Bovine tuberculosis (bTB) is a disease that seems to gather a lot of attention in the popular press, especially the highly controversial badger culling trials in England and Wales. But what is this disease, and why is it so important in this country?
Bovine TB is caused by the bacterium Mycobacterium bovis (M. bovis), a very close relative of the principle cause of human TB, M. tuberculosis. It predominantly infects cattle, but perhaps not surprisingly can also be transmitted to, and cause disease in, people making it a zoonosis (a disease which can be transmitted readily between humans and animals). In both cases the resulting disease is very serious; symptoms include persistent cough, weight loss and fever of increasing severity over weeks and months, ultimately leading to death if no intervention is applied.
A high magnification image of mycobacteria captured using an electron microscope (CDC)
Historically, human infection with bTB has been a very serious problem in the UK; records from the 1930s suggest it accounted for at least 16,000 deaths in that decade alone! The major route of transmission at the time was through people drinking the milk of infected animals. Widespread implementation of pasteurisation (the process of heat-treating milk to kill disease causing bugs) from the late 1930s onwards, combined with the routine testing of cattle for the disease and meat hygiene inspection, virtually eliminated M. bovis as a causative agent of human TB in the UK. However, in many other parts of the world, particularly developing countries (where such measures are sometimes harder to implement) human infection with bTB remains a very serious issue, with millions of people still at risk across the globe, as demonstrated by figures published by the WHO on human TB; it is estimated that in the 1990s, TB accounted for 30 million deaths worldwide, of which M. bovis is believed to have been the causative agent between 10 and 50% of cases in developing countries, the areas where TB is a particular problem1.
bTB in cattle: an ongoing saga
M. bovis is a very slow growing bug, with disease taking several months to become obvious physically. Unfortunately, during this time infected animals may remain within their herds, potentially spreading the disease with other individuals. The slow growing nature of the bacteria also makes it incredibly difficult to kill. As a consequence in the UK, and many other countries where bTB is a problem, the treatment of infected animals is not considered an option. Instead a “test and cull” policy is employed, whereby animals are tested for the disease and immediately slaughtered if found to be positive.
The test used in cattle is very similar to that used to detect TB in people (some of you may remember having this procedure as a child). It works by injecting a small amount of material taken from the M. bovis bacteria (called the “antigen”) into the skin of the cow. Animals that are actively fighting infection will react strongly to this material resulting in a large swelling in the area; this inflammation indicates that they are bTB-positive. At present all cattle in England and Wales are tested at regular intervals, typically every 1-4 years depending upon the current level of disease in the area. Farms where positive animals are found are placed under restrictions, whereby no animals may be moved off the premises until the farm’s ‘TB-free’ status is restored, in an attempt to stop the disease being spread through the movement of infected cattle to other farms.
Courtesy of Rachael Evans
Testing was originally introduced in the mid 1930s on a voluntary basis, but became compulsory from the 1950s across the entire country. There was initially a rapid reduction in disease prevalence, and throughout the 1960s and 70s it was all-but eradicated. However, from the mid-1980s onwards, and for reasons which are still not fully understood, the disease underwent a resurgence and the situation today in parts of the UK is as bad as it ever has been; between 2000 and 2010 over 250,000 cattle were slaughtered as a result of these strict measures at an estimated cost to the UK taxpayer of £500million. Currently Scotland is the only part of the country considered to be free of the disease, and while the most recent government figures suggest disease prevalence is slowly decreasing, whether this trend will continue, as hoped, remains to be seen2.
Completing the picture
One reason that bTB has continued to be an issue after so many decades is that there are a great deal of unanswered questions in connection with its resurgence and transmission. As with many diseases, more discoveries simply raise more questions- none more contentious than the issue of badgers.
It is known that badgers can act as a “reservoir host”, meaning that they are able to carry M. bovis and transmit it back to cattle. What is less clear is the significance badgers play in spreading the disease, and whether or not controlling badger populations would help in controlling bTB in cattle.
In the autumn of 2013, badger-culling trials were commenced in 2 areas of England in an experimental effort to quantify whether any benefit is afforded through a reduction of the disease in cattle. However, these trials were abandoned early in 2014, since it was not possible to achieve the target of culling 70% of the badger population; the minimum figure believed to be necessary to reduce overall disease prevalence. This is a hugely disappointing outcome, as it means that badgers have been culled for no beneficial reason, and the trial has therefore provided no new information as to whether culling badgers helps control disease in cattle- a situation neither pro nor anti-badger culling campaigners can possibly be happy with.
In addition to the link with badgers, there are many other aspects to bTB biology that are not fully understood. Two notable examples of this include the recent and rather startling news that domestic cats have infected their owners with M. bovis. This not only underlines the issue of public health surrounding bTB, but also demonstrates the variety of species which are capable of carrying infection, potentially further muddying the waters as regards reservoir host species and disease control3. A second example is that of recent research from the Department of Infection Biology, at the University of Liverpool, which has shown a new experimental link between bTB and liver fluke, a common parasitic worm that infects livestock across the UK. In this study researchers found that liver fluke were capable of ‘modulating’ a cow’s immune system, meaning they can in effect ‘turn down’ the immune response in an effort to avoid being killed by it. This appears to have knock-on consequences for the cow, since it also reduces the animal’s ability to fight off other infections like bTB. Additionally, dampening of immune responses has been shown to reduce the sensitivity of the bTB test, meaning that in areas where there is a lot of liver fluke, bTB may go un-diagnosed for long periods. This could help explain in part why certain areas of the country remain in the clutches of bTB despite our best efforts4.
Clearly then, there is a much more to bTB than the press would have you believe. The next time you see or read an article in the news about the rights and wrongs of badger culling, remember this is only one small part of a much bigger picture. For example, would the public feel differently if the government was pursuing the control of feral cat populations in an attempt to reduce the risk of TB infection in people?
This post, by author John-Graham Brown, was kindly donated by the Scouse Science Alliance and the original text can be found here.
1. Statistics worldwide: http://wwwnc.cdc.gov/eid/article/4/1/98-0108_article.htm
2. Statistics for bTB in the UK: http://www.defra.gov.uk/animal-diseases/a-z/bovine-tb/
3. M. bovis in cats: http://220.127.116.11/content/174/13/326.2.full
4. Liverpool liver fluke research and bTB: http://www.nature.com/news/bovine-tb-disguised-by-liver-fluke-1.10685