The establishment of tuberculosis in the possum population led to the disease becoming a serious threat to the New Zealand economy.

With possums being widespread and abundant (current population an estimated 30 million) and the main wildlife transmitter of TB, the control and eradication of this disease might be considered audacious, but it is possible.

The national TB Plan aims to reach freedom from bovine TB in New Zealand's livestock by 2026, achieving freedom from disease in possums by 2040 and eradication of TB from New Zealand livestock and wildlife by 2055.

Kendall's Threshold Theorem predicts that spread of a disease will rapidly increase if an infected animal infects more than one other before it dies. Conversely, a disease will rapidly decrease if an infected animal infects less than one other in its lifetime. The ratio of white to black possums represents the probability of transmission. For example, at a ratio of 2:1, an infected possum (black) is likely to transmit TB to two other possums.
Kendall's Threshold Theorem predicts that spread of a disease will rapidly increase if an infected animal infects more than one other before it dies. Conversely, a disease will rapidly decrease if an infected animal infects less than one other in its lifetime. The ratio of white to black possums represents the probability of transmission. For example, at a ratio of 2:1, an infected possum (black) is likely to transmit TB to two other possums.

The plan aims to produce 'Proof of Freedom', evidence of the probability that bovine TB has been eliminated from an area or a population of wildlife, such as possums or ferrets.

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It is not a novel concept, being based on the principles of epidemiological surveying to monitor diseases in populations. For TB, the underpinning data and theory of the Proof of Freedom model includes Bayesian probability and classical sampling methodology, TB epidemiology, population dynamics, home range and movement patterns of possums and sentinel species, and the trap-ability or detectability of possums.

Proof of Freedom is based on a calculated probability that there is less than a 5 per cent chance the possums in the area surveyed are still infected, and that the disease cannot persist at that level.

The general conceptual theory is identical for mammalian pests all over the world.

International theory as used to describe the concept of rabies eradication from foxes in Europe applies equally to TB in possums in New Zealand. It suggests that, if we reduce a population of possums below a particular threshold density — about one possum per 2ha — infected animals die before they can transmit disease to susceptible animals.

If this reduction covers a large enough land area to limit immigration of a susceptible population, the disease will die out.

The theory sits at the foundation of the National Pest Management Plan to eradicate TB, alongside the knowledge that possums are the main transmitters of TB between wildlife and farmed livestock.

Since the peak of TB herd infection in 1994, when 1700 cattle and deer herds were known to be affected, the TBfree programme has reduced infected herds to fewer than 50.
Assessment tool

The model is a tool to assess the probability that bovine TB has been eliminated from possums in a local area and was first used in 2012. Manaaki Whenua — Landcare Research scientists developed the tool in collaboration with the veterinary epidemiologists who designed the TBfree programme, now managed by OSPRI.

The general approach is that, once the model predicts probability-of-freedom (pFree) at an agreed trigger level (generally greater than 80 percent probability that TB has been eradicated), surveillance is initiated to provide evidence that TB is indeed absent
out TB — not possums

It took almost a decade to develop the Proof of Freedom framework as a major new paradigm in TB management.

Surveillance data can be also derived from surveys of 'sentinel' species such as deer, pigs and ferrets. The surveillance effort continues until a pre-defined pFree (so-called 'stopping' probability) is achieved.
At that point, possum TB freedom is declared. There are a series of 'stopping' probabilities, based on the risk and consequence of a TB freedom declaration being incorrect. Simply, the greater the risk and consequence of a false declaration of freedom, the higher the 'stopping' probability is set, therefore requiring a greater surveillance effort to achieve true TB freedom.
When the set level of confidence (probability of eradication) is achieved, an area can be reclassified from being a Vector Risk Area (VRA, areas where TB is found in wildlife) to being a Vector Free Area (VFA). At this point it can be declared that possum populations in an area have been below the level at which the disease can be maintained for a long enough period to confidently define the area as TB-free.
Vector-free areas
Since the beginning of the TBfree programme, more than 1.8 million hectares of Vector Risk Area have been declared vector-free. However, as of July 2017, a further 7.9 million hectares remain to be declared TB-free.
It took almost a decade to develop and adopt the Proof of Freedom framework as a major new paradigm in TB management. This is partly because it takes time to 'sell' new concepts to users, but mainly because the need became a priority when local TB eradication became a formal management goal.
There is now a strong focus on refinement and extension of the framework and associated software.
An example is the development of new theory by Landcare Research to allow inclusion of data from livestock TB testing and slaughterhouse inspections, which will give confidence that TB is absent from possums in particular farmed areas. That information is used only as supporting information and does not affect the calculated probabilities of TB freedom. Other research is exploring when to start TB surveillance, with the potential to greatly shorten the cost and duration of possum control required for TB freedom.
By providing an objective measure for comparing progress between areas, the Proof of Freedom tool has enabled a quantum shift in thinking from possum control to the elimination of TB from possums.
This is the key objective of TB management and is the only means by which we can successfully eradicate TB from our farmed livestock.¦

KEY FINDINGS

* Possums are the main wildlife transmitter of bovine TB in New Zealand.
* Proof of Freedom is evidence that an animal population or area is free of disease based on the probability that the disease has been eliminated.
* The TB Proof of Freedom Model uses surveillance of wildlife and livestock, as well as pest control history, to ascertain if TB has been eradicated.
* This model has shifted thinking and action from possum control to eradication of TB from possums. This is the only means by which eradication of TB in the national dairy herd can be achieved.

Kevin Crews is the national disease manager at OSPRI.
Graham Nugent of Landcare Research also contributed to this article.