Reducing TB risk

3 May 2023

As well as being a farm vet in Derbyshire, Sarah Tomlinson is a veterinary consultant with Kingshay and the technical director for the TB Advisory Service. She also sits on the Defra bTB Partnership.

Bovine TB is the most devastating disease impacting our cattle herds today. Not because of the direct animal health and welfare impacts- as most reactors are otherwise fit and healthy – but because of the massive impact a breakdown has on a farm business and the knock-on effects on animal and farmer health and welfare.

TB has many risk pathways out of our control, such as what the neighbour buys in or the area of the country in which we farm. But TB is an infectious disease and just like Johne’s, BVD, abortion, lameness and calf pneumonia, it also has ways onto the farm we can control.

Through the TB Advisory Service (TBAS) in England (and Cymorth TB in Wales), vets help farmers assess the ways TB could arrive on farm and how it could recycle once it’s there. They advise how to reduce, if not eliminate, these risks through simple, cost-effective, evidence-based interventions.

A TB Advisory Service free visit discusses six risk areas:

  • Herd’s TB history
  • Local situation
  • Incoming livestock
  • Livestock at pasture
  • Livestock in the yard
  • Business impact

The evidence behind each area is explained, finishing with four agreed recommendations.

Skin test

To appreciate the risk posed by cows, we must understand the characteristics of the skin test. The skin test is very accurate at identifying truly infected animals. It has a specificity of 99.98%, meaning you can be 99.98% certain a reactor is infected with TB. At farm level this is important due to the consequent herd movement restrictions when a reactor is identified, and the requirement for the animal to be slaughtered.

In contrast, a test’s sensitivity is the probability of correctly identifying an infected animal. On a good day, with the test carried out correctly, this is about 80%. This means 1 in
5 infected animals that should test positive actually test negative.

Whether TB lesions are subsequently identified at slaughter or not, we must trust the skin test. It is no different to subclinical mastitis. When milk records show a high cell count, you trust the test and treat the cow accordingly. Untreated, you know the infection would progress to mastitis and she could be infecting others.

What does this mean on farm?

The more recently a herd has had a TB breakdown the greater the chance of having future breakdowns. Even though a farm is officially TB free it might not be infection free. Your vet and/ or TBAS adviser can talk to you about how to identify “at risk” animals and manage them differently to limit the spread of TB within a herd, just as you would with Johne’s disease.

For incoming stock, the longer a herd has been testing clear, the more you can trust the latest test is genuinely clear. Just as you check the BVD, Johne’s, Leptospirosis status and testing history before buying cattle, ask about TB too.

What about badgers?

Most badger and cattle interactions happen indirectly, with nose-to-nose contact very rare. We do know badgers share the same environment as cattle though, ie. water sources and feed. Badgers especially like starchy food such as maize silage and cattle cake. In certain areas of the UK, we have endemically infected badger populations. Dead badger surveys have shown in some areas 1 in 4 badgers can test positive to bTB, shedding the bacteria in their urine, faeces, spit and pus from wounds. In these areas limiting badger and cattle contact will reduce your risk of a TB breakdown caused by badgers.

Badgers can squeeze through gaps of just 7.5cm so protect feed stores well. TB can also survive in water for up to 60 days, so raise water troughs to a height of 1m or use badger-proof ones. Thankfully in Scotland, and certain areas of England and Wales, we have no reason to suspect the badger populations are infected. In these areas we need to still engage in reducing cattle/badger interactions to keep the badger population TB free, as TB is much harder to manage when the badgers are infected.

Following simple evidence-based measures on farm can reduce your risk and length of a TB breakdown. Feedback from vets and farmers is telling us being part of the TB Advisory Service is rewarding and instead of waiting for the Government to do something, they feel they had taken back some control of their own TB risk.