Epidemiology of an outbreak of chronic wasting disease on elk farms in Saskatchewan
i thought some of you here might be interested in this study ;
Can Vet J. 2007 December; 48(12): 1241–1248. PMCID: PMC2081988
Copyright and/or publishing rights held by the Canadian Veterinary Medical Association
Epidemiology of an outbreak of chronic wasting disease on elk farms in Saskatchewan
Connie K. Argue, Carl Ribble, V. Wayne Lees, Jim McLane, and Aru Balachandran Western Animal Health Program Network, Canadian Food Inspection Agency, Room 654–220 4th Ave SE, Calgary, Alberta T2G 4X3 (Argue); Department of Population Medicine, Ontario Veterinary College, University of Guelph, Guelph, Ontario N1G 2W1 (Ribble); Manitoba Agriculture Food and Rural Initiatives, 545 University Crescent, Winnipeg, Manitoba R3T 5S6 (Lees); Battleford District Office, Canadian Food Inspection Agency, PO Box 1028, Battleford, Saskatchewan S0M 0E0 (McLane); Animal Disease Research Institute, Canadian Food Inspection Agency, 3851 Fallowfield Road, PO Box 11300, Ottawa, Ontario K2H 8P9 (Balachandran) Address all correspondence to Dr. Connie K. Argue; e-mail: email@example.com AbstractAn outbreak of chronic wasting disease (CWD) in farmed elk in Saskatchewan from 1996 to 2002 was reviewed to 1, determine the progression of CWD from infection to death in farmed elk; 2, assess animal risk factors for CWD infection in farmed elk; 3, assess farm management and exposure risk factors for within herd CWD transmission; and 4, assess the suitability of the Canadian Food Inspection Agency’s (CFIA) current disease control policy for CWD in light of the findings. The results from animal movement tracing, animal testing, and a farm management questionnaire were used. The duration of CWD (time from exposure to death of a CWD test-positive animal) was between a mean minimum of 19 months and a mean maximum of 40 months. Age and sex were not associated with CWD infection, except that adult elk (= 2 y) were more likely to be infected than young elk (< 18 mo) (RR = 2.3, 95% CI 1.6–3.5). Elk calves born in the last 18 mo prior to the death or diagnosis of their dam were at higher risk if their dams died of CWD (RR = 4.1, 95% CI 1.5–11.4) or exhibited clinical signs of CWD (RR = 8.3, 95% CI 2.7–25.7). Significant risk factors for transmission of CWD on elk farms were the introduction from an infected farm of trace-in elk that died of CWD (RR = 13.5, 95% CI 2.0–91) or developed clinical signs of CWD (RR = 7.1, 95% CI 0.93–54) and the elapsed time in years since the incursion of CWD (OR = 5.6, 95% CI 1.8–17.4). The assumptions on which CFIA’s disease control policies were based were validated, but based on this new information, quarantine in cases where exposure to preclinical elk has occurred could be considered as an alternative to whole herd eradication.
The mechanism and significance of finding increased risk in calves of CWD-infected elk requires further investigation. Cohort studies undertaken to determine the role of maternally associated transmission of bovine spongiform encephalopathy (BSE) suggest that bovine calves born within the last 6 mo of the incubation period of BSE-infected cows may experience a slightly higher risk of developing BSE than others of their birth cohort (20), but modelling based on these results predicts a higher incidence of BSE due to maternal transmission than has been observed in the UK. Scrapie has long been believed to be maternally transmitted, and prions have been demonstrated in the placentas of infected ewes (21,22); however, recent work indicates that lambs are infected by peri- or post-natal lateral transmission, and that infection is no more common in the lambs of infected ewes than in others of their lambing cohort (21,22). Farm management risk factors None of the farm management factors assessed were significantly associated with the occurrence of CWD transmission. This may reflect lack of power in the study due to the small number of infected farms, or it may be that none of the factors studied influenced the transmission of prion disease. This study suggests that sharing equipment and the method of feeding could influence the transmission of CWD. If shared equipment were demonstrated to be a risk factor, it would suggest that fomites could be a mechanism of transmission. Fomites encountered in this study included saws used for removing velvet antler and equipment used to administer oral medication. This may indicate blood- or saliva-borne infectivity. The tendency for feeding on the ground to be protective and the use of feeders to be a risk factor, suggests that close animal contact and contamination by saliva may be modes of transmission, while urine or fecal contamination of feed may be less important. This is consistent with recent experimental work demonstrating that CWD is transmitted by saliva and blood (8). Exposure risk factors Introduction of CWD onto a farm via an animal that subsequently died of the disease was a highly significant risk factor for subsequent within-herd transmission. Animals that eventually died of CWD probably shed greater amounts of prion, compared with animals that were euthanized, and contributed to greater contamination of the environment and greater opportunity for direct transmission to other animals. It is possible that the carcasses of elk that died of CWD may have decomposed in elk enclosures, thus serving as an environmental source of infection to other elk, as experimentally demonstrated in mule deer (10). The introduction of an animal that subsequently exhibited clinical signs of CWD was also a significant risk factor for within-herd transmission. If shedding of prions is coincident with clinical signs, elk that were euthanized would not have shed as much of the infectious agent as those that were allowed to progress to death. The introduction of a positive animal that did not express clinical signs of CWD before it was euthanized was not associated with transmission of CWD. In future, it might be reasonable to maintain such exposed animals in quarantine to determine whether or not they would eventually succumb to CWD. Substantial savings to producers and the public could be realized, and many animals spared, if herd depopulation was not required in such cases. The elapsed time between the introduction of CWD-infected animals and the depopulation of the farm was highly associated with transmission. Shorter elapsed times were associated with farms where the trace-ins had not yet developed clinical signs, reflecting tracing activity and detection of the positive animal occurring earlier in the incubation of the trace-in. Early detection and removal of clinically affected elk in a captive herd infected with CWD has reportedly led to decreased prevalence of CWD in the herd, compared with a previous outbreak where this strategy was not employed (3). Figure 3. Proposed course of chronic wasting disease (CWD) in farmed elk a Calculated mean minimum and maximum durations in Saskatchewan farmed elk b Observed youngest clinical elk and longest incubation in a preclinical elk c Observed youngest positive elk d References 1 and 7 e References 1, 3, and observed clinical elk 1248 CVJ / VOL 48 / DECEMBER 2007 ARTICLE
The CFIA’s policies for eradication of CWD in farmed cervids were successful in controlling the disease on farms in Canada (4). This analysis of the outbreak of CWD in farmed elk in Saskatchewan in 1996 to 2002 provides additional information that validates the assumptions used to develop the policy. Immunohistochemical staining is a highly reliable method to detect CWD prions (4) and could also provide information on the stage of the disease, based on grading of lesions. While animals under 12 mo of age have a low risk of infection and, therefore, may reasonably be exempted from testing, this is not true of calves of positive dams, and testing of these animals should be reconsidered. This analysis suggests that the maximum clinical period of CWD is 12 mo and that animals are infectious during the clinical stage. Analysis of this outbreak failed to provide evidence that elk are infectious prior to the development of clinical signs of CWD. Farms that were exposed to trace-ins that did not express clinical disease did not experience transmission of CWD, and calves of dams with preclinical disease were not at increased risk of CWD. The conclusion that elk are not infectious during the preclinical phase of CWD cannot be made definitively, because the shorter elapsed time between incursion and depopulation on farms exposed to such elk may have precluded the detection of infected animals that were early in the incubation period. In future, in cases of herd exposure to infected elk with preclinical CWD, maintenance of the herd in quarantine for 4 y with surveillance for clinical signs of CWD could be used to determine whether transmission occurs in this situation. If it could be confirmed that elk do not transmit CWD during the preclinical phase, substantial reduction in eradication costs, hardship to producers, and loss of animal life could be realized.
CVJ References 1. Williams ES, Miller MW, Kreeger TJ, Kahn RH, Thorne
>>>maintenance of the herd in quarantine for 4 y with surveillance for clinical signs of CWD could be used to determine whether transmission occurs in this situation<<<
a dangerous gamble i.e. the potential for letting the agent spread, expose, and infect, via sub-clinical carriers, or a sensible solution to mass slaughter i.e. extermination of cwd exposed factory farmed animals/herds ???
time will tell i suppose, but what a gamble it is. ...TSS
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