Epidemiology of chronic wasting disease in captive Rocky Mountain elk

Between June 1986 and May 1997, chronic wasting disease (CWD) was the only natural cause of adult mortality among captive Rocky Mountain elk (Cervus elaphus nelsoni) held at a wildlife research facility near Fort Collins, Colorado (USA). Of 23 elk that remained in this herd > 15 mo, four (17%) developed CWD. All affected elk were unrelated females from the founding cohort, captured as neonates and raised in 1986. The index case was diagnosed in 1989; time intervals between subsequent cases ranged from 13 to 32 mo. Initial age at onset of clinical signs ranged from about 2.9 to 8.1 yr; duration of clinical disease ranged from 5 to 12 mo (mean = 7.5 mo) prior to death. Intraspecific lateral transmission of CWD seemed the most plausible explanation for the epidemic pattern observed; neither periparturient nor maternal transmission appeared necessary to sustain this outbreak. Early detection and elimination of incubating or clinical individuals may have aided in reducing exposure or infection rates as compared to a previous outbreak in the same facility. Transmission routes and rates, pathogenesis, antemortem diagnostic tools, and the potential role of reservoirs or environmental contamination in perpetuating CWD epidemics warrant further investigation.