The epidemiology ofMycobacterium bovisin wild deer and feral pigs and their roles in the establishment and spread of bovine tuberculosis in New Zealand wildlife

Abstract

In New Zealand, wild deer and feral pigs are assumed to be spillover hosts for Mycobacterium bovis, and so are not targeted in efforts aimed at locally eradicating bovine tuberculosis (TB) from possums (Trichosurus vulpecula), the main wildlife host. Here we review the epidemiology of TB in deer and pigs, and assess whether New Zealand's TB management programme could be undermined if these species sometimes achieve maintenance host status.In New Zealand, TB prevalences of up to 47% have been recorded in wild deer sympatric with tuberculous possums. Patterns of lesion distribution, age-specific prevalences and behavioural observations suggest that deer become infected mainly through exposure to dead or moribund possums. TB can progress rapidly in some deer (<10%), but generalised disease is uncommon in wild deer; conversely some infected animals can survive for many years. Deer-to-deer transmission of M. bovis is rare, but transmission from tuberculous deer carcasses to scavengers, including possums, is likely. That creates a small spillback risk that could persist for a decade after transmission of new infection to wild deer has been halted.Tuberculosis prevalence in New Zealand feral pigs can reach 100%. Infections in lymph nodes of the head and alimentary tract predominate, indicating that TB is mostly acquired through scavenging tuberculous carrion, particularly possums. Infection is usually well contained, and transmission between pigs is rare.Large reductions in local possum density result in gradual declines (over 10 years) in TB prevalence among sympatric wild deer, and faster declines in feral pigs. Elimination of TB from possums (and livestock) therefore results in eventual disappearance of TB from feral pigs and wild deer. However, the risk of spillback infection from deer to possums substantially extends the time needed to locally eradicate TB from all wildlife (compared to that which would be required to eradicate disease from possums alone), while dispersal or translocation of pigs (e.g. by hunters) creates a risk of long-distance spread of disease. The high rate at which pigs acquire M. bovis infection from dead possums makes them useful as sentinels for detecting TB in wildlife.It is unlikely that wild deer and feral pigs act as maintenance hosts anywhere in New Zealand, because unrestricted year-round hunting keeps densities low, with far less aggregation than on New Zealand farms. We conclude that active management of wild deer or feral pigs is not required for local TB eradication in New Zealand.

Evidence of low prevalence of mycobacterial lymphadenitis in wild boars (Sus scrofa) in Poland

“…These animals are much more sensitive TB-sentinels than other wildlife species and are considered to be not only a spill-over but also reservoir hosts or even super-shedders excreting significantly higher amounts of Mycobacterium spp. bacteria than standard shedders [2]. …”

“…Tuberculosis caused by Mycobacterium bovis or M. caprae in wild boars have been reported most frequently [2, 3, 5–8], while M. microti [4] and non-tuberculous, potentially pathogenic environmental mycobacteria, have been reported less often [9, 10]. …”

Cost-based optimization of the stopping threshold for local disease surveillance during progressive eradication of tuberculosis from New Zealand wildlife

“…Although we compared only two alternative surveillance tools here (leg‐hold traps alone, or DDs (chew‐cards) with follow‐up trapping), there are other options for surveillance of residual M. bovis infection in possums in New Zealand—most notably—the use of wildlife spillover hosts as sentinels for persistent infection (e.g., feral pigs and ferrets and wild deer; Nugent, ; Anderson et al., ). In particular, feral pigs are especially sensitive detectors of persistent M. bovis infection in possums (Nugent, Gortazar, & Knowles, ; Nugent, Whitford, & Young, ; Nugent, Yockney, Whitford, & Cross, ), so where they are abundant and inexpensive to procure—a high stopping threshold would be optimal.…”

“…(), the system‐level sensitivities calculated for a VCZ of this size are SSe Trap = 0.00017 per set leg‐hold trap (if using trapping alone for surveillance) and SSe DD = 0.00019 per set DD (if using chew‐card detection with an average detection rate of 0.5% [see below] combined with follow‐up leg‐hold trapping at a rate of 4 traps/detection); the associated average item costs (as described by Gormley & Nugent, ) were $NZ 18/unit or $NZ 15/unit, respectively (Table ). The cost of recontrol was assumed to be about $NZ 1 million—this is thought to be about half the average cost of achieving local eradication of M. bovis (G. Nugent, , unpublished data), and so, it nominally represents a situation where low‐level infection was rediscovered well before prevalence had returned to pre‐control levels. The starting value for PFree prior (i.e., the level at which surveillance was initiated) was initially set at 0.70.…”

Field Trial of an Aerially-Distributed Tuberculosis Vaccine in a Low-Density Wildlife Population of Brushtail Possums (Trichosurus vulpecula)

“…A detailed wildlife disease survey conducted in the study area 4 years prior to the present study recorded a mean prevalence of gross TB lesions among resident possums of 2.7% (n = 450 [28]). Prevalence of disease among wild pigs in the area, as a result of spillover infection from possums, often exceeds 60% [33,34]. …”