Part I: Mycobacterium bovis, the English bovine tuberculosis outbreak, badgers and the culling debate.
One of the most prominent problems for public health authorities and a countries economy comes from infectious diseases that target farmed animals. There are a wide number of pathogens that affect the animals that we grow as food that present a real public health risk, such as Campylobacter jejuni in chickens, Escherichia coli O7H157 in cattle and mycobacteria like Mycobacterium avium subspecies paratuberculosis in sheep. Control of these organisms often involves improvements in animal husbandry, such as simple measures like keeping animals separated and the use of improved diagnostics or vaccinations. Where these control measures become insufficient is when the disease has other vectors, which are also in high numbers and present unique difficulties in controlling or eliminating. Such is the problem that several countries face in controlling bovine tuberculosis, caused by Mycobacterium bovis a relative of one of man-kinds earliest known pathogens, Mycobacterium tuberculosis.
M. bovis historically was a considerable risk to human health, as the organism is viable in milk and of course in the meat of infected cattle. An inquiry in 1934 determined that M. bovis was responsible for some 2,000 tuberculosis deaths (around 6% of the total, the rest being caused by the human specific M. tuberculosis). Today, M. bovis is not much of a problem in humans, as the introduction of milk pasteurization and particularly tuberculin testing has entirely eliminated bovine tuberculosis as a human and animal health threat. This is because tuberculin testing allows for the identification of animals potentially infected with M. bovis and their rapid slaughter and removal from the herd. This policy has completely eliminated M.bovis from cattle herds world wide, except in certain countries such as the
The question then has to be asked in what makes these regions so different from the rest of the world? The simple answer (but in reality not so simple answer) is the presence of other animals that serve as a vector for M. bovis. In
As a result of these vectors, which provide a large reservoir for M. bovis, it makes the typical control methods insufficient to completely eliminate the disease. This means that the continued re-infection of herds previously cleared of bovine tuberculosis becomes a considerable problem, as like its human cousin M. tuberculosis, M. bovis takes its time in causing any visible disease. This means an infected animal can fly under the radar for months exhibiting no particularly visible signs of infection and be excreting dangerous numbers of infectious bacteria in its sputum, feces and other excretions. As a result where M. bovis infection is rampant, there is a considerable economic burden on farmers in terms of more regular tuberculin testing, herd movements being locked down (put on restriction) until they pass two clear tuberculin tests and the overall loss of animals.
As many of you are probably thinking, the best way theoretically to control a vector borne disease is surely to directly control the vector and vectors that prevent the elimination of M. bovis. A particularly attractive option on the surface is to engage in a mass elimination of the potential vector from around farmland, to attempt to prevent contact between the infected vector and uninfected cattle. Unfortunately the way things work in reality is often much more difficult than theory. Compared to the large numbers and difficult terrain that the possums have on their side in
There is a considerable conservation and animal rights movement in
In solving this, numerous lines of inquiry will have to be explored to determine what the best solution that all sides could agree on would be. Firstly, it is worth looking at the biology and history of Mycobacterium bovis that make it such a successful pathogen. Then in terms of resolving this debate, it’s essential to determine the role that the Eurasian badger plays in the spread and persistence of M. bovis in the
Once the link between M. bovis and the badger as a vector has been established, an analysis to see what benefit (if any) that culling would present on reducing herd breakdowns (confirmed cases of bovine tuberculosis in a cattle herd). Finally an analysis of other potential solutions other than culling such as ‘wildlife friendly farming’ and particularly vaccinating badgers. Finally, after these factors have been considered, we’ll revisit the original question and discuss what sort of compromise between health officials, farmers and conservationists could work.
Join back tomorrow for part II of this series: What makes Mycobacterium bovis such a problem?
Although I haven’t cited anything in particular for this introduction, there are a lot of papers on this topic and there are a few that are particularly useful in understanding what will be discussed over this week here. If you have access to pubmed or other search engines, a read of some of the following papers would be recommended although not strictly required as they will be discussed in detail as the series goes on:
Gilbert M., A. Mitchell, D. Bourn, J. Mawdsley, R. Clifton-Hadley and W. Wint (2005). Cattle movements and bovine tuberculosis in
Krebs J.R., R.M. Anderson, T. Clutton-Brock,
Phillips C.J.C., C.R.W. Foster, P.A. Morris and R. Teverson (2003).The transmission of Mycobacterium bovis infection to cattle. Research in Veterinary Science, 74:1-15.