Email received August 8th 2011
Your farmer correspondent is absolutely right. (See email l) Any mammal that comes into contact with Mycobacterium bovis, and remains well, will develop antibodies and a white cell memory for the bacterium which may cause no clinical disease, but become walled up within the animal's tissues. Probably some animals (camelids and occasional cattle in a herd come to mind) have a poor ability to develop any host response to this organism and they may become overwhelmimgly infected with many lesions and then die. They represent a serious infection risk in this state but may remain undiscovered until late on in the disease process. For most animals this is not the normal course of events and they resist the organism and remain well.
The same applies to man and meeting Mycobacterium tuberculosis as in animals meeting Mycobacterium bovis.
Later on in life or under stress,( for example perturbation of badger social groups by culling) they may have a recrudecence of this infection and become clinically infected and excretors of the organisms.
Most cattle that are skin test positive have met the organism but remain healthy. But they are culled as a precaution. The problem with culling badgers is that the remainder will be stressed and perversely we may initially cause greater spillage and production of the organism into the local environment from these stressed animals.
In culling cattle who are skin reactors we are applying different standards for infection risk, to the cattle, for reasons of risk to human health.
At present there is no ideal solution. I am of the opinion that feeding and including trace elements to the badgers ( and other herbivore/omnivore wild life reservoirs) along with hormones to limit reproduction, may be a useful experiment to undertake. I have previously suggested mixed antibiotics in the feed. Maybe this is a way forward and worth a serious trial?
Dr Colin G Fink
Clinical Virologist & Hon. Senior Lecturer in Biological Sciences University of Warwick