Prevalence of and risk factors for fecal shedding of Cryptosporidium parvum oocysts in horses

Objective - To determine prevalence of and risk factors for fecal shedding of Cryptosporidium parvum oocysts among 3 populations of horses. Design - Cross-sectional study. Animals - 152 horses participating in the 1996 Texas Slate 4-H Horse Show, 144 horses examined by the veterinary teaching hospital, and 70 broodmares and their 10- to 21-day-old foals. Procedure - Information on signalment and potential risk factors for fecal shedding of oocysts was obtained. Fecal samples were evaluated for oocysts by means of acid-fast (AF) staining, immunofluorescence assay (IFA), and, for selected samples, flow cytometry (FC). Results - Results of the 3 diagnostic tests were significantly different. The best agreement was between results of the IFA and FC; AF staining and FC were more sensitive than the IFA, but AF staining was less specific. Fecal samples from 13 horses were classified as positive for oocysts. Risk factors for fecal shedding of oocysts included residence on 2 of 4 breeding farms involved in the study, age < 6 months, and history of diarrhea during the preceding 30 days. A municipal water source was implicated as a risk but could not be verified because of insufficient data. Clinical Implications - Mature horses and exposure to cattle did not appear to be important sources of cryptosporidial infection for foals. Overall prevalence of C parvum infection among these horses was low, and C parvum infection appeared to be associated with particular farms, rather than an endemic opportunistic infection in horses.