Titers and Feline Vaccination Decisions
By Christie Keith

Basic information on the immune system and titer testing was given in my article on canine titers, and the information in this article is not complete without that background. Read that article here.

Because cats get some unusual diseases, vaccination decisions are more complex than in dogs. This process is further complicated because cats can develop vaccine-associated sarcomas, or VAS. Many feline practitioners seek to minimize vaccination in cats in order to reduce the risk of this aggressive form of cancer. Titer testing is one method that is being studied in the hope of developing a safer approach to feline vaccination.

Simple Feline Viral Diseases

The three simplest feline virues are feline panleukopenia (FPV), also known as “feline distemper” or “feline parvovirus,” calicivirus (FCV) and rhinotracheitis (feline herpes virus, or FHV).

Protection against these three diseases as the result of vaccination is typically excellent and extremely long lasting (probably lifelong), although it should be noted that cats who are immune to FCV and FHV from vaccination or natural infection can still be re-infected by the viruses; they simply will not become ill, or if they do, it’s very mild.

Vaccine researcher Michael R. Lappin, DVM, conducted an extensive study of the usefulness of titer testing for these three viruses, looking at cats raised in a laboratory and also at more than 200 pet cats.

The correlation between titer levels to these viruses and protection from challenge with the disease in this study ranged from a low of 68.5 percent to a high of 100 percent, and this is sometimes cited by those casting doubt on the usefulness of titer testing. Technically they’re right, but there’s a twist.

It’s not that cats with protective or high titers got sick when exposed to the disease. No, it’s that vaccinated cats with low or no titers did not get sick. (Cats in the unvaccinated control group did.) According to the published study, “Results suggest that for cats that have been vaccinated, detection of FHV-1-, FCV-, and FPV-specific antibodies is predictive of whether cats are susceptible to disease, regardless of vaccine type or vaccination interval. Because most client-owned cats had detectable serum antibodies suggestive of resistance to infection, use of arbitrary booster vaccination intervals is likely to lead to unnecessary vaccination of some cats.” (Lappin et al, “Use of serologic tests to predict resistance to Feline Herpesvirus 1, Feline Calcivirus, and Feline Parvovirus infection in cats,” Journal of the American Veterinary Medical Association, January 2002.)

A post-vaccination titer to these feline diseases is strongly correlated with protection from the diseases, a protection that does not appear to weaken with time, even when the titer drops to low levels.

Complicated Feline Viral Diseases

Once you move beyond those three diseases, things get more complicated. That’s because cats are susceptible to some oddball illnesses, including several that specifically affect and even target the feline immune system.

Feline leukemia (FeLV) is caused by a retrovirus, the same type of virus that causes HIV/AIDS in humans. In fact, when HIV first emerged as a human pathogen, it was FeLV researchers who were the earliest HIV “experts.” Most FeLV infected cats become ill and die over a period of several years. (Some kittens who test positive for FeLV are actually infected, but others simply carry their mother’s antibodies and will eventually clear them and have normal health.)

While FeLV is deadly to kittens, most adult cats have natural immunity to FeLV. Many vets don’t recommend this vaccine be given past kittenhood, and some don’t recommend it at all, especially for indoor kittens and cats. Titer testing for FeLV is typically done only as a means of diagnosing illness or testing for exposure.

Feline infectious peritonitis (FIP) is an extremely strange disease. FIP is caused by a coronavirus, but other factors are believed to be involved, as only a tiny percentage, between one and 5 percent, of infected cats ever become ill. Cats with high titers to FIP are more, not less, at risk from the disease, and previous FIP vaccination increases the severity of the disease in some cats. Since FIP is almost exclusively found in catteries and other crowded housing situations, household pets are not typically considered to be at risk. Excellent information on FIP, including an evaluation of the vaccine and FIP titers, is here.

Feline Immunodeficiency Virus (FIV), like HIV, eventually causes the failure of the immune system. There are a number of problems with the recently-marketed FIV vaccine, but one of the biggest is that if a cat tests positive, he or she might have FIV, or might simply have been vaccinated for FIV, and there is no way to tell the difference. Like FeLV, FIV is mostly spread by bite wounds, so indoor cats are at virtually no risk.

Titer testing for these three viruses is usually done to diagnose infection, not to check immune status, and in most cases the results of a titer test will not be useful in making a re-vaccination decision.

Other Feline Vaccines

There are a number of other diseases that affect cats: Chlamydiosis and bordetella bronchiseptica, caused by bacteria; giardia lamblia, a parasite that can cause gastrointestinal illness. Vaccines are available for all these pathogens, but they are not routinely recommended, are not very effective, and titer testing is not useful in evaluating immune status.

Cats also can get rabies. Unlike dogs, rabies vaccination is not always required for cats. Titer testing of cats for rabies immunity is typically done only for purposes of travel to foreign countries that require it.