrier to cattle propagating cervid-derived prions (Raymond et al. 2000Hamir et al. 2005Tamgüney et al. 2006).

To date medical surveillance, epidemiologic data, and published laboratory studies have revealed no evidence of CWD transmission to humans although some potential for zoonotic risk is recognized (EFSA BIOHAZ Panel et al. 2017). Of secondary concern, passage of the CWD agent through cattle or other livestock hosts could lead to transformation of cervid-adapted prions to a new prion strain with greater potential infectivity for humans.

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