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Complex dynamics

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THE politics of bovine TB is obviously complex, as the past few weeks have demonstrated, but so too is the science – and not just in relation to badgers. Two recently published papers highlight the complexity of the disease in cattle and illustrate that there is still much to learn about how the disease develops and is transmitted within herds.

The first, discussing a natural transmission experiment carried out by researchers at the AHVLA – Weybridge, was published online by Veterinary Record in August and is summarised on p 448 of this print issue (Khatri and others 2012). It describes an experiment in which naturally infected cattle found positive by the skin test were housed with non-infected ‘sentinel’ cattle for a period of 12 months, after which they were slaughtered and examined postmortem. The animals were grouped in pens in a single building; all of the pens had solid walls, but all of the animals shared a common air space.

By the end of the 12 months, six of 60 sentinel animals had lesions typical of bovine TB, and Mycobacterium bovis was cultured from another two sentinels with no visible lesions. While confirming that animals with extensive pathology can be highly infectious, the study also demonstrated that even animals with limited pathology and without any lung lesions can be infectious, highlighting the fact that transmission between cattle is not confined to cattle at advanced stages of the disease. It also found evidence of an indirect route of transmission between cows in different pens, possibly either through the shared air space or by transfer of material through husbandry procedures.

The second paper was published in the October issue of the online journal PLOS Computational Biology. It describes how researchers at the University of Cambridge and the AHVLA applied mathematical modelling techniques to surveillance and testing data recorded in Great Britain from 2003 to 2005, to gain a better understanding of how bovine TB persists and is transmitted within herds (Conlan and others 2012). Among the findings was that, in the worst case scenario, up to 21 per cent of cattle herds could be harbouring bovine TB after they have been cleared from movement restrictions. The modelling suggested that the efficiency of controls was critically dependent on the time between cattle becoming infected and becoming infectious to other animals. It also suggested that larger herds may suffer a higher incidence of disease and a faster rate of spread among cattle.

The study focused on the transmission of bovine TB within herds rather than how it is introduced and, as such, cannot shed much light on the relative role of cattle movements and wildlife in spreading the disease. However, the authors estimate that there is high rate of reintroduction of infection into herds, particularly in high incidence areas. They found that eliminating the hidden burden of infection is unlikely to be sufficient to prevent recurrent breakdowns on its own, and suggest that the high rate of external infection, both through cattle movements and environmental sources, must be addressed if recurrence is to be reduced.

Taken together, the two studies serve to illustrate the complex dynamics of bovine TB transmission, and the need to tailor available control measures accordingly.

Political and media attention continues to focus on the planned pilot culls of badgers in England, as highlighted during a five-hour debate on the subject in the House of Commons last week (see p 433 of this issue). The result of the debate, in which MPs called on the Government to stop the cull, may not be binding, but it provides a further indication that the Government will continue to struggle in implementing a cull unless more can be done to get more of the public ‘on side’. People on both sides of the debate continue to pin hopes on vaccination but, as David Heath, the farming minister, made clear, this is not as simple as some have suggested. The effectiveness of vaccinating badgers has still to be demonstrated in the field and a vaccine that might be applied in cattle is, for legal and practical reasons, unlikely to be available for some time.

There is clearly a need to develop new approaches to the disease and to refine those already available. In the meantime, the need to deal with bovine TB is pressing and it remains necessary to apply all available approaches to help bring the disease under control.

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