All 2 entries tagged Tuberculosis
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March 25, 2009
I’ve had a few comments and emails since my previous entry on the Welsh Assembly Government’s proposal to cull badgers. Yesterday, Elin Jones, the Rural Affairs Minister made another announcement about her proposal, so it seems a good time to look again at what is going to happen.
There is still precious little detail in the proposal, and my request for more details on exactly what was going to be done was ignored by the Office of the Chief Veterinary Officer, who did little more than send me the URL of their website by way of reply.
There are two big problems with the planned cull that I didn’t address before. The first is that trapping and shooting is not an effective way to cull out a badger sett. We know from previous research that the harmful effects of badger culling – increased spread of TB in the surrounding area – are at least partly due to partial culls, and would probably be reduced by complete culls. Sadly, there is a shortage of humane ways to completely cull setts.
Secondly, and more significantly, they are rigging the result of their pilot study in North Pembrokeshire. As well as culling badgers (over a period of 4 years), they are going to implement “additional cattle measures” (presumably sensible things like increasing farm biosecurity, buying animals from non-endemic areas, and so on) to try and reduce the spread of TB between cattle. What this means is that the results from the pilot area will be of no value at all in assessing whether or not badger culling has been effective. If TB levels decline in the pilot area, there will be no way of telling whether that was due to the badger culling, or the cattle-based control measures.
You can be sure, though, that if TB levels do decline, it will be trumpeted by the Welsh Assembly Government as evidence that badger culling works – a classic example of policy-based evidence making!
April 15, 2008
The Welsh Assembly Government’s (WAG) Rural Affairs Minister, Elin Jones, has recently announced a £27M program to attempt to eradicate bovine tuberculosis (BTB) from Wales. The most controversial aspect of the proposals is the plan to cull badgers. I’ve had no useful response to my request for details, but there are some sketchy outlines online.
What is not controversial is that the level of BTB is rising, that this has a significant economic impact, and that badgers and cattle can transmit BTB to one another. The role of badgers in the epidemiology of BTB is, however, open to dispute. The Independent Scientific Group on Cattle TB (ISG) concluded after 10 years of research that “badger culling can make no meaningful contribution to cattle TB control in Britain”. Sir David King (then the Chief Scientific Advisor) and colleagues re-evaluated the ISG’s work, and concluded that “the removal of badgers could make a significant contribution to the control of cattle TB in those areas of England where there is a high and persistent incidence of TB in cattle”, although their terms of reference required them to ignore practical and economic considerations(!). The ISG issued a strongly-worded rebuttal to Sir David’s statement. In the light of this, there are several aspects of the Welsh plans that give me cause for concern.
The Human Health Risks
The WAG FAQ on BTB states “The main reason for Government intervention in bovine TB is in order to protect public health as well as animal health. Bovine TB is a zoonotic disease which means that it can infect and cause TB in humans”. This is rather misleading. BTB is caused by Mycobacterium bovis, whilst most TB in humans (at least in the UK) is caused by Mycobacterium tuberculosis. There were around 14 cases of TB in the UK per 100,000 population in 2007, nearly half of which occurred in London. Around 2/3 of TB cases are amongst those not born in the UK, and UK children are no longer routinely given the BCG vaccine against TB. Furthermore, the main route of transmission of BTB from cattle to humans is via unpasteurised milk, and in the UK milk is almost universally pasteurised. There are good reasons to try and control BTB, but the risk to human health is not one of them in the UK
The role of wildlife, and the importance of livestock movements
The WAG FAQ has this to say on badgers: “The [Badger Found Dead Survey] concluded that the prevalence of M. bovis infection in badgers was highest in areas of high cattle prevalence and lowest in areas of low cattle prevalence. The results of this survey are consistent with the hypothesis that the badger is an important component in the epidemiology of bovine tuberculosis in areas of high cattle incidence”. This again is seriously misleading. Cattle and badgers can transmit BTB to one another, so it’s only to be expected that areas with high incidence in one species will have high incidence in the other species. Whilst these data are consistent with the WAG’s hypothesis, they are also consistent with the hypothesis that cattle are the main drivers for BTB infection in badgers! The fact that high BTB levels in cattle are associated with high BTB levels in badgers does not establish causality in either direction.
Slightly bizarrely, the WAG FAQ cites the ISG’s work as supporting their view that there is evidence of TB transmission from badgers to cattle, yet curiously fails to mention the conclusion in the ISG’s 2006 Nature paper that badger culling has negative as well as positive effects on BTB in cattle. Whilst intensive badger culling can reduce BTB incidence in an area, the resulting disruption to badger social groups leads to increased movement of badgers, which results in greater spread of BTB in areas surrounding the cull region.
In addressing a question about TB coming from the movement of infected animals from SW England, the WAG FAQ says “There is no evidence that the increase in cattle numbers is due to movement of cattle from the South West of England.” Assuming that is meant to be a rebuttal of the suggestion, the authors would be advised to consult Gilbert et al’s 2005 Nature paper “Cattle movements and bovine tuberculosis in Great Britain”, which elegantly demonstrates that the best way to predict where BTB is going to spread is to look at where cattle from TB-endemic areas are being moved to, and highlights areas of Wales (especially in the SW of Wales) as being at high risk of importing BTB.
What exactly is proposed?
This is unclear. Whilst the Minister’s statement is clear that some badgers will be killed, the accompanying FAQ says that how many badgers will be culled, where they will be culled, when they will be culled, and what effect this will have on the badger population are all “considerations that will need to be made as part [of] the decision on a badger cull”. This seems to me to be begging the question. If none of the details have been considered yet, how can anyone be sure that killing badgers is the right way to control BTB in Wales? It seems that the policy cart is being set before the scientific horse.
Particularly, the effect on the badger population will be key to the impact on BTB levels of a badger cull – all the evidence is that it’s the increased movement of badgers post-culling that can be responsible for increasing BTB incidence around cull areas. If this cull is not to make matters worse in Wales, this should be being considered before the decision to cull is made.
The documentation available from the Welsh Assembly Government so far paints a worrying picture. Rather than carefully weighing up the evidence on the advisability of culling badgers as part of a TB control strategy, the science has been misinterpreted or misrepresented to bolster a “we will cull badgers” position, before the matter has been considered in any great detail. It might be possible to control BTB in Wales by using a strategy that includes the culling of badgers, but the devil really is in the detail: it’s wrong to just decide willy-nilly to cull badgers as appears to have happened here. To conclude that a badger cull is part of an effective BTB control plan, you need to have a clear plan of action in place – only once you have a proposal for how many badgers you plan to remove from where over what timescale can anyone assess whether it’s likely to increase or decrease BTB incidence in cattle, never mind whether it will do so in a cost-effective manner.
If anyone is to have confidence that badgers are being culled as part of a sound BTB control program, rather than for political reasons, it needs to be much clearer that science is driving policy decisions, not the other way round.