Serological tests, such as ELISA, rely on the fact that viruses and other pathogens are recognised as 'foreign' by the immune system and stimulate the production of specific antibodies. The antibodies not only help the immune system to deal with the first infection, but they remain at a low level as sentinels to identify future attempts by the same pathogen. Serological testing can not only identify an acute infection and track its progress, but can also show whether an individual animal or herd has been exposed to infection in the past.

Although serology is undoubtedly a useful diagnostic tool, it does have a number of drawbacks. It takes time for the body to generate an immune response to infection, and so serology may be of limited use in the early stages of infection. In some infections, the immune response is faster and longer lasting than others; in some, there may be very little response. This kind of knowledge is essential when ordering tests and interpreting results.

Vaccination works by stimulating the production of antibodies to a specific pathogen or pathogens, and thus raising the level of protection against infection. In many cases, serological tests cannot differentiate between vaccinated animals and those that have been exposed to natural infection. So when new arrivals are tested, it may be impossible to know for sure if they have been infected or vaccinated. The exceptions are those diseases where marker vaccines are available, for example, for IBR in cattle.

PCR-based diagnostic tests detect the presence of the pathogen itself, or rather its nucleic acid, and so can detect infection at a very early stage before antibodies are produced. As long as there are no virus particles left, vaccinated animals will test negative with PCR. However, so will those animals that have been exposed to infection in the past, but which are no longer viraemic.

Some infections produce only a short period of viraemia, and so PCR-based tests have to be applied at the right phase of infection to be useful.

It is clear that both serological tests and those based on molecular technology have a role to play in food animal production. ELISA - the most practical serological tool - is an excellent method for herd management over time; real-time PCR can be the right tool for analysis in the acute phase for individual animals because of its higher sensitivity. The key is knowing the advantages and drawbacks of each approach and thus the most appropriate technique to apply in each situation. In some cases, a combination of both may provide the most accurate and cost-effective means of protecting and improving animal health.

New tests and techniques are continually being developed and validated, so it is always worth seeking the advice or confirmation from your testing laboratory. A good laboratory will be happy to provide advice and guidance on testing for specific diseases, and help with the interpretation of results.

For more information about swine diagnostics, click here or connect to the Thermo Fisher Scientific Swine Resource Center.