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Bovine TB: recent evidence reviewed

A REVIEW of scientific literature and other reports relating to bovine tuberculosis (TB) in cattle and wildlife was published by DEFRA on November 15. The review, which was carried out by two researchers, Dr Tony Wilsmore and Mr Nick Taylor, of the Veterinary Epidemiology and Economics Research Unit at Reading university, concentrates particularly on material produced since 1997 that presents new knowledge about interactions between cattle and wildlife, especially badgers, in disease transmission. Evidence for and against the culling of badgers as a measure to control TB in cattle, as well as equivocal evidence, is reviewed, along with material covering aspects of badger ecology and behaviour that was considered relevant. The aim of the review was to assist DEFRA to address the growing problem of bovine TB in Great Britain and to allow the Government to reexamine its control policies. With regard to the role of badgers in the spread of bovine TB, the reviewers note that, in areas where bovine TB breakdowns are common in the cattle population and the disease is endemic in local badgers, molecular studies have shown that both cattle and badgers are infected with identical strains of Mycobacterium bovis. This, they suggest, indicates a connection between cattle and badgers in the transmission of the disease. They add that there is good evidence for indirect contact between badgers and cattle, and contamination of fields, cattle housing, feed stores and troughs by excreta and discharges from infected badgers. However, they point out that field studies have not been able to fully quantify the role of badgers in breakdowns of TB in cattle herds. Evidence for transmission from herd to herd that does not involve badgers is also considered, and the reviewers draw attention to data from Northern Ireland that suggest that approximately 20 per cent of breakdowns were attributable to the movement of purchased cattle on to farms, and approximately 30 per cent to spread from infected to neighbouring farms across farm boundaries. They say that there is a continued need for regular herd testing with removal of reactors and for restriction of cattle movements from areas where bovine TB is well established to areas where both cattle and badger populations are free of the disease. Turning to the ongoing randomised badger culling trial, the reviewers record concerns that the trial might not reveal statistically significant differences between proactive cull areas and nocull areas. They add: ‘That in itself would be a conclusive result, provided the trial has collected sufficient data to demonstrate a significant difference if one had existed of sufficient size to be important.’ The recent Four Areas Trial in the Republic of Ireland is also considered, and the reviewers point out that although a decrease in herd breakdowns was seen in areas where badgers were proactively culled, the Irish government’s policy is to replace culling by vaccination of badgers as soon as an efficient vaccine is developed. They also note criticisms of the trial and that the culling methodology used in the Republic of Ireland could not be employed in Great Britain. The reviewers conclude that, because a culling policy that can be implemented in Great Britain has not yet been demonstrated to be effective, it is important that research into other strategies, such as the separation of cattle from badgers through biosecurity measures, and the vaccination of the badger population, should be continued.