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Key new tool for identifying problem badger setts harbouring bovine TB

New research led by biologists at the University of Warwick has revealed a new technique that could become a valuable way of identifying badger setts that harbour TB diseased and infectious badgers in regions where TB in cattle, caused by Mycobacterium bovis (M. bovis), is problematic.

The research paper published on Thursday 23rd March 2006, in the Royal Society’s scientific journal Biology Letters entitled "Is Mycobacterium bovis in the environment important for the persistence of bovine tuberculosis?", describes the research group’s use of molecular methods (a technique called PCR) to examine the prevalence of M. bovis in soil collected from badger setts and in badger faeces collected from latrines, on cattle farms located in six endemic regions of the UK.

The researchers carried out a PCR assay targeted at the MPB70 antigen gene specific to the mycobacteria closely related to, and including, M. bovis (a group known as the Mycobacterium tuberculosis complex - MTBC). A second PCR then targeted a particular element within the MTBC group which allows the researchers to confirm that they were indeed specifically dealing with M. bovis.

The researchers detected MTBC in soil of badger setts on 78% of 60 farms sampled, and in 56% of badger latrines on 9 farms sampled. On contaminated farms an average 43% of the setts and 29% of latrines were positive. In related research, they have also shown the organism to persist in the environment for months. These results suggest that once the organism is excreted into the environment by cattle, badgers, or other wildlife, it could act as a source for further transmission.

The researchers also carried out a more detailed study of high-density badger populations in Oxfordshire and Gloucestershire, the former not located in an endemic TB region, the latter known to be endemic for TB. Here they screened setts and latrines within known badger social group territories using the PCR assays. In the Gloucestershire population, they found 100% of the examined badger setts and latrines to be contaminated with M.bovis, whereas none of the samples in the Oxfordshire population were positive.

In the Gloucestershire study they took a further step and correlated the results of environmental detection of M. bovis within the social group territories, and the infectious status of the badgers living within those social groups. Information was already available from that site on the number and frequency of badgers that were excreting M. bovis into the environment from results of mark-recapture studies (when badgers were captured, anaesthetized, sampled, and released up to four times per year). Crucially, the researchers were able to show a strong correlation between the number and frequency of excreting badgers within each social group and the proportion of environmental samples per territory that tested PCR positive.

This is of course what one might expect to find if one knew which badgers are diseased and infectious, and which setts they used - but this is precisely the problem faced by managers of bovine TB and the farmers - without technology such as this its is very difficult to differentiate "clean" setts containing uninfected badgers from "problem setts" containing infected badgers.

This study potentially opens up a much wider range of choices for all stakeholders in the badger/bovine TB debate. The lack of precise knowledge of the infectious status of badger populations on or near to cattle TB infected farms has led many to advocate blanket culling of badger populations. This action in itself has recently been shown by the Randomised Badger Culling Trial (the so called "Krebs Trial") to lead to the social disturbance amongst surviving badgers with consequential increases, rather than decreases, in TB infection rates in cattle. However, if one really was set on culling as an option, the technology employed by the Warwick researchers has the potential to allow a more targeted cull strategy of "problem setts".

Demonstration that M. bovis can survive in the environment for months also raises important questions about how infected setts and latrines should be handled even after the populations using them have been cleared.

This is clearly an important new tool and the research team is now seeking further funding to refine their analysis and explore how it could be effectively implemented to identify all contamination points (and animal sources) on the farm to improve biosecurity.

One of the lead researchers on the project Dr Orin Courtenay from the University of Warwick’s department of Biological Sciences said:

"We do not advocate culling badgers to control bovine TB, particularly in light of the scientific results emerging from the Randomised Badger Culling Trial. However if the government takes the decision to continue to cull badgers, then we would prefer that culling is targeted at diseased and infectious animals- indeed cattle, badgers or other wildlife hosts-, rather than see a policy of untargeted culling which by nature includes removal of healthy and uninfected animals. With some further scientific evaluation, a "sett test" based on state-of-the-art molecular technology could provide a tool towards achieving this aim."

Peter Dunn | EurekAlert!
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