Mycobacterium bovis diagnosis : The problems.
" I am sure that the present skin test kills perfectly healthy animals that have just met the infection but have walled it off and simply have a white cells reaction to the organism."
There are perfectly good methods of using PCR to find very small quantities of M Bovis or M tubercle or any of the many other Mycobacteria.The problems concern the habit of the infecting organisms. In man and in cattle mostly the organism travels (? via the blood) to
the lymph nodes where it lodges and may grow slowly and cause caseation. (See also http://en.wikipedia.org/wiki/Caseous_necrosis.)The organism appears in the blood only intermittently and may be within macrophages but only in low numbers. The exception is in miliary tb when it spreads throughout the body and perversely there may be only muted or no immune response so no white cells ( skin reaction ) is seen and the cattle may be TB negative on skin testing but hugely infected.
So PCR testing of blood is not very helpful. We undertook a project on a TB broken down herd and the Vet Agency sent us bloods. We could not find any TB by PCR but the herd were all known to be infected and on post mortem they had nodes that were infected.
This is the nub of the problem. I have a colleague in St Mary's Pediatrics who is looking for host proteins as specific marker of active TB
infection - The problem of diagnosis is just as important in children.In cattle, as a way of assesing the infection in an individual carcase, testing lymph nodes by PCR at post mortem, may well be useful . In order to have some idea of the sensitivity of the assay for developing a standardised approach, it would be an initially important to resolve how many lymph nodes need to be tested from each carcase - and which ones to test. A very large pilot study of infected cattle would be needed to determine the pickup ( thus sensitivity ) of any recommended method of sampling. I suppose that the carcase could be stored whilet the results were awaited. Would it then be classed as fit for the food chain? Would it be the sort of meat that has a market ? I do not know. This is not my area of expertise.
Many animals ( including man) may have a Ghon focus (in the lungs usually) (See also http://en.wikipedia.org/wiki/Ghon_focus ) which is a single area of TB infection which is then walled off and never causes any further problem - or else may break out into an infection in later life as the immune system wanes. Are such animals, which will be positive on skin test , infectious or unfit for consumption? I do not know how to answer this. I am sure that the present skin test kills perfectly healthy animals that have just met the infection but have walled it off and simply have a white cells reaction to the organism.
So it is a really difficult diagnostic, scientific and food standards problem.