To determine if the number of rectal lymphoid follicles decreases with respect to age and sex relative to diagnosis of chronic wasting disease (CWD), rectal biopsies (n = 1,361) were taken from captive Rocky Mountain elk (Cervus elaphus nelsoni) at 4 ranches in the western United States between 2005 and 2008. Rectal tissues were stained with a monoclonal antibody (F99/97.6.1), which selectively stains the abnormal isoform of the prion protein associated with CWD of elk. The number of lymphoid follicles obtained from typical biopsy tissues decreased with the age of the animal. The acceptable number of lymphoid follicles for detection of CWD was not considered to be a problem in elk up to 8.5 years of age, but in elk over 8.5 years of age, the follicle count was considered to be low. Sex of the animal had no effect on the number of lymphoid follicles observed in each age group. Rectal biopsies were an accurate test to diagnose preclinical stages of CWD in elk but may be best suited to elk that are less then 8.5 years of age.
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