Epizootiology of chronic wasting disease in free-ranging cervids in Colorado and Wyoming

Surveillance and epidemic modeling were used to study chronic wasting disea se (CWD), a transmissible spongiform encephalopathy that occurs naturally a mong sympatric, free-ranging deer (Odocoileus spp.) and Rocky Mountain elk (Cervus elaphus nelsoni) populations in contiguous portions of northeastern Colorado and southeastern Wyoming (USA). We used clinical case submissions to identify endemic areas, then used immunohistochemistry to detect CWD-in fected individuals among 5,513 deer and elk sampled via geographically-focu sed random surveys. Estimated overall prevalence (prevalence, 95% confidenc e interval) in mule deer (4.9%, 4.1 to 5.7%) was higher than in white-taile d deer (2.1%, 0.5 to 3.4%) or elk (0.5%, 0.001 to 1%) in endemic areas; CWD was not detected in outlying portions of either state. Within species, CWD prevalence varied widely among biologically- or geographically-segregated subpopulations within the 38.137 km(2) endemic area but appeared stable ove r a 3-yr period. The number of clinical CWD cases submitted from an area wa s a poor predictor of local CWD prevalence, and prevalence was typically gr eater than or equal to1% before clinical cases were first detected in most areas. Under plausible transmission assumptions that mimicked field data, p revalence in epidemic models reached about 1% in 15 to 20 yr and about 15% in 37 to 50 yr. Models forecast population declines once prevalence exceede d about 5%. Both field and model data supported the importance of lateral t ransmission in CWD dynamics. Based on prevalence, spatial distribution, and modeling we suggest CWD has been occurring in northeastern Colorado and so utheastern Wyoming for >30 yr, and may be best represented as an epizootic with a protracted time-scale.