The transmission of tuberculosis

Tuberculosis (TB) in humans is a disease of poverty. The causative agent, Mycobacterium tuberculosis, spreads between people in droplets breathed out or sneezed out. I’ll never forget the disgust on my tutor’s face when she described handling sputum samples but if your science concerns TB, you’re going to have to deal with sputum. And this is how come it’s a disease of poverty. People in overcrowded, poorly ventilated homes or shelters are at risk of droplet infection. In the rich world TB has declined dramatically but now that people move around the world so much more than we used to do, TB in the rich world is back.

What has this to do with cows (Bos primigenius) and badgers (Meles meles)? Well, humans can also be infected by the bovine species of TB, Mycobacterium bovis. When milk isn’t pasteurised people can pick up M. bovis by drinking milk. When cattle are overcrowded in barns or during transport M. bovis can spread between them by droplet infection. Here’s a list of notifiable diseases affecting livestock in Britain.

Wildlife species can be reservoirs of M. bovis. Here in Britain that’s mainly the badger (Meles meles) and there’s evidence that M. bovis is being transmitted between badgers and cattle.

Is a cull of badgers part of the answer? Well that’s a different story. I’ve noticed how emotional people get about the badger, because it’s so cute and because it’s been part of so many children’s stories. I myself, growing up with nature posters all over my bedroom wall in the 1970s and 1980s, dreamed of seeing a wild badger one day. Then when I’d grown up and M. meles had been a protected species since 1973, with protection strengthened in 1992, it was a thrill to see badgers at dusk one time when I was camping.

If I’d owned the cattle which grazed nearby, perhaps I wouldn’t have felt such thrill. But should I have invited people sanctioned by the Department for Environment Food and Rural Affairs (DEFRA) to come and shoot the badgers? Or should I have reconsidered my farm buildings?

Every farmer I know takes buildings very seriously, making sure there’s good ventilation to minimise the risk of respiratory diseases. TB is one of those diseases and there are many others affecting livestock. So no, I don’t think that those cattle will have been housed in a cramped damp barn.

But I think those cattle might have been transported too far and too often in cramped trucks. DEFRA says that livestock transport vehicles need good ventilation. Why are cattle moved around so much? That’s a topic for another blog post.

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About argylesock

I wrote a PhD about veterinary parasitology so that's the starting point for this blog. But I'm now branching out into other areas of biology and into popular science writing. I'll write here about science that happens in landscapes, particularly farmland, and about science involving interspecific interactions. Datasets and statistics get my attention. Exactly where this blog will lead? That's a journey that I'm on and I hope you'll come with me.
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11 Responses to The transmission of tuberculosis

  1. lowerarchy says:

    I’m a little surprised at the ‘cute’ description of badgers. They are the largest wild carnivore in Britain and rather fierce. Are you implying folk are against culling cute animals but wouldn’t care if the species were ugly? I’m against killing anything 🙂 The farming system itself is the problem.

  2. I think that the living conditions should always be looked at first, as in human diseases, environmental factors play a large part in transmissions. I have no problem per sé with the meat industry, however (and the UK is actually a lot better than many european countries on this), intensive farming practices are not the most healthy for the livestock, as you know, with the increase in infections etc among animals which are crowded into small spaces.

    I get the feeling the badger is being used a bit as a scapegoat, I am not sure if it is the major vector in transmission, or whether it is just an easy target. Agricultural ecology is not my area, but maybe there are other options to culling, if the badger is the main cause. Maybe dissuading badgers from entering into livestock areas may be more suitable.

    From an ecology perspective, culling of badgers may have larger ecosystem consequences, as they do fill a role within the system of reducing the population of animals which may be considered more “unwanted” such as rodents and insects, as well as aiding in soil development through their digging (like worms do).

    • argylesock says:

      I think you may be right about scapegoating the badger. My own opinion on the badger/TB story is still not certain.

      You’re certainly right that intensive farming increases certain disease risks to livestock. Not all of them, though. I think I’ve mentioned before that Toxoplasma gondii in pigs is the reason why everybody learning to cook pork and bacon learns to cook them thoroughly, but most people don’t know that the Toxo risk almost disappears if the pigs are kept indoors.

  3. This paper kind of illustrates the difficulties when dealing with transmissions between species, and the level of complexity. I think many in the media (yes, even the science journalists) do not understand it, and so when the information is passed on, it is not always interpreted correctly.

    http://www.pnas.org/content/103/40/14713.full

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