Temporal patterns and quantification of excretion of Mycobacterium avium subsp paratuberculosis in sheep with Johne's disease

Objectives To determine the frequency of excretion of Mycobacterium avium s ubsp paratuberculosis in Merino sheep with Johne's disease and to quantify excretion in a group of Merino sheep. Design a pen and laboratory experiment Procedure Seven sheep selected from an affected flock on the basis of acid- fast bacilli in the sheep's faeces were housed and total daily faecal outpu t was collected, weighed and subjected to culture for M avium subsp paratub erculosis. An end-point titration method was used to enumerate viable M avi um subsp paratuberculosis in a 15 day pooled sample from five sheep that ha d acid-fast bacilli in their faeces while housed. Results Four sheep with subclinical multibacillary Johne's disease excreted M avium subsp paratuberculosis each day for 11 days of cultural observatio n. A further three sheep were intermittent excreters but lacked other evide nce of infection with An avium subsp paratuberculosis. The average number o f viable bacteria excreted was 1.09 x 10(8) per gram of faeces while total daily excretion was 8.36 x 10(10) viable M avium subsp paratuberculosis per sheep. Examination of faecal smears stained with Ziehl Neelsen was an unre liable means of assessing daily excretion in individual animals except in t hose with severe lesions. Conclusion Excretion of M avium subsp paratuberculosis in Merino sheep with multibacillary Johne's disease occurred daily, proving that environmental contamination can be continuous on farms with endemic ovine Johne's disease . Faecal culture is a useful method for detecting infection as it does not appear to be affected by the timing of collection of a sample from sheep wi th multibacillary disease however, to maximise the sensitivity of disease s urveillance using faecal culture, sampling rates should be adjusted to take account of the proportions of multibacillary and paucibacillary cases.