One dream, one soul, one prize, one goal: controlling and eradicating bovine TB

I was fortunate enough to recently be able to attend an evening meeting at Aberystwyth University (pictured) where those present were addressed by Sir Brian May. His title was “Strategies in the fight against bovine TB”.

Dr May preceded his presentation by admitting to being rather nervous about presenting on such a subject in front of an audience of farmers and vets, and admitted to being a committed wildlife campaigner, a “badger hugger” and opposed to “the cull” (of badgers; he did not speak directly about his thoughts on culling cattle).

He also said that his intention was not to start a fight and that he is not anti-farmer, that he thought that the problem of bovine TB (bTB) is soluble, and that his aim was for a collaborative approach towards managing the disease.

He then spoke eloquently and persuasively about why badgers and badger control should not be a fundamental consideration in the control of bTB.

Whether your views coincide with Dr May’s, he is an intelligent and highly educated person with a proven ability to analyse data, and draw conclusions from it. His views, therefore, deserve consideration, regardless of whether the end result is agreement. In fact, it is unlikely that the end result will be unanimous agreement because all involved with this challenging, insidious disease will have a different perspective from which they view it and the “facts” about it.

This is because this is not so much a disease of quantifiable risks that can be targeted with entirely predictable results, but one where the epidemiology depends on relative risks, which will vary between each other depending on individual farm circumstances, both at any specific point in time and over time dependent on any management changes introduced.

Infected badgers (and no debate exists that badgers can be infected with Mycobacterium bovis) surrounding an infection-free, truly closed herd may be the major threat for the introduction of infection into that herd.

The risk from badgers may, however, be relatively lower for a neighbouring “flying” herd where all herd replacements are purchased with little regard for the bTB history of their source herd (consulting ibTB can provide valuable information to guide purchasing and reduce risk, although this may not be easy or convenient when stood in the mart bidding on an animal) or in herds where infection has already been confirmed.

The relative risk from badgers to our infection-free, closed herd will also be reduced if badger-proof fencing is erected to surround it or if badgers are culled in the locality.

Dr May’s view that the major route of M bovis transmission is in slurry is interesting and, again, worthy of consideration. In years past, when farmyard manure was composted, the heat produced, claims Dr May, would kill any viable M bovis bacteria before the manure was spread on the land as a valuable organic fertiliser.

Some authors report that viable M bovis can be detected in slurry and run-off, and for a considerable period of time (up to six months is often quoted). This might provide an alternative explanation for the spread of this organism within herds, to neighbouring herds and to badgers in the locality.

If we accept this as a potential pathway for the spread of M bovis, slurry management becomes potentially important in the control of the spread of infection.

M bovis, however, will not just appear in slurry by magic. It can only be there having been shed in the faeces of an undisclosed (the infection must, by definition, be undisclosed because, had it been identified, the infected cow would have been culled according to statute) infected animal within an endemically infected herd.

Perhaps, therefore, as well as managing other risks including purchasing behaviour, slurry management and badgers, we ought to be looking harder for infected animals in confirmed breakdown herds and in those that have recently regained Officially bTB-Free (OTF) status.

Current herd and individual animal bTB surveillance depends, because of statute, almost exclusively on the single intradermal comparative cervical tuberculin (SICCT) or “skin” test.

While the specificity of this test is extremely high (99.98%), meaning that false positive results are extremely rare, sensitivity is an issue. Even the best estimates are no higher than 80% and many authors claim 50% or even less.

This, perhaps, is of little consequence in infection-free herds where predictive value dictates an absence of false negative results. It will, however, be of significance in endemically infected herds and likely also in those that have recently returned to an OTF status (but are they actually free of M bovis infection?) where, using the best estimates of sensitivity, one-in-five infected animals will not have been identified (and it may be much higher than this).

This means that, using a detection strategy based on the SICCT test, bTB breakdowns will be destined to persist within infected herds for longer than necessary, resulting in more cattle becoming infected, and that herds will be returned to an OTF status before infection has been eliminated, allowing animals to be traded and infection introduced into previously uninfected herds.

‘Enhanced’ testing

Dr May’s views of the SICCT test, mirrored by many in the industry, are not complementary. He proposes the use of “enhanced” testing strategies involving other validated tests to identify high-risk animals that are missed by the skin test. These enhanced testing strategies should include the available serological tests – perhaps in conjunction with skin testing to maximise the sensitivity of the serological tests.

The high-risk animals identified would then be able to be managed out of the herd in an economically sustainable way, much in the way that Johne’s disease – another mycobacterial disease whose epidemiology shares many similarities with that of bTB – is currently managed, as opposed to the current statutory “test-and-cull” policy.

Whether you agree with all of Dr May’s views, it is difficult not to agree with his sentiment that the fight against bTB should not involve farmers and the wildlife lobby being at loggerheads with, and shouting at, each other. Views, of course, will inevitably differ, including among the farming industry, because of different perspectives of relative risk, but when push comes to shove, we all want the same thing – the successful control and eradication of bTB.

Perhaps a more unified approach – and pushing to allow farmers and the veterinary profession to make management decisions based on risk, assessed based on the private use of a combination of testing technologies, including more sensitive testing technologies alongside the SICCT test, and enhanced data analysis – is likely to have more success and achieve greater progress.