Co-infection – especially with liver fluke – could be desensitising existing TB tests, essentially putting control measures to waste as undetected infected animals continue to remain alongside the healthy.
Many TB-infected animals are potentially being missed by existing skin tests, delegates at the Agri-food and Bioscience Institute (AFBI) 2018 Scientific Outlook conference heard.
Residual infection
Prof. Simon More from University College Dublin said: “At the six-month clearance tests between 10% and 15% will break down again – much of that is related to residual infection.
“One year after herds have been released the risk for those herds is up to four times greater than herds which never broke down.
“Work from Cambridge using two different models showed 5-20% of herds will still have infected animals.”
The findings seem to be significant in terms of controlling bovine TB.
Dr. Andrew Byrne from AFBI told the conference that research had showed gamma interferon positive animals who had skin tested negative were 2.3 times more likely to fail a future TB test than gamma negative herd-mates.
Testing the tests
Identifying infection early on and taking it out of these herds was a key part in controlling the disease, Byrne explained.
He said: “We are interested in characterising these transmission events and identifying how we can cut the chains and break down this overall system.
“We looked at the movement networks, the wildlife aspect, test evaluation, the variation in risk across herds and also co-infection.
“There may be some circumstances where infected animals are missed and you might be able to pick them up through antibody responses or serological respponses.”
Hidden infected
“We asked the question, of the animals that are skin test negative but gamma interfon positive, do they harbour any future risk?” he said.
“Because right now – or at least up until recently – it was the choice of the farmer as to whether to retain these animals or not.
We looked at around 1,000 gamma interfon positive animals within around 225 herds in Northern Ireland over five years and saw what was their future risk within those herds.
Byrne explained AFBI research had showed gamma interferon positive animals who had skin tested negative were 2.3 times more likely to fail a future TB test than gamma negative ones.
“Over 18 months, 22.6% of gamma interferon positive animals were likely to fail a test compared to 6.1% – even in higher risk herds,” he said.
“Obviously this is a huge risk to the farmer.”
Co-infection
Diseases such as BVD, Johnes and Liver Fluke could interact with the TB diagnostics or the lesions causing either mis-diagnosis or progressive spread.
He said: “Co-infection is the principal that if you have multiple infections within the same host it can ultimately affect your ability to detect and can exacerbate infection resulting in increased transmission.
“It ultimately has a negative impact on TB eradication efforts. It’s undermining our trust, I suppose, in the skin test.
“One of the more interesting things we found was that animals infected with liver fluke were likely to produce smaller lumps meaning that they were more likely to test negative.”
