Number of Cryptosporidium parvum oocysts or Giardia spp cysts shed by dairy calves after natural infection

Objective-To determine the total number of Cryptosporidium parvum oocysts a nd Giardia spp cysts shed by dairy calves during the period when they are m ost at risk after natural infection. Animals-478 calves naturally infected with C parvum and 1,016 calves natura lly infected with Giardia spp. Procedure-Oocysts or cysts were enumerated from fecal specimens. Distributi on of number of oocysts or cysts versus age was used to determine the best fitting mathematic function. Number of oocysts or cysts per gram of feces f or a given duration of shedding was computed by determining the area under the curve. Total number of oocysts or cysts was calculated by taking the pr oduct of the resultant and the expected mass of feces. Results-Intensity of C parvum oocyst shedding was best described by a secon d-order polynomial function. Shedding increased from 4 days of age, peaked at day 12, and then decreased. An infected 6-day-old calf would produce 3.8 9 X 10(10) oocysts until 12 days old. Pattern of shedding of Giardia spp cy sts was best described by exponential functions. Intensity of shedding incr eased from 4 days of age, peaked at day 14, and then decreased. An infected calf would produce 3.8 X 10(7) cysts from day 50 until day 56. Conclusions and Clinical Relevance-The large number of oocysts and cysts sh ed indicates that shedding by dairy cattle poses a risk for susceptible cal ves and people. Estimates reported here may be useful to aid in designing c ost-effective strategies to manage this risk.