Often we know when we have a sick horse that our veterinarians talk about the antibody titers, but sometimes we really don't know what that means. In this information from the University of Kentucky College of Agriculture, David Horohov, PhD, discusses what the term can mean, and when it can be used to give us important information for our horses' health.
One of the most often used, and least understood, immunological terms is antibody titer. But what does titer mean, and how can this information be used in assessing the immune status of a horse?
A titer is defined as the concentration of specific antibodies in the blood that recognize a particular agent, such as equine influenza virus. The titer is determined by serially diluting the serum fraction of blood and assaying (testing) each dilution for the antibody of interest (for example, equine influenza virus antibodies). The last dilution of a serum sample that responds in the assay determines the titer. The greater the concentration of the specific antibody in the serum sample, the higher the titer. For example, a titer for an influenza hemagglutination inhibition assay of 1:10 would be very low; a titer of 1:320 would be high. A low or undetectable titer indicates very little antibody present in the serum.
This information has several important uses, including the following:
Animal exposure to pathogen: Prior to exposure, the antibody titer is very low or undetectable. Following exposure, the immune system produces antibodies, resulting in an increase in the titer. A rising antibody titer in paired sera samples collected two to four weeks apart provides evidence for exposure to the agent. This evidence is particularly useful when it is not possible to identify the agent, such as Neorickettsia risticii (Potomac horse fever).
Disease diagnosis: Very high antibody titers can help diagnose purpura hemorrhagica and internal abscesses (“bastard strangles”) caused by Streptococcus equi.
Vaccine efficacy: In the case of some infectious diseases (for example, equine influenza virus), protection from infection often requires that a certain antibody titer (determined experimentally) be obtained following vaccination. This information is then used to formulate vaccines so that they stimulate the necessary antibody response and achieve the desired titer. Since antibody titers decay over time, re-vaccination is necessary to boost the titer back to these protective levels.
Can titer levels be used to determine when it is necessary to vaccinate horses? While this might seem to be more efficient, several factors preclude the use of such an approach. The major impediment is that for many infectious diseases, the protective titer is not known precisely enough to make a vaccination decision. With rabies, animals with very low or undetectable titers to the virus may actually be protected from infection, but the assay lacks sufficient sensitivity to register the immunity. In contrast, EHV-1 has no known level of antibody that accurately predicts protection.
This lack of correlation between antibody titer and protection is likely due to the fact that cell-mediated immune responses are more important in preventing some infectious diseases. Currently it is not possible to determine a numerical measure of protection provided by cell-mediated immunity.
Even for those diseases in which protective antibody titers can be determined, the time and costs associated with performing these assays currently makes this approach impractical. Further, the time-dependent decay in antibody titers following vaccination is fairly predictable, meaning that regular revaccination schedules can be developed without having to determine titers. Nevertheless, future developments in veterinary immunology may lead to an increased use of titer information in the development of vaccine recommendations.