Epidemiology of equine Cryptosporidium and Giardia infections

Summary Prevalence and infection patterns of Cryptosporidium and Giardia infections in horses were studied by a direct immunofluorescence staining method. Faecal examinations of 222 horses of different age groups revealed Cryptosporidium infection rates of 15–31% in 66 foals surveyed in central Ohio, southern Ohio and central Kentucky, USA. Only 1 of 39 weanlings, 0 of 46 yearlings, and 0 of 71 mares were positive. Giardia infection was found in all age groups, although the infection rates for foals were higher (17–35%). Chronological study of infection in 35 foals showed that foals started to excrete Cryptosporidium oocysts between 4 and 19 weeks and Giardia cysts between 2 and 22 weeks of age. The cumulative infection rates of Cryptosporidium and Giardia in foals were each 71%. Some foals were concurrently infected with both parasites and excretion of oocysts or cysts was intermittent and long‐lasting. The longest duration of excretion was 14 weeks for Cryptosporidium and 16 weeks for Giardia. Excretion of Cryptosporidium oocysts stopped before weaning, while excretion of Giardia cysts continued thereafter. Infected foals were considered the major source of Cryptosporidium infection in foals, whereas infected mares were deemed the major source of Giardia infection in foals. The high infection rate of Giardia in nursing mares suggested a periparturient relaxation of immunity. The results indicated that Cryptosporidium and Giardia infections are common in horses.