How does chronic TB affect possum movements?
Significant new insight into the movement of possums in relation to their role as vectors of bovine tuberculosis (TB) has been revealed in recent research by Graham Nugent and his colleagues.
The team now has a deeper understanding of how far possums range in forested and semi-arid habitats; how much of their range they use on a daily, weekly or monthly basis; their short-term foraging patterns; and how their movement patterns change in response to recently-depopulated buffer zones.
One thing not yet known is how TB affects possum movement patterns. In this article, we explain why this information is important, and summarise a mix of new and old data that provides fresh insight into the question. An improved understanding of possum movements will help further improve the proof-of-freedom framework now being used to declare areas free of TB. This framework relies on a Possum-TB model that assumes possums remain fully active (and in a steady-state of infectiousness) throughout the whole of the disease course. Is that likely?
Recent research suggests that the disease course in possums from infection to death – at least for artificially-infected possums – averages 18 weeks (range 6–28 weeks). It also suggests possums with late-stage disease are likely to be severely debilitated, so chronic TB will affect possum movements – but to what extent? Recently, Ivor Yockney and his colleagues sought to answer this question on Muzzle Station in North Canterbury, by radio-tracking wild possums fitted with GPS collars (Fig.1) Before release, each possum was artificially infected with TB by injecting Mycobacterium bovis directly into their paws, an inoculation method that produces a disease pattern closely matching that seen in naturally-infected possums. Ivor, Graham and Jackie Whitford then tracked the possums over the ensuing months and eventually recovered the collars, capturing some possums along the way to chart the progress of the disease, allowing some to run close to the full course of disease before recapturing them, or simply collecting carcasses of animals that died from TB.
Analysis of this possum movement data by Cecilia Latham and Frank Cross indicated that the majority of artificially infected possums showed routine and steady-state movement behaviour for the first 8–10 weeks post-infection (Fig.1a, b), which corresponds to the pre-clinical and early clinical stages of TB identified from earlier studies: at this time TB lesions are likely to be developing in the possums’ axillary and inguinal regions but not in their lungs. Then from weeks 10–12 onwards, possum movement became progressively curtailed (Fig. 1a, b), mostly likely reflecting internal disease spread and, in particular, development of severe and increasingly debilitating lung disease. This morbidity phase lasted 2–4 weeks, before the animal died, which could be either pin-pointed to a day and time from collar downloads (Fig. 1a) or estimated from collars emitting a ‘mortality signal’ due to cessation of possum movement (Fig. 1b).
But not all possums followed this pattern. Some seemed to succumb to disease rapidly (Fig. c), while others maintained ‘normal’ movement behaviour for extended periods, despite developing severe disease (Fig. 1d); still others seemed to suffer few ill effects and were possibly short- or medium-term ‘survivors’ (Fig. 1e), something that these and other studies have shown can occasionally occur.
What this means for the role of possums in TB transmission is that the course of disease and its effects on possums is not only variable across time but also probably differs widely between individuals, and that accordingly an individual possum’s infectiousness is likely to vary over time. For example, the first clinical symptom of severe disease (both artificially-induced and naturally-acquired) is that inguinal and axillary lesions rupture to release infectious material to the exterior though a sinus. Earlier work by Dan Tompkins in conjunction with Bryce Buddle of AgResearch showed that lesions at this stage can contain in excess of 10 million TB bacteria per gram of tissue, yet during this time some possums maintain full or near-full mobility, and can potentially create an infection risk over most of their home range. In contrast, another period of infectiousness is likely to occur near death (peri-mortem), when possums have developed extensive lesions throughout their lungs and viscera so are likely to be excreting enormous numbers of bacteria. By this stage, however, our results suggest they are barely moving, so the risk is geographically much more restricted. Such animals are ‘sitting ducks’ for both other scavenging wildlife (e.g. pigs, ferrets) and for other possums who may attack, scavenge or even attempt to copulate with them.
These results confirm that the assumptions in the current possum-TB spatial model (i.e. that infected possums present a more or less constant infection risk over their whole range for the duration of the disease course) is possibly too simplistic. We now need to complete more detailed analyses of the movement data from tuberculous possums, to explore how big an effect this new understanding has in changing the patterns of disease predicted by the Possum-TB model.