Low frequency of PrP genotype 225SF among free-ranging mule deer (Odocoileus hemionus) with chronic wasting disease

The prion protein (PrP) gene was characterized in 1482 free-ranging mule deer (Odocoileus hemionus) from Wyoming and Colorado. Using DNA sequences from 363 deer, dimorphisms at codons 20 (aspartate/glycine) and 225 [serine (S)/phenylalanine (F)) were found; silent changes occurred at codons 131 (tyrosine) and 247 (isoleucine). The remaining samples were surveyed for codon 225 genotype and all were characterized for chronic wasting disease (CWD) infection status. A total of 112 deer with the genotype 225SF or FF were found, of which one was CWD-positive; 1370 were 225SS, with 289 positive for CWD. Among CWD-negative deer, the frequency of 225SF/FF genotypes was 9-3% but among CWD-positive deer it was only 0.3%. For all samples combined, CWD status was not independent of codon 225 genotype (P<0.0001). The odds that a deer of the 225SS genotype was CWD-infected were 30 times greater (95% confidence intervals =4-213) than for a 225SF deer. The proportion of 225SF animals in sampled subpopulations varied from 0 to 18%; the CWD prevalence varied from 0 to 25%. However, no relationship was observed between genotype frequency and CWD prevalence in different areas. The PrP sequences of experimentally infected mule deer were analysed from pre-existing projects and 10 animals were found with 225SF genotypes, all of which were positive for CWD. Data available from some of these animals suggest that the 225SF genotype could be associated with longer incubation periods in CWD infection compared with the 225SS genotype.