Bovine tuberculosis in cattle and badgers

Badger Encounters in the Wild book
Superb book of Jim Crumley's encounters with badgers in the wild in Scotland. The quality of the writing is superb. A great  read. Click here to buy: Encounters in the wild

http://www.teaching-biomed.man.ac.uk/uploads/public/bio/BS369/website/lecture-10.htm

What is the problem?

The story so far...

• Mycobacterium bovis is a bacterium which is closely related to the human form (Mycobacterium tuberculosis) and the avian form (Mycobacterium avium).
• It primarily infects cattle.
• All types of mycobacterium cause tuberculosis or TB. In the 1930s there were 50,000 cases of human TB each year. 4000 of these were caused by Mycobacterium bovis, resulting in 2000 human deaths.

1923

Milk was tested and sick cattle were slaughtered, however this didn't prove too successful in combating the illness.

1935

Testing of cattle was carried out by injecting tuberculin antigen into each cow. The individuals carrying the disease would react to the antigen-these animals were slaughtered.

An investigation then began to see how widespread the disease was in the Badger population :

• Badgers are most numerous in the south-west of England, as are cattle. Due to this, bovine tuberculosis in Badgers is mainly a south-west problem, it isn't as serious in the rest of the country.
• Badgers could only be checked for TB at post mortem, so unfortunately all Badgers both infected and uninfected were culled. Gassing (by cyanide) was an efficient way to kill them, but the Badgers died in their setts and could not be tested for TB. Cyanide poisoning was later checked, and was found to take a long time to kill the badgers so was banned because cruel. Cage trapping the Badgers is less cruel, but requires more time and man power, so is less efficient as a control method.
• When Badgers were killed, the incidence of TB in cattle decreased. When no Badgers were being killed, the incidence of TB in cattle increased. How was TB being spread between cattle and Badgers?

The success of eradicating bovine TB in Britain was not universal. Especially in SW England there were regular outbreaks. These were not only new cases but repeat cases in farms where cattle herds were thought to be have been cleared.

1971

A badger was found dead in the open in Gloucestershire, on a farm where there had recently been bovine TB. An autopsy was carried out on the Badger, and bovine TB was discovered. Other Badgers too were examined - 22% of Badgers in the area had bovine TB.

• Of a large sample of Badgers from the south-west, 14% had TB. However, this was not representative of the whole of the UK, as the area was where there were known to be outbreaks previously.
• The roles of other wildlife in bovine tuberculosis is not known so well. Other carriers of the disease may be mink, fox, mole, rat, ferret and wild deer.
• TB isn't very infectious in Badgers and isn't always fatal. It also tends to stay within a Badger group.
• TB is not a major cause of death in Badgers-52% of Badgers die in road traffic accidents.
• The main mode of TB transmission is respiratory - especially from mothers to cubs within setts.

1985

The situation in the south-west still wasn't much better. In the early 1980s, each TB breakout had cost £5,700. However, when the Badgers were killed, the maximum saving was £1.9 million, but it cost MAFF (i.e. the taxpayers) £11.3 million. Thus Badger control is not economic.

Today...

• It is still uncertain how TB spreads between Badgers and cattle, and in which direction it goes.
• With humans, TB is more prevalent in dense populations. This doesn't appear to be the case with Badgers. The incidence of TB is not related to Badger density.
• It is not necessarily the Badgers that need to controlled - a better option would be more secure housing for cattle (to ensure Badgers can't mix with them), better ways of feeding them (so Badgers can't raid their feed troughs), and keeping cattle away from setts.