Dispersal of a non-motile species: the story of bovine TB’s spread in New Zealand
Dispersal is easy for most land mammals. They walk to wherever they need to go. In contrast, parasitic bacteria such as Mycobacterium bovis (the cause of bovine tuberculosis; TB) have to rely on their hosts for dispersal. M. bovis is part of a broader group of related bacterial species that cause different forms of tuberculosis. This group has been extremely successful in dispersing around the globe – the single progenitor of the group is believed to have developed as a disease of humans in Africa perhaps 40,000 years ago and subsequently spread with humans when they first colonised the Middle East. There it split into two main lineages, one carried by humans and the other mostly by animals. The shift to animal hosts is likely to have occurred with the domestication of animals about 13,000 years ago. It is now accepted that humans gave tuberculosis to animals, the reverse of that once believed.
Since then, M. bovis has spread around the world as people colonised new lands and took their livestock with them. Without doubt, M. bovis arrived in New Zealand from Europe early in the 1800s – by 1880 4–7% of cattle slaughtered in Wellington were considered to be infected. Eff orts to control TB in cattle in New Zealand were largely ineffectual until the mid-1900s. Between 1930 and 1951 ~35,500 cattle annually were classed as tuberculous at slaughter (compared with just 270 per year in recent years). Oddly, even though M. bovis has a very wide host range, the disease did not appear to spread to wild mammals during this time.
Tuberculosis was first recorded in New Zealand wildlife (a wild deer) in 1954, then in a wild pig in 1964. Although possums are now the main host of M. bovis in New Zealand, TB in possums was not recorded until 1967. This is unexpected given that possums had lived alongside infected cattle for nearly a century. Curiously, despite the long initial lag, the jump from livestock to possums then appears to have occurred independently in numerous places in both the North and South Island between 1967 and 1981. By 2004, M. bovis had become established in wildlife populations in ~10 million hectares or ~40% of New Zealand.
It was inevitable, after it first established in possums, that most of the subsequent spread of M. bovis would be the result of direct (possum–possum) or indirect (possum–other wildlife host–possum) transmission between possums. Paul Livingstone analysed the locations at which TB was found in possums during the 1980s. The spatial patterns indicated that infection was spreading within the wildlife populations in contiguous forest or bush habitats, and to farmland along the edges of rivers and from bush–pasture margins. By analysing the pattern of new outbreaks of infections in cattle that appeared to have been caused by possums, Paul estimated that TB was spreading in extensive forest lands by 1.6–2.3 km per year. In less rugged country with a mixture of native and exotic forest, the disease was spreading by 1.4–4 km per year, and by 2.5–5.0 km per year in more open or tussock-covered country.
Because forest-dwelling possums have home ranges only a few hundred metres wide, and because the frequency of transmission between possums is quite low, these rates of spread suggest that the principal mechanism of TB spread is likely to have been through dispersal of infected animals in a series of migrations. Infected possums dispersed, establishing infection in new possum populations, which then triggered further waves of expanding infection. Additionally, or alternatively, infection may also have been spread through indirect transmission from possums to far more wide ranging species such as wild deer, feral pigs or ferrets, which later transmitted the disease back to possums in distant uninfected areas. In North Canterbury during the 1990s, for example, wildlife TB appears to have spread southward too rapidly to have been solely due to possums, suggesting ferrets or wild pigs were probably also involved. It is possible that pig hunters may have inadvertently contributed by discarding offal from infected carcasses in another (previously clean) area on their way home.
The continued spread of M. bovis has been largely halted by intensive control of possums at the fringes of infected areas.However, there is a recent exception. In 2011, M. bovis was detected in the Rolleston Range, Canterbury, a long way from any known infected possum population. Although the source of the outbreak is unknown, it is possible that M. bovis was carried across the Southern Alps by a dispersing deer. Another possibility is that hunters illegally released pigs (to create a hunting resource) they had obtained from an infected area, accidentally introducing TB.
Even though the relative importance of the various mechanisms by which M. bovis spreads to new possum populations may never be known, the bacteria is very effective in finding ways of piggybacking on mammals. Without continued intensive efforts to prevent the spread of TB, it is highly likely that M. bovis would spread through all possum populations in New Zealand within a few decades.