Chronic Wasting Disease of Elk and Deer

Filed Under: Diseases

First reported in the late 1960’s in captive mule deer found in Colorado, Chronic Wasting Disease (CWD) has now been found in cervids (hoofed ruminant mammals whose males of the species have horns) in much of the northwestern United States and several Canadian provinces.  Surveillance studies in hunter-harvested animals from 1996-1999 found that CWD infected approximately 5% of the mule deer, 2% of the white-tailed deer, and <1% of the elk in the Colorado and Wyoming areas of the United States.  The geographical area and number of animals infected continues to grow. 
 
The disease is caused by a prion, and the subsequent disease that is produced belongs to a group of disorders known as a transmissible spongiform encephalopathy, or TSE.  All TSEs attack the central nervous systems, of their victims causing small holes to form in the brain which finally ends with the death of the infected individual. 
 
CWD is highly transmissible within the deer and elk populations.  Prions are historically known to spread through contaminated foodstuff.  The mode of transmission of CWD has not been positively identified but has been theorized to include direct animal-to–animal contact as well as contaminated food and water sources.
 
CWD prions have been shown to be released from both the salivary and alimentary tracts of sick deer.  The CWD prions can bind to soil particles and increase their infectivity several hundredfold.  Wildlife scavengers such as crows can excrete the CWD prion following ingestion of the infected carcass.  A deer carcass has been shown to take over 50 days to fully decompose, and during this time no fewer than 18 mammalian and 21 bird species have been observed consuming the carcasses.  The prion has been shown to remain active in infected soil for at least 16 years.
 
The incubation period, or the time from infection to the onset of symptoms, is typically a minimum of 15 months. 
 
Infected animals may not show any symptoms in the early stages of the infection.  As the disease progresses, infected animals eventually become emaciate;  they may walk in repetitive courses, stagger, have a wide-based stance, or appear to be drowsy.  These animals may lose control of bodily functions, salivate or drool, exhibit dysphagia (difficulty eating), urinate excessively, and carry their head and ears lowered.  Infected animals are often found near water where they tend to drink large amounts of liquid.  In the late stages of the disease the symptoms may be confused with epizootic hemorrhagic disease, or EHD.
 
Surveillance of CWD in Wisconsin has shown that the incidence of infection increases with age.  Less than 1% of  fawns have tested positive, with that level rising to 2-3% for yearling females and males.  The rate increases further to 4-5% of the females and up to 10% of male deer that are 2 years or older.
 
To date there has not been a proven case of transmission of CWD to man.  One study of 14 macaques infected orally or intracerebrally with CWD were found to be symptom-free of the disease 6 years following exposure.  The macaque study would seemingly suggest that humans are resistant to infection.  Creutzfeldt-Jakob Disease, or CJD, is the TSE syndrome that occurs in humans.  Most commonly CJD is associated with Bovine Spongiform Encephalopathy, or BSE.   Since CJD can occur 10 years or later following infection, the study looking for any susceptibility of CWD to humans is ongoing.   A similar study in squirrel monkeys found that13 out of 15 animals became infected within 69 months following infection.  It is said that squirrel moneys are more distantly related than are macaques to humans.
 
In 1999 there was a confirmed death of a man from Creutzfeldt-Jakob Disease (CJD) who was a resident of Minnesota and participated in wild game feasts.  He eventually succumbed to dementia, speech abnormalities, and myoclonic jerking.  Two additional deaths of men with dementia who frequented these feasts could not be confirmed as CJD although they died from similar central nervous disorders.  Could there be many more cases of CJD that are often misdiagnosed as dementia?  At least one pathologist from MI thinks so, revealing to this author that he felt a number of undiagnosed cases of death from dementia are actually undiagnosed cases of CJD.  The source of this case and other CJD cases may not be a result of CWD but also of BSE, or mad cow disease, that has been shown to infect cattle and humans.   
  
Hunters are advised to check with wildlife agencies to assess their risks from eating deer and elk, especially if the animals appear sick or test positive for CWD.  Testing for CWD may be conducted by submitting the head, retropharyngeal lymph nodes, or spleen, depending on the lab used.  Testing of the animal is available from most state wildlife agencies but testing may take as long as 12 weeks.  Hunters are further advised to wear gloves when field-dressing their kills, bone-out the meat, and minimize the handling of brain and spinal cord tissues.
 

References:
 
http://www.cdc.gov/ncidod/dvrd/cwd/
 
http://www.state.tn.us/twra/pdfs/cwd.pdf
 
http://dnr.wi.gov./org/land/wildlife/whealth/issues/CWD/
 
http://wildlife.state.co.us/NewsMedia/.Videos/CWDSamplingDemonstration.htm
 
http://www.sciencecentric.com/news/article.php?q-09073127-species-barrier-may-protect-macaques-from-chronic-wasting-disease
 
MMWR Weekly.  February 21, 2003/52(07);125-127.
 
James, Paul.  “Both Sides of the Fence:  A Strategic Review of Chronic Wasting Disease.”  A Report Prepared for the Canadian Wildlife Federation, Canadian Plains Research Center, University of Regina.  October 2008. 
 
Pekoc, Ken.  Chronic Wasting Disease Alliance.  National Institutes of Health (NIH). July 29, 2009. 

Topics: deer, elk, mad cow, wasting

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