Roisin Conneely
It’s that time of year when reindeer are everywhere, and whilst some of them may have magical luminescent noses and get looked after by Father Christmas, some are not so lucky. In recent years, a devastating disease has taken its toll on deer species, spreading even to Norway’s reindeer populations. So, what is this destructive disorder, and what could it mean for Rudolph and his pals?
Chronic wasting disease (CWD) is a contagious neurodegenerative disease caused by prions which affects members of the Cervidae family (aka, deer). Prions are misfolded proteins which form cysts in the brain and central nervous system (CNS). Prions are abnormal forms of the Prp protein, encoded by the PRNP gene. Researchers are still unsure as to the exact function of the Prp protein, however, it is believed to be involved in the functioning of the nervous system, with proposed roles in the growth of new neurones and the formation of synapses. When prions become misfolded, their new structure renders them resistant to degradation by enzymes in the body, as well as high temperatures, and conventional disinfection methods. These resistant prions are referred to as Prpres, Prpd (disease-causing), or specifically in CWD, as PrpCWD.
Find out more about the biology of prions in this video from Osmosis:
CWD is It is a transmissible spongiform encephalopathy (TSE) and currently has no cure. Another famous example of a TSE that you might be more familiar with is BSE, bovine spongiform encephalopathy, more commonly known as Mad Cow Disease. BSE is the only form of TSE known to be spread to humans, made famous by the 1990’s epidemic throughout Britain.
CWD was first observed in mule deer (Odocoileus hemionus) during studies by the University of Wyoming and Colorado State University in 1967. It has since spread to 23 US states, two Canadian provinces, and recently, to South Korea and Europe. Symptoms vary, but signals include behavioural changes, such as; isolation, hyperexcitability and lethargy. Additional symptoms include; difficulty swallowing, hypersalivation, polydipsia and polyuria (excessive thirst and urination, respectively). Disease-causing prions may persist in the environment for extended time periods, allowing for easy transmission between deer, which is why the disease is such a huge problem.
But why should we be worried about deer?
Deer play hugely important roles in their ecosystems, regulating plant grown, aiding seed dispersal, as well as being prey species for apex predators such as wolves (Canis lupus), coyotes (Canis latrans), or lynx (Lynx lynx) in European populations. Studies have also shown that deer herbivory has positive effects on other species, including red-backed salamanders (Plethodon cinereusand) and garter snakes (Thamnophis sirtalis). Hence, CWD-induced cervid declines may have effects on several other species.
The first case of CWD outside of North America appeared in South Korea in 2001, in elk (Cervus canadensis) imported from Canada in 1997. The deer were traced to a CWD infected farm in Canada, and it was found that nine of the 144 imported deer were infected. Eradication attempts have consisted of mass culling of imported deer and their offspring, plus individuals that may have come into contact with them. However, up to 2018, five outbreaks of the disease have been recorded, hence it seems that the mass-slaughter approach isn’t working, and may lead to declines in South Korean cervid populations.
Recently, CWD has appeared in Norway. Norway contains what is believed to be the last remaining population of wild tundra reindeer (Rangifer tarandus tarandus) in Europe, hence, CWD in Norway could become an extinction threat for this species. CWD was first observed in Norway in March 2016, when a herd of 400 wild reindeer were approached as part of another study. A diseased deer was found away from the herd and died shortly after being discovered. An autopsy revealed it was suffering from CWD.
So how did the disease arrive in Norway and begin infecting our beloved reindeer? Well, the origins of CWD in the country have been strongly debated; it has been proposed that it came from deer importation, as was the case in South Korea. However, this seems unlikely as the importation of deer is prohibited in Norway. One explanation could be the importation of urine baits from North America for hunting. Urine used for such lures is not processed, hence may contain infective prions. However, as previously discussed, prions are resistant to chemical treatment, hence processing of the product is unlikely to reduce its infectivity anyway. It could also be that CWD developed spontaneously in a single deer and spread throughout the population.
Could CWD spread to humans?
The short answer is, at the moment, no. The longer, scarier answer is, who knows? It was once believed that BSE couldn’t be transmitted to humans, and we all know how well that worked out. The disease has been artificially induced in several non-deer species, such as pigs, ferrets, and macaque monkeys, so never say never…
Further research is crucial to allow us to understand and manage the spread of the disease in reindeer. After all, who else will help Santa get all his presents delivered?
Don’t forget to follow us on Twitter and Instagram to keep up to date with our latest posts!
