Feb 12, 2003
The Science of CWD:
A Chronology of Conclusions on Chronic Wasting Disease
Anthony C. Grabski, Ph.D
1980, Journal of Wildlife Diseases
Presented is a clinical characterization of CWD and pathological
evidence supporting the conclusion that the disease is a specific spontaneously
occurring form of spongiform encephalopathy. Currently, studies are underway
to characterize fully the clinical and pathological features of CWD, and
to determine its epitzootiology and transmissibility.
1997, Journal of Wildlife Diseases
The source of spongiform encephalopathy in free ranging cervids is undetermined.
The mode of transmission of spongiform encephalopathy also is unknown.
Equally little is known about other aspects of the epizootiology of spongiform
encephalopathy in wild cervids. The number of case submissions has increased
since the first case was diagnosed in 1981. We believe this increased
detection rate is mostly due to greater efforts to locate affected animals.
Although the occurrence of spongiform encephalopathy in wild cervids could
be on the rise in this area, preliminary analysis of deer and elk harvest
data from 1990 to 1994 do not reflect such a trend. .
1998, Journal of Wildlife Diseases
In 1985, all cervids residing in the FWRF and its satellite facilities
were killed in an attempt to eradicate CWD. Paddocks where affected cervids
had resided were treated with 1,000 ppm calcium hypochlorite solution
via mobile sprayer and helicopter, plowed to a depth of 0.3 m, and treated
a second time; shelters, feed bunkers and automatic waterers were either
replaced or hand cleaned twice with 1,000 ppm calcium hypochlorite solution.
An outbreak of CWD occurred between 1986 and 1997 with an incidence rate
of 17%. Our observations provide compelling circumstantial evidence for
lateral transmission of CWD among elk. We also acknowledge the possibility
that an unidentified point source of infection could have independently
given rise to the outbreak. Alternatively, environmental contamination
could have been a factor infecting elk with CWD. If an environmental source
was involved, then it apparently survived our extensive disinfection procedures
in 1985 and would likely hamper elimination of CWD from the FWRF and other
infected facilities in the future :
Polymorphisms in the PrP gene are associated with variation in relative susceptibility in both free-ranging and farm-raised elk.
2000, Journal of Wildlife Diseases
Within species, CWD prevalence varied widely among biologically or geographically-segregated
subpopulations within the 38,137 km2 endemic area but appeared stable
over a 3-year period. The duration of CWD occurrence in free-ranging cervids
remains as enigmatic as its geographic origin. CWD may have been present
in free-ranging deer in both Colorado and Wyoming since the early 1960s,
if not earlier.
Effective strategies for controlling or eliminating CWD in wild deer and elk have not been identified. It follows that random culling via harvest or other means may be relatively ineffective in reducing CWD prevalence.
2001, Journal of Wildlife Management
Once introduced and established, most infectious diseases are
extremely difficult to eliminate from free-ranging populations. It follows
that a new or emerging wildlife disease should be carefully evaluated
early on to assess both its potential importance and prospects for effective
management. The biological mechanisms underlying CWD transmission are
poorly understood, and as a result model mechanisms are at best a collection
of educated guesses.
Selective culling may offer the greatest promise of reducing CWD prevalence, particularly when infected populations are detected early in the course of an epidemic and tested aggressively for several decades.
2002, Journal of Wildlife Management
Although most captive deer residing in endemic research facilities eventually
contract CWD, individuals occasionally survive a lifetime. Genetic resistance
associated with the PNRP genotype has been demonstrated in free-ranging
and farmed elk and is being investigated in deer.
The apparent persistence of PrPCWD in contaminated environments may represent a significant obstacle to eradication of CWD from either farmed or free- ranging cervid populations. Perhaps most important, impacts of CWD on population dynamics of deer and elk are presently unknown.
A quote from a 1990 issue of Ecology seems an appropriate capstone to this review of the state of knowledge on CWD:
"Not only is the science incomplete, the system itself is a moving target, evolving because of the impacts of management and the progressive expansion of the scale of human influences on the planet. Hence, the actions needed by management must be ones that achieve ever-changing understanding as well as the social goals desired. That is the heart of active experimentation at the scales appropriate to the question. Otherwise the pathologies of management are inevitable-increasingly fragile systems, myopic management, and social dependencies leading to crises."
- - - Anthony Grabski, Ph.D.
References
- Williams, E.S. and Young, S., (1980) Chronic wasting disease of captive mule deer: A spongiform encephalopathy. J. Wildlife Diseases, 16: 89-98.
- Spraker, T. R., Miller, M. W., Williams, E. S., Getzy, D. M., Adrian, W. J., Schoonveld, G. G., Spowart, R. A., O'Rourke, K. I., Miller, J. M., and Mertz, P. A., (1997) Spongiform encephalopathy in free-ranging mule deer, white-tailed deer, and rocky mountain elk in North Central Colorado, J. Wildlife Diseases, 33: 1-6.
- Miller, M. W., Wild, M. A., and Williams, E.S., (1998) Epidemiology of chronic wasting disease in captive rocky mountain elk. J. Wildlife Diseases, 34: 532-538.
- O'Rourke, K. I., Besser, T. E., Miller, M. W., Cline, T. F., Spraker, T. R., Jenny, A. L., Wild, M. A., Zebarth, G. L., and Williams, E.S., (1999) PrP genotypes of captive and free-ranging Rocky Mountain elk with chronic wasting disease. J. General Virology, 80: 2765-2769.
- Miller, M. W., Williams, E.S., McCarty, C. W., Spraker, T. R., Kreeger, T. J., Larsen, C. T., and Thorne, E. T., (2000) Epizootiology of chronic wasting disease in free-ranging cervids in Colorado and Wyoming. J. Wildlife Diseases, 36: 676-690.
- Gross, J. E. and Miller, M. W., (2001) CWD in mule deer: a model of disease dynamics, control options, and population consequences. J. Wildlife Management, 65: 205-215.
- Williams, E. S., Miller, M. W., Kreeger, T. J., Kahn, R. H., and Thorne, E. T., (2002) Chronic wasting disease in mule deer and elk: A review with recommendations for management. J. Wildlife Management, 66: 551-563.
- Walters, C.J. and Holling, C.S. (1990) Large-scale management experiments and learning by doing. Ecology 71: 2060-2068.
PostScript: An e-mail I received recently from Rob Harstad highlights the dangers of intervening in nature with an inadequate understanding of the phenomena in question. He began with a quote from Lewis Thomas writing about DNA in The Medusa and the Snail:
"If our kind of mind had been confronted with the problem of designing a similar replicating molecule, starting from scratch, we'd never have succeeded. We would have made one fatal mistake: our molecule would have been perfect. Given enough time, we would have figured out how to do this, nucleotides, enzymes, and all, to make flawless, exact copies, but it would never have occurred to us, thinking as we do, that the thing had to be able to make errors."
"The capacity to blunder slightly is the real marvel of DNA. Without this special attribute, we would still be anaerobic bacteria and there would be no music...If we had been doing it, we would have found some way to correct this, and evolution would have been stopped in its tracks."
Rob then comments:
"When I read this it occurred to me that the WDNR is trying to stop evolution in its tracks. . . .Even if they were successful in eliminating the disease from Wisconsin, or the continent for that matter, by killing all the deer, both resistant and susceptible individuals, the species is left wide open to future, and inevitable it seems to me, outbreaks of this disease. And are not deer, like mankind, afflicted with any number of potentially fatal diseases, communicable and not? How was it decided that this particular disease would be the last for all the deer? . . . ."
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