What is measured by the ELISA tests?
What is measured by the ELISA tests?
Answered by: Enric Mateu I Published on: July 18, 2016
Most animals infected with the porcine reproductive and respiratory syndrome virus (PRRS) develop antibodies in a 7 to 14 days range period from the start of infection; A small percentage - usually less than 5% - will develop those before day 21st.
These initial antibodies are directed against the N protein of the virus, which is one of the most antigenic, but those antibodies do not have direct relationship with the protection since they are devoid of virus neutralization capacity. Most commercial ELISAs use this N protein for diagnosis as it allows a quite early detection. On the other hand, N protein shares epitopes that are common to type-1 and type-2 PRRS virus, allowing the ELISA based on this protein to be universal. Some commercial kits have added other antigens (eg GP5) in order to gain detection spectrum. Current ELISAs cannot differentiate between infected and vaccinated animals.
Most PRRS ELISAs are based on a format known as indirect ELISA, although there are other types on the market. The indirect ELISA is based on attaching the virus antigen to the microtiter plates used to perform the test, and adding afterwards the serum sample to be tested. The reaction is usually revealed by the addition of an antibody to porcine IgGs that it is bound to an enzyme. When a suitable substrate is added, if the antigen-antibody reaction has occurred and the enzyme is present, it will change colour. In this ELISA type, the colour intensity that the substrate develops is proportional to the amount of antibodies, which allows a semi-titration of the serum, that it is usually expressed as the amount of colour of a particular serum compared to that used as a standardized positive.
This is what is referred as the S / P ratio. The S / P value of a particular serum varies depending on the brand of the kit being used and, therefore, value comparison cannot be made directly. It is often mentioned that the S / P ratio is higher in infected animals than in those vaccinated. Although this is generally true, it cannot be assumed in an individual basis since there are field strains that induce low S / P ratios but cause disease. In other words, the prediction of virulence cannot be made on the basis of S/ P.
When we use the ELISA in a previously PRRS virus free population that it is not being vaccinated, seroconversion detection is excellent and allows an adequate diagnosis. In these cases, S / P ratios are usually high and the change from negative to positive (healthy to infected) is very evident. On the contrary, in animals that are already being vaccinated this seroconversion is not so clear. In fact, a significant percentage of multivaccinated sows did not increase their S / P ratio after a booster dose of the vaccine. This may be due to various circumstances.
One explanation could be the saturation of the immune response, that is, there comes a point where no higher levels of antibodies will be produced irrespectively of the number of booster doses applied. Another reason is that the immunodominant antigens can change as the response evolves. Finally, it cannot be ruled out that some animals become unresponsive to N protein when they are vaccinated many times (which does not mean that they lose protective immunity).
In summary, ELISA results should generally only be used to determine if an animal was in contact with the virus - vaccine or field virus - and not to predict the virulence of the strain that infected them or to determine the degree of immunity of an animal or herd.