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Chronic wasting disease

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Chronic wasting disease
This deer visibly shows signs of chronic wasting disease.
SpecialtyVeterinary medicine

Chronic wasting disease (CWD), commonly referred to as zombie deer disease,[1] is a transmissible spongiform encephalopathy (TSE) of mule deer, white-tailed deer, elk (or "wapiti"), moose, and caribou. CWD primarily affects members of the deer family, although transmission of CWD to squirrel monkeys and mice and rice with human genes was shown.[2] In 1967, CWD was first identified in mule deer at a wildlife research facility in northern Colorado, United States. It was initially recognized as a clinical "wasting" syndrome and then in 1978, it was identified more specifically as a transmissible spongiform encephalopathy (TSE). Since then, CWD has spread to free-ranging and captive animal populations in 26 US states and three Canadian provinces.[3] CWD is typified by chronic weight loss leading to death. No relationship is known between CWD and any other TSEs of animals or people.

Although reports in the popular press have been made of humans being affected by CWD, a study by the Centers for Disease Control and Prevention suggests, "[m]ore epidemiologic and laboratory studies are needed to monitor the possibility of such transmissions".[4] The epidemiological study further concluded, "[a]s a precaution, hunters should avoid eating deer and elk tissues known to harbor the CWD agent (e.g., brain, spinal cord, eyes, spleen, tonsils, lymph nodes) from areas where CWD has been identified".[4]

Signs and symptoms

Most cases of CWD occur in adult animals; the youngest animal diagnosed with natural CWD was 17 months. The disease is progressive and always fatal. The first signs are difficulties in movement. The most obvious and consistent clinical sign of CWD is weight loss over time. Behavioral changes also occur in the majority of cases, including decreased interactions with other animals, listlessness, lowering of the head, tremors, repetitive walking in set patterns, and nervousness. Excessive salivation and grinding of the teeth also are observed. Most deer show increased drinking and urination; the increased drinking and salivation may contribute to the spread of the disease.[5]

Cause

The cause of CWD (like other TSEs, such as scrapie and bovine spongiform encephalopathy) is believed to be a prion, a misfolded form of a normal protein, known as prion protein (PrP), that is most commonly found in the central nervous system (CNS) and peripheral nervous system (PNS). The misfolded form has been shown to be capable of converting normally folded prion protein, PrPC ("C" for cellular) into an abnormal form, PrPSc ("Sc" for scrapie), thus leading to a chain reaction. CWD is thought to be transmitted by this mechanism. The abnormality in PrP has its genetic basis in a particular variant of the protein-coding gene PRNP that is highly conserved among mammals and has been found and sequenced in deer. The build-up of PrPd in the brain is associated with widespread neurodegeneration.[5][6][7] However, the prion cannot be detected in about 10% of CWD cases, leading some to suspect something else causes the diseases and that the prion is just a marker of infection. Another theory that has gained some support since 2017 is that the cause is a cell wall-deficient bacterium called Spiroplasma. Frank Bastian of Louisiana State University has long been unsuccessfully trying to grow Spiroplasma from the brains of infected animals, where it would grow for about 10 hours, then die. Bastian claims to have successfully cultured Spiroplasma from the brains of 100% of deer with CWD and sheep with scrapie after changing the growth medium. As a result, a Pennsylvania hunting agency, Unified Sportsmen of Pennsylvania, and scientists at the University of Alabama have taken interest in his work. Spiroplasma possesses many of the properties of the CWD agent, including resistance to heat and radiation, and it is thought that the body may trigger prion formation as a defense mechanism against Spiroplasma. The Prion may also protect Spiroplasma from attack by the adaptive immune system.[8][9][10]

Spread

As of 2016, no evidence has been found of transmission to humans from cervids, nor by eating cervids, but both channels remain a subject of public health surveillance and research.[7]

As of 2019, the Centers for Disease Control and Prevention has stated that 251 counties and 24 states in America have reported cases of Chronic Wasting Disease in deer.

Direct

CWD may be directly transmitted by contact with infected animals, their bodily tissues, and their bodily fluids.[11] Transmission may result from contact with both clinically affected and infected, but asymptomatic, cervids.[12]

Recent research on Rocky Mountain elk found that with CWD-infected dams, many subclinical, a high rate (80%) of maternal-to-offspring transmission of CWD prions occurred, regardless of gestational period.[12] While not dispositive relative to disease development in the fetus, this does suggest that maternal transmission may be yet another important route of direct CWD transmission.

Experimental transmission

In addition to the cervid species in which CWD is known to naturally occur, black-tailed deer and European red deer have been demonstrated to be naturally susceptible to CWD.[13] Other cervid species, including reindeer and caribou, are also suspected to be naturally vulnerable to this disease.[11] Many other noncervid mammalian species have been experimentally infected with CWD, either orally or by intracerebral inoculation.[11] These species include monkeys, sheep, cattle, prairie voles, mice, and ferrets.[14]

An experimental case study of oral transmission of CWD to reindeer shows certain reindeer breeds may be susceptible to CWD, while other subpopulations may be protective against CWD in free-ranging populations. None of the reindeer in the study showed symptoms of CWD, potentially signifying resistance to different CWD strains.[15]

Indirect

Environmental transmission has been linked to contact with infected bodily fluids and tissues, as well as contact with contaminated environments. Once in the environment, CWD prions may remain infectious for many years. Thus, decomposition of diseased carcasses, infected "gut piles" from hunters who field dress their cervid harvests, and the urine, saliva, feces, and antler velvet of infected individuals that are deposited in the environment, all have the potential to create infectious environmental reservoirs of CWD.[5]

One avian scavenger, the American crow, was recently evaluated as a potential vector for CWD.[16] As CWD prions remain viable after passing through the bird's digestive tract, crows represent a possible mechanism for the creation of environmental reservoirs of CWD.[16][17] Additionally, the crows' extensive geographic range presents ample opportunities for them to come in contact with CWD. This, coupled with the population density and longevity of communal roosting sites in both urban and rural locations, suggests that the fecal deposits at roosting sites may represent a CWD environmental reservoir.[16] Conservative estimates for crows' fecal deposits at one winter roosting site for one winter season ranged from 391,552 to 599,032 kg.[16]

CWD prions adhere so tightly to soil surface particles that the ground becomes a source of infection and may be a major route of transmission due to frequent ground contact when cervids graze.[5]

Diagnosis

Diagnosis is based on post mortem examination (necropsy) and testing; examination of the dead body is not definitive, as many animals die early in the course of the disease and conditions found are nonspecific; general signs of poor health and aspiration pneumonia, which may be the actual cause of death, are common. On microscopic examination, lesions of CWD in the CNS resemble those of other TSEs. In addition, scientists use immunohistochemistry to test brain, lymph, and neuroendocrine tissues for the presence of the abnormal prion protein to diagnose CWD; positive IHC findings in the obex is considered the gold standard.[5]

As of 2015, no commercially feasible diagnostic tests could be used on live animals. Running a bioassay, taking fluids from cervids suspected of infection and incubating them in transgenic mice that express the cervid prion protein, can be used to determine whether the cervid is infected, but ethical issues exist with this, and it is not scalable.[5]

A tonsillar biopsy technique has been a reliable method of testing for CWD in live deer,[18] but it only seems to be effective on mule deer and white-tailed deer, not elk.[19] Biopsies of the rectal mucosa have also been effective at detecting CWD in live mule deer, white-tailed deer, and elk, though detection efficacy may be influenced by numerous factors including animal age, genotype, and disease stage.[20][21][22][23]

Epidemiology

The origin and mode of transmission of the prions causing CWD is unknown, but recent research indicates that prions can be excreted by deer and elk, and are transmitted by eating grass growing in contaminated soil.[24][25]

Animals born in captivity and those born in the wild have been affected with the disease. Based on epidemiology, transmission of CWD is thought to be lateral (from animal to animal). Maternal transmission may occur, although it appears to be relatively unimportant in maintaining epidemics. An infected deer's saliva is able to spread the CWD prions.[26] Exposure between animals is associated with sharing food and water sources contaminated with CWD prions shed by diseased deer.[27]

The disease was first identified in 1967 in a closed herd of captive mule deer in contiguous portions of northeastern Colorado. In 1980, the disease was determined to be a TSE. It was first identified in wild elk and mules in 1981 in Colorado and Wyoming, and in farmed elk in 1997.[5][6][7]

In May 2001, CWD was also found in free-ranging deer in the southwestern corner of Nebraska (adjacent to Colorado and Wyoming) and later in additional areas in western Nebraska. The limited area of northern Colorado, southern Wyoming, and western Nebraska in which free-ranging deer, moose, and/or elk positive for CWD have been found is referred to as the endemic area. The area in 2006 has expanded to six states, including parts of eastern Utah, southwestern South Dakota, and northwestern Kansas. Also, areas not contiguous (to the endemic area) areas in central Utah and central Nebraska have been found. The limits of the affected areas are not well defined, since the disease is at a low incidence and the amount of sampling may not be adequate to detect it. In 2002, CWD was detected in wild deer in south-central Wisconsin and northern Illinois and in an isolated area of southern New Mexico. In 2005, it was found in wild white-tailed deer in New York and in Hampshire County, West Virginia.[28] In 2008, the first confirmed case of CWD in Michigan was discovered in an infected deer on an enclosed deer-breeding facility. It is also found in the Canadian provinces of Alberta and Saskatchewan.

Chronic wasting disease in North America

In February 2011, the Maryland Department of Natural Resources reported the first confirmed case of the disease in that state. The affected animal was a white-tailed deer killed by a hunter.[29]

CWD has also been diagnosed in farmed elk and deer herds in a number of states and in two Canadian provinces. The first positive farmed-elk herd in the United States was detected in 1997 in South Dakota. Since then, additional positive elk herds and farmed white-tailed deer herds have been found in South Dakota (7), Nebraska (4), Colorado (10), Oklahoma (1), Kansas (1), Minnesota (3), Montana (1), Wisconsin (6), and New York (2). As of fall of 2006, four positive elk herds in Colorado and a positive white-tailed deer herd in Wisconsin remain under state quarantine. All of the other herds have been depopulated or have been slaughtered and tested, and the quarantine has been lifted from one herd that underwent rigorous surveillance with no further evidence of disease. CWD also has been found in farmed elk in the Canadian provinces of Saskatchewan and Alberta. A retrospective study also showed mule deer exported from Denver to the Toronto Zoo in the 1980s were affected. In June 2015, the disease was detected in a male white-tailed deer on a breeding ranch in Medina County, Texas. State officials euthanized 34 deer in an effort to contain a possible outbreak.

In February 2018, the Mississippi Department of Wildlife, Fisheries, and Parks announced that a Mississippi deer tested positive for chronic wasting disease.[30] Another Mississippi whitetail euthanized in Pontotoc County on Oct 8, 2018 tested positive for CWD. The disease was confirmed by the National Veterinary Services Lab in Ames, IA on Oct. 30, 2018.<Times Daily Florence, AL 11/12/18

Species that have been affected with CWD include elk, mule deer, white-tailed deer, black-tailed deer, and moose. Other ruminant species, including wild ruminants and domestic cattle, sheep, and goats, have been housed in wildlife facilities in direct or indirect contact with CWD-affected deer and elk, with no evidence of disease transmission. However, experimental transmission of CWD into other ruminants by intracranial inoculation does result in disease, suggesting only a weak molecular species barrier exists. Research is ongoing to further explore the possibility of transmission of CWD to other species.

By April 2016, CWD had been found in captive animals in South Korea; the disease arrived there with live elk that were imported for farming in the late 1990s.[31]

In the summer of 2018, cases were discovered in the Harpur Farm herd in Grenville-sur-la-Rouge, Quebec.[32]

Europe

In 2016, the first case of CWD in Europe was from the Nordfjella free-ranging reindeer in southern Norway. Scientists surveyed the diseased female reindeer until the reindeer died and used the carcass to isolate the prions. The main origin of CWD to Norway is still unknown, whereas importation of infected deer was the contamination source in South Korea. Norway has strict legislation and rules not allowing importation of live animals and cervids into the country. Norway has had a scrapie surveillance program since 1997; while no reports of scrapie within the range of Nordfjella reindeer population have been identified, sheep are herded through that region and are a potential source of infection.[33]

In May and June 2016, infected wild moose were found around 300 km north from the first case, in Selbu.[34][35] By the end of August, a fourth case had been confirmed in a wild reindeer shot in the same area as the first case in March.[36]

In 2017, the Environment Agency of the Norwegian government released guidelines for hunters hunting reindeer in the Nordfjella areas. The guidelines contain information on identifying animals with CWD symptoms and instructions for minimizing the risk of contamination, as well as a list of supplies given to hunters to be used for taking and submitting samples from shot reindeer.[37]

In February 2018, Finnish Food Safety Authority EVIRA stated that the first case of CWD in Finland has been diagnosed on a 15-year old European elk that had died naturally in the municipality of Kuhmo in the Kainuu region. Before this case in Kuhmo, Norway was the only country in the European Economic Area where CWD has been diagnosed. The elk did not have the transmissible North American form of the disease, but it had the Norwegian form of CWD, which occurs incidentally in individual animals of the deer family. In Finland, the occurrence of the disease has been studied already since 2003 and none of the roughly 2,500 samples analyzed so far have tested positive for the disease. The exportation of live animals of the deer family to other countries has been temporarily banned as a precautionary measure to stop the spread of the CWD, and elk hunters are going to be provided with more instructions before the start of the next hunting season, if appropriate. The exportation and sales of meat from the animals of deer family will not be restricted and moose meat is safe to eat as only the brain and nervous tissue of infected moose contains prions.[38]

Research

Research is focused on better ways to monitor disease in the wild, live animal diagnostic tests, developing vaccines, better ways to dispose of animals that died from the disease and to decontaminate the environment, where prions can persist in soils, and better ways to monitor the food supply. Deer harvesting and management issues are intertwined.[39]

See also

References

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