I’ve spent the last five years trying to persuade students to do a research project with me looking at issues around the scientific evidence concerning bovine tuberculosis and badgers – without success, no takers (they all want to do projects about HIV). Fortunately, now they don’t have to as the Royal Society has published a comprehensive (by which I mean yes, everything) review of the subject:
A restatement of the natural science evidence base relevant to the control of bovine tuberculosis in Great Britain. Proc. R. Soc. B August 2013 doi: 10.1098/rspb.2013.1634
Abstract: Bovine tuberculosis (bTB) is a very important disease of cattle in Great Britain, where it has been increasing in incidence and geographical distribution. In addition to cattle, it infects other species of domestic and wild animals, in particular the European badger (Meles meles). Policy to control bTB is vigorously debated and contentious because of its implications for the livestock industry and because some policy options involve culling badgers, the most important wildlife reservoir. This paper describes a project to provide a succinct summary of the natural science evidence base relevant to the control of bTB, couched in terms that are as policy-neutral as possible. Each evidence statement is placed into one of four categories describing the nature of the underlying information. The evidence summary forms the appendix to this paper and an annotated bibliography is provided in the electronic supplementary material.
Because you are unlikely to read this landmark paper O gentle reader, I’ve read it for you, and here are the highlights:
Why should we care about bovine tuberculosis (bTB)?
Bovine tuberculosis (bTB) is a major disease of cattle that can also affect humans, and many other livestock and wild animal species. Human infection has not been a major public health problem in developed countries since the introduction of milk pasteurization. Advanced cases in cattle experience loss of condition, and this directly affects the economic value of the animal, but in most developed countries detection of infection leads to movement restrictions being placed on the herd, mandatory slaughter and economic losses for farmers. The English and Welsh governments estimate that they have spent £0.5 billion in the last decade on testing, compensation and research with further costs being borne by the agricultural industry.
Why not just vaccinate cows against (bTB)?
EU law currently prohibits the vaccination of cattle as it can mask the detection of infection. Vaccination of badgers is the subject of intense current research, and vaccination has been under way in Wales since 2012. The main protective effect for cattle vaccinated with BCG is to reduce the severity of disease. This is measured experimentally at post-mortem by comparing the extent of infection within the bodies of vaccinated and control cattle. A recent field trial in Ethiopia found that the carcasses of 13 vaccinated calves had 56–68% less disease than was seen in 14 control calves. If vaccinated cattle do become infected, it is likely that a reduction in the extent of disease will limit their infectiousness, reducing onward transmission to cattle and to wildlife.
Which got TB first, the badger or the cow?
Cases of bTB in cattle occur more frequently in regions that support higher densities of both badgers and cattle. Most studies that have looked for an association between high badger densities and elevated cattle TB incidence have not found one. Transmission occurs within wild badger populations; there is insufficient evidence currently available to say definitively whether the disease can persist in British badger populations without on-going transmission from cattle. Little is known about how M. bovis is transmitted between badgers and cattle. Transmission may be indirect; for example, through contamination of pasture, feed and drinking water. Alternatively, direct transmission via aerosol droplets at close contact may occur, possibly inside farm buildings as well as outdoors.
Why not just kill the infected badgers?
(Well, apart from the moral issues around this…) Badger culling was used routinely in the past, and its effectiveness was the subject of a major experiment, the Randomised Badger Culling Trial (RBCT), which ran from 1998 to 2006. The RBCT found that annual proactive culling, as conducted in the trial, resulted in a relative reduction in new confirmed cattle herd breakdowns inside culling areas, which persisted after the final culls in 2005 but subsequently diminished over a 6-year period. While culling was being carried out, there was an increase in the incidence of confirmed herd breakdowns on land surrounding (within 2 km) the RBCT proactive culling areas, though this rapidly waned after culling stopped. Reactive culling was discontinued in 2003 because bTB in these areas were significantly higher than in no-cull areas. Culling badgers is known to disrupt badger social structure, and this has been shown to cause badgers to move more frequently and over longer distances. This effect is known as perturbation. The idea that perturbation may result in increased disease transmission. It is not currently known whether alternative culling methods (e.g. shooting of free-ranging badgers or snaring) could reduce badger densities more or less effectively in Great Britain than the cage trapping used in the RBCT, nor how different reductions in badger numbers inside culling areas would influence impacts on cattle bTB on adjoining land.
What about better testing?
The gamma interferon (IFNg) test is used as an auxiliary test to the cervical tuberculin test (SICCT or ‘skin’ test) and has lower relative specificity (median animal-level specificity of 98%). IFNg identifies some exposed cattle not identified by the skin test and has a median estimated animal-level sensitivity of 67%. The IFNg test requires only a single farm visit and is then conducted in the laboratory, where it can be more consistently interpreted.
In other (i.e. my) words, we could probably control bTB using a combination of vaccination and better testing methods. If we wanted to.