The Voice of Reason: Badgers and BTB

With cold wintery weather and regular snow showers, this week has felt more like December than March. A combination of this unseasonable weather and days of hard graft (depending on one's definition of what hard graft is) I have had little time for YETI'ing. Fortunately I have been undertaking a number of badger surveys (which has dangerously got me thinking. . .) and thought with not a lot happening let's take a look at what is one of Britain's most iconic mammals and the well documented association with BTB.

A question very much on every ones lips at the moment is the proposed cull of badgers in Britain with experimental trials having occurred in some places i.e. Gloucestershire. Recent news suggests that the government is about to give the green light to a full scale badger cull in England, which could see in excess of 100,000 animals slaughtered. As someone familiar with badger ecology and biology, I thought I would give my opinion and look at the facts while things remain quiet on the Yeti front. DEEP BREATH . . . . . (Sorry . . . .I don't have any photos of actual badgers).  

Active badger sett showing, small amounts of bedding in entrance (probably caught on the animals fur). 

So what exactly is Bovine TB? Well, Bovine Tuberculosis is a bacterial disease caused by the bacillus Mycobacterium bovis and is similar in many ways to Mycobacterium tuberculosis the human counterpart of the bacterium. In-fact these two diseases are very similar and it is likely that M.bovis evolved from M.tuberculosis and was originally spread from humans to cattle following their domestication, as early as the Neolithic period. Cattle then passed what was now a mutated disease back to humans and on to other mammals, including badgers. The disease basically consists of the formation of tiny tubercles on internal organs, most often found within the lungs. Within these tubercles the bacteria quickly multiply. When one such tubercle bursts it spills the bacteria out spreading the disease around the remainder of the hosts body.

TB often causes the animal to become weak, lethargic and eventually die. What makes it complex, certainly in the case of  the badger is some animals appear not to show signs and suffer, where the disease lays dormant, resulting in the animal acting as a reservoir for the disease. Badgers seem to have a remarkable tolerance to the bacterium and generally between 50% and 80% of tuberculous individuals have no observable lesions. In some studies badgers with TB and ones without it co-exist in the same sett over years without it spreading between them. Some animals do succumb to it and often leave the sett in a very poor condition, to live their remaining days alone often above ground. Approximately 35,000 cattle are slaughtered each year as a result of Bovine TB, making it one of the key issues currently facing British cattle Farmers.

Badger fur caught on barbed wire. Badgers have distinctive cream fur and longer guard hairs which are black/grey with cream tips. 

So why are badgers such an issue in regards to BTB? People often argue that mammals other than badgers carry TB and could contribute to the spread of Bovine TB and act as reservoirs. This is true, however the significance of this is unlikely to be high in most species as they do not interact with cattle, are not social and are poor hosts for the disease. One of the exceptions is perhaps deer. A host species needs to be able to carry the infection and interact with cattle regularly. Deer populations are on the increase and could certainly become a more significant reservoir for the disease in the future, especially given that they are capable of ranging further more quickly in comparison to badgers, which are relatively sedentary especially in areas with high population densities. However deer have not always been as numerous in comparison to how prevalent bovine TB has historically been, especially the large species like red deer and fallow deer, which is suggestive that they are not a significant player in this saga.

Badgers are undoubtedly a key reservoir for the disease as they are relatively vulnerable to infection and their biology means they often come into closer contact to cattle, in comparison to many other wild mammals. Badgers regularly feed in grazed pasture where they are directly close to cattle, they also urinate on the pasture causing an indirect risk, they regularly feed on stored cattle food, drink from water troughs and some radio-tracking studies have even noted them as physically interacting with cattle. In Britain, badgers are the only known maintenance host for M. bovis, although there are some “spill-over hosts” (i.e. populations in which infection will persist only where a maintenance host is present in the ecosystem), including Red foxes. Despite this the exact transmission from badger to cow and back from cow to badger is still poorly understood and may involve a number of transmission route ways. It is widely accepted that cow to cow routes are the most significant way in which cattle spread the disease, although cattle are less likely to act as long term reservoirs due to regular health checks.       

Badger footprint, showing the typical bear shape. Four (five) parallel toes with a kidney shaped pad behind. 

So what do we know about culling badgers as a means of protecting cattle from TB? With the evidence suggesting that Bovine TB was linked to badgers, the idea of culling as a control measure was investigated. This has occurred almost continually in one way or another since the 1970's, usually in the form of local trials. Thankfully unselective and inhumane methods such as the use of hydrogen cyanide to gas setts was soon outlawed and is no longer used as a culling method. This was replaced by live trapping and shooting, which still operates to an effect today. In the 90's Professor John Krebs began a Randomised Badger Culling Trial, this trial had a key aim to determine the extent to which the removal of badgers reduced TB in cattle. The trial used two culling regimes, Proactive - where badgers were eliminated as far as possible from the area and Reactive - where culling occurred in a small area as a direct result of a TB outbreak. In addition areas used as a control where no culling took place, were also studied, something which previous trials had not investigated in sufficient detail.

The results of this trial offered conclusive and interesting evidence, which appears is being consistently overlooked. The Reactive culling resulted in an increase in the rate of TB in cattle by on average 27 per cent. The proactive cull resulted in a lower rate of TB in cattle in culled areas, than in control areas, showing the removal of badgers did reduce TB. However only by a rather low 20%, not a lot given the time and money spent culling. What makes matters even more interesting is the rate of bovine TB actually increased by 25% (on average) in a 2km band around the culling area. The increase in TB surrounding the cull area effectively cancels out the results of culling inside the cull area.

The reason for this is known as the perturbation effect. Culling alters the spatial social organisation of badgers, which results in abnormal behaviour, not only in culled areas but in areas surrounding the cull site. Badgers are territorial animals which more of less constantly remain within the territory, or in close proximity to its boundary. Culling causes badgers to increase their territories size, range further and disperse more frequently. Larger territories often have more poorly defined boundaries increasing the (indirect and direct) interaction of animals from neighbouring setts. All of this moving around also causes TB to move around and spread at an increased rate, which is what the data from the trials found.   

In my opinion this is conclusive enough data to abort a national cull and look at other ways forward, and look at a genuine ways to control the disease in cattle. Culling trials in Ireland did however show that by culling 100% of badgers in an area you can reduce TB, this is unethical and not a practical solution to the problem.

So how do we go forward? I personally think the most realistic way of reducing the disease in cattle is to deal directly with the cattle rather than badgers. Cattle are easier to work with, easier to conduct research on and easier to access. Research into vaccinating cattle needs to be continued and looked at as a realistic way forward. However it is not as straight forwards as it would seem in that so-far no 100 effective vaccine has been found. Vaccinating cattle would also mean that you would no longer be able to conduct TB tests on vaccinated herds as they would be diagnosed as TB-positive. So any development of a vaccine would also require the development of a new TB test. Recent promising advances have been made using Bacille Calmette-Guerin (BCG), the vaccine used on humans. To make things awkward however, EU Legislation forbids vaccination of cattle as it may increase the spread of the disease, as vaccinated cattle cannot be reliably tested. Trials are ongoing in Gloucestershire to test the efficiency of vaccinating badgers.

In my opinion administering drugs to a wild animal like a badger would be very difficult, it would involve applying an oral vaccine to bait, which would be impractical as it would be impossible to ensure the whole sett had fed on the relative quantity of vaccine. This would then have to be regularly carried out to vaccinate young animals weaned after the initial bait was set up and animals new to the sett/area, which have dispersed from other areas. In addition to cattle vaccination, increased bio-security of stock sheds and food stores would help reduce the risks of infection i.e. increase ventilation and have lower numbers together, as would better fencing where possible (a method much cheaper than a national cull believe it or not). Certainly stricter farm bio-security, continued research into a more efficient vaccination for cattle and tighter measures of cattle movement would be positive steps forward. 

Regardless to the economic implications of the disease, do we not have a duty in protecting our wildlife from disease spread from our livestock? 

Latrine site showing droppings. This feature is typical of badgers and are used to mark territory boundaries and provide social messages between individuals.