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.

What Happens to Animals Removed from Herds due to TB?

Once an animal is interpreted as positive to an approved test, no further on-farm analysis is carried out prior to mandatory removal from the establishment. Animals which are positive on either the blood or skin test plus those designated as very high-risk cohort animals (“negative in contacts”) are subject to compulsory removal.

The vast majority of these animals are removed from herds under DAERA’s “Haulage and Slaughter” contract to an abattoir for slaughter. A minority of animals are either destroyed on farm (usually because they are unfit for transport for welfare reasons) or die on farm for other reasons before they can be collected for slaughter. These are generally disposed of by normal fallen stock collection procedures.

A very small number of the animals destroyed on farms are taken to the laboratory if samples or laboratory post-mortem examination are required from an animal that is unfit for transport.

All animals that arrive at an abattoir must be accompanied by Food Chain Information (FCI) and are subject to an ante mortem inspection by an official Veterinarian and a post-mortem examination on the kill line. Any animal presenting with clinical signs of tuberculosis at antemortem inspection must not be slaughtered for human consumption. A detailed post-mortem inspection is carried out on skin test reactors, inconclusive reactors, and negative contact cattle.

Regular mandatory testing means that infected cattle are generally identified at an early stage of infection, often prior to the development of advanced tuberculous lesions. In addition, due to the food safety controls that all cattle undergo when they are presented for slaughter   there is negligible risk of consumers contracting tuberculosis from the consumption of meat and meat products.