Isolation of Mycobacterium avium subsp paratuberculosis from environmental samples collected from farms before and after destocking sheep with paratuberculosis.

OBJECTIVE: To determine whether Mycobacterium avium subsp paratuberculosis could be isolated from soil-pasture, faecal, water and sediment samples on farms before and after removal of sheep with paratuberculosis. A feasibility study and subsequent field survey. PROCEDURE: First the analytical sensitivity of radiometric culture of the organism from two types of soil was determined relative to faeces. Then soil-pasture, faecal, water and sediment samples were collected for culture from a range of sites from 6 farms with paratuberculosis affected sheep and goats. Similar samples were collected from 20 farms at least 9 months after removal of infected stock.
RESULTS: The analytical sensitivity of culture of M a paratuberculosis from soil samples was 2 orders of magnitude less than that from faeces, and environmental samples required longer incubation periods to yield significant growth in radiometric culture (BACTEC) medium. However, the organism was recovered from approximately 20% of 163 soil-pasture, water and sediment samples from 6 properties with clinically-affected animals with paratuberculosis. The positive samples were from a range of topographic sites, including open exposed and dry areas, however, low lying areas tended to have larger numbers of organisms. When the same sites were sampled again about 5 months later, only 1 was culture positive, and none were culture positive > 12 months later. Of 17 water and dam sediment samples collected from farm 6, which had long-standing high prevalence OJD infection, only one water sample and one sediment from the same dam were culture positive. None of the 5 water samples from the other farms were culture positive. Of 96 water samples, 90 sediment samples and 93 soil samples from farms that had been destocked of infected sheep/goats for 9 to 24 months, one sediment sample from a farm in Victoria (destocked for 12 months) and two sediment samples from a farm in New South Wales (10, 19 months) were culture positive. Recontamination from cattle or water could not be excluded as a cause of the positive cultures from the second farm.
CONCLUSION: M a paratuberculosis can be detected by radiometric culture in environmental samples from farms grazed by sheep or goats with paratuberculosis. There is a relatively low likelihood of recovery of the organism from water samples from such farms, and at 5 or more months after removing stock with paratuberculosis the likelihood of positive cultures from environmental samples is very low. Although the analytical sensitivity of culture from environmental samples is less than that from faeces, surveys of environmental sites are nevertheless feasible. However, improved culture methods are needed for critical surveys and to study the movement and fate of the organism in the environment.