bTB Testing
The tuberculin skin test is the primary screening test for TB in cattle in Great Britain. It has been in use in Great Britain since 1942. The test is used throughout the world to screen cattle and is considered to be the best test currently available for detecting TB in live animals.
It involves injecting a small amount of both avian and bovine tuberculin into the skin of an animal. If an animal is infected, the animal will display a localised allergic reaction (in the form of swelling) a few days after the injection. Therefore, a fold of skin in the area the animal is injected is measured both before and 72 hours after the injection is administered.
The gamma interferon (g-IFN) blood test is being used more widely since October 2006 in England and Wales, alongside the skin test. Unlike the tuberculin skin test it can be carried out as often as necessary, and there is no need to wait 60 days between tests. The g-IFN has a good sensitivity and appears to detect infected animals earlier than the skin test. The g-IFN and tuberculin skin tests detect marginally different groups of animal, and are therefore frequently used in conjunction with the skin test to improve detection of infected animals.
The g-IFN test is not a replacement for the tuberculin skin test. European legislation stipulates that Member States must use the skin test for routine herd testing.
The Gamma Interferon test (g-IFN) has been introduced to improve testing of cattle alongside the tuberculin skin test. One of the key challenges in controlling bTB lies in detecting infection in individual live animals. The onset of signs of TB in cattle is usually slow, and infected animals may be infectious for months or years before a diagnosis based on clinical signs can be reached. As a result, diagnosis of bTB must rely on detecting infection with the causative bacterium, rather than disease by the use of immunological tests. Routine screening for bTB relies on one of the variants of the intradermal tuberculin test, also known as the skin test, which has been in use in Great Britain since 1942.
There is much debate over the testing of cattle which has led to many sceptics thinking there should be more widespread use of the g-IFN test, which has good sensitivity, appears to detect infected animals earlier than the skin test and can be repeated as often as necessary without the need to wait 60 days between tests. Indeed there are some people who suggest that veterinarians gain from the continuous trade that they receive from testing and welcome repeatedly returning to farms to retest inconclusive or negative reactors.
Meanwhile farms are closed down restricting movements and herd numbers increase year on year, stretching resources, for example stocking rates pushed well past their productive limits and valuable calves are unable to be sold into the market.
It is clear that in infected herds animals testing positive to the g-IFN test should be removed as the immunological response demonstrates they have been exposed to infection and represent a potential risk to other animals.
In addition, work in the Republic of Ireland shows that these animals are 7 to 9 times more likely than a g-IFN test negative animal to disclose as a skin test reactor at subsequent skin tests if not removed from the herd.
Information to consider...
The skin test has been used for many years with success in many countries. Both tests depend on the tested animal producing an immunological response with the g-IFN test being more sensitive. The g-IFN test produces many more reactors as it picks up more immune reactions.
Once an animal has mounted a successful immune response to infected organisms; it is bound to fail a test that depends on detecting an immune response, when slaughtered very often the only visible signs are in the lymph nodes – part of an animals immune defence - which proves no significance at all.
What is important is that if cattle is to be vaccinated it will become immune to the disease and therefore the testing regime that we have come accustomed to will be derelict.