What is Bovine TB?

Mycobacterium bovis

Bovine TB is caused by the bacterium Mycobacterium bovis (M. bovis). It is difficult to diagnose, particularly in the early stages. Clinical signs (which are very rare) may include: emaciation, lethargy, weakness, anorexia, low-grade fever, pneumonia with a chronic moist cough, lymph node enlargement and visible or microscopic lesions in infected organs.

Any mammal can contract Bovine TB, but surprisingly little is known about how it is spread, what makes animals susceptible to it and how it develops in the host animal.
It is very unusual for humans to catch Bovine TB in the UK. In 2009, for example, it accounted for only 0.5% or about 45 of the 9,040 human cases of TB in the UK. Most cases of TB in humans are caused by a different bacterium, Mycobacterium tuberculosis.

Virtually all the few human cases of Bovine TB are either in older people who have reactivated old lesions acquired before widespread pasteurisation of milk, or in people who were infected overseas. Thus transmission to humans actually taking place in the UK is negligible (1).

The test used for TB in humans is similar in mechanism and function to the skin test used on cattle. Interestingly humans who react to it are classed as immune unless signs of disease can be detected, whereas cattle are classed as reactors and slaughtered, whether symptoms exist or not.

In the case of cattle, most authorities on the subject (including Defra(2)) believe that Bovine TB is most likely to enter previously uninfected areas as a result of cattle movement, and then primarily spread between cattle in respiratory secretions.

1. Professor Paul R Torgerson (co-author of ‘Public health and bovine tuberculosis: what’s all the fuss about?’), email 22/02/11 tells us that virtually all these cases are either in old people who probably have reactivated old lesions that were acquired before there was compulsory milk pasteurisation or immigrants who were infected overseas. Thus transmission to humans in the UK is virtually zero at the present time.
2. DEFRA: Department for Environment, Food and Rural Affairs

  • Misguided rules
  • What are our Options?
  • Let's Rethink bTB
Cattle grazing on a farm

According to Defra, the skin test misses 1 in 5 cattle that it should identify as reactors. (This is the ‘sensitivity’ of the test). For every four ‘reactors’ slaughtered in the belief that they are or will become infectious or infected, one more remains undetected and potentially infectious in the herd or worse still, moved to infect another herd or area. If one or more reactors have been found in the herd, a further test is done 60 days later and it may then detect the missed reactors, or maybe not.

In many countries this shortcoming is recognized and the skin test is used as a herd test. All animals in the herd are tested individually as in Britain, but if a single reactor is found, the entire herd is slaughtered and restocking is delayed.

After sixty painful and expensive years of testing and slaughtering cattle, and an intervening period of relatively few incidents of Bovine TB, we are still several decades from any chance of ‘official TB free’ status. As the Bovine TB Advisory Group concluded in its report to Defra; “Bovine TB has been a difficult and demanding problem for many years. There are reasons for believing that it can be controlled and finally eradicated but this will require a long-term commitment by all stakeholders and take at least 20 years.”

Can we afford such cost and will farmers tolerate another 20 years of movement restrictions, disruptive and inaccurate testing and compulsory cattle culling?

1. Continuing with the current policy, even with marginal changes, cannot be considered a serious option.
2. Drastically increased severity applied to the current policy, along the lines of measures adopted in Australia and the USA, might increase effectiveness:
* Changing from culling of individual animals to complete herd depopulation, dealing with wildlife reservoirs and delaying restocking of cattle.
* Increased risk based and out of area movement controls.
* More frequent testing of cattle, and use of alternative tests.
Clearly the political and financial cost of such measures would be prohibitive and attainment of ‘official TB free’ status would still take decades to achieve.
3. Acceptance that Bovine TB is not a significant human health risk in the UK and that farmers know best what will work in their circumstances.
* Farms would be free to choose to vaccinate cattle and/or various degrees of compulsory vaccination could be introduced.
* Milk would continue to be pasteurised.
* Inspection at abattoirs would continue.
* Farms would be free to continue routine testing and acquire herd TB free status or to choose vaccinated status, in response to market demand or farm preference.
* Any animal showing actual symptoms of Bovine TB would be tested and either slaughtered, or in appropriate cases, could be isolated and treated.
* Farmers would have the freedom to choose (within guidelines) the most suitable means for Bovine TB control in their circumstances. This is how most animal health problems are successfully managed.
The principle objection to vaccination is that, according to Defra (Options for vaccinating cattle against bovine tuberculosis, June 2007 16); “Not all vaccinated animals would be protected from TB and therefore vaccination alone will not be sufficient to demonstrate disease free status without testing and allow trade in those animals”. This is a disingenuous argument, as use of the skin test to demonstrate TB free status and select potentially infectious cattle is subject to the same shortcoming.

In our most recent discussion paper we consider current Bovine TB policy in England and Wales, and propose alternatives that we believe to be both practical and cost effective.

We have drawn our evidence from a variety of sources, but in particular from Defra (the Department for Environment Food and Rural Affairs) and from earlier work by Professor Paul Torgerson and Professor David Torgerson.

Where numerical or statistical arguments are necessary to show what is actually happening on farms, we have explained the argument as clearly as possible. We have also explained that some widely held views are supported only by statistical correlation (open to many alternative explanations), rather than any evidence of a physical link between effect and presumed cause.

Click the picture or click here to read the document in its original format

Wildlife reservoirs?

There is some evidence of a statistical link between the prevalence of Bovine TB infection in cattle and in wildlife but no direction or means of transmission has been established with any certainty. Possible explanations for the statistical correlation include cross infection between the species (either cattle to badger or badger to cattle) or a locally occurring common risk factor, causing both species to be independently susceptible. The matter is likely to remain shrouded in uncertainty and controversy. Even less is known about whether other domestic and wild mammal species are implicated in cattle infection.

Even in areas where it is rife, most wild and domestic animals fight off the disease. This is the natural and correct response of the immune system. A few animals may succumb to infection and develop symptoms (i.e. become ‘ill’).
The economic lifespan of cattle varies but, with the exception of breeding stock, tends to be relatively short. Following initial challenge by M. bovis bacteria, infection may be held in a dormant state by the immune system for many years or indefinitely. Hence the fact an animal is a ‘reactor’ does not mean it will go on to develop the symptoms of having the disease Bovine TB, be infectious, or become ill. It may simply have immunity.

DEFRA Needs Convincing

Defra

Defra’s stated reasons for the current policy, principally protection of human health, exports and animal welfare, do not stand examination.

The policy relies on a flawed diagnostic test that even Defra describes as ‘imperfect’** . It leaves potentially infected animals in the herd, and falsely condemns large numbers of healthy animals.

The policy is causing widespread losses and distress to farmers, and is a burden on the taxpayer. After 60 years of cattle testing and culling, further decades of compulsory slaughter separate us from an uncertain chance of TB free status. Defra admits the policy is failing and that the reasons are not known.

** See page 13 of the linked document.