Has the role of scavenging been overlooked?

BOVINE TB (bTB) (caused by Mycobacterium bovis) remains endemic in parts of New Zealand. Since the late 1960s, the introduced brushtail possum (Trichosurus vulpecula) has been regarded as the principal wildlife reservoir, implicated in recurrent infections of farmed deer, cattle and among other wild mammals. Broad-scale poisoning of them has been the main countermeasure to reduce bTB transmission risk. Although the UK focus has revolved around badgers, it has been suggested that wild deer may also be underestimated reservoirs of bTB (VR, 16/23 November 2024, vol 195, pp 402–403). According to the BVA's policy, several deer species can maintain M bovis infections and transmit them to other deer or cattle.1 By the early 1990s, New Zealand epidemiologists had also determined that deer may be more effective than cattle at transmitting M bovis to other species, and that deer had infected previously bTB-negative possum populations.2, 3 New Zealand's bTB policy has consistently attributed most new and persistent herd infections in endemic areas to wild animal vectors, especially possums.4 While possums are highly susceptible to M bovis irrespective of the exposure route, and tuberculosis (TB) in possums is progressive and fatal usually within months, latency and recrudescent infections are not considered features of naturally acquired infections in possums. Following British colonisation in the 1840s, settlers introduced numerous mammals, including red deer (Cervus elaphus) for hunting and possums for the fur trade, and by the mid-20th century wild deer were abundant and widespread. The first confirmed bTB in wild deer preceded discovery in possums by at least a decade5, 6 and high prevalences (up to 40 per cent) were recorded in some populations.7 From the 1930s to the 1980s, wild deer were heavily harvested for bounty, skins and venison. Shot deer were often eviscerated in remote field locations, leaving offal accessible to scavengers. Recent Australian studies show that brushtail possums are not just herbivores but opportunistic scavengers of fresh carcases.8, 9 Early transmission experiments in Australia showed that brushtail possums could develop generalised TB from eating material with tuberculous lesions. Under the conditions outlined, the oral pathway of exposure may explain the early discoveries of TB in possum populations in New Zealand.10 It appears that possums first became infected with M bovis by scavenging carcases of infected deer, establishing a persistent sylvatic cycling involving other scavenging animals. Within a decade of the first discovery of bTB in possums in New Zealand, more than 20 separate locations with infected wild possums were recorded.11 Possums and badgers are prominent in the bTB narratives of New Zealand and the UK. As natural mesoscavengers and amplifier hosts of bTB, a reassessment of their role is warranted. Managing the wildlife transmission cycle of bTB in the UK and New Zealand requires stricter regulation of hunting practices to minimise possums and badgers accessing discarded viscera and carcases of high-risk species such as deer. Instructing licensed shooters and landowners on hygiene practices is a practical and inexpensive measure to enhance overall bTB control strategies in both countries.