Some California Water districts have excluded recreational use of horses within watersheds contributing to their reservoirs out of fear that equine manure could contaminate water supplies and lead to human disease, despite a near absence of scientific data to support such a policy.

In order to determine if horses used for recreational riding on public lands in California backcountry are a significant source of Cryptosporidium parvum or Giardia duodenalis, Rob Atwill and colleagues conducted a pilot study of 91 horses. None of the animals were shedding parasite eggs after having been ridden in the backcountry, which led to the conclusion that recreational riding does not appear to pose a significant risk for contamination of regional surface water supplies by these microbes.

The investigation has been expanded by funding from the Center for Equine Health to study the occurrence and distribution of these microbes in all types of California horses. The American Water Works Association, Centers for Disease Control and Prevention, U.S. EPA and other groups have expressed interest in the results of current research, which includes development of a DNA fingerprinting technique to distinguish between strains of C. parvum shed by horses, humans, wildlife or other livestock, as the strain found in horses may not be the same strain that infects humans.