Bovine TB and badgers – why it all mattersJanuary 17, 2012
Before I start I should mention that this post will not be about the political aspects of the cull decision. Nor will it be about the scientific data that is being used to support and oppose the government’s plans. I am not an expert in these areas and so do not feel competent enough to blog about the issues. However, please feel free to discuss these things in the comments (please keep it polite and attack the argument rather than the person you disagree with).
What I want to write about today is why it is actually important to reduce the levels of bovine TB in our cattle and wildlife. There is a good review article here (unfortunately behind a paywall) and it is from this that much of what will follow is taken.
Bovine tuberculosis (bTB) is caused by Mycobacterium bovis – a species closely related to Mycobacterium tuberculosis which is the most common cause of human TB. M. bovis can infect and potentially cause disease in most mammals, and is zoonotic (so can infect humans too).
Humans and Mycobacterium bovis
People can be infected by three main routes:
- Infected milk – this was the most common route before pasteurisation and is still a potential hazard in places where milk is not routinely pasteurised
- Inhalation of aerosolised infectious droplets – ie. when an infected cow with tuberculous lung disease breathes out there may well be droplets that carry the Mycobacterium. Again, this was a reasonably common route historically. The main group of people at risk from this route are farm workers. (I was actually wondering whether this could potentially be a problem from places like children’s farms where more of the population could be exposed to infected animals. I’ve not seen any information on it. It’s probably not a problem at the moment but if the TB levels in the cattle herd keep increasing perhaps it could be in the future?)
- Infected meat – this is a very rare route of infection as gross signs of disease are easily spotted at meat inspection.
The World Health Organisation has this to say about bovine TB:
TB due to M. bovis often occupies sites other than the lungs (it is extra-pulmonary), but in many cases is clinically indistinguishable from M. tuberculosis infection. However, patients with M. bovis often do not respond to the drugs commonly used to treat TB, sometimes resulting in a fatal outcome.
In general there is a period of latency after exposure to the organism – in this period there are no clinical signs of disease. Some (but not the majority) of latent infections will then go on to develop active disease which if not treated can be fatal. This progression is more common if the person is immunosuppressed.
The picture in cattle is reasonably similar to that in humans. Animals most commonly get infected via aerosolised droplets but there is the potential for calves to be infected via milk. Some animals may never show signs of disease, in some the signs of disease may start off being quite vague (loss of appetite, weight loss) but cases can then develop into tuberculous lung disease.
Badgers and Mycobacterium bovis
Badgers, like cattle, are considered a “maintenance host” for bovine TB – the organism multiplies and infects the population from generation to generation and even if badgers never came into contact with any other possible Mycobacterium host the organism would still survive in the population.
Again, some badgers may never show any signs of clinical disease but some may suffer from “florid disease” (as the paper by Gallagher and Clifton-Hadley – see below – beautifully describes it) including tuberculous lung disease and tuberculous lesions in other body sites. That same paper proposes that it is those badgers with the most advanced disease that are the main reservoir of infection for other animals species such as cattle.
Other animals and Mycobacterium bovis
Some other species have also been proposed to act as maintenance hosts including deer and possibly goats. Some species are thought to act as “spill-over” hosts (in these species the infection is self-limiting and they can be considered a “dead-end” host unless they happen to infect a maintenance host). Currently considered as spill-over hosts are pigs, cats and dogs. These animals may also show signs of disease (although again it can take a long time for disease to develop).
Importance in the UK
Because of milk pasteurisation and the control methods currently implemented in cattle the human cases of M. bovis are extremely rare. This disease is much more of a problem in the developing world where these control mechanisms may not be in place.
However, this does not mean the the disease should be forgotten about.
The incidence of the infection in cattle seems to be increasing (see the HPA link below) so there’s more chance of people coming into contact with it, if not directly from cattle, then from some of the spill-over hosts like cats and dogs.
Another extremely important reason not to forget this disease is because the number of immunosuppressed people in the population is increasing (this includes people suffering from diseases like AIDs and also includes people on immunosuppressive drugs like chemotherapy) and as I mentioned before, this leads to greater chance of an infection becoming active and clinical symptoms developing.
It may be (this is my speculation here) that in the future the most common picture for human infection (although it will still be extremely rare) will be an immunosuppressed person getting infected via a spill-over host like a pet cat or dog.
As I’ve discussed, this disease is not only bad for us but it is also bad for cattle, for badgers, and for many other animals should they become infected. In the end, it is in all (and I’m including animals in this ‘all’) of our best interests to eradicate this disease from our country.
And this is generally accepted. However, what still remains to argue about is “How?”
Defra’s policy statement on bovine TB control from December 2011 (also PDF)
Health Protection Agency information on bovine TB
Animal Health information on bovine TB
GALLAGHER, J. (2000). Tuberculosis in badgers; a review of the disease and its significance for other animals Research in Veterinary Science, 69 (3), 203-217 DOI: 10.1053/rvsc.2000.0422
de la Rua-Domenech, R. (2006). Human Mycobacterium bovis infection in the United Kingdom: Incidence, risks, control measures and review of the zoonotic aspects of bovine tuberculosis Tuberculosis, 86 (2), 77-109 DOI: 10.1016/j.tube.2005.05.002