Wednesday, October 10, 2007

Gamma IFN - "Extreme Caution" is needed.

In the final piece of John Daykin and Dr. Lewis Thomas's Opinion in Veterinary Times they offer views on the use of Gamma IFN, a blood test which allegedly finds bTb 'earlier' and more accurately that the intradermal skin test. We say ' allegedly' because in practise, this most blunt of blunt instruments, succeeds only in piling up dead cattle higher and quicker, while doing absolutely nothing about the source of their infection. Mr. Daykin and Dr. Thomas have this to say on its use:
Despite the ISG’s protestation that gamma IFN testing will significantly improve the detection rate of reactors, we would urge extreme caution in this respect. Potentially a huge number of non-lesioned (NVL) cattle will be killed as reactors with no certainty that they would ever have presented an infectious threat to other cattle. Irish work informs that there is a seven to nine-fold increase in the likelihood of the skin test detecting gamma IFN positive animals at the next tuberculin test. However, the ISG, when asked whether they had any data to show whether or not gamma IFN positive animals would become skin test positive at a subsequent test, suggested that a large number would not. Once again, no proof for this opinion was offered, and the Irish work was not referred to. Before the gamma IFN test is rolled out into the field, it is surely imperative to know what magnitude of skin test negative/gamma IFN positive animals are likely to test positive at a subsequent skin test. If, as we believe, the problem lies in the wildlife reservoir, the removal of large numbers of NVL cattle that will inevitably result from the widespread use of the gamma IFN test will have absolutely no impact on the epidemic. But what it will do is swell the ever-increasing compensation bill enormously. Has the ISG conducted a cost-benefit analysis for their proposed package of increased cattle control measures?
We doubt it. But more worryingly, various farmer organisations are enthusiastic - as long as Defra pay. However, that was before the zoning of much of SE England, combined with regular blood testing for Bluetongue virus, brought the livestock industry to its knees - as we described in our posting below.

Many trials have been done with gamma IFN. Some are referred to in current research projects, but running through most is an assumption of 'early detection'. Earlier than what? Skin tests every 60 days? And?

A herd under Tb restriction has to pass such a skin test, the internationally recognised diagnostic tool before trading restrictions are lifted anyway - despite what gamma IFN may or may not show. Work using experimentally infected calves, found that gamma showed positive results but also that the skin test found all the candidate animals as well. But more worrying is continued reference to Irish work which kept postive gamma IFN animals for several months before slaughter. When they were found to be infected, the assumption was made that the blood test had picked up evidence of infection at a 'very early stage'. What was not made clear was that these cattle were on 'an endemically infected farm', and during that wait-and-see period, were not removed from any possible further exposure from any other source. This is not 'science' it is extremely sloppy assumptions. John Daykin and Dr. Thomas conclude in rather less descriptive terms:
Professor Bourne and his colleagues proffered a worryingly large diet of opinion and assumption in answering vital questions on epidemiology and testing which belied a failure to understand the basic field epidemiology of bTB. They also had to admit that their opinions on the poor sensitivity of the skin test, which they put at 66%, could not be backed up with any data, and appear to be seduced by a false optimism for the prospects of the gamma IFN test. We strongly caution against the widespread use of this test until far more data on its sensitivity and specificity are available. Further, we suggest that the inherent expense of this test, the logistical difficulties involved in its application and the potentially massive costs of compensation for gamma IFN positive animals should be considered before we accept the ISG’s advice on its widespread use.

This excellent piece concludes with the gloomy prediction that "the reduction of testing intervals, the refining of skin test protocols, pre-movement testing and the use of the Gamma Interferon test will do little or nothing to stem the increasing tide of bTB".

With that conclusion, we would agree. Cattle measures alone do not work. Others have tried, and comprehensively failed. And VLA's spoligotype maps do not support a spread of bTb across the country from cattle movements. But the addiction of 'boys' for 'new toys', and the absolute mind set against culling infectious wildlife, seems to indicate that even with highly questionable gamma IFN blood assays, which Parliamentary Questions confirmed were giving many 'false postives', even more cattle will die totally unecessarily.

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