This study evaluated fecal shedding of Cryptosporidium parvum from California cow-calf herds with respect to age, geographic region, temporal effects, and association with watery feces in order to help focus watershed management plans designed to minimize contamination of source water with this parasite. 3.9 percent of all cattle in the study were shedding C. parvum oocysts, while point prevalence of shedding among all calves in the study was 5.6%. Adjusting for month of fecal collection, the odds of shedding among 2-month-old calves were 41 times greater than among cattle over 4 months old. Adjusting for age of the animal, odds of shedding C. parvum among cattle tested in May were 8.7 times greater than among cattle tested during summer. Results showed that calves 1 to 4 months old are most likely to shed oocysts. Therefore, the risk of contamination of drinking water attributable to nearby cow-calf operations is likely limited to those months when young calves are on the watershed, with little risk when the annual calf crop exceeds 4 months old. There was a low etiologic fraction for the entire study population. Only 9% of cattle with watery feces had C. parvum infection.
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