Wednesday, June 20, 2007

Bourne and the ISG - "An Instant Response"

The single most important thing to come out of the RBCT dispersal trials is that the transmission of bTb from badgers to cattle appears at last to be accepted. That John Bourne has very effectively illustrated how not to cull the badgers responsible, is the point from which the debate must move on.

Lines on a map indicating cattle testing areas may be a heroic gesture, but ten years ago, the RBCT was centred on bTb hotspots. Unless the ISG's much publicised 'edge' effect extended 100 miles, the incidence where no badger control was undertaken - that is the rest of the country except Krebs' Proactive and for a short time Reactive - is now at least as bad , if not worse than those original 'hotspots'.

To walk away, is not an option. Another reason why walking away is not an option, we explored in our posting of September 2004. Lurking in an EU drawer, somewhere in the bowels of the European Commission is this veterinary certificate which no doubt, Defra would like to forget. Nevertheless, it exists already. Poised for action.

We are grateful for his instant response to the 'findings' of the RBCT sent to us by Andrew Proud, BVSc, MRCVS, DVSM which we quote in full:

An Instant Response to the ISG
Everyone now knows what the ISG has decided although practically nobody has yet had time to read its final report, let alone ponder it. I shall be attempting that exercise shortly but by the time I have done so it will no longer be news so an instant comment is indicated
Cabinet ministers have said that they will be guided by the science but there is a very real danger that they will be blinded by the science unless certain obvious facts are considered.
A generation of Government Veterinary Scientists, now largely retired, assembled evidence over many years with even leaner resources than have been accorded to Prof. Bourne and his colleagues, put their evidence to the Krebs Committee. They argued that a reservoir of disease in badgers was the major, probably the only, reason why bovine tuberculosis was persisting in British cattle, that the measures taken up to 1986 had been highly effective and that the measures permitted since the Dunnett Report of 1986, although far from ineffective, were insufficient to control the disease.
Those measures were:
1. (From 1974 to 1981) Gassing of badger setts where there was good evidence that cattle were being infected by badgers.
2. (1981-1986) The “Clean Ring” policy, where, instead of gassing setts, all social groups of badgers using the infected farm were identified and as many as possible (in a prolonged operation) were trapped, shot, and examined post mortem. Subsequently, any social groups of badgers contiguous to groups in which tuberculous badgers were found were subjected to the same treatment until a clean ring of uninfected social groups had been removed around the infected farm.
3. (1986-1998) Trapping badgers only on land which had been grazed by confirmed reactors where there was good reason to believe that the reactors had been infected by cattle.
It was argued that the dramatic disappearance of disease from three large scale operations, most notably that around Thornbury between 1975 and 1980, was proof that removal of badgers was effective in preventing disease in cattle.
Critics of the policy argued that this evidence was inadmissible because no controls had been included in the experiment, conveniently ignoring numerous unofficial controls, as for example on the border of the M5 East of the Thornbury experimental area.
The Krebs Committee described the evidence as “compelling,” but recommended that a new experiment should be undertaken which included a control.

It has already been pointed out that the post Krebs experiment was fatally flawed; there is no comparison between the Thornbury 5 year gassing programme and the feeble attempts at trapping in the Krebs trial. Even in the years of trapping between 1981 and 1998 the trapping operations were prolonged, intensive, and, where necessary repeated. Eight days of trapping, followed by a year’s absence in the Krebs trial was hopelessly inadequate. The year’s interruption during the Foot and Mouth epidemic was most damaging.
If the Krebs trial tells us very little about the validity of the large scale proactive operations, it tells us even less about the huge number of lesser, reactive operations which took place between 1986 and 1998 and nothing at all about the clean ring operations of 1981-6. The Krebs trial included nothing remotely resembling the latter.

A moment’s thought will be sufficient to show that an actively pursued clean ring operation would greatly limit the perturbation effect which the Krebs trial demonstrated in its short-lived reactive area studies.

But there is another consideration which has been totally ignored: the Krebs trial was confined to the worst TB “hotspots” in the country. Perversely, the word “hotspot” is now being used with a meaning far removed from that which it carried in 1998. Then it referred to the areas of highest concentration of herds with confirmed reactors to the tuberculin test. Three factors would have contributed to an area being a “hotspot.”
1. Density of cattle population
2. Density of badger population
3. Proportion of badgers which were tuberculous.
Even if the Krebs trial tells us anything meaningful about the effect of removing badgers in areas where those factors combined to make that area a “hotspot,” it tells us nothing about the effect of badger removal in areas where disease has only recently entered the badger population, where the cattle herds are widely separated by land not occupied by cattle, or where the badger population is thin and/or minimally infected.
It was in such areas, where disease in cattle was newly being identified (now misleadingly being called “hotspots”) that the old (1986-1998) badger removal operations seemed to be most effective; in the 10 years since such operations ceased there has been the most dramatic increase in the spread of disease in new areas.

It is possible that the ISG is right and that, in the areas where the density of reactors to the tuberculin test is highest, badger removal will not be a cost effective answer to the problem (although it has yet to be proved that vigorous, intensive and prolonged badger removal operations would not be dramatically effective). That does not mean that it would not be cost-effective to prevent the establishment of disease in new areas by immediate action to eliminate infected social groups of badgers.

Whether or not it is cost effective to control the disease by removing the most heavily infected populations of badgers, the Government needs to face another question: is it cost effective to control bovine tuberculosis by constantly testing cattle and removing reactors while cattle cannot be protected from infection from a reservoir of disease in an ever increasing population of a protected species?
The cost is easy enough to calculate, but what is the benefit? There is no Public Health benefit: pasteurisation of milk and cooking of beef and venison (even without meat inspection) are sufficient measures to prevent human infection (1)

Is there an Animal Health cost? I strongly doubt it. Not only are there compelling reasons for believing that the disease only spreads with difficulty between cattle (2), but I am now (after over 30 years close involvement with the subject) being forced to the conclusion that most reactors, if left, would either recover, or contain the disease without showing any symptoms at all for the rest of their working lives.
Few, if any, animals being raised for slaughter would develop clinical disease; the only benefit in testing them is to disclose the presence of infected badgers.
Undoubtedly, among breeding animals, a small proportion of reactors would become clinical cases if left, but historical evidence strongly suggests that the mere culling of clinically diseased animals is a remarkably effective control measure(3).

Andrew J Proud BVSc MRCVS DVSM
June 2007

1. I am aware of one recent case of farming people being infected; had the Public Health Laboratory Service staff who wrote up the incident discussed the case with the SVS staff who dealt with the case they might well have concluded that the infected humans had been infected by badgers, not by cattle, but the Official Secrets Act precludes me from explaining why!

2. The need for brevity prohibits me including these reasons here but I am preparing another document that sets them out.

3. My scrutiny of historic government reports is not yet complete but I already have enough evidence to support this claim and intend to publish in due course.

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