Improved quantitative estimates of low environmental loading and sporadic periparturient shedding of Cryptosporidium parvum in adult beef cattle

Our primary goal was to generate an accurate estimate of the daily environmental loading rate of Cryptosporidium parvum oocysts for adult beef cattle, using immunomagnetic separation coupled with direct immunofluorescence microscopy for a highly sensitive diagnostic assay. An additional goal was to measure the prevalence and intensity of fecal shedding of C. parvum oocysts in pre- and postparturient cows as an indicator of their potential to infect young calves. This diagnostic method could detect with a ≥90% probability oocyst concentrations as low as 3.2 oocysts g of feces^-1, with a 54% probability of detecting just one oocyst g of feces^-1. Using this diagnostic method, the overall apparent prevalence of adult beef cattle testing positive for C. parvum was 7.1% (17 of 240), with 8.3 and 5.8% of cattle shedding oocysts during the pre- and postcalving periods, respectively. The mean intensity of oocyst shedding for test-positive cattle was 3.38 oocysts g of feces^-1. The estimated environmental loading rate of C. parvum ranged from 3,900 to 9,200 oocysts cow^-1 day^-1, which is substantially less than a previous estimate of 1.7 × 10⁵ oocysts cow^-1 day^-1 (range of 7.7 × 10⁴ to 2.3 × 10⁵ oocysts cow^-1 day^-1) (B. Hoar, E. R. Atwill, and T. B. Farver, Quant. Microbiol. 2:21-36, 2000). Use of this highly sensitive assay functioned to detect a greater proportion of low-intensity shedders in our population of cattle, which reduced the estimated mean intensity of shedding and thereby reduced the associated environmental loading rate compared to those of previous studies.