Archive for the ‘Mycobacterium bovis’ Category

Although I have been in the land of the blogging dead for some time now, I’ve still been following and taking a great amount of interest in the British Governments response to the bovine tuberculosis epidemic in England. My previous posts and some background can be found on my old blog (Part I, Part II, Part III and Part IV), but a quick summary before I go into the meat of this post is probably a good idea (and nobody wants to follow around a billion links all over the place!).

Essentially, England has somewhat of a crisis on its hand as the incidence of bovine tuberculosis, caused by Mycobacterium bovis rises dramatically and the tuberculin skin testing method used to control the organism in most places fails. Why does a method that has worked to eliminate M. bovis from the cattle population worldwide successfully fail in England, New Zealand and a few other specific places? It turns out that M. bovis does a clever trick of infecting a secondary animal that provides it with an external reservoir making it much more difficult to eliminate. In New Zealand, bovine tuberculosis is found in possums, which often live near bush on the edges of farmland and subsequently die in a paddock where cattle may fossick around the infected corpse. In Britain, the principal animal that carries bovine tuberculosis is the badger, but unlike in New Zealand where we’re quite happy to shoot possums on sight as they are a pest, the badger is a beloved native animal and has a considerable conservationist movement behind it. This of course, does not sit well with farmers that are watching bovine tuberculosis rates go through the roof cutting into their already strained profits. Due to the emotions, public debate and several scientific studies that severely muddied up the water, the British government is finding a tough time to deal with the issue.

Jumping forward to more recent times, I noticed a relatively recent Nature editorial that discussed the issue and took an opportunity to have a go at David King, the UK governments chief scientific advisor:

Back in February 1998, the Independent Scientific Group on Cattle TB (ISG) was set up under the chairmanship of John Bourne, a prominent animal-health specialist, to advise the government department that was responsible for the issue at the time. After much deliberation and the submission of several peer-reviewed papers (such as C. A. Donnelly et al. Nature 439, 843–846; 2006), the ISG issued its final report on 18 June this year. Its conclusions were robust: “Badger culling cannot meaningfully contribute to the future control of cattle TB in Britain.”

King then proceeded to consider the ISG’s report along with, in his words, “other scientific evidence”, with the help of five specialists of his choosing. On 30 July he gave his report to the secretary of state with a startlingly different conclusion. “Removal of badgers,” it states, “should take place alongside the continued application of controls on cattle.” This report was made public on 22 October.

“Other scientific evidence” indeed. If I had to think of what this was, straight of the top of my head, it was more than likely the “four counties” trial in Ireland, which in stark contrast to the C.A. Donnelly et al. paper, found that complete badger culling was actually a highly effective way of reducing the incidence of bovine tuberculosis. Noticing that there was a response to this, I was a little disappointed to see that the Irish studies on badger culling were not the reason for the decision, but rather the interpretation of the existing studies was. David King responds:

 Although the scientific conclusions produced by my experts differ from the main conclusion of the ISG report, they nonetheless follow directly from the ISG’s data, which clearly show that carrying out badger removal over a large area and a sustained period of time, together with cattle removal and other controls, would deliver an overall reduction in TB incidence in cattle herds. This is the only effective course of action until efficacious vaccines become available.

Interesting enough, as the original Donnelly et al. papers did demonstrate that badger culling did have a positive effect in reducing bovine tuberculosis incidence. The contention really was between completely exterminating all badgers in the zone you wanted to control bovine tuberculosis in; or if you could get away with only culling a certain amount of the population: not culling as a general rule, which even the original studies demonstrated was effective. It is somewhat curious to me why Nature were so critical of David King and made the implication that he had ignored the data, because the data as I read it did support a badger cull to begin with: it just depended on how many badgers were culled. Of course, I’ll state immediately that it’s been some time since I read these studies in depth and had a really good feel around the subject area, so I may have missed something in the period between my original posts and now. This does give me a good excuse to look at the original data again and write that long since overdue fifth part of the series that looked specifically at this issue (boy, do I wish I had it done now).

David King also makes a pretty salient point:

On the basis of the scientific evidence, I do not believe that we can control TB in cattle — and badgers — without removing the sources of infection in both species. Other countries have been unable to control TB in cattle without addressing the wildlife reservoir (N. E. Tweddle and P. Livingstone Vet. Microbiol. 40, 23–39; 1994)

Quite true.


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