Bovine TB vaccine cuts spread through herds by 80%

Bovine TB is a dairy farmer’s worst nightmare. Controlling the disease, which is very similar to the form of TB contracted by humans and can actually infect us too, involves rigorous screening, and culling of any animals found to be harbouring the bug. This approach to management, though, which is common across the EU and related countries, has made it very hard to gather adequate evidence about the potential effectiveness of the vaccine we have for TB and which might provide a superior way to control the condition. Now researchers at the University of Cambridge have managed to do the requisite trial, by working in Ethiopia. Vaccinating cows, they’ve found, doesn’t prevent infection, but it does dramatically reduce the severity of the disease in affected animals, and it cuts the rate of spread through herds by over 80%. The study’s author is Andrew Conlan…

Andrew - Tuberculosis in cattle is known to be associated with reductions in productivity. And what I mean by that is costs for farmers due to the animals producing less milk or having less quality meat. Historically, the risk to us from cattle has been from drinking unpasteurised milk. That was responsible for killing around 2000 children every year back in the 1920s before we brought in pasteurisation. And that risk is actually one of the main motivations for why we want to control bovine tuberculosis. Although as long as you pasteurise milk, actually that risk is probably quite small.

Chris - Farmers dread this arriving on their farm among their herd though, don't they? Is that because the control measures are quite draconian?

Andrew - Certainly in the UK the costs associated with the disease are almost entirely down to the costs associated with control. If a farmer gets TB on their herd, they have to go through a very punishing series of tests of every animal in their herd at regular intervals to demonstrate that the herd is clear from infection. And that's incredibly costly to farmers because of the animals that they lose, the time that they lose in doing the testing and the interruption to their business because they can't trade animals while they're under restrictions.

Chris - We do have vaccines that can combat tuberculosis. We used to give them routinely to children in the UK and other countries. We don't anymore. Are they any good?

Andrew - So the vaccine that we have for tuberculosis is the same vaccine that we've had for over a hundred years. But when it has been tried in the past using experimental challenges, where we actually artificially infect animals with tuberculosis, it has a very low protection. But the vaccine has been known for a long time to reduce the amount of disease in animals and the rate at which they become sick.

Chris - And is that what you set out to test here then to try and get some physical numbers around this?

Andrew - The argument for using this vaccine was always that, by reducing the rate of progression of the disease, it could also reduce the rate of transmission within herds. But that hadn't actually ever been measured before. And it was only by using a very clever natural transmission design where we compared the rate of transmission, actually measured it directly for animals that had been vaccinated and then infected and animals that had been not vaccinated and infected, that we could actually measure directly what this effect on transmission had. And what we found was that in terms of the rate of transmission or the protective effect on transmission, BCG is very good, has up to 80% reduction in transmission, compared to about a 20% reduction in terms of having lesions. Now it's important to remember those animals still have evidence of infection but are much less extensive, much less progressed and than animals that weren't vaccinated.

Chris - Where did you do this and how?

Andrew - So these studies were done by our colleagues at the Animal Health Institute in Ethiopia. There's a good reason for doing these studies in Ethiopia. So when we tried to do natural transmission studies in the UK, they were basically a failure because we didn't see enough transmission. And that's because in the UK, as soon as we find an animal that tests positive to TB, we kill it. In Ethiopia, which has endemically infected herds, there's already a supply of these animals and the facilities there allowed us to do these studies over a period of two to three years.

Chris - And you find that although animals pick the disease up, they don't develop this significant severe disease and they don't transmit it among themselves at anything like the pace they would've done if they weren't vaccinated.

Andrew - That's right. The fundamental readout of the experiment was that we compared animals that had been vaccinated and infected and those that had not been vaccinated put them in contact with fresh animals. And we saw that the animals that hadn't been vaccinated had pretty much the same rate of transmission as the animals that we recruited from the field. Those that had been vaccinated at a dramatically lower rate of transmission

Chris - Is the argument then that it's a good idea to vaccinate your cattle, but you accept that we will see infection, but we won't see anything like the rate of disease transmission through a herd.

Andrew - So bovine tuberculosis control is what we call a wicked problem. The biggest barrier to the use of vaccination has been that we know that it interferes with the current tests that we use to detect infection. So in countries like the UK where we actually have quite extensive control measures, vaccination could actually cause more problems if it undermines the action of the controls that we have. Because of this context, vaccination isn't currently recommended by international bodies. And that's a shame because actually in countries that don't have control measures at all at the moment, like Ethiopia with emerging dairy markets and increasing risk of transmission of bovine TB vaccinations could actually be a very cost effective way to reduce the risks and potentially increase the health of animals in those farms.