Chronic Wasting Disease Flashcards
Importance Chronic Wasting Disease
Chronic wasting disease (CWD) is a neurodegenerative disease caused by a prion that affects cervids including deer, elk and moose.
CWD is now found in up to 26 US states, parts of Canada, South Korea (outbreaks captive cervids 2004-2010), and Norway (wild cervids in 2016).
CWD can be devastating in farmed herds. This disease is always fatal once the clinical signs appear, and most or all of the herd can eventually become infected. It is one of the most difficult prion diseases to control: CWD prions are transmitted from animal to animal, and they can also be spread from contaminated environments for up to two years or more.
Thousands of captive or wild deer and elk have been killed in the U.S. and Canada in control efforts.
Potential for CWD to affect other species, including humans.
Cooking does not destroy prions, and ingestion of another prion, the agent that causes bovine spongiform encephalopathy (BSE), has been linked to a fatal human neurological disease. CWD prions have been found in muscle (meat), as well as other tissues of cervids, and could enter the food supply.
The evidence so far suggests that CWD does not affect humans, livestock or wild predators of cervids; nevertheless, the possibility that it could be zoonotic has not been ruled out.
Etiology Chronic Wasting Disease
Prions are infectious proteins that appear to replicate by converting a normal cellular protein into copies of the prion. The cellular protein, which is called PrPc, is found on the surface of neurons. Pathogenic isoforms of PrPc are designated PrPres; PrPCWD or PrPTSE are other names for this protein. There may be more than one variant of the CWD prion
Species Affected Chronic Wasting Disease
Chronic wasting disease affects a number of cervid species.
Naturally acquired cases have been reported in mule deer (Odocoileus hemionus), black-tailed deer (O. hemionus columbianus), white-tailed deer (O. virginianus), Rocky Mountain elk (Cervus elaphus nelsoni), captive red deer (Cervus elaphus elaphus), moose (Alces alces) and wild reindeer (Rangifer tarandus tarandus).
Fallow deer (Cervus dama dama) might be relatively resistant to CWD.
There is currently no evidence that CWD prions have infected any animals other than cervids in nature
Geographic Distribution Chronic Wasting Disease
To be updated:
Chronic wasting disease was originally reported only from a limited area of the U.S. encompassing northeastern Colorado, southwestern Nebraska, and southeastern Wyoming, but it has now spread extensively in North America
Transmission Chronic Wasting Disease
- Horizontally between cervids by direct contact, environmental contamination or a combination of these routes.
- Grazing is thought to be important in acquiring these prions from the environment. In deer, prions have been detected in saliva, blood, urine, feces and antler velvet, and some sources (e.g., saliva, feces, urine) can contain these agents before the animal develops clinical signs. (concentration of prions in urine and feces is very low, the volume of these excretions could contribute significantly to environmental contamination).
- Prions have also been detected in the skeletal muscles and fat of deer, and in heart muscle from white-tailed deer, elk and experimentally infected red deer. The occurrence of prions in blood and lymphoid tissues suggests that no tissues from infected cervids should be considered prionfree.
- Vertical transmission has, to date, been documented in the offspring of wild elk, as well as in experimentally infected Reeves’ muntjac deer.
- Whether CWD prions can be shed in milk is unknown.
- CWD prions can be transmitted on contaminated fomites, even from animals not yet displaying clinical signs, and seem to persist for a few years in the environment (scavengers or predators).
- Prions can bind to soils, and soil-bound prions are infectious for cervids. Their persistence differs between soil types. Repeated cycles of wetting and drying soil in the laboratory are reported to decrease, though not necessarily eliminate, infectivity.
- CWD prions have been transmitted experimentally between deer by the oral and intranasal routes, in aerosols and by blood transfusion.
which species transmit CWD less efficiently?
Elk, which have smaller amounts of prions in lymphoid tissues than deer, are thought to transmit CWD less efficiently.
Disinfection Chronic Wasting Disease
very resistant to most disinfectants, including formalin and alcohol. They are also resistant to heat, ultraviolet radiation, microwave irradiation and ionizing radiation, particularly when they are protected in organic material or preserved with aldehyde fixatives, or when the prion titer is high. Prions can bind tightly to some surfaces, including stainless steel and plastic, without losing infectivity. Prions bound to metal seem to be highly resistant to decontamination.
A 1-2 N sodium hydroxide solution, or a sodium hypochlorite solution containing at least 2% (20,000 ppm) available chlorine, has traditionally been recommended for equipment and surfaces. Surfaces should be treated for more than 1 hour at 20°C (68°F). Overnight disinfection is recommended for equipment
autoclaving at 134°C (273°F) for 18 minutes. Some reviews also recommend 132°C (269oF) for 1 hour (gravity displacement sterilizer)
composting may reduce or eliminate CWD and other prions in tissues, while another suggested that soil microorganisms might degrade prions in buried carcasses.
Incubation Period Chronic Wasting Disease
The minimum incubation period is thought to be approximately 16 months, and the average incubation period is probably 2 to 4 years. The peak incidence occurs between the ages of 2 and 7 years
Clinical Signs Chronic Wasting Disease
Chronic wasting disease is always fatal.
typically, cervids develop progressive weight loss, lassitude and behavioral changes over several weeks to months, with many animals becoming severely emaciated before they die. Ataxia, head tremors, teeth grinding, repetitive walking of the enclosure’s perimeter, fixed gaze, hyperexcitability when handled, or other neurologic signs may be seen.
A few apparently asymptomatic animals, or animals with mild clinical signs (e.g., mild ataxia, weight loss), may die suddenly after handling (assoc. particular genotype).
Diagnostic Tests Chronic Wasting Disease
Histological examination of the brain (medulla oblongata at the level of the obex).
This disease is usually confirmed by detecting prions in the CNS and/or the lymphoid tissues, especially the retropharyngeal lymph node, at necropsy.
In live animals, chronic wasting disease can be diagnosed with palatine tonsil (deer) or rectal lymphoid biopsies (deer and elk). Rectal lymphoid biopsies could also identify some experimentally infected reindeer
Immunohistochemistry (‘gold standard’). Immunoblotting (Western blotting) and rapid tests including enzyme-linked immunosorbent assays (ELISAs). Also: protein misfolding cyclic amplification (PMCA) and quaking-induced conversion or real-time quaking-induced conversion. can sometimes find prions in blood, saliva, urine, feces or other samples from subclinically infected animals, as well as cerebrospinal fluid of animals with clinical signs
Prevention Chronic Wasting Disease
The risk of introducing chronic wasting disease can be reduced by maintaining a closed herd or minimizing outside purchases of stock. If replacement animals must be added, they should be from herds known to be negative for this disease. Voluntary and/or mandatory programs to control or manage CWD in farmed cervids have been established in the U.S. and Canada, with eradication from captive populations as the ultimate goal. Control programs are based on the identification of individual animals, fencing, restrictions on herd additions, and CWD testing of cervids that die on the farm or are slaughtered. Further details on these programs are available online at federal websites (see Internet Resources), as well as from individual states and provinces. Whether it would be possible to breed captive cervids with genotypes more resistant to CWD, without increasing the risk of asymptomatic prion shedding (and consequent environmental contamination), is not yet clear.
When CWD is found in captive cervids, the herd is usually quarantined. Herd and premises plans are developed and, in some cases, the herd is depopulated. Carcasses from CWD-infected animals cannot be used as food for humans or other animals and must be destroyed. Korean authorities have attempted to eradicate CWD by slaughtering all imported deer, as well as indigenous deer that had been in contact with these deer or were born from them. Controlling CWD is very difficult in wild cervids. Some states have banned practices that encourage cervids to congregate in certain areas (e.g., feeding or baiting), in an attempt to decrease transmission rates between animals in close contact. Some states have also culled their herds to reduce population density and decrease the spread of disease. Evidence for the effectiveness of such culling programs varies, although some studies suggest that they have been beneficial in some cases. Culling programs might be able to eradicate CWD from a limited area if it was introduced recently. Infected captive cervids should be kept from contact with wild cervids. Many states and provinces also have restrictions on the transportation of tissues from hunter-killed cervids in CWD endemic areas. These guidelines are available at the Chronic Wasting Disease Alliance website (see Internet Resources), state websites and other sources
