REVIEW ARTICLE Chronic Wasting Disease · mule deer (Odocoileus hemionus), white-tailed deer (O....

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Vet Pathol 42:530–549 (2005)

REVIEW ARTICLE

Chronic Wasting DiseaseE. S. WILLIAMS1

Department of Veterinary Sciences, University of Wyoming, Laramie, WY

Abstract. Chronic wasting disease (CWD) is a unique transmissible spongiform encephalopathy (TSE) ofmule deer (Odocoileus hemionus), white-tailed deer (O. virginianus), and Rocky Mountain elk (Cervus elaphusnelsoni). The natural history of CWD is incompletely understood, but it differs from scrapie and bovine spon-giform encephalopathy (BSE) by virtue of its occurrence in nondomestic and free-ranging species. CWD hasmany features in common with scrapie, including early widespread distribution of disease-associated prionprotein (PrPd) in lymphoid tissues, with later involvement of central nervous system (CNS) and peripheraltissues. This distribution likely contributes to apparent efficiency of horizontal transmission and, in this, issimilar to scrapie and differs from BSE. Clinical features and lesions of CWD are qualitatively similar to theother animal TSEs. Microscopically, marked spongiform lesions occur in the central nervous system (CNS)after a prolonged incubation period and variable course of clinical disease. During incubation, PrPd can beidentified in tissues by antibody-based detection systems. Although CWD can be transmitted by intracerebralinoculation to cattle, sheep, and goats, ongoing studies have not demonstrated that domestic livestock aresusceptible via oral exposure, the presumed natural route of exposure to TSEs. Surveillance efforts for CWDin captive and free-ranging cervids will continue in concert with similar activities for scrapie and BSE. Erad-ication of CWD in farmed cervids is the goal of state, federal, and industry programs, but eradication of CWDfrom free-ranging populations of cervids is unlikely with currently available management techniques.

Key words: Chronic wasting disease; elk; mule deer; prion disease; transmissible spongiform encephalop-athy; white-tailed deer.

The transmissible spongiform encephalopathies(TSEs) are unusual infectious diseases of animals andhumans. The TSEs, including chronic wasting disease(CWD), are designated prion diseases because of theirassociation with aberrantly refolded isoforms of theprion protein, a normal cellular glycoprotein (PrPC).120CWD-associated prion protein (PrPCWD or PrPd)82 iswidespread in the lymphoid tissues and the CNS fromearly in the incubation phase until death.98,132,137 Scra-pie, the first TSE identified, was the focus of consid-erable research and even controversy concerning itsorigin and nature (genetic versus infectious) for manyyears.2,114,115 However, it was the recognition that priondiseases were transmissible and affected humans aswell as animals that stimulated much of the currentscientific interest in the nature of these diseases.Scrapie has been recognized for hundreds of years,

transmissible mink encephalopathy (TME) for more

1 Deceased December 29, 2004. This paper was in revi-sion at the time of Dr. Williams’s death. Final edits werecompleted by her collaborators, Dr. Jean Jewell at the Uni-versity of Wyoming and Dr. Mike Miller of the ColoradoDivision of Wildlife.

than 50 years, CWD for more than 30 years, and bo-vine spongiform encephalopathy (BSE) for fewer than20 years. Although CWD is now well known, it wasjust a few years ago considered an obscure disease ofmule deer (Odocoileus hemionus) and elk (Cervus ela-phus nelsoni) in a limited area of North America. Thefirst published reports of CWD were in the early1980s,157,158 but well before that time, biologists work-ing in facilities in which this disease occurred recog-nized CWD as a syndrome that limited the lifespan ofthe captive mule deer they were studying. It was yearsbefore it was determined that this wasting syndromeof deer was contagious. Since the BSE epidemic, fol-lowed by recognition of the associated variant Creutz-feldt-Jacob disease (vCJD) of people, there has beenmuch greater scientific and public attention paid toTSEs in general and concordant increased interest inunderstanding CWD. This has resulted in studies ofCWD that now span the spectrum from pathogenesisto spatial epidemiology.CWD occurs among two very different populations

of cervids: deer and elk that are raised and managedin much the same way as other ruminants in agricul-tural systems and free-ranging deer and elk popula-

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Vet Pathol 42:5, 2005 531Chronic Wasting Disease

Fig. 1. Known historic distribution of chronic wastingdisease in farmed and free-ranging cervids in North Amer-ica, 1977–2004. Dark gray jurisdictions indicate whereCWD has been identified in farmed cervids; to date all buta few identified farmed cervid herds known to have hadCWD have been depopulated. Red foci are areas whereCWD has been diagnosed in free-ranging cervids. The yel-low focus indicates occurrence of CWD in mule deer in azoological garden. In March–May 2005, the New York StateDepartment of Agriculture and Markets announced five pos-itive cases of chronic wasting disease in captive white-taileddeer from two closely connected herds in central New York(Oneida County) and two positive cases in wild white-taileddeer from the same area.104

tions that are under much less human control. Under-standing differences in epidemiology and managementstrategies that can be employed to control or eradicateCWD for these two populations of animals is critical.In this review, I will survey the rapidly accumulatingknowledge about CWD with a focus on informationmost relevant for veterinary pathologists.

Clinical and Epidemiologic FeaturesDescriptions of clinical CWD are based primarily

on observations of captive animals; the progression ofCWD in free-ranging animals is less well known be-cause of the inherent difficulties in their study. Fea-tures of CWD in terminal clinical stages are readilyappreciated, even by untrained observers and havebeen described.155,159 Qualitatively, they are similar tosheep with terminal scrapie40 or cattle with BSE,151 al-though disruptions and alterations of locomotion canbe more subtle in CWD. The salient clinical featuresof progressive CWD disease in adults is weight lossand behavioral changes that typically span weeks ormonths. In addition to these general nonspecific fea-tures, signs that might only be present in some casesinclude odontoprisis; sialorrhea, or excess salivationbecause of difficulty swallowing; ataxia and headtremors; esophageal dilation and regurgitation; and as-piration pneumonia. Terminal physiologic and behav-ioral alterations can include polydipsia and polyuria;syncope; periods of lack of awareness; fixed stare;changes in interaction with herdmates and handlers(decreased or increased flight distances); alteredstance, often with lowered head; repetitive walking ofperimeters of enclosures; and hyperexcitability whenhandled. In general, the signs of terminal CWD aremore subtle in elk than in deer. Elk more often havedisturbances in locomotion and less often display poly-dipsia than deer.Pruritus with hair loss, commonly observed in ter-

minal sheep scrapie, is not a feature of advancedCWD. However, the hair coat of affected animals canbe rough and dry,158 with patchy retention of the winterhair coat in summer.10,159 This appears to be a reflectionof poor body condition rather than a primary effect ofthe disease.No specific clinical diagnostic feature of CWD ap-

pears during the early to midphase of disease. Behav-ioral changes can be subtle and fall within the normalrepertoire of cervids. Similarly, normal seasonalchanges in body mass occur in free-ranging and cap-tive cervids; thus, evaluation of body condition musttake into account time of year, nutritional or habitatquality, and reproductive status.Polydipsia and consequent polyuria in terminal cas-

es of CWD are probably associated with damage tothe supraoptic and paraventricular nuclei and diabetes

insipidus.157 With the exception of low urine specificgravity, found in terminally affected cervids with freeaccess to water,157,158 clinical chemistry and hematol-ogy are not useful in diagnosis of CWD. Alterationsfrom normal cervid values reflect nonspecific changesfrom poor body condition, aspiration pneumonia, oranother intercurrent disease process. Specific physio-logic studies have not been conducted on CWD-af-fected cervids.Deer with subclinical or early clinical CWD are sus-

ceptible to sudden death after handling100 and are morelikely to die following immobilization or misadventurethan unaffected deer155 (L. Wolfe et al., personal com-munication; W. Cook et al., personal communication).Although the mechanism of this posthandling mortal-ity has not been identified, functional lesions in theparasympathetic innervation of the heart have beenidentified in BSE;7 similar processes might occur inCWD, accounting for acute mortality following sig-nificant stress.Aspiration pneumonia can occur early or late in the

clinical course of CWD155,159 and could be responsiblefor apparent rapid death, even in the absence of othersigns of CWD. Thus, CWD should be considered inany adult cervid with aspiration pneumonia.156 Aspi-ration pneumonia presumably follows loss of effective



532 Vet Pathol 42:5, 2005Williams

motor control over swallowing associated with regur-gitation or rumination due to central damage.159Subclinical or clinically apparent changes in behav-

ior could increase the likelihood of an affected free-ranging deer or elk being harvested during the fallhunt,35 succumbing to collisions with vehicles,90,100 orbeing killed by predators90 (M. Miller, personal com-munication).Duration of clinical disease is extremely variable

and in part might reflect difficulty in determining theonset of clinically apparent CWD. Astuteness in de-tection of subtle clinical signs might require consid-erable familiarity with the individual animal; this is notpossible in many captive herds and is essentially im-possible in free-ranging cervids. Sudden death mightoccur rarely in captive deer,100 and short duration ofdisease (days) is occasionally observed in elk (W.Schultz and T. Kreeger, personal communication).More often, a slowly progressive clinical course of‘‘ain’t doing right’’ is noted over weeks or months.Death typically occurs within 4 months, although afew animals survive as long as a year.155 Death of clin-ically affected animals might follow environmentalstresses, such as periods of extreme cold. Because ofa compromised ability to forage, find water, and pos-sibly avoid predators, clinical duration is likely to beshorter in free-ranging cervids compared with cervidsmaintained in captivity.Because of prolonged incubation periods, which in

the natural disease is a minimum of approximately 16months, fawns do not develop clinical CWD, althoughevidence of infection can be detected by a variety ofmethods through much of the incubation period.111,132Under experimental conditions, disease-associated pri-on protein (PrPd) was found in lymphoid tissues of thealimentary tract in mule deer by 42 days postoral ex-posure.132 Only rarely are yearlings clinically affected.Maximum incubation periods of naturally exposedfree-ranging animals are difficult to determine becauseit is impossible to determine exactly when an animalis exposed, but average incubation periods probablyrange from 2 to 4 years. However, CWD has beendiagnosed in an elk �15 years of age155, a mule deer�12 years of age (unpublished data), and in white-tailed deer of �5 years of age111 residing in CWD en-demic facilities with high prevalence of the disease.These cases might represent animals exposed to theCWD agent as adults or could indicate prolonged in-cubation periods. The influence of dose of agent onduration of incubation is not clear in CWD, but pre-liminary observations of experimentally and naturallyexposed captive elk suggest that, to a degree, higherdoses result in shortened incubation periods (E. S. Wil-liams, unpublished data; T. Kreeger, personal com-munication). Incubation periods in rodents exposed to

scrapie48,88,121 are inversely related to dose of TSEagent, and a similar trend appears to be present in cat-tle orally exposed to BSE agent (G. Wells, personalcommunication).Seasonality is not a significant feature of CWD at

the individual level because animals can develop clin-ical CWD at any time of the year. However, on a pop-ulation level, affected animals are more commonly re-ported in the fall, perhaps because of an observationalbias, or in the winter, possibly reflecting increased en-vironmental stress.Epidemiologic features should be considered when

investigating possible cases of CWD. In farmed herdsnaturally affected by CWD, rarely will more than oneanimal be affected at a time,87,117,134 although there isoften herd history of ill thrift in adult animals. Deerand elk from captive herds that have a history of fre-quent purchases from unmonitored sources or fromherds in known CWD endemic areas (Fig. 1) are atgreater risk for CWD than animals from closed andmonitored herds and from herds outside CWD endem-ic areas.Within CWD endemic areas, the vast majority of

CWD cases in free-ranging deer and elk detected inthe course of surveillance activities are subclinical(�97%).98 Clinically affected free-ranging cervids arerelatively rare, and even within endemic areas onlyabout a third to half of clinical suspects examined indiagnostic laboratories have CWD (E. S. Williams, un-published data). However, surveillance programs infree-ranging cervids that target deer and elk that showclinical signs compatible with CWD have been suc-cessful at identifying CWD in a more efficient andmore cost effective manner than widespread hunter-harvested animal surveillance.100,103,156Prevalence of clinical or subclinical CWD infection

as detected by immunohistochemistry (IHC) of lym-phoid tissue or brain in captive herds varies consid-erably from �1% in some farmed herds with recentintroduction of the disease to essentially 100% inCWD endemic research facilities.111,117,157 Likewise,prevalence varies widely in free-ranging populationsfrom �1% in deer and elk to �30% in some localpopulations of deer103 (W. Cook, personal communi-cation; Wyoming Game and Fish Department and Col-orado Division of Wildlife, unpublished data). Preva-lence of CWD in elk is lower than in sympatric deer.101

Gross PathologyAs in the other animal TSEs, individual gross le-

sions of CWD are nonspecific, but in combination,they may be helpful in diagnosis of CWD. Lesions thatmight be present reflect the clinical signs: rough, dryhair coat that might not have been appropriately shedand megaesophagus. It is important to realize that




Table 1. Published reports of tissue distribution of histologic lesions, PrPd, or infectivity detected in cervids with CWDby histopathology, immunohistochemistry, enzyme-linked immunosorbent assay, western blot immunoassay, or bioassay.

Species Tissue Detection Method*

Mule deerBrain Histopathology, IHC, WB, infectivi-

ty52,106,123,125,132,135,137,138,157,160,161Spinal cord Histopathology, IHC135,137,138,160Tonsil IHC, ELISA98,112,132,136,138,150,163Retropharyngeal lymph node IHC71,98,132,138Alimentary tract–associated lymphoid tissue IHC132,138Peripheral lymph nodes IHC132,138Spleen IHC138Retina IHC138Peripheral nervous system IHC131Enteric nervous system IHC131Endocrine organs IHC131,138

White-tailed deerBrain Histopathology, IHC, WB, infectivity92,123,125,135,153Spinal cord Histopathology135Tonsil IHC150Retropharyngeal lymph node IHC, ELISA71

ElkBrain Histopathology, IHC, WB51,106,117,123,125,135,158,160Spinal cord Histopathology, WB133,160Tonsil IHC, ELISA134Retropharyngeal lymph node IHC, ELISA71,134

* IHC � immunohistochemistry; WB � western immunoblot; ELISA � enzyme-linked immunosorbent assay.

body condition of animals in subclinical or early clin-ical stages of CWD can be normal; good body con-dition does not rule out CWD. In the terminal stagesof disease, typically the animals are emaciated; mightor might not have aspiration pneumonia; often havewatery rumen contents, which can be frothy or containincreased amounts of sand and gravel; and have diluteurine if water was freely available.155,159 Carcasses offree-ranging deer and elk might be dehydrated, pre-sumably due to a lack of ready access to water. Otherpotential causes of emaciation and pneumonia shouldbe ruled out during the course of postmortem exami-nation of cervids suspected of having CWD.

Histopathology and Electron MicroscopyThe histopathology of CWD in clinically affected

animals is typical of the ruminant TSEs;53,138,153,160 spe-cific histologic lesions are only observed in gray mat-ter of the CNS. Qualitatively, lesions observed inbrains of natural and experimental (intracerebral ororal routes of exposure) cases of CWD in natural hostsare the same; duration of clinical disease does not sig-nificantly affect lesion distribution or severity,160 andlesions of CWD in free-ranging and captive mule deerare similar.137 Some variation in severity can occuramong individuals but the anatomic distribution of le-sions is quite constant. CNS lesions are bilaterally

symmetrical and spongiform change is obvious; vac-uolization occurs in neuronal perikarya and neuronalprocesses. Neuronal degeneration and astrocytic hy-perplasia and hypertrophy occur53,160 but are not prom-inent features. As with the other TSEs, inflammatorycell response is not apparent unless associated withintercurrent disease.Microscopic lesions are most striking in the dien-

cephalon, olfactory cortex, and nuclei of the medullaoblongata, prominently the dorsal vagal nucleus, al-though milder lesions are widespread in brain and spi-nal cord, with relative sparing of the basal ganglia,cerebral cortex, and hippocampus.137,160 In clinically af-fected cervids, examination of well-fixed medulla ob-longata at the level of the obex is considered sufficientfor diagnosis of CWD,160 and sections at this levelwere used for CWD surveillance before availability ofimmunohistochemistry.Amyloid plaques are relatively common and can be

detected on hematoxylin and eosin (HE)–stained brainsections, most prominently and with decreasing fre-quency, in white-tailed deer, mule deer, and elk (E. S.Williams, unpublished data). These appear as slightlypale fibrillar eosinophilic areas of neuropil in HE prep-arations (Fig. 2a) and can be surrounded by vacuoles(‘‘florid plaques’’).94,153,160 These are more easily de-tected with silver stains,160 with Congo red staining,8




Fig. 2. Thalamus; white-tailed deer with chronic wasting disease. Amyloid plaques. Fig. 2a. Formalin-fixed, paraffin-embedded tissue, HE. Amyloid plaques delineated with arrows. Bar � 10 �m. Fig. 2b. Immunohistochemical stain ofamyloid plaques. Bar � 10 �m.

or, most dramatically, by immunohistochemis-try51,52,94,95 for detection of PrPd (Fig. 2b). Amyloidplaques have been reported in a variety of other TSEs,including kuru,56 sporadic CJD,13 murine scrapie,29BSE in macaques (Macaca fascicularis),93 and rarelyin BSE and scrapie,14,53,82,165 and they are a salient le-sion of vCJD in humans.152 Thus, the presence of am-yloid plaques is not reflective of a particular strain ofagent, but rather, it presumably reflects the interplaybetween host and pathogen.Ultrastructurally, the lesions of CWD are similar to

the other TSEs.49,50,95 Membrane-bound vacuoles, somecontaining secondary vacuoles and curled membranefractions, are present in neurons and neuronal pro-cesses. Dystrophic neurites can contain degeneratingmitochondria, pleomorphic membrane-bound struc-tures, and electron-dense bodies.Usefulness of IHC in diagnosis of TSEs, for study

of natural and experimental pathogenesis of these dis-eases and for strain typing, is well estab-lished51,65,82,97,146,149 and offers a sensitive method fordetection of PrPd while maintaining structural context.Obtaining brains from free-ranging and extrinsicallymanaged captive cervids free of significant autolyticchange can be difficult which often precludes use ofhistopathology alone for diagnosis of CWD. In addi-tion, histopathology is only sensitive for diagnosis ofclinically affected cervids; it is insensitive in subclin-ical CWD98 because spongiform encephalopathy oc-curs at about the same time clinical signs develop (E.S. Williams, unpublished data).Patterns of PrPd deposition in CWD-affected cervid

brains include perineuronal accumulation, extracellularplaques and granular deposits, perivascular accumu-lation, and subependymal and subpial deposi-tion.52,94,137 In clinical CWD, widespread staining ofPrPd occurs throughout the brain in susceptible species(Table 1).136,138,153 Typically, correlation is good be-tween deposits of PrPd and regions of spongiformchange (Fig. 3a, b);136 however, PrPd can be found inareas of the brain with no spongiform change. Sprakeret al.137 exhaustively documented deposition of PrPd inmule deer brain in various stages of CWD infection.Deposition of PrPd occurs widely in lymphoid tis-

sues during CWD incubation in the absence of histo-logic lesions in these tissues.112,132,153 In this way, CWDclosely resembles scrapie.4,68,69,146,149 Lymphoid tissuestherefore are very useful for diagnostic purposes andsurveillance. Only rarely have deer been found to havebrain but no lymphoid PrPd deposition136 (�1%; E. S.Williams, unpublished data); this occurs more fre-quently in elk (�10–15%;134 E. S. Williams, unpub-lished data). Thus, use of lymphoid tissues alone forsurveillance of CWD in elk could miss a significantnumber of infected animals134 but is useful in surveysdesigned to detect CWD-infected cervid populations.71Specific granular PrPd labeling can be present in the

light and dark zones of germinal centers (Fig. 4), butstaining is less often identified in the follicular mantle.In studies of tonsils of mule deer early and late inincubation by triple label immunofluorescence andconfocal microscopy, PrPd was detected primarily inextracellular locations in association with folliculardendritic and B-cell membranes.130 Aggregates of PrPd




Fig. 3. Tonsil; mule deer with chronic wasting disease.Formalin-fixed, paraffin-embedded tissue. Immunohisto-chemical stain demonstrating accumulation of disease asso-ciated prion protein in germinal centers. Bar � 100 �m.

Fig. 4. Dorsal vagal nucleus and hypoglossal nucleus in obex region of the medulla oblongata; mule deer with chronicwasting disease. Fig. 4a. Formalin-fixed, paraffin-embedded tissue, HE. Spongiform encephalopathy in the dorsal vagalnucleus (upper half of photomicrograph) and unaffected hypoglossal nucleus (lower half of photomicrograph). Bar � 50�m. Fig. 4b. Immunohistochemical stain. Marked staining of the dorsal vagal nucleus with a sharp line of demarcationwith the unaffected hypoglossal nucleus. Bar � 50 �m.

also were found in tingible body macrophages withinfollicular germinal centers.Disease-specific prion protein is detected by IHC in

a variety of tissues of subclinically and clinically af-fected cervids131,137 (E. S. Williams, M. Miller, K. Fox,and J. Jewell, unpublished data; Table 1). In peripheraltissues other than lymphoid tissues, PrPd depositionappears as scattered coarse granules to relatively dif-fuse staining. Coarsely granular staining can be foundin and around myenteric and submucosal plexus neu-

rons, around nerve fibers, and in satellite cells.131Staining occurs in islets of Langerhans in the pancreas,in the adrenal medulla, and in the pars nervosa andintermedia of the pituitary gland131 (E. S. Williams,unpublished data).Recent studies in laboratory rodent models of scra-

pie demonstrated PrPd in skeletal muscles22,103,141,142,and two studies found infectivity for rodents in mus-cle.22,142 PrPd has also been demonstrated in skeletalmuscles of humans with CJD45 and of sheep with scra-pie,6 suggesting skeletal muscles as a source of infec-tivity for TSEs. No evidence of PrPd was found insections of skeletal muscle from deer with natural138and experimental60 CWD by IHC, but additional stud-ies are warranted and are underway.Classification systems for the stages of CWD infec-

tion in naturally affected mule deer and elk based onhistologic lesions and deposition of PrPd have beenproposed98,134, 137. The essence of these systems is that,during the course of incubation, PrPd is typically de-tected first in lymphoid tissues followed by depositionin the lateral aspect of the dorsal vagal nucleus in themedulla oblongata in the absence of spongiformchange. Spongiform change then occurs in the vagalnucleus, probably at about the onset of clinical disease,followed by more widespread deposition of PrPd andspongiform change in the brain. Variations in this pat-tern occur in individual animals. The usefulness formanagement purposes of such stage description sys-tems has yet to be shown, but they have been used in




Table 2. Antibodies and epitopes used for diagnosis ofCWD by immunohistochemistry or immunoblotting.

Antibody PrP Epitope Recognition

Monoclonal antibodyF89/160.1.5* IHFG (bovine residues 150–153)106F99/97.61* QYQRES (bovine residues 220–

225)1366H4† DYEDRYYRE (human residues 144–

152)131R35‡ CGQGGTHGQWNKPSK (residues

101–115)123R505§ SQWNKPSKPKTN (cervid residues

100–111)1253F4� MKHV (hamster residues 109–112)94

Polyclonal antibody6800 III# WGQGGGTHSQWNKPSK (human

residues 81–96)94R522§ GQGGSHSQWNKP (bovine residues

94–105)125,130

* VMRD Inc., Pullman, WA.† Prionics, Zurich, Switzerland.‡ Laboratory of Persistent Viral Diseases, Rocky Mountain Lab-oratories, Hamilton, MT.§ ID-Lelystad, The Netherlands.� New York State Institute for Basic Research in DevelopmentalDisabilities, Staten Island, NY.# H. Diringer, Berlin, Germany.

attempting to evaluate the influence of genetics onpathogenesis.111

Diagnostic TechniquesThe search for simple and reliable antemortem tests

for TSE agents continues. Biopsy of the lymphoid tis-sue of the third eyelid105,107,109 or the tonsil129,146 andexamination of the tissue by IHC for PrPd have provenuseful in diagnosis of scrapie in live sheep. Tonsil bi-opsy of live mule deer and white-tailed deer for CWDtesting has been useful in a research context112,150,163(W. Cook, D. Edmonds, and E. S. Williams, unpub-lished data; N. Mathews, personal communication)and, more recently, in evaluation of management strat-egies (test and removal) for a local population of free-ranging mule deer in Colorado.164 Tonsil biopsy is notpractical on a large scale because it requires anesthesiaof the animal before sampling. Sampling lymphoid tis-sue from the nictitating membrane of deer and elk hasnot proven to be as useful as in sheep (K. O’Rourke,personal communication; L. Wolfe, personal commu-nication).Immunohistochemistry is considered the ‘‘gold stan-

dard’’ for diagnosis of CWD.112,145 Protocols for IHCemploy various techniques for antigen retrieval and avariety of antibodies (Table 2) and chromagens.106,130,136For regulatory purposes, standard IHC protocols areused in US Department of Agriculture–approved lab-

oratories and include hydrated autoclaving, formicacid treatment of sections, specified antibody, and au-tomated staining of retropharyngeal lymph node sam-ples from deer and retropharyngeal lymph node orobex samples, or both, from elk and deer. Retropha-ryngeal lymph nodes are used in deer because of earlylymphoid deposition of PrPd at this location, makingtesting more sensitive than if brain alone was used.71,98For maximum sensitivity, multiple sections of lymphnode should be examined because of patchy PrPd de-position in early infection.71 Retropharyngeal lymphnode is positive in the majority of elk with CWD in-fection; however, 10–15% of elk have detectable PrPdin brain but not retropharyngeal lymph node134 (E. S.Williams, J. Kreeger, and H. Edwards, unpublisheddata), leading to recommendations to test both lymphnode and obex samples in elk. Rapid screening testsfor the TSEs, including CWD, are in increasingly wideusage. Five rapid tests have been licensed by the USDepartment of Agriculture, Center for Veterinary Bi-ologics, for CWD testing (enzyme-linked immunosor-bent assays [ELISAs]: Bio-Rad Chronic Wasting Dis-ease Test, BioRad Laboratories, Hercules, CA;HerdCheck CWD Antigen EIA Test, IDEXX Labora-tories, Westbrook, ME; Enfer TSE Test, Abbott Lab-oratories, Abbott Park, IL; and CWD Dot Blot ELISATest, VMRD Inc., Pullman, WA; and a strip test: PDLCWD Rapid Antigen Test, Prion Development Labo-ratory, Buffalo Grove, IL). Sensitivity of IHC com-pared with these rapid detection methods depends onthe techniques and tissues used, and there are few con-trolled direct comparisons. In a large study of hunter-harvested cervids in Colorado and with IHC results asthe gold standard, one ELISA (Bio-Rad) was 98.3%and 100% sensitive on retropharyngeal lymph nodesfrom mule deer and elk, respectively.71 Specificity wasessentially 100% for both species.

Differential DiagnosesDescribing the full range of differential diagnoses

for CWD is beyond the scope of this review, but manydiseases should be considered when attempting to de-termine cause of illness or death of cervids with signscompatible with CWD. As for other infectious diseas-es, epidemiologic aspects, as well as clinical and path-ologic findings, should be brought to bear in deter-mining the cause of cervid illness and mortality. Im-portant infectious diseases of cervids have been re-viewed.154In much of North America, hemorrhagic disease

(epizootic hemorrhagic disease and bluetongue) cancause clinical signs compatible with CWD. Occasion-ally, encephalitis can occur in white-tailed deer and elkwith acute hemorrhagic disease, leading to clinicalneurologic manifestations that could slightly resemble




Table 3. Substitution polymorphisms in the protein coding region of the prion gene (Pnrp) in deer and elk.

Species Codon No.Amino Acid Encoded by

Major AlleleAmino Acid Encoded by

Minor Allele

Mule deer (Odocoileus hemionus) 20 Aspartate (D) Glycine (G)24,66225 Serine (S) Phenylalanine (F)

White-tailed deer (Odocoileus virgi-nianus) 95 Glutamine (Q) Histidine (H)86,111

96 Glycine (G) Serine (S)116 Alanine (A) Glycine (G)

Rocky Mountain elk (Cervus ela-phus nelsoni) 132 Methionine (M) Leucine (L)106,108

signs of CWD. Chronic hemorrhagic disease can resultin emaciation of white-tailed deer and, rarely, muledeer, and affected animals are occasionally observedin late fall or winter. The lesions and epidemiology ofepizootic hemorrhagic disease and bluetongue in cer-vids have been reviewed74; histopathology, IHC, virusisolation, and serology can be used to distinguish thesediseases.In eastern North America, meningeal worm (Pare-

laphostrongylus tenuis) infection is relatively commonin populations of white-tailed deer, the reservoir host;although uncommon, CNS disease occasionally occursin this species. Mule deer and elk can be seriouslyaffected. The nematodes migrate through spinal cordand brain, resulting in local traumatic and inflamma-tory damage. The lesions and epidemiology of diseasecaused by this parasite have been reviewed.91 Menin-geal worm only occurs in a few areas in which free-ranging populations of elk and mule deer exist, butfarmed elk and mule deer are at risk of infection inareas in which this parasite is present.Loss of condition and behavioral changes occur in

elk with locoweed intoxication,162 and these signs canresemble CWD. The microscopic lesions of locoism inthe CNS1 are easily differentiated from CWD. Natu-rally occurring locoism has not been described in deer.

PathogenesisStudies of the pathogenesis of scrapie in natural

hosts and in laboratory rodents have proven to be goodmodels for investigating CWD. Before the availabilityof immunoassays for tracking the presence of PrPd,bioassay of tissues for scrapie agent was by intrace-rebral inoculation of mice,55,89 an expensive andtime-consuming method. More recently, elegantstudies on scrapie agent by IHC have tracked accu-mulation of PrPd throughout the body during incuba-tion.4,15–17,67–70,146–149 Scrapie agent in naturally and oral-ly exposed animals enters via the alimentary tract; ac-cumulation of PrPd occurs in gut-associated lymphoidtissue—in particular, in germinal centers innervated bysympathetic fibers and in myenteric and submucosal

plexuses—and moves to the CNS via sympathetic andparasympathetic nerves to the intermediolateral col-umns of the spinal cord and the parasympathetic vagalnucleus (dorsal nucleus of the vagus nerve) in the obexregion of the medulla oblongata. From there, PrPd canbe found in increasing amounts and widening distri-bution in lymphoid tissues and CNS. A similar patternappears to occur in deer with CWD, with involvementfirst of tonsil and gut-associated lymphoid tissues,presence of PrPd in the enteric nervous system, fol-lowed by initial involvement of the CNS at the vagalnucleus and thoracic spinal cord and concurrent dis-tribution in peripheral lymphoid tissues98,131,132,137 (Wil-liams et al., unpublished data). More detailed studiesof CWD pathogenesis in all three naturally susceptiblespecies are underway.Recent studies have detected prion infectivity in

blood of sheep with scrapie and BSE73,75 and has beensuspected in humans with vCJD.96,116 Occurrence ofthe agent in blood might account for the widespreaddistribution of PrPd that occurs in lymphoid tissues inscrapie and possibly in CWD.

GeneticsThe influence of genetics on scrapie susceptibility,

resistance, and incubation period has been recognized,investigated, and debated for decades.2,14,76–80,110,114,115At least 10 amino acid polymorphisms in the proteinencoding region of the sheep PrP gene have been re-ported,23 and alleles at codons 136 and 171 stronglyinfluence susceptibility and resistance. Now determi-nation of PrP genotype and selection for resistant ge-notypes form an important aspect of programs to con-trol this disease in North America (http://www.aphis.usda.gov/vs/nahps/scrapie/) and in some countries inEurope (http://www.defra.gov.uk/animalh/bse/othertses/scrapie/nsp/index.htmlh).39Investigations of possible genetic influences on

CWD susceptibility are less extensive but ongoing (Ta-ble 3). The first such studies reported nucleotide se-quences of the PrP gene and deduced amino acid se-quences and compared them to those of domestic live-




stock or humans.33,66,128 O’Rourke et al.108 determinedthat elk with methionine homozygosity at codon 132(132MM) were overrepresented among elk with CWDcompared with elk heterozygous for methionine andthe minor allele, leucine (132ML) or homozygous forleucine (132LL) at that locus. Although naturally ac-quired CWD has been diagnosed in elk with thesethree genotypes132 (J. Jewell, personal communica-tion), evidence indicates that incubation period mightbe lengthened in elk with 132ML or 132LL genotypes(T. Kreeger et al., personal communication; E. S. Wil-liams, unpublished data). If these observations are con-firmed and expanded, some degree of genetic manip-ulation of farmed elk herds might be useful in man-agement of CWD.Experimental studies of elk exposed to scrapie agent

by intracerebral inoculation resulted in somewhat lon-ger incubation periods in elk of the 132ML genotypecompared with elk homozygous for methionine.62However, influence of genotype on brain and lymphoidtissue distribution of PrPd was not detected in elk.132Three substitution polymorphisms occur in white-

tailed deer PrP (Table 3) and two in mule deer, ofwhich one (codon 20) is removed during processingand one (codon 225) remains in the mature polypep-tide. The existence of a processed pseudogene of highsequence identity to the functional PrP gene in allmule deer and both black-tailed deer subspecies (O.hemionus columbianus and O. hemionus sitkensis)24,85(J. Jewell, personal communication) and about 25% ofwhite-tailed deer111 (J. Jewell, personal communica-tion) complicated initial sequence studies of the deerprion protein. Thus, some deduced amino acid se-quences reported before recognition of the pseudogenelist a polymorphism at codon 138 (serine/asparagine);only serine is encoded at that locus by the functionaldeer PrP gene. In one population of white-tailed deerwith high prevalence of CWD, presence of the pseu-dogene did not appear to influence presence or absenceof disease.111 Pseudogenes are rarely expressed, butfunctional pseudogenes have been described,9 war-ranting more investigation into the role that pseudo-genes might play in CWD.Results to date indicate that deer of all common PrP

genotypes are susceptible to CWD. CWD was diag-nosed in all major genotypes of white-tailed deer stud-ied in Wisconsin and Nebraska;86,111 however, fewerdeer homozygous for serine at codon 96 were foundwith CWD than statistically expected. Captive muledeer heterozygous for serine/phenylalanine at codon225 (225SF) are susceptible to experimental oral ex-posure but have prolonged CWD incubation periodscompared with homozygous (225SS) deer.84 A studyof free-ranging mule deer in the CWD endemic areasof Wyoming and Colorado found that the 225SF ge-

notype is less frequent among CWD-positive animalsthan in the overall population84; thus, 296 positive an-imals included one SF and 295 SS genotypes, whereas22 SF samples were expected statistically. In one Col-orado mule deer herd sampled extensively, about 18%were 225SF. However, only 4% of the CWD-infectedmule deer from the population that included this herdwere of the SF genotype (J. Jewell, personal commu-nication; M. Miller, personal communication).Thus, in all three natural cervid hosts, the influence

of genetics on CWD incubation periods and pathogen-esis requires further understanding. Although no cer-vid genotype may provide complete resistance toCWD, if incubation periods and pathogenesis of CWDare significantly affected by genotype, as occurs insheep scrapie, this might influence CWD epidemiolo-gy and have implications for development of manage-ment strategies for this disease.

Host RangeThe natural and experimental host ranges of CWD

are shown in Table 4. Only three species—mule deer,white-tailed deer, and elk—are known to be naturallysusceptible to CWD. Black-tailed deer (O. hemionuscolumbianus), a subspecies of mule deer, are suscep-tible to CWD.157 It is likely, because of close taxonom-ic relationships, that all subspecies of white-tailed deer,mule deer, and elk (which includes red deer, C. elaphuselaphus) would be susceptible to CWD if appropriate-ly exposed.Because of concerns that CWD could spread to cat-

tle on ranges shared by deer and elk, collaborativestudies were begun in 1997 to evaluate susceptibilityof cattle. Five of 13 (38%) mixed breed beef cattleintracerebrally inoculated with a pool of brains frommule deer with CWD developed evidence of diseasetransmission approximately 2–5 years postinocula-tion.58,59 Microscopically, spongiform encephalopathywas equivocal, but PrPd was identified by IHC in manyparts of the brain except the cerebellum PrPd wasfound by immunoblotting, and scrapie-associated fi-brils were detected by negative stain electron micros-copy. Deposits of PrPd were multifocal and centeredaround or in astrocytes; neurons were seldom in-volved. Occasional small plaques occurred in the neu-ropil. The lymphoid tissues examined were negativefor PrPd staining. No evidence of transmission wasfound in the remaining intracerebrally inoculated cattlekilled 7 years postinoculation. In contrast, transmissionof scrapie occurred in nine of nine (100%) cattle in-oculated intracerebrally and maintained for more than1 year.37,38 Lesions of scrapie in cattle in this studywere subtle, spongiform change was minimal, and PrPdstaining was within neurons. In another study of scra-pie transmission to cattle, three of 10 (30%) animals




Table 4. Natural and experimental host range for the transmissible spongiform encephalopathies of cervids.

Transmissible SpongiformEncephalopathy Species

Chronic wasting diseaseNatural hosts

Mule deer (Odocoileus hemionus hemionus),157 black-tailed deer (O.hemionus columbianus)

White-tailed deer (Odocoileus virginianus)135Elk (Cervus elaphus nelsoni)158

Experimental hostsOral inoculation Moose (Alces alces shirasi; E. S. Williams, unpublished data)Intracerebral inoculation Cattle58

Domestic sheep (A. Hamir, personal communication)Domestic goat159Laboratory mice27Domestic ferret11Mink (R. Marsh et al., personal communication)Hamster (following passage in ferret)11Squirrel monkey (Schiurus saimiri; R. Marsh et al., personal communi-cation)

ScrapieIntracerebral inoculation Elk61

inoculated with scrapie agent had incubation periodsbetween 2 and 4 years. The lesions in these affectedanimals also were very subtle.126 No transmission ofscrapie to cattle occurred when they were exposedorally to the scrapie agent in rendered feed,36 and thereis no epidemiologic evidence that scrapie can be hor-izontally transmitted to cattle. These observations sug-gest that if CWD is similar to scrapie, it is unlikelythat cattle will contract CWD by natural routes, eventhough a small percentage might be susceptible by in-tracerebral inoculation. Cattle orally exposed to a highdose of brain from mule deer with CWD and cattleliving in CWD endemic facilities remain clinicallyhealthy more than 7 years postinoculation or -exposure(Williams et al., unpublished data). In addition, a sur-vey of 262 adult culled cattle that resided in CWDendemic areas for at least 4 years was negative forevidence of a TSE.47Domestic sheep are susceptible to CWD agent by

intracerebral inoculation; Suffolk sheep of a genotypehighly susceptible to scrapie (homozygous for gluta-mine [Q] at codon 171 [171QQ]) developed a diseaseindistinguishable from scrapie following inoculation(A. Hamir, personal communication). Susceptibility ofdomestic sheep to CWD agent by oral exposure hasnot been studied. One domestic goat developed CWDapproximately 6 years after intracerebral inoculation ofdeer CWD brain.159 Clinical signs included intensepruritis and loss of body condition. Microscopically,

widespread spongiform encephalopathy resembled thatof scrapie.Currently, no widely available laboratory animal

model exists for the study of CWD. Mink and ferretsare susceptible to CWD by intracerebral inoculationbut have only been used in a limited number of CWDstudies11,92 (R. Marsh, personal communication; C.Sigurdson et al., personal communication; E. S. Wil-liams, unpublished data). Conventional wild-type lab-oratory mice and hamsters are poor hosts for CWDagent; it transmits with low efficiency on first passagein mice27 and only to hamsters following passage indomestic ferrets.11 Lack of a suitable laboratory modelfor CWD has hampered study of some aspects ofCWD biology. However, transgenic mice expressingnormal cervid cellular prion protein (PrPc) are beingdeveloped and validated by several laboratories26 (M.Oldstone, personal communication; R. Rubenstein,personal communication) and availability of these an-imals in the future will be very useful in the study ofCWD.Based on epidemiologic investigations, there is cur-

rently no evidence that humans are susceptible toCWD,18–20,167 although it is impossible to prove thathumans are not susceptible to this agent.21 A few squir-rel monkeys (Sciurus saimiri) were susceptible toCWD by intracerebral inoculation (R. Marsh and S.Young, personal communication), and expanded stud-




ies of nonhuman primate susceptibility to CWD areunderway (R. Race, personal communication).Deer and elk in CWD endemic areas are hunted for

venison and consumed by local as well as nonresidentfamilies. Because it is not possible to provide absoluteassurance that venison is without risk, hunting ordersand Web sites in states and provinces with CWD incervid populations provide information and guidancefor the public about CWD (e.g., http://www.dnr.state.wi.us/org/land/wildlife/whealth/issues/CWD/, http://wildlife.state.co.us/CWD). Hunters are advised toavoid harvesting deer and elk that appear ill, to debonemeat during processing, to wear latex or rubber gloveswhen dressing the carcass, and to avoid contact withbrain, spinal cord, and lymphoid tissues. Regulationsconcerning requirements for harvested deer and elk tobe tested for CWD vary among jurisdictions; in mostlocations, this is voluntary and available through statelaboratories. Recommendations are that animals test-ing positive for any prion disease should not be con-sumed by humans or other animals.167

Origin and Strain TypingThe origin of CWD is not known and might never

be known. Three hypotheses have been suggested. Per-haps the most plausible is that CWD derives from scra-pie. Support for this hypothesis comes from the mod-erate ability of PrPd from CWD-affected deer and elkto convert PrPc from sheep to the abnormal isoformPrPd 125 and from comparisons of glycoform patternson western blots of PrPd from CWD-affected deer andelk and sheep scrapie.123 In addition, scrapie agent in-oculated intracerebrally causes a disease in elk similarto scrapie.61,62 However, CWD is unlike any character-ized scrapie strain in rodent models;27,31 it does notappear to be transmissible to raccoons (Procyon lotor)as is scrapie,63 and incubation period in an intracere-brally inoculated goat159 was longer (6 years) than ex-pected for scrapie. Alternatively, CWD might be a dis-ease of mule deer after spontaneous alteration of PrPcresulting in formation of an infectious prion protein.Although spontaneous generation of prions might betheoretically possible and has been suggested as theorigin of some cases of sporadic CJD of hu-mans,44,119,120 it would be exceedingly difficult if notimpossible to prove retrospectively. A third possibilityis that CWD is derived from another, currently un-identified, source of infection. Although it seems high-ly unlikely, a human source of CWD can not be com-pletely dismissed; Hadlow54 reminds us that, at leastexperimentally, TSE agents are transmissible from hu-mans to animals.The epidemiology of CWD is most compatible with

a single strain that originated in mule deer and theninfected elk and white-tailed deer.159 CWD was first

recognized as a clinical syndrome in mule deer manyyears before it was detected in elk housed in facilitieswith CWD-affected mule deer.159 The first recognitionof CWD in white-tailed deer was in free-ranging deersympatric with CWD-affected mule deer and elk byretrospective examination of tissues when IHC forPrPd became available.155Characteristics of histologic lesions have been used

for decades to distinguish forms of human and naturaland experimental animal TSEs.31,42,113 Typing TSEstrains in rodents is based on intracerebral inoculationof genetically characterized mice with the agent of in-terest and then comparing lesion profiles on the basisof vacuolation scores in specified areas of the brain ofaffected mice and incubation periods.42 Multiplestrains of scrapie have been characterized inmice.28,30,43 One mule deer source of CWD (captivemule deer from Colorado) was tested in this system,and it was found to be unlike any scrapie strains, BSE,TME, vCJD, or strains of sporadic CJD.27,31 The muledeer CWD source examined differed from other TSEsources studied in mice, giving a unique lesion profileand presence of marked perivascular amyloid depos-its.27 Microscopic characterization of BSE and scrapiestrains in sheep has been evaluated recently with pat-terns of PrPd deposition by IHC46,83 and a panel ofantibodies recognizing different PrP epitopes, provid-ing yet another way of potentially evaluating strainsof the TSE agents.Another proposed system for distinguishing animal

TSE strains uses intracerebral inoculation of rac-coons.57,64 Incubation time of TME in raccoons wasvery short (5–6 months), incubation time of scrapiewas moderate (about 2 years), and at the time the studywas published, raccoons had not developed evidenceof CWD infection. However, many more sources ofTSE agents would need to be tested in raccoons beforethis could be considered a useful system for strain typ-ing.Recently, comparison of the relative amounts of di-,

mono-, or unglycosylated PrPd detected by immuno-blotting, or glycoform pattern analysis, is receivingconsiderable attention as a method of determiningstrains of TSE agents and as a potential epidemiologictool, although a plea for caution in the use of thistechnique alone was recently published.118 Multipleforms of PrPd appear to be produced by different celltypes after infection with the same TSE agent, sug-gesting that structural differences in PrPd do not in-variably code for different strains of scrapie;82 thiscould also be the case for CWD. The diglycosylatedforms of PrPd predominate in cervids with CWD123(Fig. 5).Glycoform pattern analysis of PrPd was one of the

techniques used to help determine the relationship be-




Fig. 5. Western immunoblot of chronic wasting disease–associated prion protein; brain from mule deer (left) andwhite-tailed deer (right). The strongest signal is in the upperdiglycosylated band and the weakest in the lower unglyco-sylated band.

tween BSE and vCJD34,72 and is a component used incharacterization of forms of classical CJD.81,113 Com-parison of PrPd from a small number of mule deer,white-tailed deer, and elk with CWD, sheep with scra-pie, and cattle with BSE, did not reveal glycosylationpatterns that could be used to reliably distinguish theseagents; however, some differences found between PrPdfrom individual mule deer and elk suggested the pos-sibility of different CWD strains.123 No differenceswere found in PrPd derived from six distinct regionsof brain from clinically affected mule deer or elk.123Although the origin of CWD remains obscure, the sim-ilarity of glycoform patterns in scrapie-affected sheepand CWD-affected cervids was felt to provide supportfor the hypothesis that CWD was derived from sheepscrapie.123Other molecular techniques for investigation of TSE

strains have been applied to CWD. In vitro analysis ofthe ability of cervid PrPd to convert PrPc of variousspecies to the abnormal isoform indicated a greaterdegree of conversion of ovine PrPc to PrPd than eitherbovine or human PrPc but less than conversion of ho-mologous PrPc.125 Conformation-dependent immuno-assay, an assay that recognizes PrPd without the needfor proteinase treatment, has also been applied to PrPdfrom a small number of CWD-affected cervids.127 Inthese analyses, PrPd from elk could be distinguishedfrom that of mule and white-tailed deer, suggestingpossible strain differences; the explanation for thesedifferences has not been determined. Links betweenscrapie and CWD have been proposed on the basis ofsimilarity of ribosomal DNA sequences from Spiro-

plasma mirum in sheep with scrapie and elk withCWD;12 however, the evidence provided was weak andassociation of S. mirum with the TSEs is not widelyaccepted.

TransmissionAlthough the exact method of transmission of CWD

is not known, horizontal and indirect transmission ap-pear to be the most important routes of spread.99 Epi-demiologic observations suggest that in addition totransmission within species, CWD can be transmittedfrom elk to mule deer and white-tailed deer, from muledeer to elk, and from mule deer to white-tailed deer.156Maternal transmission, if it occurs, does not appear toplay a significant role in the epidemiology of the dis-ease.99 To date, PrPd and infectivity have not beenidentified in placentas of deer and elk. This is in con-trast to scrapie, in which high levels of infectivity re-side in the placenta,5,122,143,144 and epidemiologic inves-tigations have found that transmission at the time oflambing appears to be important in maintaining scrapiein flocks.Results of a series of studies investigating CWD

transmission have recently been published.102 Directtransmission of CWD between animals occurred, al-though the exact mechanism was not determined. Therole of environmental contamination in maintaining in-fectivity is not entirely understood; however, con-trolled studies showed infectivity remained on pasturesin which CWD-affected deer resided approximately 2years previously. These studies were conducted in pas-tures presumed to be highly contaminated. Thus, ex-trapolation to field situations should be done with care,but these data suggest caution in managing pastures orpaddocks that have housed CWD-affected cervids. Inaddition, mule deer were infected by contact with skel-etal remains of CWD-affected deer and surroundingground and vegetation. This information bolsters theneed for caution when moving portions of harvestedcervids that contain the highest amount of infectivity,such as the head and the spine, to areas where CWDdoes not exist.

Regulatory ConsiderationsConcomitant with growing concerns about CWD

has been an increase in guidelines and regulations gov-erning farmed cervids and management of free-rang-ing deer and elk. Following recognition of CWD infarmed elk, states, provinces, and federal animal healthregulatory agencies developed voluntary and/or man-datory programs to control or manage the disease, withthe ultimate goal of eradication of CWD from theseindustries. Details of these programs are available on-line at the various state and provincial Web sites andfrom the Canadian Food Inspection Agency (http://




Table 5. Summary of best management practices for handling chronic wasting disease carcases and tissues in animalhealth laboratories (adapted from the American Association of Veterinary Laboratory Diagnosticians Best ManagementPractices3).

Laboratory Situation Handling Procedures Disposal and Disinfection Recommendations*

Necropsy laboratory (carcasses,unfixed tissues, and surfaces)

Biosafety level 2 Incineration (�900–1,000�C)

Use of disposable or dedicatedequipment

Digestion (alkali hydrolysis units)

Facial protection Landfill (in accordance with applicable regu-lations)

Waste reduction 1 : 1 chlorine bleach solution for 1 hour(�20,000 ppm free chlorine)1 N NaOH for 1 hourAutoclave at 134�C for 4.5 hours10% aqueous acid phenol solution† for �30minutes or 1% solution for 16 hours

Histology laboratories Biosafety level 2 Waste solutions treated at a final concentra-tion as described above, neutralized or di-luted, and disposed of down the drain

Use of disposable or dedicatedequipment

Incineration or alkaline hydrolysis for tissuewastes

Consider treating fixed tissueswith formic acid for 1 hour be-fore processing

* See Brown et al.,25 CDC and NIH,32 Ernst and Race,41 Race and Raymond,124 Taylor,139 Taylor et al.,140 and WHO.166† Environ LpH, Steris Corporation, Mentor, OH.

www.inspection.gc.ca/english/anima/heasan/disemala/cwdmdc/certnorme.shtml) and the US Department ofAgriculture (http://www.aphis/usda.gov/vs/nahps/cwd/farmed-cwd.html).These programs are based on individual animal

identification, annual census and visual inspection, andtesting of animals dying on the farm, when harvested,or at slaughter, to establish certified herds after years(typically 5 years) of monitoring. If CWD is identified,herds are quarantined, they might be depopulated withindemnity, and herd and premises plans are developed.At the time of writing, only two elk herds (Colorado)and one white-tailed deer herd (Wisconsin) were underquarantine for CWD.Management of CWD in free-ranging herds is more

difficult than in farmed cervids and varies dependingon jurisdiction and history of CWD in that location.For example, an ambitious program of white-taileddeer population reduction is being pursued in the rel-atively small CWD endemic area in Wisconsin(www.dnr.state.wi.us/org/land/wildlife/whealth/issues/cwd/index.htm). In the much larger geographic area inwhich CWD occurs in Wyoming, Colorado, and con-tiguous states, eradication is impossible with currentlyavailable techniques. Management of CWD in theseareas varies among states and can include surveillance,attempts to prevent geographic spread, and efforts to

reduce prevalence through population reduction. Moststates or provinces provide guidance to hunters or haverestrictions concerning transport from CWD endemicareas of tissues of highest infectivity from harvesteddeer and elk, particularly skull containing brain andvertebral column containing spinal cord. Most juris-dictions recommend transporting only deboned meat,cleaned skull cap and antlers, and hides. A list of stateand provincial regulations related to CWD has beencompiled and is available online (http://www.cwd-info.org/index.php/fuseaction/policy.regulations).Concerns about safe handling of animals and tissues

suspected of having CWD or scrapie led the AmericanAssociation of Veterinary Laboratory Diagnosticians(AAVLD) to develop best management practices forhandling animal TSE agents classified as biosafety lev-el 2 agents, including CWD, scrapie, and TME. Detailsare available to members on the AAVLD Web site(http://www.aavld.org/aavld-3/Current�news.jsp)3 or tothe public on the Wyoming State Veterinary Labora-tory web site (http://wyovet.uwyo.edu/WSVL/updates/UPDATES�BY�YEAR/Updates�2004.htm) and shouldbe consulted if working with these agents in diagnosticlaboratories. The primary features of these guidelinesare shown in Table 5.Formic acid is used in many laboratories for pre-

treatment of tissues possibly containing TSE agents




after cutting into blocks but before processing into for-malin.25 This treatment greatly decreases or abrogatesinfectivity before sectioning of tissue blocks. Becausetreatment with full-strength formic acid can result inbrittleness of the tissues, difficulty in sectioning, andalteration of staining qualities, adequate washing is im-portant. However, good quality histologic preparationscan be obtained readily.

SummaryCWD is a unique TSE and has inherent challenges

beyond those of the domestic animal TSEs. Transmis-sion readily occurs among cervids; no complete ge-netic resistance has been identified, and environmentalcontamination might be important in maintenance ofthe disease. Although CWD probably can be con-trolled in farmed cervid industries, management ofCWD in free-ranging populations of deer and elk isproblematic, and it is likely the disease will continueto spread. Veterinary pathologists need to be aware ofthe features of CWD and will be important participantsin surveillance programs and investigation of CWDnatural history and biology in the future.

AcknowledgementsI gratefully acknowledge the mentorship of Dr. Stuart

Young, and this review is dedicated to his memory. Throughmany exciting discussions and untold hours of working onthe microscope, he was an inspiration. He will be missed. Ialso acknowledge a long-standing collaboration and innu-merable discussions about CWD with Dr. Mike Miller withthe Colorado Division of Wildlife and Dr. Tom Thorne withthe Wyoming Game and Fish Department. Discussions withDr. Jean Jewell about genetics and molecular biology havebeen particularly helpful. Collaboration and discussion withtoo many individuals working with the TSEs to mention in-dividually are gratefully acknowledged. Partial support wasprovided by the Department of Veterinary Sciences, Univer-sity of Wyoming; National Science Foundation–National In-stitutes of Health Grant DEB-0091961; Department of De-fense Grant DAMD 17-03-1-0542; and the National Cattle-men’s Beef Association.

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154 Williams ES, Kirkwood JK, Miller MW: Transmissiblespongiform encephalopathies. In: Infectious Diseases ofWild Mammals, ed. Williams ES and Barker IK, 3rded., pp. 292–301. Iowa State University Press, Ames,IA, 2001

155 Williams ES, Miller MW: Chronic wasting disease inNorth American deer and elk. Rev Sci Tech 21:305–316, 2002

156 Williams ES, Miller MW, Kreeger TJ, Kahn RH,Thorne ET: Chronic wasting disease of deer and elk: areview with recommendations for management. J WildlManag 66:551–563, 2002

157 Williams ES, Young S: Chronic wasting disease of cap-tive mule deer: a spongiform encephalopathy. J WildlDis 16:89–98, 1980

158 Williams ES, Young S: Spongiform encephalopathy ofRocky Mountain elk. J Wildl Dis 18:465–471, 1982




159 Williams ES, Young S: Spongiform encephalopathies ofCervidae. Rev Sci Tech Off Int Epizool 11:551–567,1992

160 Williams ES, Young S: Neuropathology of chronicwasting disease of mule deer (Odocoileus hemionus)and elk (Cervus elaphus nelsoni). Vet Pathol 30:36–45,1993

161 Williams ES, Young S, Marsh RF: Preliminary evi-dence of transmissibility of chronic wasting disease ofmule deer. Proc Annu Meeting Wildl Dis Assoc, 1985

162 Wolfe GJ, Lance WR: Locoweed poisoning in a north-ern New Mexico elk herd. J Range Manag 37:59–63,1984

163 Wolfe LL, Conner MM, Baker TH, Dreitz VJ, BurnhamKP, Williams ES, Hobbs NT, Miller MW: Evaluation ofantemortem sampling to estimate chronic wasting dis-

ease prevalence in free-ranging mule deer. J Wildl Man-ag 66:564–573, 2002

164 Wolfe LL, Miller MW, Williams ES: Feasibility of ‘‘testand cull’’ as a strategy for managing chronic wastingdisease in urban mule deer populations. Wildl Soc Bull32:500–505, 2004

165 Wood JLN, Done SH: Natural scrapie in goats: neuro-pathology. Vet Rec 131:93–96, 1992

166 World Health Organization: Infection control guidelinesfor transmissible spongiform encephalopathies. Reportof a World Health Organization consultation. WorldHealth Organization, Geneva, Switzerland, 2000

167 World Health Organization: WHO consultation on pub-lic health and animal transmissible spongiform enceph-alopathies: epidemiology, risk and research require-ments, p. 52. World Health Organization, Geneva, Swit-zerland, 2000

An electronic copy of this review may be viewed at: http://wyovet.uwyo.edu in Updates on Diseases.



REVIEW ARTICLE Chronic Wasting Disease · mule deer (Odocoileus hemionus), white-tailed deer (O....

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