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Abstract
Oral-delivery Mycobacterium bovis bacillus Calmette-Guérin (BCG) vaccine in a lipid matrix has been shown to confer protection against M. bovis infection and reduce the severity of tuberculosis (TB) when fed to brushtail possums (Trichosurus vulpecula), the major wildlife vector of bovine TB in New Zealand. Here we demonstrate the feasibility of aerial delivery of this live vaccine in bait form to an M. bovis-infected wild possum population, and subsequently assess vaccine uptake and field efficacy. Pre-trial studies indicated a resident possum population at very low density (<0.6 possums/ha) at the field site, with a 5.1% prevalence of macroscopic TB lesions. Pilot studies indicated that flavoured lipid matrix baits in weather-proof sachets could be successfully sown aerially via helicopter and were palatable to, and likely to be consumed by, a majority of wild possums under free-choice conditions. Subsequently, sachet-held lipid baits containing live BCG vaccine were sown at 3 baits/ha over a 1360 ha area, equating to >5 baits available per possum. Blood sampling conducted two months later provided some evidence of vaccine uptake. A necropsy survey conducted one year later identified a lower prevalence of culture-confirmed M. bovis infection and/or gross TB lesions among adult possums in vaccinated areas (1.1% prevalence; 95% CI, 0–3.3%, n = 92) than in unvaccinated areas (5.6%; 0.7–10.5%, n = 89); P = 0.098. Although not statistically different, the 81% efficacy in protecting possums against natural infection calculated from these data is within the range of previous estimates of vaccine efficacy in trials where BCG vaccine was delivered manually. We conclude that, with further straightforward refinement to improve free-choice uptake, aerial delivery of oral BCG vaccine is likely to be effective in controlling TB in wild possums. We briefly discuss contexts in which this could potentially become an important complementary tool in achieving national eradication of TB from New Zealand wildlife.
Highlights
Aerial delivery of vaccine-laden baits has been used in Europe [1,2] and in North America [3] to control viral diseases among wildlife, such as classical swine fever and rabies
We sought to develop a practical tool for reducing or eliminating TB from a wildlife host living within remote areas of difficult terrain where the usual approach is not feasible or not socially-acceptable
We considered that the successful development and laboratory testing of the lipid matrix as an effective carrier for live bacillus Calmette-Guerin (BCG) [45], its ability to be doi:10.1371/journal.pone.0167144.g002
Summary
Aerial delivery of vaccine-laden baits has been used in Europe [1,2] and in North America [3] to control viral diseases among wildlife, such as classical swine fever (in wild suids) and rabies (in mesocarnivores). In this paper we document the development and experimental field testing of aerial delivery of a bacterial vaccine (Mycobacterium bovis bacillus Calmette-Guerin; BCG) aimed at helping control bovine tuberculosis (TB) caused by virulent strains of M. bovis among wildlife in New Zealand. The risk to human health is diminished by pasteurisation of milk and slaughterhouse inspection to prevent affected meat from entering the food chain, combined with efforts to eliminate the disease from livestock [4]. In New Zealand, the major wildlife maintenance host of TB is the introduced brushtail possum (Trichosurus vulpecula) [6]
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