To TB test an animal, a small dose of purified proteins derived from TB bacteria (called tuberculin) is administered into the skin. This will provoke a reaction if an animal is carrying Mycobacterium bovis.
An animal previously exposed to bovine TB is likely to respond with a localised swelling at the site of the injection. This is the visible and palpable sign that the disease could be present.
OSPRI Disease Manager Jane Sinclair works at the front line of disease management for New Zealand farmers. She outlines what happens when an animal returns a positive result to a TB test.
"Most New Zealand farmers are familiar with TB testing. Depending on where a farm is, most farmers will receive a visit from a TB tester who performs a skin test on animals in dairy, beef or deer herds. A farm’s disease area status will determine whether that occurs annually, or every two or three years, and may include extra tests when stock moves between farms.
Once an animal tests positive to a skin test, a blood test is conducted. This is done as many of the skin test reactions are caused by other Mycobacteria and can produce a ‘false-positive’ reaction. If one animal in a herd has TB, it might mean others do too. That depends on several factors:
how long the animal has been infected and how long since the herd was last tested.
An animal that returns a positive blood test is called a ‘reactor’ and must be slaughtered so a post-mortem examination can be carried out. If a diagnosis of TB is then made, the herd will be deemed ‘Infected’ and a restriction on the animal moving off the farm is imposed via a Restricted Place Notice (RPN).
A TB infection is difficult news for a farmer to receive, and for the OSPRI team to deliver.
It has serious implications for most farmers — for farm management, confidence in work practices, and the value of livestock. Because of that, there is a lot of support available for the farmer, the family and the neighbours through the next steps in the process.
Farmers are usually angry. Good farmers know and love their animals and everything they do each day is for the benefit and health of their livestock. Often, they express disbelief. They want to know — as soon as analysis is available — how their herd might have become infected. Did a TB-infected possum wander onto the farm and spread the bacteria directly? Did livestock brought onto the farm from another area transport the bacteria and infect its new herd-mates?
However bovine TB arrived, it means the farmer will see much more of the OSPRI disease management team than they are used to. An Area Disease Manager will visit to deliver the Restricted Place Notice and start an investigation to work out, using movement records available from OSPRI’s traceability system NAIT, where the infection has come from and, importantly, where it might have moved to.
Once a Restricted Place Notice is delivered, the case manager must inform neighbours on the immediate boundary. To keep herds separate, and stop the TB infection spreading through ‘over the fence’ contact, stock might be shifted to paddocks further away, or electric fences erected within a boundary fence to create a safety buffer.
A Restricted Place Notice can have huge implications where farmers were expecting to sell stock, because that is no longer allowed. Infected animals are slaughtered, and the herd is regarded as infected until case managers can see two clear whole-herd tests six months apart.
Back at OSPRI, decisions are taken according to the source of infection. If the disease has moved onto the farm ‘on the back of a truck’, there are no implications for neighbours. Infection can be contained and managed without affecting the wider community.
If an infection has come from wildlife, there are much wider ramifications for neighbours and greater community information needs. Some herds remain infected for years. From the declaration of ‘Clearance’ (when the infection has been removed from the herd) to the status of “C10” (10 years clear) is a decade.
That is a long time for a farming family. It is important at this stage to keep talking to each other and talking to the neighbours. There is no need for a farmer to hide or think it is shameful to have one’s herd infected by TB.
The good news is that OSPRI’s disease management strategy is working to remove TB from all New Zealand’s cattle and deer herds. OSPRI works with every farmer affected to help manage any infection and keep farm businesses running successfully.
There are 37 herds under active management in New Zealand at the time of writing. This is a fraction of the almost 1700 infected herds at TB’s peak in the mid-1990s, and the good news is: ultimately, it will be gone.
Bovine TB will be eradicated from livestock by 2026. By 2040, TB will be removed from possums. And by 2055, bovine TB will be eradicated. Exports can then be proudly promoted as coming from a TB-free New Zealand."