TB, the disease responsible for almost 2 million animal deaths worldwide per year, is thought to latently and pathogenically infect almost a third of the world’s human population. The extent of its prevalence in other animals is unknown and practically immeasurable but we do know that both human TB (Mycobacterium tuberculosis) and ‘animal TBs’ can infect humans. The zoonotic potential of TB-causing Mycobacteria makes veterinarians pivotal cogs in the TB prevention and control mechanism, both in the UK and across the world. The main concern comes from Mycobacterium bovis, the species that is most pathogenic in bovines.
The prevalence of disease-causing TB infection in humans in the UK is very low and the proportion of that caused by M. bovis is even lower (1%). This is due to high standards of healthcare, availability of necessary medicines, doctors, and food safety process. Pasteurization of milk has been crucial to preventing a larger number of people becoming infected with M. bovis. Other countries are not so fortunate. In many parts of the world, pasteurization is not common practice and as many as 10% of TB infected patients in Nigeria are infected with bovine TB
Unfortunately, the classical test (the tuberculin skin test) for TB used in cattle is very ineffective as it’s sensitivity is so low (~60%), which has meant that eradication of bovine TB from the UK has been a long-fought battle with little prospect of victory. The lack of a DIVA vaccine (Differentiation of Infected from Vaccinated Animals) and multiple wildlife vectors have further hampered eradication efforts. Badgers are often cited as the leading reason for bovine TB’s spread and persistence within the UK national herd. However, recently 25 dogs from a pack of foxhounds have been destroyed due to bovine TB infection (enraging the members of the public who oppose the methods of vector control in the UK) which has initiated more discussion and research into which vectors do cause the greatest number of infections in cattle and people in the UK. Deer, sheep, pigs, cats, wild boar, possums and even elephants have been reported to be infected with M. bovis.
This picture was famously photo shopped to use as lobbying propaganda for tougher stances on vector control in the UK. It has been shown that direct contact between badgers and cattle is an incredibly rare phenomenon and unlikely to occur in daylight.
For the UK, eradication should be relatively straightforward: we work on an island with tight meat and animal import restrictions and regulations and a large healthcare workforce with world-class equipment and networks. Australia has successfully eradicated bovine TB, so it has been proven to be possible. But the UK has unique challenges: the weather and climate are ideal for transmission, and the astronomically high stocking densities and proximity of livestock to wildlife vectors make environmental contamination much more likely. The efforts to control bovine TB in the UK are greatly influenced by politics and very little policy is based on science. Disease control has been devolved to the Welsh Government, where the move from control to eradication strategy was initiated, is mostly based around increased testing frequency and biosecurity but experimental randomized badger vaccination has also been trialed. (The prevalence of TB in farms across Wales has decreased by around 25% in 6 years under this strategy).
The tide is turning though. More regular mandatory testing frequency is increasing in areas where TB is endemic (highly prevalent and persistent in an area) and the method of testing is moving away from the classical tuberculin skin test and vets are now using a more accurate blood test as more laboratories equipped to analyze the results are being built. The voices of animal welfare charities and eminent scientists are beginning to put chinks in the armor of the government’s vector control strategies with a U-turn in policy on the (distant) horizon. Vaccination of wildlife vectors is increasingly popular as an experimental strategy for reducing bovine TB prevalence and baited oral vaccines are being found to be effective.
All of this good news is, for the moment, overshadowed by the terrible lapses in food safety, welfare and common sense that is extending bovine TB’s grasp on the UK but once it has been eradicated I hope that the scientists, farmers, environmentalists, shop owners and policy makers fighting this terrible disease can share what they have learned with the world and try to unite to end TB.