Does ‘No Visible Lesions’ Mean No Bovine TB? Understanding bTB Test Results

It is not uncommon for animals who are classed as a reactor to the skin test on the reading day of a TB test to be reported as having ‘no visible lesions’ at post-mortem inspection. Some people find this a confusing and frustrating result, assuming this means that the animal was not actually infected with TB.

However, the purpose post-mortem inspection is not primarily to assess the presence of TB in that animal but rather the chronicity, and likely route of infection and so the result of ‘no visible lesions’ could even be good news from a farmers point of view - but understanding this requires some knowledge of the TB skin test and could help you to better understand the process of TB infection and testing in your animals.  

The TB skin test has a reputation for being a ‘bad test’, but how ‘good’ it is depends on what question you want it to answer. It is true that the TB skin test is only 80% ‘sensitive’ – meaning it will pick up approximately 80% of animals that are infected with TB as positive on the test. This means that it is actually leaving some infected animals behind as ‘false negatives’.

It is over 99% ‘specific’ meaning that when an animal does test positive on the test that animal is actually infected with TB over 99% of the time. In short, the TB skin test is right about reactors having TB over 99% of the time.  

So, what does it mean when these animals then come back as having ‘no visible lesions’? When a TB reactor is culled the carcass undergoes specific inspection by abattoir vets to assess common sites of infection (lymph nodes in the head and chest, the lungs etc).

To do this the inspectors make cuts into these tissues to look for M.Bovis abscesses with the naked eye. This relies on the inspectors making enough cuts in the tissues (or having good enough eyesight!) to see the abscesses. Of course, the smaller the abscesses the less likely they are to be seen.

Since TB is a chronic disease, the abscesses will gradually get bigger with time, so it is likely that early in the disease process any abscesses present will be too small to be seen and that only long-standing chronic forms of the disease will be seen as visible abscesses which are found at post-mortem.

In-fact only 40% of reactors have visible lesions at post-mortem, even though more than 99% are infected with TB.  

So, if an animal is a reactor at the skin test but reported as ‘no visible lesions’ on the post-mortem this is likely to be good news as it indicates that the animal is likely to be in the early stages of the disease, and so the risk that it has been carrying TB for a long time and transmitted TB to other animals in your herd is lower.

If there are visible lesions seen, while a chronic infection in an animal is not good news, the location of the lesions can be helpful to assess how that animal acquired TB or what are the risks of it spreading it to other animals, e.g.: a cow with TB in her udder is more likely to transmit TB in the milk to calves being fed her milk than if the lesions are found in the lungs.

Again, this can help to assess transmission risk to other animals (or yourself!) and allow you to act to reduce the risk of spread of TB through your herd. 

If you have reactors found on your farm, there is advice available through the ‘TBAS’ (TB advisory service) who can provide tailored farm-specific biosecurity advice for your farm.

At the practice we have TBAS accredited vets who can come and assess your risks and discuss in more detail steps you could consider. Contact he practice for more information or to speak to one of our vets. 

TB hub is also a great resource for advice and further information