Mycobacteria and TB have been in the news a lot recently. In fact, one particular species has been hogging the limelight: Mycobacterium bovis. As its name suggests, it likes to infect cows, but as we’re recently all too aware, it’s quite happy in badgers too.
There are about 120 species of mycobacteria. They’re rod-like bacilli with a thick, waxy cell wall. The “human” member of the Mycobacterium family (using this word conversationally, as Mycobacterium is of course a genus, not a family, taxonomically) is M tuberculosis. From its point of view, it’s very successful. From our point of view, it’s the most important bacterial disease that afflicts us, causing about one-and-a-half million deaths a year.
This is how TB is caught: you breathe in tiny droplets of fluid containing just a few bacilli. In your lungs, your own immune cells move in and swallow up the bugs – but this is exactly what they want. Inside the immune cell, the bacilli replicate and pile up. More immune cells pile in. If you’re lucky, the bacilli are held tight in this lump, this tubercle. If you’re unlucky, the bugs get out and the disease spreads – through your lungs (and you start coughing up droplets with bacilli in, ready to infect someone else), through your whole body, even getting into your bones.
But this isn’t the story written in the DNA of the bacilli. In 1999, an early genetic study cast doubt on the ancestor-descendant relationship between the bovine and human forms of TB. More studies confirmed the new story. If anything, the ancestor of both human and bovine forms must have been closer to the human form – with its larger chromosome. It’s even possible that cows caught TB from us (or at least, from another mammal that had caught “human” TB).
Using molecular clocks to date the age of M tuberculosis, looking for the last common ancestor of current versions of the bug, is problematic, and has produced a great range of dates from 15-40,000 years ago. All of these easily predate farming. However, this date is likely to just record a population crash in TB, probably because of a crash in the numbers of its host. Humans (and their ancestors) could have been suffering from TB for hundreds of thousands of years before then.
Earlier this year, filming for BBC2’s Prehistoric Autopsy series, I visited Göttingen, and the lab of Professor Michael Schultz. He showed me a fascinating fossil, a piece of a Homo erectus skull, found in a travertine tile factory in Turkey. Scientific articles can be dry, stuffy things, but the one in which Michael described the fossil includes this fantastic quote: “Given the nature of its discovery in a factory workshop, the hominin was unfortunately reduced to a standard rough-cut tile thickness of 35mm.”
Despite the rough treatment of the fossil, the bone was very well preserved, and on the inner surface of the skull, Michael showed me clusters of small pits – things that just shouldn’t be there in normal bone. They were quite clearly pathological, and Michael believed that the best explanation for them, given their appearance and their position inside the skull, was meningitis caused by M tuberculosis. Here was evidence for a human ancestor suffering from TB, half a million years ago.
Back to the present, and TB has scarcely been out of the news for the past few weeks. The Great Badger Cull has become one of the hottest political potatoes of the year. So what is the scientific evidence? Well, it seems pretty clear that badgers do help spread bovine TB. But that also seems to be where the certainty ends. Bovine TB in the UK has been going up and up – but how much of that is due to better diagnosis? And could culling badgers really help to reduce it? A study published in the journal Nature in 2006 showed that culling badgers reduced the rates of TB among cattle in the area where the cull took place – but increased it in neighbouring areas. In 2011, based on the results of previous trials, scientists advised the current government that culling 70% of badgers in large areas could result in a 16% reduction in bovine TB. For the government, that was enough.
But some scientists are now concerned that the cull – particularly if carried out by free shooting, which hasn’t been trialled, or if targets are missed – could make matters worse.
For this winter, the badgers are safe. Like Caesar presiding over a bizarre gladiatorial contest, environment secretary Owen Paterson granted the badgers a stay of execution, at the eleventh hour. There are just too many of them to make a 70% cull achievable this late in the year.
So the debate continues. It’s an argument about science, politics and economics. It centres on protecting food animals from harm, just as our ancestors have done since farming first got started. But, to me, it also raises interesting questions about how we see ourselves and other animals. It’s about how much we see ourselves as a “dominant” species, entitled to subjugate the needs of other animals beneath our own. It’s about how much room we demand as a human population (with a taste for milk and beef) and how much room we’re prepared to make for wildlife.
And let’s not forget, if it hadn’t been for us, cattle and badgers might not have had TB in the first place.