Skin and Interferon-gamma testing
Testing for TB is challenging. Unlike many other organisms that cause disease, the bacteria that cause TB are difficult to isolate and do not lead to infected animals having strong or easily detectable immune responses. The main way of diagnosing TB is through a skin test or through a blood test called the interferon-gamma test.
The skin test measures the animal’s immune response to the TB organism. Cells of the immune system that have been exposed to the TB organism develop a memory of the infection. When these cells come into contact with the TB organism or components of it they respond by multiplying and developing a localised response. The TB skin and blood tests measure this immune memory and responsiveness.
Extracts of the TB organism (known as tuberculin), when injected into the skin of an infected animal, will trigger cells with a memory of being exposed to TB to enter the skin where the injection has been placed, leading to a swelling of the area which can be measured. When this swelling is seen it is an indication that the animal is infected.
Within NI, the skin test used is a comparative one. This is because other closely related bacteria can generate similar immune responses that cross-react with TB. So for example an animal that has been exposed to avian TB (which does not cause disease in cattle) may respond to TB tuberculin injected into their skin. To avoid these ‘false positive’ reactions, animals that are skin tested are injected with bovine tuberculin and avian tuberculin. Animals infected with bovine TB will have a larger response to bovine tuberculin than to avian tuberculin.
The interferon-gamma blood test measures the same thing and is also a comparative test. In this case white blood cells are mixed with the tuberculins. If there are cells with a memory of being exposed to TB they will generate a chemical of the immune system called interferon-gamma. This chemical can then be measured in the lab. Animals that are bTB infected will generally produce more interferon-gamma to bovine tuberculin than to avian tuberculin.
Post-mortem tests
TB infection usually leads to localised infections in specific areas of the body. Most commonly these are in centres of the immune system known as lymph nodes. When an infection is established in one of these areas, it can lead to changes in the lymph node that are characteristic of TB. These changes are often called lesions and can sometimes be seen with the naked eye. Because TB is usually a respiratory infection, the lymph nodes usually affected are those associated with the lungs. During meat inspection these lymph nodes are examined visually and if a lesion is seen it is suggestive of TB being present. The lymph node can then be examined by microscopy to see if the changes are indeed TB and by bacteriology or molecular tools to establish whether the TB organism is present.
Other tests
Antibody tests
Most methods used for detecting infections use antibody tests to detect evidence of infection. This is because many infections, especially viral infections, generate strong immune responses, in infected animals and people, that produce proteins known as antibodies. Antibody responses can be very easily measured in, for example, blood. However, TB doesn’t generate strong antibody responses. Conventional antibody tests are not sufficiently sensitive to be useful for TB because of the low levels of antibody TB infection normally leads to. However, newer more sensitive antibody tests are being developed for TB that use novel technologies. It is possible that these tests might become available in NI in the future. For that to happen will require extensive validation work to ensure they are sufficiently accurate and practicable for widescale use.
Phage tests
A novel approach to TB detection has been the use of viruses known as bacteriophages (commonly referred to ‘phages’). These tests use phages to identify the presence of TB bacteria directly. One of the potential advantages of these tests is that in principle they are a rapid means of identifying the TB organism (compared to conventional bacteriology which can take several weeks). However, to date, these tests remain largely experimental and have not been subjected to sufficient validation to allow them to be adopted by TB control programmes.
How good are the Tests Currently Used for bTB?
Estimates of how good the skin and blood tests are that are currently used for bTB vary; how tests perform will vary in different situations and in different herds. In general the comparative skin test is very specific. This means it is very unusual for it to give false positive results. Any animal that tests positive with the skin test is very likely to be infected. Often these animals do not show lesions at slaughter. This does not mean they are not infected, just that the infection was identified at an early stage, before the development of visible changes.
However the comparative skin test has a relatively poor sensitivity. While estimates vary, it is generally agreed that it probably misses on average around 1 in 4 infected animals during each testing cycle. This means that for any round of testing in a herd that has infection, there is a significant likelihood that some infected animals will be missed at the initial skin test. Therefore all TB programmes that use the skin test rely on repeated testing. Repeating the test increases the likelihood of detecting all the infected animals in a herd. However even then, it is possible that a herd that is repeatedly negative on the skin test may have some residually infected animals present that have not been detected.
The interferon gamma blood test is more sensitive than the skin test, missing fewer infected animals (perhaps 1 in 5 infected animals). Therefore it can be useful in trying to detect infection where there is a suspicion of residual infection that the skin test is failing to detect. Nonetheless it also can fail to detect some infected animals. A greater constraint of this test is that it will yield a larger proportion of false positive results (perhaps 1 in 20 tests yielding on average a false positive). For this reason it can only be used in a targeted way and not as a widescale surveillance tool.
Post-mortem surveillance is a much less sensitive test for bovine TB. It depends on inspectors seeing lesions consistent with TB infection. This is challenging in the context of a commercial abattoir which limits the degree to which an examination can be carried out and within NI where most infected cattle are in the earlier stages of infection and therefore less likely to have obvious lesions present at slaughter. Nonetheless abattoir surveillance does detect some infection within NI. Typically around 8% of all animals that are TB culture positive are detected this way each year.