chronic wasting disease chronic wasting disease chronic wasting disease (cwd), sometimes called zombie deer disease, is a transmissible spongiform encephalopathy (tse) affecting deer. tses are a family of diseases thought to be caused by misfolded proteins called prions and include similar diseases such as bse (mad cow disease) in cattle, creutzfeldt-jakob disease (cjd) in humans and scrapie in sheep. in the united states, cwd affects mule deer, white-tailed deer, red deer, sika deer, elk, caribou, and moose. natural infection causing cwd affects members of the deer family. experimental transmission of cwd to other species such as squirrel monkeys and genetically modified mice has been shown. chronic wasting diseaseother nameszombie deer diseasedeer with signs of chronic wasting diseasespecialtyinfectious disease, neurology, veterinary medicinesymptomsbehavioral changes, weight loss, decreased interactions with other animals, tremors, repetitive walking in set patterns, increased drinking and urination, excessive salivation and grinding of the teeth, loss of fear of humans, confusionusual onset18–24 months after initial exposurecausesa type of priondiagnostic methodsuspected based on symptoms, confirmed by examination of the brainpreventionexterminating infected deer, incineration and disposal of infected deer carcassestreatmentnoneprognosisalways fatal in 1967, cwd was first identified in mule deer at a government 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 tse disease. since then, cwd has been found in free-ranging and captive animal populations in 30 us states and four canadian provinces. in addition, cwd has been found in one minnesota red deer farm, one wild reindeer herd in norway (march 2016) as well as in wild moose. single cases of cwd in moose have been found in finland (march 2018) and in sweden (march and may 2019, september 2020). cwd was found in south korea in some deer imported from canada. cwd is typified by chronic weight loss and clinical signs compatible with brain lesions, aggravated over time, always 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, by 2004 a study for the centers for disease control and prevention (cdc) suggested, "ore epidemiologic and laboratory studies are needed to monitor the possibility of such transmissions". the epidemiological study further concluded, "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". history chronic wasting disease was first discovered by american wildlife veterinarian beth s. williams. williams performed necropsies on deer and elk that had died of an unknown syndrome. she recognized that the brain lesions in these animals were consistent with transmissible spongiform encephalopathy (tse). in 1978, she and neuropathologist stuart young cowrote the first scientific paper that named the disease and described it as a tse. in february 2003, the centers for disease control published a report of "fatal degenerative neurologic illnesses in men who participated in wild game feasts--wisconsin, 2002". it read in conclusion that "although no association between (animal) cwd and (the feastors in wisconsin who happened to have) cjd was found, continued surveillance of both diseases remains important to assess the possible risk for cwd transmission to humans." in september 2003, hoey pointed out that one of the three patients (the 55 year-old) "presented with a 3-month history of difficulty in writing and unsteadiness of gait, followed by dementia, speech abnormalities and myoclonic jerking. pathologic examination of the brain at autopsy 3 months later revealed widespread subcortical spongiform lesions consistent with cjd." as of 2013 patrice klein the cwd program manager at usda/aphis rejected the somewhat inconclusive findings of hoey and stated that no evidence had been found of transmission to humans from deer, nor by eating animals in the deer family, but both channels remain a subject of public health surveillance and research. quebec farm outbreak the mffp ministry in quebec practiced 9500 tests in the period between 2007 and autumn 2018 before they detected a seropositive case in september 2018. the september 2018 discovery of cwd on a managed operation in grenville-sur-la-rouge quebec prompted a wholesale slaughter of 3500 animals in two months before the enterprise shut down permanently. the cfia ordered the cull, as well as the decontamination of 25 centimetres (9.8 in) of soil in certain places on the 400-hectare (990-acre) operation. post-discovery, each animal was tested for cwd by the cfia before it was released onto the market. other quebec producers lamented the glut of supply. a 400 km quarantine area was declared, in which all hunting and trapping activities were banned. the government slaughtered hundreds of wild animals over a two-month period. the routine cull for market was between 70 and 100 animals per week. when the producer was forced to close, the weekly slaughter neared 500 animals per week. one year later, 750 wild specimens had been culled in the 45 km-radius "enhanced monitoring area", and none tested positive for cwd. it came to light in august 2019 that prior to 2014 in canada, all animals on cwd-infected farms were buried or incinerated. since 2014, however, the cfia has allowed animals from cwd-infected farms to enter the food chain because there is "no national requirement to have animals tested for the disease". from one cwd-infected herd in alberta, 131 elk were sold for human consumption. for the fall 2019 hunting season in western quebec, the provincial ministry relaxed the rules for the annual white-tailed deer (wtd) hunt, in an effort to curb the spread of cwd. any wtd can be hunted with any weapon in certain municipalities in the outaouais valley and the laurentides. the mffp hopes thereby to receive more samples to test for cwd. the quarantine around grenville was still in place, and the ministry specifically prohibited (only) the "removal" from the quarantine "enhanced monitoring area" zone of "the head, more specifically any part of the brain, the eyes, the retropharyngeal lymph nodes and the tonsils, any part of the spinal column, the internal organs (including the liver and the heart), and the testicles." signs and symptoms due to cwd taking 18–24 months after initial exposure to onset, most cases of cwd occur in adult animals; the youngest animal to exhibit clinical symptoms of the disease was 15 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. loss of fear of humans and appearance of confusion are also common. the aphis summarized it as: behavioral changes, emaciation, weakness, ataxia, salivation, aspiration pneumonia, progressive death. cause the cause of cwd (like other tses, such as scrapie and bovine spongiform encephalopathy) is 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. genetics the allele which encodes leucine, codon 132 in the family of elks, is either homozygous ll, homozygous mm, or heterozygous ml. individuals with the first encoding seem to resist clinical signs of cwd, whereas individuals with either of the other two encodings have much shorter incubation periods. in white-tailed deer, polymorphisms at codons 95 (q->h) and 96 (g->s) dramatically affect cwd progression and prion strain specification. deer containing the 96s allele have delayed progression to clinical disease onset. when deer with the 95h polymorphism become infected, they select for altered strains of cwd. spread researchers in july 2019 stated that "with all the research on the malignity of prions, and the permanence of prions in the wider environment, and their resistance to destruction and degradation, it is necessary to reduce the potential sources of exposure to cwd." in fact an aphis scientist observed that, while the longevity of cwd prion is unknown, the scrapie prion has been measured to endure for 16 years. the prpcwd protein is insoluble in all but the strongest solvents, and highly resistant to digestion by proteases. prpcwd converts the normal protein prpc into more of itself upon contact, and binds together forming aggregates. prusiner noted in 2001 that: in the prion diseases, the initial formation of prpsc leads to an exponential increase in the protein, which can be readily transmitted to another host however, it is noted that as of 2013, although cwd prions were transmissible within the cervidae family, cwd was not considered transmissible to humans or to cattle. how the prions that cause cwd spread 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. animals born in captivity and those born in the wild have been affected with the disease. 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. exposure between animals is associated with sharing food and water sources contaminated with cwd prions shed by diseased deer. direct cwd may be directly transmitted by contact with infected animals, their bodily tissues, and their bodily fluids. spread may result from contact with infected deer regardless of if they are symptomatic. recent research on rocky mountain elk found that with cwd-infected cows, many subclinical, a high rate (80%) of maternal-to-offspring transmission of cwd prions occurred, regardless of gestational period. 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. modern news stories and anecdotal evidence from treating physicians suggest that a human may have contracted creutzfeldt-jakob disease via the consumption of squirrel brains infected with cwd. 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. other cervid species, including caribou, are also suspected to be naturally vulnerable to this disease. many other non-cervid mammalian species have been experimentally infected with cwd, either orally or by intracerebral inoculation. these species include monkeys, sheep, cattle, prairie voles, mice, and ferrets. 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. 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. in 2013, researchers at the national wildlife research center in fort collins, colorado successfully inoculated white-tailed deer with the misfolded prion via the nasal passage, when the prions were mixed with clay. this was important because the prions had already been shown by 2006 to bind with sandy quartz clay minerals. one avian scavenger, the american crow, was recently evaluated as a potential vector for cwd. as cwd prions remain viable after passing