Chronic Wasting Disease and the Myth of Human Susceptibility

 

Transmissible spongiform encephalopathies (TSEs) are relatively recently characterized diseases, so called because they cause abnormal changes in nervous tissue, often resulting in a ‘spongy’ appearance of brain material taken from deceased individuals who had the disease.  TSEs are typically understood as being caused by prions, or abnormal proteins which cause modification of existing proteins within an organism into more prions.  This cycle usually leads to protein conglomerations and always to neurodegenerative processes and ultimately death.  Some research has suggested connections to bacterial infections caused by Spiroplasma, although the complete molecular underpinnings of these diseases are not yet fully understood.  Since the interspecies transmission of prions is fundamentally limited by genetic constraints, TSEs are often classified by the species which they affect; for example, scrapie affects sheep, mad cow disease affects cattle, and chronic wasting disease (CWD) affects members of the family Cervidae (deer and elk).  However, prions have been known to spread to a new species, as is the case in Creutzfeldt–Jakob disease (CJD).  Variant CJD was first identified in 1996 when an epidemic of mad cow disease in the UK leading to tainted beef was associated with a small number of CJD cases in humans.

It is because of this potential for the zoonotic spread of TSEs, that a wave a fear has erupted in areas affected by CWD.  Many hunters, outdoorsmen, and wildlife observers are concerned that human exposure to CWD-afflicted animals could lead to the transmission of a TSE disease into humans.  This notion is perpetuated by the perceived similarity to mad cow disease and the incidences of variant CJD.  Many recent blog posts, magazines, and news articles have suggested a connection between CWD and human-TSEs, or simply stated that contact with or consumption of CWD-afflicted animals will lead to the contraction of a TSE.  While the zoonotic transmission of a TSE is certainly possible, these statements regarding CWD specifically are simply not supported by any experimental evidence to date.

Research has been conducted on mice to examine human susceptibility to CWD.  These experiments have focused on the particular gene (prnp) which codes for the prion protein; this normal protein is what is modified by invading CWD-prions, ultimately turning them all into CWD-prions. Thus, this gene and the protein it codes for are fundamental elements in the transmission of CWD.  Members of the Cervidae family, mice, and humans all have a copy of prnp, but in slightly modified forms.  Mice who were homozygous for the cervid version of the gene were found to be susceptible to the CWD prions.  On the other hand, mice with only the human version of prnp were completely immune to the same prions and did not contract any form of a TSE disease.  Additionally, variations of the cervid version of prnp resulted in differing levels of infection; this suggests varying levels of immunity within the susceptible species.

6950b057ac9b490e8e86b5cabe908b00FWCWDafflictedDeer.jpg
A deer with Chronic Wasting Disease. (Taken from delware.gov)

Ultimately, there is a risk of CWD transmission into other species.  In order to avoid any chance of zoonotic transmission, hunters have been instructed by the Center for Disease Control and Prevention (CDC) to avoid contact with or ingestion of cervid CNS tissue.  Additionally, animals infected with CWD should not be consumed.  However, this miniscule risk should not be overstated.  To date, no evidence exists that suggests that humans are susceptible to CWD; both the CDC and the CWD Alliance have made public statements which confirm as much.  The widespread notion that CWD is a serious risk to human health flies in the face of research that suggests otherwise.  Scientists continue to research the disease, but their efforts are focused on management and prevention of the condition within cervid species.  The general public should be aware that while CWD is a concerning disease from a wildlife preservation and ecological standpoint, it does not pose a serious risk to human health.

Sources referenced:

http://www.cwdmyths.com/myths/cwd-deadly-humans

http://www.cwdmyths.com/news/deer-hunters-stop-cwd-hysteria-please

http://jvi.asm.org/content/81/9/4533.full

http://www.virology.ws/2015/03/11/is-chronic-wasting-disease-a-threat-to-humans/

Spiroplasma spp. from transmissible spongiform encephalopathy brains or ticks induce spongiform encephalopathy in ruminants. PMID: 17761489

(https://www.ncbi.nlm.nih.gov/pubmed/17761489)

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