Chronic Wasting Disease of Deer
CWD is one of the fatal neurodegenerative diseases classed as transmissible spongiform encephalopathies (TSE). It appears to infect only members of the Cervidae and seems to be unrelated to any other TSE. It has so far only been seen in deer of N American birth.
CWD was first recognised as a clinical entity in the late 1960's in captive mule deer and elk ( i.e.wapiti, Cervus elaphus Canadensis) in a research facility in Colorado and Wyoming. Disease modelling techniques suggest it may have been present in wild deer in the 1950's but it was not diagnosed in wild deer until the 1980's when diseased white-tailed deer, mule deer and elk were seen in Colorado and Wyoming. Since that time the disease has been diagnosed in wild deer in Wisconsin, South Dakota, Colorado, New Mexico and Illinois and, in Canada in some free ranging white tailed deer in Saskatchewan. Amongst farmed deer it has been found in 26 elk farms and two deer farms in the USA and in Canada in farmed deer and elk herds in Saskatchewan and Alberta.
Experimental inoculation has succeeded in infecting sheep, goats and ferrets but so far natural transmission to other species has not occurred. Cattle fed CWD infected material remained healthy until at least six years.
Deer species are not normally affected until at least 18 months of age with peak incidence at 3 – 5 years. Most clinically affected animals die within twelve months. Signs are emaciation, excessive salivation, drooping of head and ears, protruding eyes, increased thirst and urination, pneumonisa and trauma induced injuries. Behavioural signs are loss of fear of humans, ataxia, repetitive movements, and an inability to judge space and distance.
Pathologically lesions in the supraoptic and paraventricular nuclei are thought to reduce the synthesis of antidiuretic hormone leading to polyuria and polydipsia. The dorsal motor nucleus of the vagus nerve is the site first affected and it has been suggested that infection travels up the vagus.
The transmission routes of CWD remain unclear but it seems certain that the disease can be transmitted by saliva, urine or most probably faeces. Healthy deer introduced into paddocks vacated by infected animals have succumbed in about 18 months. This unlike other TSE's although scrapie can be transmitted through infected birthing fluids and placentae.
Control of CWD has been pursued vigorously. In Canada, by July 2003, 8,700 deer had been destroyed as a result of tracings, yielding 232 positives of which only 24 were clinically affected. In addition screening of 35,500 culled deer by immunohistochemistry of the obex took place between 1996 and 2002.
The economic costs go very far beyond the public funding of the immediate costs of tracing, slaughter and disinfection. From a very buoyant elk farming industry in which elk cows in Canada were being sold for 20,000 Canadian dollars prices fell rapidly to a few hundred dollars. Those farms that were found to have been infected are forbidden to ever carry livestock again.
The potential for damage to the wild deer industries is colossal and it is believed that both hunting and eco-tourism have already been adversely affected: in Wisconsin in 2002 a 10% drop in hunting caused a 100 million $US dip in a 1.7 billion $ industry.
The size of the US hunting and eco-tourism industries is enormous: in 2001 an estimated 66 million people spent 38.5 billion US dollars watching wildlife whilst a further 11 million hunters spent another ten billion US dollars. These fgures can probably be doubled allowing for re-circulation of the money. In Michigan in 2003, 775,000 hunters killed 360,000 deer and contributed 1.2 billion to Michigan's economy.