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TB SPECIAL: 7 bovine TB myths dispelled by experts

With bovine TB often at the heart of misconceptions and scaremongering, Olivia Midgley asks vet Den Leonard and Defra farmer liaison on bTB, Ian McGrath, to dispel some of the myths.


Olivia   Midgley

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“I have not had TB so my badgers are clean and I do not want to get rid of them”

“I have not had TB so my badgers are clean and I do not want to get rid of them”

In areas of the UK where badgers are not infected there is no point in culling them and they should indeed be left alone.

 

The theory about leaving ‘clean badgers’ in relatively infected areas arises primarily from research in the 1980s which suggested badger social groups, even in a relatively small area, tend to be either clean or dirty and largely stay as such.

 

This was true particularly where there was some degree of stability of the social groups.

 

However, this work was done when population densities of badgers were much smaller than we have today.

 

What we have seen over the years is a steady increase in badger populations, based on the observations of landowners as well as assessments of populations in scientific work on culling and vaccination since the 1992 Protection of Badgers Act.

 

Badgers tend to spread infection during territorial fights via bite wounds, and the pressure of a higher population density makes these territorial battles more frequent.

 

In the early 1990s only one in 400 roadkill badgers in Cheshire were found to be infected with TB.

 

This would suggest there was not enough infection in the badgers then to be a contagious problem between setts.

 

However with the TB prevalence in Cheshire badgers having risen to one in four badgers, alongside a swell in the population, the infection pressure is now sufficient to cause the epidemic spread of TB in badgers.

 

This picture does vary across different parts of the UK, so an increase in surveillance of wildlife would be beneficial to make sure any control measures used are proportionate and targeted.

 

The knowledge sporadic small scale, poorly delivered culling will disrupt social groups and potentially increase TB through perturbation is why culling is only done under license and with strict conditions to reduce these risks.

 

Where the badger population is high and infected, it would appear leaving apparently clean setts would not solve the problem with the available control methods.

“Badgers should be taken off protected list and farmers allowed to control them on their land”

“Badgers should be taken off protected list and farmers allowed to control them on their land”

There is no prospect whatsoever Parliament would change the law and allow badgers to be included on a general licence applying to other disease carriers and predators which can be culled legally in the UK.

 

Even should such a nationally unpopular decision be granted then we know low level uncoordinated ineffective badger control makes TB much worse.

 

If you want to reduce TB then badger population management has to be done quickly and efficiently, working over a large area with hard boundaries to limit perturbation and ingress and including as many of the badgers in the zone as possible.

 

This is why license conditions are as they are.

 

There has not been a change in the law to allow badger culling, there has always been provision in the Protection of Badgers Act 1992 under license.

 

The change is there has been an update of the scientific understanding of the effectiveness of badger culling on TB in cattle based on the long-term results from the Randomised Badger Culling Trial.

 

This is what forms the condition of the licenses.

 

The initial culls in Gloucester and Somerset were to test a methodology to deliver these requirements, they were not an experiment to assess whether culling reduces TB in cattle; this was already agreed by top TB scientists.

 

Rather than hope farmers will one day be allowed to legally kill cull badgers in the same way they can foxes or rabbits, farmers should get on with the well proven licensed culls to reduce TB where it persists in their local wildlife.

“What we need is better TB tests – the current ones do not work”

“What we need is better TB tests – the current ones do not work”

Farmers get tremendously frustrated when their TB positive animal is taken from them only for a letter to arrive saying no TB was found in the animal.

 

This is one of the poorest understood myths in our industry.

 

The skin test is one of the best tests at accurately telling us an animal is definitely infected with TB.

 

In fact, if you tested 5,000 uninfected cattle, only one would (inaccurately) skin test positive.

 

This seems crazy when only 40 per cent of skin test reactors show TB in the abattoir.

 

This happens as TB takes a long time after infection to cause visible lesions in the animal, so the test is removing cows before they get chance to grow lesions.

 

If we left them alive they would go on to develop lesions or become infectious.

 

Where the skin test is less good is as an individual one-off test.

 

This is because its sensitivity is relatively low and, as such, it will miss about 20 per cent of infected animals.

 

Using it across herds and areas, it finds where infection resides.

 

Once we find TB we restrict movements and test repeatedly and widely until we are sure we have found all these hidden carriers to overcome the sensitivity limitations of doing one test once.

 

Success

 

We also increase the skin test sensitivity by doing ‘severe interpretation’ and by adding the gamma interferon blood test where appropriate to catch more of these truly infected animals.

 

The success of our TB tests can be seen by analysing how effectively new breakdowns in our low risk area (LRA) are cleared using these tests, so those areas do not see long-term repeat breakdown problems.

 

If the tests were poor we would see recurrent breakdown of TB in the LRA as there are plenty of new breakdowns from cattle movements into the LRA.

 

However there are virtually none.

 

This is detailed in the recurrence maps and statistics in the Government’s surveillance reports.

 

This is in stark contrast to the high risk area where, because of the constant reinfection of cattle by wildlife, the tests have no chance of clearing up infected areas at all and recurrent and persistent breakdowns are increasingly common.

“What we need is a vaccine”

“What we need is a vaccine”

Millions of pounds have been spent over nearly a 100 years trying to find a good vaccine for TB in cattle and badgers.

 

Although a badger vaccine has been granted a temporary licence in the UK, there is no actual evidence it will be beneficial enough to be of use in infected areas.

 

The study to assess the effect of the vaccine to obtain a licence showed some reduction in the severity of TB in vaccinated badgers, but the badgers do still become infected when challenged.

 

In this study, the TB prevalence in the assessed wild badger population started off at 53 per cent.

 

At the end of the four-year study the prevalence had only dropped to 33 per cent in the vaccinated badgers, but it also fell to 37 per cent in the unvaccinated.

 

This information was not highlighted by the authors of the published work, but is in the raw data of the study.

 

In terms of cows, several large-scale experiments over decades have consistently shown vaccination will not deliver sufficient protection to form a meaningful contribution to control.

 

This is particularly true when, in the absence of wildlife reservoirs, the current testing and control measures in cattle rid them of TB successfully around the world.

 

Economic

 

This was also true in our country before the Protection of Badgers Act 1992.

 

It is highly questionable whether pursuing a cattle vaccine makes economic sense when throughout the non-wildlife-infected areas of the UK we can control cattle just as well as the rest of the developed world.

 

Vaccination is now being used in the Republic of Ireland, but this tends to follow a reduction of the badger population by culling.

 

They have calculated what the population density of badgers needs to be to stop TB being epidemic in badgers and once the population is reduced to this level the vaccine has a much greater chance of being effective and is much cheaper to deliver.

“Badgers will be made extinct by culling”

“Badgers will be made extinct by culling”

The evidence, from the Randomised Badger Culling Trial (RBCT) and from earlier cull zones in this country as discussed in the Krebs report, is the impact of TB reduction in cattle is greatest when a high number of badgers are removed.

 

Indeed, when badgers were removed entirely from the Thornbury area by gassing all setts every three months for six years starting in the late 1970s, TB in cattle dropped from 5.6 per cent over the 10 years before culling, to zero for the 10 years following the cull.

 

Badger numbers started to recover in Thornbury within two years after culling ceased, but TB did not reach its pre-cull levels there for a further 30 years.

 

Strategy

 

In the 1997 Krebs report, it was noted the badger population in the South West rose during the period culling had taken place as part of the Government’s strategy.

 

Despite all this, when the RBCT was being designed the Government did not allow the complete removal of badgers from cull areas, fearing local extinction.

 

The RBCT data shows removing 70 per cent of the badger population will lead to a significant and long-lasting reduction of cattle TB and will not endanger the badger population.

 

To this end, Natural England set target numbers which reflect a 70 per cent reduction as part of the new licences which are granted.

 

The target is presented as a range, with a minimum and maximum, as there is a degree of uncertainty about the true population size.

 

The issue for the cull companies is commonly a difficulty in achieving the minimum number of this target.

 

This speaks of the difficulty of using the current methodology of removing significant badger numbers and certainly tells us badgers will not be eliminated either locally or nationally.

 

Natural England also monitors the number and location of removed badgers as the culls progress and adjusts its targets to reflect what the companies see on the ground, both to ensure the population is not removed entirely, but also to make sure sufficient removal over the whole area is achieved.

 

It is ironic the people who complained the initial pilot cull zones in Gloucester and Somerset were not effective enough as they failed to hit the minimum target numbers are the same people who try to convince the public the badger is going to be made extinct by the policy.

“The Low Risk Area is only tested once every four years”

“The Low Risk Area is only tested once every four years”

All animals slaughtered in the UK are checked for TB for visible lesions in a carcase inspection.

 

Not all cattle with TB have lesions, but this quite low sensitivity screening will find herds where TB has become developed.

 

As such, it is contributes to the overall countrywide surveillance, whether the cattle are from a low risk area (LRA), high risk area (HRA) or edge area.

 

Also, testing in a geographical area will be staggered so, while cattle are checked each year, even in a small area, not everyone will be having their four yearly test in the same year.

 

As such, the testing is far more frequent than annual in a smaller area.

 

Couple this with dealer herds, heifer rearers and raw milk sellers who have to test annually, and the average interval between test in the LRA is about three years.

 

When disease is found in the LRA, most breakdowns, unless the reactors are recently bought in and have never been grazed, trigger a 1.8-mile (3km) radius of testing, again contributing to a greater overall surveillance than every four years.

 

Importantly, it is targeted surveillance around known disease.

 

Reassuringly, in about 100 radial tests since 2013, lateral local spread of the same TB strain has only been found once.

 

And finally, to test the LRA annually would be £14 million of a bTB budget of £100 million, so this would be a huge proportion of the total.

 

The testing regime in the LRA is working as vets are not finding lesioned cattle from the LRA at slaughterhouse surveillance.

 

Apart from one case, the radials are not showing lateral local spread and the level of TB in the LRA is static if not decreasing.

"The 60-day wait before post-movement tests is a waste of time"

"The 60-day wait before post-movement tests is a waste of time"

The reason the post-movement test is carried out 60 days after an animal has left the edge or high risk area (HRA) is purely down to an understood incubation period from contact with TB to earliest chance the animal will react to the TB test.

 

So, if an animal was moved on March 1 and had its pre-movement test carried out on January 14, simply allowing 60 days from the pre-movement test would suggest the post-movement test could be carried out after March 17.

 

But this would be only 17 days after the animal was last potentially in contact with TB bacteria, whether in wildlife, the environment or other cattle in the HRA or edge. This would not be long enough.

 

So we have to allow 60 days from when the animal was last potentially in touch with TB bacteria.

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