Tuberculosis (TB)

Mycobacterium bovis can infect a wide range of mammals including deer, cattle, possums, pigs, ferrets, other mustelids and humans.

In New Zealand, the risk of bovine TB infection to humans is very low. But historically and in some developing countries today, human (M. tuberculosis) and bovine TB infections are significant public health issues.

Possums are the only NZ wildlife species in which TB infections are maintained. They account for about half the herd infections in the areas where TB is present in wildlife. Other wildlife species, such as ferrets, may infect deer and cattle, but are ‘spillover’ hosts that do not spread the disease within their own species.

Other cases of herd infection arise from the movement of deer or cattle carrying undetected TB infections between farms. Infected animals may be silent ‘carriers’ of the disease, showing no symptoms for months or years after infection.

To reduce the risk of TB and other diseases spreading to their deer – and to protect native biodiversity – farmers are encouraged to actively control possums, ferrets and other feral animals on their farms. When buying deer, check that the property they come from is free of TB and other diseases of concern.

Because there is no effective treatment for TB in farm animals, control of the disease is based on the identification and slaughter of animals that are effected.

Because on-farm testing and post-mortem inspection of carcases at deer slaughter plants has been so effective, the clinical signs of TB are now rarely seen on farms. However, in the early days of the industry when TB was widespread, deer in the later stages of the disease often suffered weight loss. There was also evidence of production losses in deer herds with long-standing infections.

The statutory body responsible for TB control is TBfree New Zealand, a subsidiary of OSPRI. OSPRI also manages the National Animal Identification and Tracing (NAIT) programme.

If you farm cattle and/or deer, even one animal, you must register your location and contact details with OSPRI. All animals are required to be tagged with NAIT-approved RFID ear tags and registered in the NAIT system.

NAIT tagging weaner deer on a Wairarapa farm. Photo: OSPRI

Most cattle and deer have traditionally been tested for TB at intervals of between one and three years, depending on the TB risk in a herd or an area.

Because the disease is now much better controlled in possum populations, the risk of livestock catching the disease has reduced. In areas of lowest risk, deer herds no longer have to be regularly tested, but all deer carcases continue to be closely monitored at deer slaughter premises for signs of infection.

In areas where deer farmers are required to test their herds, farmers are notified in advance of their next test by the national testing contractor, AsureQuality. In these areas, farmers that send a high proportion of their deer for slaughter each year can apply for ‘closed herd’ deer test status by calling OSPRI. This means skin testing of their herd will be required only once every three years.

In addition to complying with the TBfree testing requirements, farmers must comply with the rules of the NAIT programme. This records and traces the movements of cattle and deer throughout their lives. Traceability is important because it enables outbreaks of diseases like TB to be quickly isolated and controls put in place.

TB tests measure an animal's immune response to bovine TB, using the MCT (mid-cervical test). This involves a single injection of tuberculin in a closely clipped skin patch on the neck (see photo). Three days later the site is ‘read’. Any swelling at the site is regarded as a positive test.

All animals that test positive are given an official orange reactor ear tag. The animals may be re-tested with either the CCT (comparative cervical skin test) or blood test (ETB), or they may be directed to slaughter.

The CCT and ETB tests can differentiate between a genuine bovine TB infection and those reactions caused by exposure to M. avium (avian TB) or M. paratuberculosis (Johne’s disease) – bacteria that are closely related to M. bovis.

If avian TB is detected no action is needed as it rarely causes clinical disease. On the other hand, Johne’s disease does cause clinical disease, but it is not covered by the TBfree programme. For more information, see Johne’s disease >>

Reactor tags must remain in place until reactor animals are slaughtered or cleared on a re-test. If the animals are cleared, the tags must be removed before the animals are moved off the farm.

Each herd is given a TB status – Infected, Suspended or Clear (with a number indicating the number of years they have held that status, for example, C4). All deer herds in the lowest risk areas are now classified as CM (Clear Monitored) as they are only monitored through slaughter surveillance.

Areas with a high TB risk are classified as Movement Control Areas (MCAs). A TB test within 60 days or an official permit is required before any deer or cattle in an MCA are moved to another property.

Special pre-movement tests may also be required for herds with an Infected (or Suspended) status, whether or not they are in an MCA. These herds are not required to have a pre-movement TB test if they are being sent directly to slaughter at a deer slaughter premise, but they may first require a permit to move.

In certain circumstances an official exemption to move stock without a pre-movement test may be granted. Call OSPRI on 0800 482 463 to apply.

The ability to rapidly and accurately trace animals from their farm of origin to eventual slaughter is a vital biosecurity and food safety tool.

The National Animal Identification and Tracing (NAIT) programme provides this service. It keeps records of the movements of cattle and deer throughout their lives.

It is important for farmers to comply with NAIT requirements, because it enables outbreaks of diseases like TB to be quickly isolated and for controls to be put in place. Thanks to the efforts of deer farmers, compliance levels are steadily improving, as reflected in the graph from OSPRI (below).

Keep up the good work: 

• Tag all cattle and deer with NAIT-approved ear tags
• Register them in the NAIT system within 180 days of birth or before they move off the farm for the first time – whichever comes first  
• Record all off-farm movements of stock on the NAIT database within 48 hours of the movement
• Replace missing tags with NAIT-approved tags and record them in the NAIT system.