Cattle represent a reservoir for Giardia and Cryptosporidium and may contaminate water sources.

To determine the distribution of Cryptosporidium and Giardia on dairy farms and in water bodies near the farms.

Twenty dairy farms and 20 wells and 13 surface water samples associated with dairy farms.

Proportions of samples positive for Cryptosporidium or Giardia were determined by a direct immunofluorescence assay. Fecal and water samples were taken at different times.

Thirty‐two (95% CI: 29–35%) and 14% (95% CI: 12–17%) of fecal samples, and 100 (95% CI: 96–100) and 55% (95% CI: 32–77%) of herds, were positive for Giardia and Cryptosporidium, respectively. Giardia duodenalis assemblage E was detected in high proportions (90%) of fecal samples. Cryptosporidium bovis predominated (51%) in all cattle. C. andersoni predominated in adult cattle (53%), whereas the predominant species in animals < 2 months and 2–6 months was C. bovis, respectively. Only calves < 2 months of age were positive for C. parvum. In 46% (95% CI: 19–75%) and 85% (95% CI: 55–98%) of surface water, concentrations of Giardia cysts and Cryptosporidium oocysts were higher in downstream, than in upstream, locations of farms, whereas only 1 groundwater sample was positive for Cryptosporidium.

This sample of dairy cattle was predominantly infected with nonzoonotic species and genotypes of Cryptosporidium, Giardia, or both. More studies are needed to determine if the presence of Giardia or Cryptosporidium in surface water was associated with shedding in animals from nearby farms.