Assessing the Effectiveness of Tuberculosis Management in Brushtail Possums (Trichosurus vulpecula), through Indirect Surveillance of Mycobacterium bovis Infection Using Released Sentinel Pigs.

In New Zealand, wild pigs acquire Mycobacterium bovis infection by scavenging tuberculous carrion, primarily carcasses of the main disease maintenance host, the brushtail possum (Trichosurus vulpecula). We investigated the utility of captive-reared, purpose-released pigs as sentinels for tuberculosis (TB) following lethal possum control and subsequent population recovery. Within 2-3 years of possum control by intensive poisoning, TB prevalence and the incidence rate of M. bovis infection in released sentinel pigs were lower than in an adjacent area where possums had not been poisoned. Unexpectedly, TB did not decline to near zero levels among pigs in the poisoned area, a fact which reflected an unanticipated rapid increase in the apparent abundance of possums. Monitoring infection levels among resident wild pigs confirmed that TB prevalence, while reduced due to possum control, persisted in the poisoned area at >20% among pigs born 2-3 years after poisoning, while remaining >60% among resident wild pigs in the nonpoisoned area. When fitted with radio-tracking devices, purpose-released pigs provided precise spatial TB surveillance information and facilitated effective killing of wild pigs when employed as "Judas" animals to help locate residents. Sentinel pigs offer value for monitoring disease trends in New Zealand, as TB levels in possums decline nationally due to large-scale possum control.

Percutaneous interdigital injection of Mycobacterium bovis as a model for tuberculous lesion development in wild brushtail possums (Trichosurus vulpecula).

Brushtail possums (Trichosurus vulpecula) are the major wildlife reservoir of Mycobacterium bovis, the causative agent of bovine tuberculosis (BTB), in New Zealand. Primary diagnosis of BTB in wild possums is by palpation to detect peripheral lymphadenomegaly followed by necropsy examination, which frequently identifies gross tuberculous lesions in the peripheral lymph nodes and lungs. Experimental infection studies were conducted with wild possums in an attempt to emulate field BTB, focussing on percutaneous administration of virulent M. bovis in the paws. In a preliminary study, viable M. bovis bacilli were recovered from lymph nodes draining fore- or hindlimbs 12 days after percutaneous injection. Subsequently, 21 wild possums were injected interdigitally with 500 colony forming units (cfu) of M. bovis, radio-collared and released; 17/18 possums recaptured 8 weeks later had an established M. bovis lymphatic infection, with 16 having culture-positive gross lesions in the superficial and/or deep axillary lymph nodes. A dual-site infection model was established, involving simultaneous interdigital injection of 100 cfu of M. bovis into front and rear paws of 19 wild possums; this identified that the average degree of lymphadenitis involved 30-fold enlargement of the draining lymph node by 7-8 weeks post injection (wpi). A time-course study demonstrated establishment of M. bovis infection in peripheral lymph nodes of 9/11 possums at 3-5 wpi of doses ranging from 60 to 190 cfu, but with no development of gross lesions; by 7 weeks, 8/8 animals injected similarly had both an established infection and gross lesions of peripheral lymph nodes. The incidence and progression of peripheral lesion development, together with indications of sequential infection of the lungs, liver and mesenteric lymph nodes(MLNs), indicates that a low-dose percutaneous M. bovis infection model is likely to emulate natural disease in possums.

Mycobacterium avium subsp. paratuberculosis infection in wildlife on three deer farms with a history of Johne's disease.

AIM: To determine the prevalence of Mycobacterium avium subsp. paratuberculosis (Map) infection in wildlife, in pastoral landscapes with a recent history of clinical Johne's disease in livestock. METHODS: A total of 449 wild mammals and birds from three farms in the South Island of New Zealand with recent histories of clinical Johne's disease in their deer herds were trapped and examined for gross pathological changes in the gastrointestinal tract. Additionally, individual mesenteric lymph nodes from 380 mammals, and segments of gastrointestinal tract from 32 birds were excised, homogenised and cultured for viable Map bacilli. The prevalence of Map infection was then calculated for the various species. Faecal samples from those mammals which had culture-positive tissues were further cultured for the presence of Map. RESULTS: Gross pathological changes were identified in the gastrointestinal tract of four brushtail possums, one cat, six ferrets, 12 hares, six hedgehogs, three rabbits, one stoat, and one paradise shelduck. Infection with Map in the gastrointestinal tract was confirmed in only three of these cases, one each of brushtail possums, hares and hedgehogs. In contrast, Map infection in the absence of gross pathological changes was frequently recorded in enteric tract tissues of mammals and birds. Among mammals, Map infection was recorded in 18/73 (25%) brushtail possums, 4/23 (17%) cats, 15/42 (36%) hedgehogs and 29/113 (26%) rabbits. Among birds, intestinal tract tissue Map infection was recorded in 3/17 (18%) paradise shelducks. Among 64 of the 74 mammals which had Map culture-positive tissues, 38% (n=5) of hedgehogs and 11% (n=3) of rabbits also had culture-positive faecal samples. CONCLUSIONS: This study is the first to identify that Map infection can be prevalent in wildlife in New Zealand. There was a high prevalence of Map infection among both scavenging and grazing wild animals. Both mammals and birds are capable of harbouring viable Map organisms in their gastrointestinal tract; further, viable Map was excreted into the environment via faeces by hedgehogs and rabbits. CLINICAL RELEVANCE: Previous studies overseas have postulated a role of wildlife as reservoirs of Map infection and possible vectors of Johne's disease to livestock. Here, brushtail possums, hedgehogs and rabbits and in particular were identified as potential wildlife hosts for Map infection in New Zealand. This suggests that several wildlife species could contribute to the persistence of Map infection within a wildlife/livestock complex, and potentially, perhaps more importantly, to the spread of infection between farms.

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 R(0) 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.