Conducta prosocial en la adolescencia

Bovine TB is a zoonotic infection of cattle caused by the bacterium Mycobacterium bovis. The bacterium that causes tuberculosis in humans, M.

tuberculosis is closely related to M. bovis, as is M. avium ssp. paratuberculosis which causes a wasting enteric disease in cattle, Johnes disease, and is

implicated in the human condition Crohn‟s Disease.

M.bovis can infect all land mammals (Defra 2005b, p.1) although these hosts vary in their ability to pass on the infection. Badgers are an important wildlife reservoir of M.bovis, although their role in transmitting the disease to cattle is little understood and remains the cause of significant controversy. Deer also can become infected and transmit the disease within their own population and to other species.

M.bovis is a slow growing microaerophilic organism. This makes the lungs an important site of infection. The fact that it is so slow growing, taking six to eight weeks in optimised laboratory conditions to grow, makes routine bacterial culture useless as a diagnostic method. A further difficulty in culturing M.bovis is that infected animals do not excrete organisms in a steady flow. On some days animals may excrete huge numbers of bacteria (super excretors), while the same animal may, on other days, not excrete observable numbers of bacilli.

In addition to the lungs, bacilli may survive within T lymphocytes. The immune response to infection by M.bovis attempts elimination by phagocytocis, yet the survival of bacilli within cells makes the complete elimination of infection highly unlikely. Sites of infection are thus walled off by the body in tubercles. While such tubercles may remain latent for many

months or years, they may grow and spawn secondary tubercles throughout the body (Hancox 2000, p.88).

As bacterial culture is ineffective in diagnosing bovine TB, although used in confirming a diagnosis, diagnostic tests rely on the detection of an immune response. In the UK, a skin test (the Single Intradermal Comparative Cervical Tuberculin (SICCT)) is presently the definitive test in cattle. Inactivated proteins from M. bovis are inoculated in the skin of an animal‟s neck. A second site on the neck is inoculated with proteins from M. avium. This is because cattle may have become exposed to this cause of avian TB, and will produce an immune response to it, although M. avium does not go on to cause disease in cattle. Three days post inoculation, the sites are examined. A positive test, one which results in an animal being classed as a „reactor‟, is one in which the reaction to the site of M. bovis inoculation is 5mm or more than the reaction to M. avium inoculation (Defra 2005b, p.4). In a herd test, these reactor animals are slaughtered and examined post mortem. Lesions typical or suggestive of TB are then cultured under laboratory conditions. Lesions are most likely to occur in the lungs or in the lymph nodes of the lung and throat. A reactor becomes a confirmed case of bTB when M.bovis is successfully isolated from these cultures. Should this occur then the herd from which the animal was

removed is deemed to have experienced a „breakdown.‟ Skin tests from that

herd are then re-examined and any animal where the M.bovis reaction is 3mm or more than the M.avium site is now deemed also to be infected with M.bovis (Defra 2005b, p.4).

The SICCT has a drawback that is common to many tests which attempt to detect an immune response in an animal. The drawback is that such tests cannot be 100% accurate. There is a balance to be struck between specificity, the certainty that the organism that has provoked the immune reaction is indeed the one that is being tested for, and sensitivity, that is the likelihood that the test will pick up a positive. The greater the sensitivity the greater the chance that the test will pick up a reaction that is a false positive. The greater the specificity, the greater the chance that a positive animal will be missed and that it will give a false negative result. The sensitivity of the SICCT, that is the ability of the test to detect all positive animals, is believed by Defra to vary between 77% and 95% based on the results of a number of studies. Sensitivity between these parameters means that for every 100 infected animals tested, the test will fail to pick up between 5 and 23 cases depending on the sensitivity of the particular batch of tests being performed (Defra 2005b, p.4). Sensitivity of the test is thought to be very good, at least 99%. This means that of 100 uninfected animals tested only 1 will give a false positive reaction (Defra 2005b, p.4).

M. bovis used to be a significant cause of disease in humans. In the 1930s there were some 2,000 deaths annually from bovine TB (Hancox 2002, p.224). The consumption of raw milk was the usual source of infection, but as pasteurisation of milk became routine, the number of cases dropped. In 2001, only 40 cases of tuberculosis in humans were the result of infection with M. bovis. Doctors consider that most of these were contracted abroad or were the reactivation of lesions pre-dating milk pasteurisation (Defra 2005b, p.7). So

although bovine TB is a zoonotic disease, the risk of infection to humans today is very low. Those working in direct contact with infected animals are at the highest risk, while the risk to the general public is exceedingly small.

While the human impact of the disease has been reduced, the impact of bovine TB on the farming industry remains significant. In 2003, 6% of cattle herds suffered TB breakdown (Defra 2005a, p.20), mostly concentrated in the southwest and west of England, and the southwest of Wales (Reynolds 2006, p.119). New herd breakdowns are running at an average of 18% increase per annum (Defra 2005a, p.20). The economic cost to the taxpayer has risen from £38.2m in 1999/00 to £88.2m in 2003/04 (Defra 2005a, p.26). This figure is mostly made up of the costs of herd testing and compensation to farmers. However, £14.3m was spent on research into bTB including the costs of the Randomised Badger Culling Trial (RBCT). The remaining sums were spent on a variety of other bovine TB related expenditure. The cost of testing and compensation alone for 2003/04 was £67.6m. The 2001 FMD epidemic appears to have had an adverse affect upon bovine TB with areas of the country previously with a low incidence of bovine TB experiencing new breakdowns following restocking. In Cumbria, for example, the vast majority of new TB cases can be linked to restocked herds post 2001 FMD outbreak, or to bought in infected animals (Defra 2005a, p.23). It is worth noting that during the FMD epidemic, routine bovine TB testing was in abeyance.