DEER

Q: What are Chronic Wasting Disease and Tuberculosis and what do they have to do with deer?
A:  Chronic Wasting Disease (CWD) is a disorder of the nervous system, and part of a group of diseases called Transmissible Spongiform Encephalopathies (TSE’s).   The TSEs include diseases of sheep such as scrapie and the infamous ‘mad cow disease’ (bovine spongiform encephalopathy), a variant of which is suspected to cause Creutzfeldt-Jakob Disease (CJD) in humans.   CWD is similar to BSE and scrapie, but affects only cervids (deer and elk).   The precise cause of CWD was unknown until very recently.   Some authorities had suggested that CWD was caused by a bacterium, while others postulated that it could've be an unconventional incomplete virus called a “virino”.   However, more recent data suggest that CWD is caused by an infectious abnormally folded protein called a “prion”.   Prions collect in the brain, causing the death of brain cells and the formation of microscopic holes in the brain tissues.   These prions seem to be capable of resisting the enzymes and chemicals that might normally breakdown infectious proteins.   There is an additional complication too, because infectious prions have the ability to cause normal proteins to re-fold in their own image, leading to rapid increase of infectious prion numbers.  

CWD is always fatal, and there is normally a long incubation period before the symptoms -- which include weight loss, behavioural changes (i.e. isolation), blank facial expressions, nervousness, excessive salivation, teeth grinding and repetitive patterns of movement -- appear.   CWD is currently raging through deer and elk populations in the United States – according to an article in the Guardian recently, in the Rocky Mountains (Colorado) alone, as many as one-in-ten deer and one-in-twenty elk are thought to be infected.   There are currently several teams of scientists from across the US and Canada looking into how the disease is transmitted and, ultimately, how to stop it.   At present, culling elk and deer, along with imposing fines for people feeding them (as this draws deer from miles around, increasing the potential for infection) are the only ways that authorities have found to get a handle on the situation.   Contrary to popular misconception, there is currently no evidence that CWD can be transferred to humans, although the consumption of meat from infected animals is not advised.

Tuberculosis (frequently shortened to TB) is an infectious disease that most often attacks the tissues of the lungs (although it is capable of attacking any part of the body).   The most common form of TB is caused by the bacterium Mycobacterium tuberculosis, although there are various other species and subspecies of Mycobacterium that can cause different forms of TB.

It is not particularly easy for humans to become infected with TB -- close contact for a prolonged period is required in order for the TB bacteria to jump host -- although the situation is somewhat less clear for other species.   Similarly, contracting TB bacteria does not necessarily equate to the development of a TB infection.   According to the American Lung Association, about ten million Americans have contracted TB germs, yet only 10% of these will develop a TB infection.   Having the germs but having no symptoms, infection or potential to spread infection is referred to a “latent TB”.   Symptoms of TB include persistent cough, tiredness, weight loss, appetite loss, fever and the coughing up of blood.

In deer, it is the paratuberculosis form of TB that is found.   Paratuberculosis (PTB) is a chronic inflammatory disease of the intestinal tract.  The disease-causing agent is the bacterium Mycobacterium avium paratuberculosis, which lives in the intestinal cells and lymph nodes, causing progressive thickening of the bowel wall of the lower intestine and the upper large intestine.   PTB is an incurable wasting disease, variants of which affect humans and livestock.   According to a paper published in the Journal of Wildlife Diseases during 2002 by a team of scientists at the Universitat Autonoma de Barcelona, PTB was found in a population of about 1000 free-ranging Fallow deer (Dama dama) between 1997 and 1998 in Spain.   Five of the eight deer studied had diarrhoea and lung lesions, and M. a. paratuberculosis was cultivated from two.

It should be noted that the incidence of TB in deer is very low, as are records of deer carrying anthrax, foot-and-mouth disease and lime disease.   Bovine TB is currently a very hot topic in cattle and badger biology.

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