TB Background Information

What causes TB in cattle?

Bovine tuberculosis (TB) is an infectious disease of cattle. It is caused primarily by a bacterium called Mycobacterium bovis (M. bovis). M. bovis can also infect and cause disease in many other species most notably badgers, deer, cats, dogs, pigs and camelids (llamas and alpacas).

Can TB in cattle spread to humans?

Bovine TB is a zoonotic disease. This means that it can be transmitted from animals to humans.

Northern Ireland has a comprehensive TB eradication programme in place for cattle. This together with other public health controls (milk pasteurisation and meat inspection) means cases of M. bovis in humans are now uncommon.

Why do we test cattle for TB if the risk to humans is low?

An effective TB programme is needed for 3 main reasons:

• To protect public health – If cattle were not tested the disease incidence would increase dramatically and this would increase the risk to human health.
• To protect trade – The NI agri-food sector is highly dependent on exports to other countries. Without a comprehensive TB eradication plan in place our meat and dairy products would not be accepted by our key export markets.
• To protect animal health – in countries without an effective TB control programme in cattle, clinical disease is commonly seen and is a major cause of lost productivity.

What are the symptoms of TB in cattle?

Most infected cattle show no clinical signs and appear perfectly healthy.

There are 2 main reasons for this:

• The disease usually progresses very slowly, and many animals are slaughtered for beef or detected at a TB test, long before symptoms develop.
• Some animals may develop a “latent” infection which may never progress to cause clinical disease.

In cases where the disease does become very advanced, the most common signs are chronic weight loss, coughing and laboured breathing. TB can also occasionally cause a chronic form of mastitis which does not respond to treatment.

• Legislation requires that all reactor animals undergo a clinical examination by the testing veterinary surgeon to check for any clinical signs.

How do cattle become infected?

To become infected, cattle must be exposed to a source of M. bovis. There are several ways this can occur:

• Direct Spread between cattle

  As TB is mainly a respiratory disease, bacteria can be passed out of the infected animal’s body in its breath or in discharges from the nose or mouth. Other cattle can then become infected by breathing in these bacteria.

  TB may also be found elsewhere in the animal, such as in the gut or udder. In such cases, bacteria can also be shed in dung and milk. Other cattle may then become infected by drinking infected milk or ingesting contaminated feed.

• Indirect Spread

  As the bacteria can survive for long time periods outside the host, TB can also be spread via contaminated objects. For example – contaminated wellies, vehicles, and farm machinery / cattle trailers.

  The bacteria can also survive in slurry and farmyard manure and there is also a risk of spread through this source.

• From wildlife

  Infected wildlife, particularly badgers (and possibly deer in some areas), can also transmit the disease to cattle.

  Exposure may occur at pasture or when wildlife has access to winter housing or feed. As with cattle to cattle spread, transmission of the disease from wildlife to cattle can occur through direct nose to nose contact or through contamination of the environment with respiratory excretions, urine, and faeces.

Disease can also spread in the other direction – from cattle to wildlife.