Tuberculosis due to Mycobacterium bovis in humans in the south-west region of Ireland: is there a relationship with infection prevalence in cattle?

OBJECTIVE

To compare the incidence of tuberculosis due to Mycobacterium bovis in humans to the prevalence of M. bovis infection in cattle in south-west Ireland and discuss possible links between them.

SETTING

In the south-west region of Ireland, a mixed urban and rural community (pop. 536,000), there is a residuum of human tuberculosis caused by M. bovis.

METHODS

A retrospective analysis of the incidence of culture-positive M. bovis disease in humans in south-west Ireland from 1983 to 1994 and of the results of tuberculin testing in cattle from 1978 to 1994 for the same region.

RESULTS

One to five cases of human tuberculosis due to M. bovis were recorded per year while the overall prevalence of bovine infection fell gradually during the period of study from 467 tuberculin-positive animals per 100,000 cattle tested in 1983 to 158 per 100,000 in 1994.

CONCLUSION

The low incidence plateau of human tuberculosis due to M. bovis together with the decline in prevalence of animal infection in the overall period studied suggest a cut-off in the animal to human chain of infection at two points; the animal source and the ingestion of (now pasteurized) milk. This would suggest that disease in humans is now due to reactivation of previous foci of infection which were acquired when milk pasteurization was not compulsory. Based on this, we would anticipate a further reduction and possible elimination of human tuberculosis due to M. bovis in this region in the next 10-15 years.