Why it’s unlikely Geronimo the alpaca’s TB test gave a false positive

10 August 2021
What was claimed

Bovine TB blood tests can give a false positive result if the animal has previously been tested with a skin test.

Our verdict

This seems very unlikely.

What was claimed

Bovine TB blood tests detect bovine TB protein itself, not the immune response.

Our verdict

False.

Environment secretary George Eustice has been criticised for an article he wrote in the Mail on Sunday about an alpaca due to be culled after testing positive for bovine tuberculosis (bTB).

The alpaca named Geronimo first tested positive in 2017, but his vet has argued this may have been because his antibody levels were “falsely elevated before the test” rather than a sign of a real infection. 

In his column, Mr Eustice claimed that this was not true, because the test detected bTB itself, rather than the immune response to bTB. 

This itself is incorrect. The test does detect antibodies, not bTB itself.

However, an expert we have spoken to told Full Fact that Geronimo’s earlier tests would not have made him incorrectly test positive afterwards.

What is Bovine TB?

Bovine TB, as the name suggests, is a bacterial disease which affects cattle, but can also infect other animals including badgers, deer, goats, pigs and camelids (like alpacas and llamas). 

Once infected, an animal almost never completely clears the bacteria on their own. Dr Neil Watt who co-created the “Enferplex” bTB test used in alpacas told Full Fact: “It’s very unusual for any animals to completely ‘self cure’ TB...although whether the infection is active (bacteria replicating and lesions developing) or latent (bacteria in a non-replicating form in lymph nodes and other organs) varies over time.”

The infection can be cured with antibiotics, but this is a long, complex and uncertain process that is rarely chosen with animals.

The risks to humans are small, but the disease can be quite serious

There are a few ways to test for the disease, the most common of which is the “skin test”. 

Proteins (called tuberculin, or purified protein derivative (PPD)) extracted from dead bovine TB bacteria and avian TB are injected under the skin of an animal. This is not the live pathogen itself.

If the animal has already been infected with TB, its immune system triggers a strong inflammatory response, causing the area where the bovine tuberculin was injected to swell up. 

The test is highly “specific” which means that it almost never returns a positive result for an uninfected animal. 

Another test for the disease is the aforementioned Enferplex test. It works by testing blood (which may be exposed to tuberculin beforehand to boost the antibody count) for seven antibodies animals can develop in response to a TB infection. 

Geronimo, who is from New Zealand, tested negative multiple times using skin tests in his home country, but since arriving in the UK has reportedly tested positive twice using the Enferplex test

Why the controversy?

Geronimo’s vet has suggested the recent positives may be false positives caused by the tuberculin used in earlier tests. 

Mr Eustice claimed this wasn’t true because the Enferplex tests detect bovine TB protein itself, not antibodies. However, Dr Watt, told us: “He [Mr Eustice] was wrong on that. The test detects the antibodies, not the antigen.”

That aside, it appears highly unlikely that earlier tests gave Geronimo any false positives. 

Dr Watt says: “The work hasn’t been done in alpacas specifically to show that bovine TB tests don’t induce a positive test, but there’s two reasons against that theory.

“One is that PPD is not a live extract. An animal won’t develop antibodies unless it’s been infected.

“Also there has been research done on cattle in France which were tested at intervals of 60 days, and the specificity remained. If the test was causing cattle to test positive, you’d have seen loss of that specificity.

“There are plenty of cattle who get repeat tests and don’t test positive unless they get TB itself.”

The Department for Environment, Food and Rural Affairs has said the injection of tuberculin does not induce a false positive and that the positivity rate of “at-risk” alpaca herds with these primed antibody tests remained very low.

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