The main host of TB is …possum!
It has recently been suggested (both through media and by an MP) that possums are ‘not the single most important vector [of bovine tuberculosis (TB)] as official channels are fond of repeating’, based on only 54 (0.04%) possums being diagnosed as infected with TB from 124 213 necropsies conducted by TBfree New Zealand over nine years.
In stark contrast, in their 2015 review of the role of possums as hosts of TB, Graham Nugent and his colleagues use much of the same data as strong evidence that major progress is being made in locally eliminating TB from possums and other wildlife. This necropsy data is very largely collected in ‘freedom’ surveys conducted in areas in which possums have long been controlled to very low densities, to confirm that there is a high probability TB has been locally eliminated from possums. By definition, such freedom surveys are not conducted in areas where TB is known or suspected to occur in the possum population. Thus, finding very few infected possums is a measure of the substantial success of the TB control strategy, not of the insignificance of possums as hosts of TB. Such surveys have been central in helping TBfree New Zealand declare over one million hectares free of infected wildlife, enabling them to stop possum control in these areas.
Nonetheless, the disparity in the inferences drawn from the same data set is thought- provoking. How strong is the evidence that possums are not only a host for TB, but also a crucially important one? Graham’s 2015 review answers that question ‘scientifically’, but some simple historical observations are equally compelling.
Possums in some places undoubtedly do get TB (Fig.1) – in the most extreme case, in 1990, Jim Coleman and colleagues found 62% of possums living in rough grazing land at Flagstaff Flat in Westland were infected. Possums usually have small home ranges, so those living more than a few hundred metres from where cattle graze seldom interact with them. Most possums that get TB die within 4–6 months, so sustained presence of TB in possums in deep forest indicates that the disease is cycling within their populations – and there are good examples of TB persisting in possums in deep forest. In the Hohonu Range in Westland, Jim Coleman and Peter Caley recorded a TB prevalence of 13% in 1973/74, falling to 3% in 1989/90, then rising to 9% in 1997, with prevalence unrelated to nearness to farmland. Likewise, in the Hauhungaroa Range in the central North Island, in 1997–2000, Graham recorded a 6% prevalence in an uncontrolled possum population in forest >3 km from farmland – higher than the 2% recorded in the wider area more than 15 years previously.
While it is almost impossible to observe TB being transmitted between wild animals, there is now incontrovertible proof that possums can and do pass TB to one another. In the Orongorongo Valley in Wellington in an area away from farmland, Carlos Rouco and Dan Tompkins tracked possums known to have a unique strain of TB, and found that strain had passed to other possums that shared the same patch of forest.
It is also obvious that cattle only occasionally pass the disease to possums – possums shared rough farmland habitat with often heavily infected cattle herds during the early part of last century, but TB was not recorded in possums until the 1960s. In contrast, there is very strong historical evidence that TB passes from possums to cattle far more readily. Within a decade of TB being found to be common in some local populations of possums, possum control was applied near and on farms in which the level of TB in cattle had stayed stubbornly high despite intensive quarterly testing and culling of infected animals. Within two years of such possum control, TB in cattle had dropped to very low levels (Fig.1), strongly implying that possum control had largely stopped TB transmission from possums to cattle. This pattern and response to control was also observed in other places in the 1980s, leaving no need for further scientific confirmation.
Thus, modern scepticism about the role of possums as vectors of TB is perhaps understandable, given the relative rarity of infection in possums nowadays, but it is misplaced. The rarity of infection in possums, and therefore the low rate at which they now pass TB back to cattle, is a measure of local eradication success – it demonstrates that TB is being progressively eliminated from possums in more and more areas. These data have been the key in convincing government and agricultural stakeholders that national eradication of bovine TB is feasible and affordable, and should be adopted as New Zealand’s TB management goal. The slightly daunting aspect of the national eradication goal is that it aims to achieve a prevalence in possums that is considerably lower than 0.04% mentioned above. The goal is less than 0.000003% – less than one infected possum in 30 million (i.e. none).
The review referred to above was funded by TBfree New Zealand, and this article was funded by Landcare Research.