Antibody Titers

You might have heard your veterinarian say, "Let's run a titer on him," when referring to your horse and whether he's protected against disease, or to figure out what might be causing particular clinical signs. What exactly does "titer" mean?

In immunological terms, titer refers to the concentration of specific antibodies in a blood serum sample. The titer is determined by serially (repeatedly) diluting the serum and assaying (quantitatively analyzing) each dilution for the activity (for example, how many virus-neutralizing antibodies are present). The last dilution of a sample that responds in the assay determines the titer.

The greater the concentration of the specific antibody you're looking for in the serum sample, the higher the titer.

This information has several important uses. In clinical practice it can be used to determine if an individual has been exposed to an infectious agent. Prior to exposure the antibody titer would be very low or undetectable. Following exposure the immune system produces antibodies, resulting in an increase in the titer. Rising antibody titers, as determined using paired sera samples collected days or weeks apart, provide evidence for exposure to the infectious agent. This is particularly useful when it is not possible to isolate the agent for identification, as is the case for Potomac horse fever (you cannot isolate Neorickettsia risticii, the bacterium that causes Potomac horse fever).

Very high antibody titers are also useful in the diagnosis of purpura hemorrhagica (an immunologically mediated condition characterized by swelling of the limbs and widespread skin hemorrhages, varying in severity from a mild transient reaction to a severe fatal condition) and metastatic abscesses caused by Streptococcus equi, the bacterium that causes strangles infection.

Another use of titer is to determine vaccine efficacy. Protection from infection often requires that a certain antibody titer be obtained following vaccination. These levels are determined experimentally. This information is then used to formulate vaccines so that they stimulate the necessary antibody response and achieve the desired titer. Since antibodies slowly disappear from the circulation over time, re-vaccination is necessary to boost the titer back to these protective levels.

The exception is strangles, for which vaccination is contraindicated in horses with high antibody titers.

Should we be vaccinating horses against other diseases without first determining their titers?

While this might seem more efficient, the time and cost associated with determining titers prior to vaccinating makes this approach impractical at this time. Further, the time-dependent decay in antibody titers is fairly predictable, meaning that regular revaccination can be recommended without having to determine titers.

About the Author

David W. Horohov, PhD

David W. Horohov, PhD, is a professor and the William Robert Mills Chair in Equine Immunology in the Department of Veterinary Science at the University of Kentucky's Gluck Equine Research Center. His research program focuses on identifying and characterizing cytokines ("messenger molecules" by which cells of the immune system signal and instruct one another) and their role in protective and pathologic (disease-causing) immune responses. He's particularly interested in the effect of age on immune responses in the horse.

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