Longitudinal study of the spread of ovine Johne's disease in a sheep flock in southeastern New South Wales, Australia

摘要

On-farm investigation and monitoring for ovine Johne's disease was performed in a flock of approximately 3000 sheep between 1997 and 2002. The study aimed to better understand ovine Johne's disease prevalence, distribution and spread on this recently infected farm and to plan practical disease control and intervention strategies. The AGID, pooled faecal culture (PFC) and histopathology were used for partial and then whole flock testing using PFC annually three times. Faecal shedding of Mycobacterium paratuberculosis (MAP) 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 a 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 MAP during the study period. These results provided indirect evidence of the finite duration of survival of MAP on pasture and the influence of age on susceptibility of sheep to developing patent MAP infection. A further feature of the epidemiology in this flock was the slow transmission of MAP, related to the long incubation period (exposure to shedding interval) of five years and the absence of clinical signs of OJD throughout the study period. The findings suggest that management practices such as partial flock culling, selective grazing management and vaccination might have reduced mycobacterial contamination on this farm, possibly to a level at which patent MAP infection no longer occured. Better understanding of disease spread within flocks over time through flock profiling using PFC will help devise 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.