Modelling the regional dynamics and management of bovine tuberculosis in New Zealand cattle herds

A previous model addressed the within-herd transmission of bovine Tb (Mycobacterium bovis) in New Zealand cattle herds. This complementary model considers transmission between herds, in an area of New Zealand's North Island (the Waikato) largely free from wildlife reservoirs. The model is used to account for the observed pattern of Tb breakdowns and to assess the likely impact of changes in management on the percentage of herds on movement control between 1988 and 1993 (an average of 0.76% in the Waikato region). It partitions the region into two sub-areas: one small, with significant infection from wildlife; the other much larger and lacking external sources of infection. The observed pattern of herd breakdowns (in different parts of the Waikato and in the region as a whole) could be accounted for in the model by infection from wildlife reservoirs in the southern sub-area (Waitomo) and infection due to movement of cattle from infected but undiagnosed herds to uninfected herds throughout the whole region. During the above period, the model suggests that about 0.8% of herds became infected each year, that most new infections (81% of the total) occurred between herds in the larger sub-area, and that about 13% of the total occurred through infection from wildlife in the smaller sub-area. The whole situation appears to be in a state of near-neutral balance (even without input of new infection), with removal through testing of new infections almost exactly balancing the rate of infection by between-herd movement. Therefore, if the external input is removed, the number of infected herds and of herds of movement control in the whole region declines so slowly that the decline is almost undetectable. Other management changes had a more dramatic effect--particularly, reducing the testing interval to a maximum of 1 year in the whole district. This also appeared to be a necessary condition for achieving < or = 0.2% herds on movement control within 10 years (whatever other strategies were employed). The greatest reduction in the percentage of herds on movement control came from yearly testing plus a 50% reduction in infection from wildlife. A further but smaller additional improvement was gained by reducing cattle movements throughout the region by 50%.