Objective

To better understand ovine Johne's disease prevalence, distribution and spread within a recently infected sheep flock, through whole flock testing over several years and to plan practical disease control and intervention strategies.

Material and Methods

On farm investigation and monitoring for ovine Johne's disease was performed on the study farm between 1997 and 2002. Tests used were serology (AGID), pooled faecal culture (PFC) and histopathology. From 1997 to 2000 partial flock testing was done and from June 2000 to October 2002 annual whole flock testing using PFC was performed to determine distribution and prevalence of infection (infected flock profile). Flock size was around 3000 sheep.

Results

Faecal shedding of Mycobacterium paratuberculosis (M. ptb.) commenced in home bred sheep around six to seven years after a single introduction of a mob of 410 infected sheep in 1993. For at least seven years there was clustering of infection and shedding within one or two age groups only. Sheep in these age groups appeared to have been exposed to mycobacterial contamination at an early age (<12 months) and commenced shedding at five years of age or older. Groups that were exposed to contamination as adults did not shed detectable levels of M. ptb. during the study period. Clinical signs of OJD were absent throughout the study period.

Conclusion

Clustering of patent infection in age groups of sheep that were exposed as lambs was a feature of the epidemiology of M. ptb. transmission on this farm, providing indirect evidence of finite duration of survival of M. ptb. on pasture and influence of age on susceptibility of sheep to develop patent M. ptb. infection. Transmission of M. ptb. on this farm occurred very slowly, related to the long incubation period (exposure to shedding interval) of five years. The findings suggest that partial flock culling, selective grazing management and vaccination could lead to a reduction in mycobacteria contamination on farm, possibly to a level at which patent M. ptb. infection no longer occurs. Better understanding of disease spread within flocks over time through flock profiling using PFC will also help in devising surveillance strategies (including testing protocols for market assurance testing) that account for clustering of infection as well as very slow transmission of infection through a flock.