Chronic wasting disease: Emerging prions and their potential risk

Prions cause fatal neurodegenerative diseases in humans and animals by converting the cellular prion protein PrP C into aggregation-prone PrP Sc .Chronic wasting disease (CWD) is a prion disease or transmissible spongiform encephalopathy (TSE) of free-ranging and farmed cervids.CWD is highly contagious and transmitted through horizontal transmission enabled by the shedding of prions in excreta and their persistence in the environment.The disease is undergoing a dramatic spread across North America, has been found in South Korea, and, recently, has been identified for the first time in Europe in free-ranging reindeer (Rangifer tarandus tarandus) and moose in Norway. CWDCWD first appeared in North America in captive animals in 1969 when biologists conducting a physiological study recognized that their animals often died with a syndrome of weight loss and behavioral changes.The biologists thought that the animals were probably dying of a nutritional deficiency due to toxin exposure or stressful conditions of captivity.In 1978, histological examination of brains of affected deer from wildlife research facilities in Colorado and Wyoming, United States, allowed us, for the first time, to recognize TSE features and define CWD as such [1].Since then, CWD has had an impressive expansion and a very rapid spread in North America; the geographic range of CWD includes 21 US states and 2 provinces in Canada (Saskatchewan and Alberta) and is most likely to continue to grow [2].Disease prevalence in free-ranging deer can be as high as 40% in the most endemic areas in Colorado and Wyoming [3].More recently, CWD has been described in Northern Europe [4].CWD is a slowly developing disease, with prolonged incubation periods.Experimental inoculation of mule deer has confirmed a 2-to 4-year incubation time [3].The disease has been found in yearling deer and elk as well as in adult animals as old as 15 years.Clinical symptoms include physical and behavioral changes [5].All these symptoms can be subtle early in the disease or fall within the normal repertoire of behavior or seasonal body mass fluctuations.Therefore, diagnosis based on clinical signs is not reliable and pathological or biochemical analyses of brain or lymphatic tissue are necessary.These will reveal neuronal vacuolation and spongiform changes, astrocytosis, and PrP Sc accumulation in the brain or PrP Sc deposits in lymphoid tissues.In most prion diseases, PrP Sc and infectivity are restricted mainly to the brain.However, in CWD, PrP Sc can be found in many extraneural tissues, body fluids, and excreta, facilitating horizontal transmission.