Thanks to extensive studies over many years, it is now known that the immune system is exceedingly complex. Its purpose is simply to protect the body against ‘foreign’ substances or invaders and, as such, is of major importance in procedures such as blood transfusion and tissue and organ transplantation, where compatibility is all important with its effects often having to be suppressed, especially with the latter. The major role of the immune system regarding our dogs (and usually ourselves too) is combating infectious agents, firstly by limiting any damage they are causing and then killing the invaders off completely if possible. One particular area that worries many owners is immunity resulting from vaccination, especially to parvovirus and sometimes distemper, and the rest of this article has been written with those viruses in mind.
To understand this we need to look briefly at how the immune system works. Basically, and rather simply, we have two fundamental components both associated with the lymphocytes, a type of white blood cells; firstly cell-mediated immunity produced by T-lymphocytes which will attack infected cells specifically, and humoral immunity produced by B-lymphocytes which secrete antibodies, specific proteins which bind to and hopefully kill off infecting agents. In fact with many viral infections cell mediated immunity is actually more important than humoral immunity but we tend to look at the latter simply because antibodies are much easier to detect and quantify.
Following natural infection (this may take several days depending on incubation period) or vaccination, an antibody called IgM (M for macro as it is a large molecule) appears and reaches peak levels, or titre, quite quickly before starting to decline, disappearing usually after a few weeks or a month or two. About the time of the IgM peak, IgG, or gammaglobulin, is seen at a low level in the blood. Like IgM, the IgG titre increases to a maximum and eventually declines too.
However IgG does not disappear completely and settles at a long-lasting resting level in the blood stream. Its presence in the blood thus denotes previous natural infection or vaccination and consequently immunity, especially with viruses like the two mentioned above.
There are several types of tests for measuring antibodies but details of the actual methods are of no relevance here. What does concern us is the interpretation of the results. For example, some tests may involve double-diluting the test serum starting at a low dilution, e.g. from 1:8, and the titre given as the highest dilution giving a positive reaction, so if 1:128 is the highest the titre is simply given as 128. Any sample giving a titre of less than 8 is deemed negative. Others tests may work on strength of reaction, expressing the antibody levels in units with samples giving less than ten units being considered negative. The cut-off point for all such tests errs on the side of caution to take any non-specific factors into account. Fortunately most titres are above the cut-off point and any ‘grey area’, so problems of interpretation are few.
The question owners may ask, is ‘does the antibody titre really matter?’ It may surprise many to know that the answer is actually ‘No’! A dog with a titre of, let’s say, 200units is not twice as immune as one with only 100units and so forth. A ‘Yes/No’ answer is all that is required! Some tests may simply provide such an answer. Even if antibody levels decline to undetectable levels, the immune memory is such that re-infection is most unlikely to cause serious problems. I do not know if re-infection, or natural infection after vaccination, with viruses like parvovirus or distemper has been documented but we have an example in humans. Re-infections with rubella (German measles) virus have been reported. However symptoms have been invariably mild or totally absent and even when a pregnant woman has been re-infected there was no threat to the unborn child, unlike primary infection in early pregnancy when there is a high risk of foetal abnormality.