The prevalence of shedding of Cryptosporidium and Giardia spp. based on a single fecal sample collection from each of 91 horses used for backcountry recreation.

Cryptosporidium parvum and Giardia duodenalis are now recognized as primary enteric pathogens in animals and humans. Regulatory agencies, such as the Environmental Protection Agency are under increasing pressure to reduce the concentration of these protozoa in surface waters. Given the popularity of recreational riding of horses on public land in California backcountry, concerns have been raised by various regulatory agencies as to whether horses used for backcountry recreation are a significant source of C. parvum and G. duodenalis for the environment. The prevalence of fecal shedding of Cryptosporidium and Giardia in horses with a history of being ridden in California backcountry during 1993 and 1994 was estimated. Using both direct fluorescent antibody and levitation centrifugation tests, none of 91 single-collection fecal samples from throughout California had Cryptosporidium oocysts or Giardia cysts. Horses ranged from 4 to 24 years of age. Because none of the 91 samples, collected 1 time from each horse, were positive and assuming that the sensitivity and specificity of the test methods employed were 100%, the highest probable prevalence of shedding for either protozoal pathogen was < 3.2% for the cohort of horses studied.