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PCR, M. bovis and badgers

01 October 2010, at 12:00am

RICHARD GARD reports on latest developments in the fight against bovine TB

A VERY short time after taking office, the new Minister of Agriculture attended the Devon County Show and made a statement about badger culling.

If he was reported correctly, he was intending to base any action on science and the detection of TB in badgers using the “polymerase chain reaction” (PCR) tool. 

The regional press has recently trumpeted that the application of a magic black box is the way to eradicate TB in cattle. The idea being put forward is that a probe is inserted into the earth near a badger hole, or into a latrine, and the box indicates whether M. bovis is present. If the organism is identified, then you shoot the badgers.

Anyone who has worked with the application of PCR will quickly point out that there are very great scientific difficulties with what is being talked up.

A whole raft of information is available (e.g. and in summary the current situation is described by Dr Colin Fink of Micropathology Ltd as there having been “a quantum leap in sequencing mixed bacterial or virus samples and pulling out the quantitation of what pathogens and commensals are present in, say, soil samples or in throat samples. The new problem is that the technology is now so sophisticated that information overload is the new disease.

“There is very good field technology for PCR single round where the pathogen is single and likely to be there in large quantities (foot-and-mouth comes to mind as the ideal example). The problem is that there are many soil Mycobacteria and deciding what is M. bovis as an animal pathogen and what is simply there as other species is not yet easy to separate. “This is why there is so much ‘noise’, which is difficult to interpret. The new technology could be applied to this problem and I suspect would give interesting data. It is not suitable for use in the field.”

A commercial field-based battery- operated product is being talked up in farming circles, produced by Enigma Diagnostics at Porton Down. Further developments are awaited.

A key aspect for PCR is phrased as detecting clinically significant material. The basic result from running a test is a number. How that number is interpreted becomes an important issue. A low level of M. bovis could be residual on the ground for years. The badger that left that marker could well be dead or already evicted from the badger community and moved on.

The concern then is about “fresh” samples and significant quantities of the organism. Maybe this interpretive work has already been done but if not, linking scientific detection with badger culling could raise various challenges. The indications are that there are badgers that are not a risk to the cattle.

It has been said at TB meetings that a single cell of a Mycobacterium is sufficient to infect a bovine. Is this an active cell or a passive cell? Does a cell from the environment have the same infective ability as a cell from a clinical source? If such science is available, now may be a good time to offer clarification before veterinary practices are asked by clients to provide advice on environmental TB detection.

Unrealistic expectations?

Much rests on a means to control TB, both politically and to reduce the burden on cattle farmers. If PCR cannot be currently used accurately in the field, then an advisory note needs to be sent out to reduce unrealistic expectations. Maybe, if enough money and resources are applied, the problems of environmental noise may be overcome.

The technical developments for M. bovis identification are one obstacle, interpreting the results a second but an effective way of culling the badgers is the third obstacle and fortunately the easiest to solve.

In endemic TB areas, the virtues of local badger exterminators are being extolled: people who, for a fee, will shoot badgers. In all species, stress and other ill health are recognised as triggers for Mycobacteria infectivity.

Disruption of badger communities by filling in setts, road kill, rat poison, paracetamol, antifreeze and random killing are all included in the list of actions expected to cause badgers to either move into other territories and fight, or feel unwell. Both outcomes are expected to increase M. bovis excretion. Shooting a few badgers on one farm could well increase local badger stress and bring down the cattle herds at farms nearby. The idea of simply reducing the bacterial load by reducing the badger population only applies if the M. bovis excretors are removed from the land.

The Minister is probably now aware of all the obstacles to the effective control of TB. He is also aware that it is practical to identify the location of sick badgers, living outside healthy badger communities. Removing them from an area in one day during the winter is the way to improve TB in cattle grazing land that was occupied by badgers.

A practical, cheap, sustainable, targeted method is gas, used in a way that matches the realities of badger accommodations. Unhealthy badgers are not necessarily in traditional setts.

The sooner the use of carbon monoxide is approved to cull sick badgers, and applied in an effective way in local areas, the sooner a resolution to TB in cattle can be seriously anticipated. Vets may wish to encourage urgent approval for the use of the smoky tractor, in a controlled, effective way to best effect, as part of a wildlife management programme.

A practical, successful and rapid testing method to assess the concentration and effectiveness of carbon monoxide gassing of badgers is available. Approval need not be a lengthy or expensive process.

Achieving healthy badgers and healthy cattle in a local area can be a reality. Clients expect practices to be involved in the forthcoming reduction of herds undergoing 60-day testing with trading restrictions. It will be important that the detailed TB records of these herds are kept up to date.

Where short interval testing is brought back in-house by DEFRA this winter, it is hoped that the results of the tests will be provided to the veterinary practice that has been carrying out the testing and may do so again.

The data will become more important for disease reduction as time goes on in addition to the role with disease surveillance and compensation. Relying on information from the client may be effective for herds that are routinely visited but in many counties the majority will be beef herds and information may be harder to obtain.

If the disease reduction programme can be effectively activated, then the concerns about the DEFRA budget will be alleviated. During the next three years significant financial gains can be made for the farmers and for the TB budget.

The BCVA congress in Torquay from 14th to 16th October will provide an opportunity to define the practical ways forward.

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