Age, geographic, and temporal distribution of fecal shedding of Cryptosporidium parvum oocysts in cow-calf herds

Objective-To evaluate fecal shedding of Cryptosporidium parvum from Califor nia cow-calf herds with respect to age, geographic region, temporal effects , and association with watery feces. Animals-Cows and calves from 38 beef cow-calf operations. Procedure-Fecal specimens were collected and examined for C parvum oocysts, using immunofluorescent microscopy. Associations between age, geographic r egion, month of collection, watery feces, and likelihood of shedding C parv um were evaluated. Results-3.9% of cattle were shedding C parvum oocysts. Prevalence of sheddi ng among calves ranged from 0 to 13%, and was 0.6% among cattle greater tha n or equal to 12 months old. The odds of shedding C parvum among 2-month-ol d calves were 41 times greater than among cattle > 4 months old. The odds o f shedding C parvum among cattle tested in May were 8.7 times greater than among cattle tested during June, July, or August; The odds of infected indi viduals having watery feces were 3 to 4 times greater than for noninfected individuals, but the etiologic fraction was only 8 to 9%. Conclusions and Clinical Relevance-Substantial fecal shedding of C parvum b y cow-calf herds was limited to calves 1 to 4 months old, with low prevalen ce detected in older animals. Risk of contamination of watersheds with C pa rvum was limited to those periods when young calves were in the herd. Altho ugh the odds of having watery feces were greater for animals infected with C parvum than for noninfected animals, the low etiologic fraction suggests that most calves with watery feces were not infected with C parvum.