INTRASPECIFIC TRANSMISSION OF MYCOBACTERIUM BOVIS AMONG PENNED FERAL PIGS IN NEW ZEALAND

We investigated intraspecific transmission of Mycobacterium bovis (etiologic agent of bovine tuberculosis [Tb]) among penned pigs (Sus scrofa) in New Zealand. Between 2002 and 2006, we conducted two trials, each involving two 1-ha pens in natural habitat in which uninfected sentinel pigs were kept with wild-caught feral pigs, many of which were infected with M. bovis. The rate of M. bovis transmission to sentinels was used to assess whether intraspecies transmission between live pigs could explain the high levels of infection in the wild population. In trial 1, no new infection was detected in 18 penned sentinels after total exposure of 3.9 yr (2.6 mo/sentinel); three of 11 sentinels in the wild became infected. In trial 2, a more heavily infected source pig population (94% prevalence compared with 42% in the first trial) was used, and one (4%) of 25 penned sentinels became infected. However, 75% of the eight sentinels released to the wild became infected. Combining trials, the difference in apparent annual incidence was significant (mean ±95% confidence interval = 0.03±0.12 for penned sentinels vs. 1.06±6.74 for released sentinels; t= –3.51, P=0.04). In the pens, infected pigs were kept in contact with susceptible sentinels for 7 yr in total, but only one transmission event was detected. Taking pig longevity into account, the R₀ value (the basic reproductive rate of disease) for intraspecies infection between live pigs seems unlikely to exceed 0.25, even under highly conducive conditions. We suggest that live pigs are unlikely to transmit M. bovis to wild conspecifics often enough to sustain Tb in pigs by this route of transmission alone. The high prevalences seen in some New Zealand feral pig populations are likely to result from transmission from another host or route.