What Are Titers? Can They Show What Vaccinations a Cat Needs?


I love veterinary conventions because they combine two of my favorite things: learning and travel. I recently returned from the Pacific Veterinary Conference in Long Beach, California. I attended many interesting lectures, but one experience really stood out from the others. I submitted my blood for a rabies titer.

First, rabies is an occupational risk for veterinarians. Almost all of us have been vaccinated for it. My most recent rabies vaccine was in 1997. The purpose of running the titer was to determine whether I should get a booster shot.

As I filed through the drab room in the convention center basement set up for drawing vets’ blood, several thoughts crossed my mind. The first was that vets definitely don’t react well to being on the wrong end of the needle. Pass-out beds, candy, juice, and comforting music were provided — and got heavy use. Many of my fellow vets were various hues of green. My rationalization for my own queasiness, namely that I had run into my all-time No. 1 vet school drinking buddy the night before, may or may not have been true for the others.

But I had a more important thought. Here was a group of veterinarians who had signed up for titers in order to determine whether we needed to be vaccinated. Yet almost none of us recommend titers in our patients, even though, relative to us, our feline patients have a much higher likelihood of serious adverse events (such as cancer) following vaccines. Are we a bunch of hypocrites?

The short answer is yes and no, but mostly no.

What are titers? Titers are quantitative measurements of antibodies in the bloodstream. They are supposed to offer an approximation of an individual’s immunity to a disease. But to truly understand titers one must understand a bit about how the immune system works. What follows is a massive simplification, but it should suffice for this discussion.

The purpose of the immune system is to “reject” (which usually means kill) “foreign” things. Foreign is defined as anything that does not belong in the blood or tissues of the body, and it includes viruses, bacteria, fungi, and non-living debris (such as a splinter or foxtail) that makes its way into the body.

When the immune system detects something foreign in the body, it reacts in part by producing antibodies. Antibody production can take a significant period of time — up to weeks — to really get going. The antibodies then bind to the foreign thing, which is a signal to cells in the immune system to digest, kill, or otherwise attempt to eliminate the foreign thing.

Vaccination is useful because it allows the body to skip the the above-mentioned lag time in producing antibodies and reacting to foreign things. If a susceptible cat is exposed to panleukopenia, she will probably get sick and die before her immune system can mount a sufficient reaction. However, if she has been adequately vaccinated (which would make her non-susceptible) her body will be primed to fight the virus immediately and no disease will occur.

So, the point of titers is to measure the immune system’s preparedness. But all they measure is antibody levels circulating in the blood. They do not measure the other aspects of preparedness, such as the intervention of immune system cells to kill pathogens that have been tagged with antibodies or the body’s ability to rapidly produce additional antibodies in response to a pathogen.

Titers are therefore at best an approximation of immune system preparedness. That said, it turns out that the approximation is more accurate for some diseases than for others.

The all-time most dreaded disease in cats is panleukopenia. Although it is sometimes called feline distemper, it is unrelated to the canine distemper virus (although it is closely related to canine parvovirus). Recent research suggests that titers offer a good approximation of immunity to panleukopenia. Cats with high titers against the virus are not likely to become sick if exposed to it, and they probably don’t need to receive booster vaccinations against the disease.

Panleukopenia vaccines usually come bundled into a single shot (the so-called FVRCP) that also protects against feline calicivirus and feline herpesvirus. Titers are commercially available for all three of these diseases. Unfortunately, titers do not appear to offer a good approximation of true immunity against calicivirus and herpesvirus.

Therefore, although commercially available titers appear to be appropriate for measuring whether an adult cat needs to be vaccinated against panleukopenia, they do not appear to be appropriate for making this determination for the other two pathogens in the FVRCP.

Sadly, at this time titers also appear to be of little use in determining whether cats require other vaccines, such as those against rabies, coronavirus (FIP), or feline leukemia virus. This is partly because titers for these diseases are not readily available in a commercial setting, and partly because less is understood about whether titers offer an accurate approximation of immunity for these diseases. Bear in mind that rabies vaccines in cats are often mandated by law, and that most jurisdictions will not consider titers in lieu of vaccination.

I hope that better ways to measure immunity will be forthcoming in the future, but for now, titers appear to offer little benefit to cat owners for all but panleukopenia. (Dogs are a somewhat different story.)

My feline vaccination recommendation remains what it has always been. Cat owners should find a good vet who will take time to discuss the risks and benefits of each vaccine in light of a cat’s age, lifestyle, geographic location, and individual life circumstances. Most vaccines should be given no more often than every three years.

And how about me? What am I going to do with my rabies titer results? First, bear in mind that I won’t get them for another month or longer. Rabies titers have very long turnaround times, and I don’t yet know what mine is. However, if it’s low I will get a booster vaccine (which will not make me happy — those things are phenomenally expensive). If it is high, I will not.

However, even if my titer were sky high I would not rely on it as a completely accurate picture of immunity to rabies. If I am ever bitten by — or even tangentially exposed to — a rabid animal I intend to undergo a complete post-exposure rabies vaccine series. Given that rabies is the most deadly transmissible disease known in the world, no other option would be reasonable.

You can read more super up-to-date information on titers, written by one of the world’s most respected experts, here.

Other stories by Dr. Eric Barchas:

Got a question for Dr. Barchas? Ask our vet in the comments below and your topic might be featured in an upcoming column. (Note that if you have an emergency situation, please see your own vet immediately!)

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