A cohort study was performed to investigate cryptosporidial prevalence and species distribution in 13 organic and 13 conventional dairy herds. Faecal samples were collected from 221 calves and 259 cows. Management routines were recorded at farm inspection and through a questionnaire. Samples were concentrated using sodium chloride flotation and cryptosporidial oocysts were detected by epifluorescence microscopy. Molecular analysis was used to determine species and subtypes. A multivariable model for factors associated with calves being Cryptosporidium spp. positive was built. Cryptosporidium spp.-positive animals were identified in all herds. Prevalences were similar in organic and conventional calves (44·7% vs. 52·3%), as well as in cows (3·1% vs. 3·8%), P > 0·05. Cryptosporidium bovis, C. ryanae and C. parvum were identified. C. ryanae was identified in a calf younger than the described prepatent period. The multivariable model included four significant variables; calf age, cleanliness of bedding, cleaning routines for group pens and farmers' attitudes towards biosecurity.
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