All 3 entries tagged Cattle

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August 23, 2011

Who mooves where?

Writing about web page http://www.biomedcentral.com/1746-6148/7/31

As a consequence of The BSE outbreak in the late 1980s in the UK, the UK government set up a computerised system for recording the births, deaths, and movements of cattle. Spurred on by the realisation that the 2001 foot and mouth disease outbreak was initially spread around the country by people buying infected animals, these data have been made available to researchers. This means that we now have a comprehensive picture of the lives, locations, and deaths of cattle in the UK for over a decade – some 43 million animals, and 157 million animal movements!

This is a great resource for epidemiologists, as every trade in live animals is risky – if I sell you cattle from my farm, then if there are infectious diseases circulating on my farm, there’s a chance that you’ll buy infected cattle from me, and thus I have transmitted infection from my farm to yours. But, trading animals is essential to the farming industry in the UK, so there will always be a balance to be struck between economics and infectious disease control.

So what do all these data tells us? Well, for the full details, have a look at my recent paper in the journal BMC Veterinary Research, but since it’s open access, I can show you a couple of the figures here. In common with other European countries, British cattle tend to not move that often during their lives, and to not move great distances, although a few cattle really rack up the miles:

Of course, the other thing you can do with a decade’s worth of data is to look at trends across time. If we look at the number of movements per month, you can can clearly see the troughs caused by foot and mouth disease in 2001 and 2007:

(The peak in 2000 is an artifact of “Cattle Count 2000”, not a real surge in movements)

A seasonal trend is pretty clear, too – more animals are moved in the spring and autumn, co-inciding with the main calving times. Beyond that, there isn’t much change over time; if we look at the numbers of cattle each farm moves on and off in a year, there was some increase between 2002 and 2005, but things have rather levelled off since:

(the boxes are median and quartiles, whiskers are the central ninety-five percentiles, the solid lines are the means)

This, actually, shows the value of taking a long-term view: some scientists looked at the changes between 2002 and 2005 and concluded that there was a real increasing trend in movement numbers (and some associated disease-risk measures); we can now see that this was only a short-term issue. These data, as well as providing an insight into the structure of cattle farming in the UK, might well be the basis for a predictive model of livestock movements. That would be really useful in trying to understand the impact of proposed regulatory changes – rather than introducing things like pre-movement testing for tuberculosis and hoping they’re beneficial, we could attempt to model what changes would occur, and so design better regulations to keep the UK cattle industry safe.

You might be wondering where the infectious disease modelling is in this work – how has the UK’s vulnerability to an invading disease of cattle changed in the last decade? I’ve wondered that too, and have been doing some work on it, but that paper’s still with referees, so you’ll have to wait a little longer before I can write about it here…


March 25, 2009

Policy–Based Evidence Making in Wales

Follow-up to The Welsh Badger Cull from Counting Cows

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 Badger Cull

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.

Conclusions

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.


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