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Researchers find some mule deer more susceptible than others to CWD
Wyoming
Researchers from the University of Wyoming College of Agriculture's Department of Veterinary Sciences and the Colorado Division of Wildlife have found that certain mule deer were 30 times more likely to have chronic wasting disease (CWD) than other deer which have a slightly different form of the protein, PrP, that creates the disease.
Their paper, "Low frequency of PrP genotype 225SF among free-ranging mule deer (Odocoileus hemionus) with chronic wasting disease," was selected for rapid publication ahead of print at Journal of General Virology Direct and can be read in full at http://vir.sgmjournals.org/misc/direct.shtml.
Jean Jewell, a researcher in the Department of Veterinary Sciences, had wondered whether genetic differences could be found between free-ranging mule deer that were infected with CWD and those that were not.
Prior research had shown some sheep are more resistant to scrapie, a disease similar to CWD, than others. Research found there was a slightly different protein sequence in their prion protein (PrP) gene.
"We wanted to look at normal mule deer PrP sequence and compare that to CWD-infected mule deer to see if anything similar was operating in mule deer," said Jewell.
She noted that elk also have a different protein sequence that appears to allow some to be less susceptible to CWD. So far, nothing similar has been found in white-tailed deer.
Jewell said mule deer are the most numerous of the three natural hosts of CWD--mule deer, white-tailed deer and elk--in the areas of southeastern Wyoming and north central Colorado where CWD is endemic.
In CWD-infected animals, the disease-associated prions, or protein part, enter an animal through the digestive system, accumulate in tonsils and lymph nodes and promote conversion of normal cellular protein to an abnormal form. They eventually travel the peripheral nervous system to areas of the brain and, once there, cause nerves to degenerate and create the vacuoles or holes in brain tissue that are characteristic of the spongiform encephalopathies. The disease is fatal to infected cervids (antler-bearing animals).
There is no evidence CWD, which was first detected in mule deer in Colorado about 40 years ago, is a health risk to humans.
Jewell and the late Beth Williams of the Department of Veterinary Sciences, along with Mary Connor from the Department of Forest, Range and Wildlife Sciences at Utah State University in Logan, and Lisa Wolfe and Michael Miller of the Colorado Division of Wildlife, Wildlife Research Center in Fort Collins, analyzed the PrP genotypes of 1,482 free-ranging mule deer from 13 herds in southeastern Wyoming and from the Estes Park area in Colorado and categorized them according to CWD infection status.
Tissues were made available through collaboration with the Wyoming Fish and Game Department.
They extracted chromosomal DNA from each sample. Researchers found only one place in the final protein sequence where a change occurred--at a genetic location called codon 225. They identified 225SF and 225SS genotype deer, and found that 225SS deer were 30 times more likely to be CWD-infected than 225SF deer.
A few CWD-infected SF deer have been found in the wild, and captive SF research animals were infected experimentally; however, in experimentally infected deer, the SF deer with CWD lived 12 months longer than the SS deer. Among SF deer, it also took longer for the prions to invade the central nervous system and the disease progressed more slowly.
"These deer are not resistant to CWD," Jewell says adamantly, "but might be more difficult to infect. At this point, we don't truly know if it is caused by geography or genetics."
Jewell is cautious lest the study results could prompt someone to start breeding 225SF deer to try to solve CWD in the wild.
"It won't work," Jewell says. "It's a simplistic answer. You are always going to have 225SS deer produced in the wild, and if you try to selectively breed for one trait, you don't know what else you are taking away that may be of importance to that animal in the wild. It's just a simplistic notion."
Date: 6/23/05
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