Bovine tuberculosis (known as TB) in deer and cattle is caused by the bacterium Mycobacterium bovis. The presence of TB in New Zealand (NZ) farmed deer is very low (with only four herds infected in 2012) but remains a threat to meat (including venison) and velvet exports. This is because it can be transmitted from infected herds to humans. This is very rare, but the risk of it happening is used as a trade barrier by some countries that import meat and co-products.
The carcass inspection process in NZ is based around identifying TB. If TB is suspected on a farm, consult a veterinarian immediately. Visit www.ospri.co.nz for more detailed information on TB.
The possum is the most important vector of TB for domestic livestock. Transfer of the disease to deer can occur when deer sniff a sick possum. Pigs and ferrets can also have TB, but they are ‘end stage hosts’ meaning they cannot maintain the disease within their own population. TB testing on pigs and ferrets is useful to indicate if TB is circulating in an area.
Clinical symptoms are typically only seen in the terminal stages, with enlarged lymph nodes and weight loss. Animals with bronchial disease may cough.
Infected animals are typically identified early by testing, before any impact on deer production. However if deer test positive a key effect on farm profitability is an inability to sell young or store stock from infected herds. Animals within a herd that has had reactor animals (tested positive to an Animal Health Board approved test) must go to slaughter directly from the farm. This has impacts on management, especially feed supply, as farmers have to finish deer on-farm or slaughter at lighter carcass weights.
There is no effective treatment for TB.
If the animal tests positive as a reactor, the value of the meat to the farmer is nil and there is no compensation. All deer testing costs are paid directly by farmers. Some retesting may be subsidised by the Animal Health Board (AHB).
Bovine TB in NZ is monitored and managed by the AHB. Deer in Movement Control Areas need to be tested before moving off the farm. Herds in Special Testing Areas need to be tested annually or every two years but do not need testing before stock movement. Herds in Surveillance Areas are considered to be low risk and only need to be tested every three years. A new Closed Herd Deer TB testing policy was introduced 1 March 2012. This voluntary programme allows for herds of 100 or more to be eligible for triennial testing.
The first stage of diagnosing TB infection in deer is the mid-cervical skin test. This is carried out by accredited vets or AsureQuality technicians. A protein from the TB organism is injected into the neck and the reaction recorded 72 hours later. If a lump appears TB may be present. As deer are exposed to a number of similar bacteria present in their environment, a false positive reading can occur (typically 15%).
In low risk herds a deer testing positive can then have a secondary comparative skin test (known as CCT) to confirm infection. In herds with a history of TB or where there is a risk of TB being introduced, the CCT test cannot be used as it lacks sensitivity. In these herds an ELISA test (known as ETB) is the better option.
Advice on the most appropriate tests can be obtained from the AHB’s District Disease Control Manager and local testing officer.
To reduce the risk of TB, control possum, pig, ferret and stoat populations around the farm. Only purchase TB tested deer and contact the AHB before moving deer.
If a deer reacts positively to an ETB test and is considered to be infected with TB then it must be tagged and slaughtered. A Permit to Move will be needed from the AHB before sending to slaughter. A TB infected herd must achieve two clear tests at least six months apart in order to regain a Clear status.
Information on Tuberculosis in deer is available in a convenient DINZ Deer Fact sheet (November2015). Download your own copy here >>