CRYPTOSPORIDIUM-PARVUM - OOCYST EXCRETION AND VIABILITY PATTERNS IN EXPERIMENTALLY INFECTED LAMBS

Cryptosporidium parvum infections of domestic animals can have a consi derable economic impact and as oocysts are voided in the faeces of inf ected hosts, environmental contamination with agricultural waste has a lso become a matter of concern. Since only viable oocysts are potentia lly infectious, the numbers of oocysts excreted during infection can h ave important implications for both veterinary and public health. Duri ng the course of infection in experimentally infected lambs, oocyst vi ability was assessed by a fluorogenic vital dyes assay and by a maximi zed in vitro excystation assay. The excreted oocyst populations contai ned a higher proportion of viable oocysts 5-11 days post infection (d. p.i.) than later in the infection. Oocyst viability declined consisten tly 11-15 d.p.i. and coincided with periods when peaks in serum and in testinal anti-Cryptosporidium antibodies have been reported to occur. Infected lambs excreted a mean of 4.8 (standard error [S.E.]+/-0.4) x 10(9) oocysts per g of faeces, of which half were non-viable and there fore of no significance for disease transmission. This study demonstra tes that the numbers of viable oocysts excreted by infected lambs is s maller than previously suspected.