We have different approaches to rabies in dogs and cats versus humans. The ultimate goal is still the same: preventing this almost invariably fatal infection. However, between humans and animals there are differences in who we target for vaccination, frequency of vaccination, utility of rabies antibody titres, and how we respond to potential rabies exposures, to name a few.
There are good discussion points for all of those, but today I’ll focus on how we deal with rabies antibody titres.
What is a rabies titre?
A rabies titre is a test that measures the amount of anti-rabies antibodies in a person or an animal’s blood. In general, antibodies indicate whether an individual’s immune system has responded to either infection or vaccination; in the case of rabies, because infection is almost always fatal, a titre usually indicates response to a vaccine. The higher the titre, the more antibodies are present, which usually means a stronger response. We can periodically recheck an individual’s titre to see how long those antibodies persist over time.
However, interpreting the test results can be a challenge.
While we have reasonable cutoffs for what titre we think indicates a good response to a rabies vaccine, we don’t truly know what is a “protective titre” – because that would involve testing titres and then seeing which individuals get rabies after being exposed. That’s not the type of research that’s ever going to be done in people (and rarely even in animals, due to risk, ethics and expense, among other things).
Rabies titre monitoring in people
Traditionally, it has been recommended that individuals at higher risk of rabies exposure (like veterinarians and animal control personnel) have their rabies titre checked every 2 years, and that they get a booster vaccine if their titre is below the recommended cut-off (although compliance with that is pretty low). However, in 2022 the US CDC’s Advisory Committee on Immunization Practices (APIC) updated their recommendations for the prevention of human rabies. Under the new guidelines, veterinarians fall under category 3 “people who interact with, or are at higher risk to interact, with mammals other than bats that could be rabid, for a period longer than three years after they receive PrEP” (PrEP stand for pre-exposure prophylaxis, which simply means pre-exposure vaccination). The recommendation for individuals in this group is to either have their titre checked 1-3 years after their initial 2-dose vaccination series (with a booster if the titre is low) or get a one-time booster 3 weeks to 3 years after their initial vaccination series. Routine repeat testing every two years is no longer recommended, presumably because they’ve found over time that the vast majority of people who respond to the vaccine initially will have a very good titre for a long time (because human rabies vaccines are very good vaccines).
Rabies titre testing in dogs and cats
We approach titre testing in pets differently and more cautiously than in humans. While not cheap, we can measure rabies titres in dogs and cats and we have a cutoff for what is considered “acceptable” in terms of demonstrating response to previous vaccination. However, as discussed above, an acceptable titre is not a guarantee of protection against infection if an animal is exposed to rabies. (Also, while there are some excellent rabies vaccines available for pets, there are a lot more rabies vaccine products for pets than there are for people, and they are not all of equal quality.)
The best way to ensure pets are protected is to use a high-quality vaccine, and vaccinate them according to the manufacturer’s instructions, because research has been done to demonstrate that this will actually protect the pet from (experimental) infection with rabies.
Many jurisdictions where rabies is being controlled require pets (or at least dogs) to be currently vaccinated against rabies, which typically means vaccination every 1-3 years depending on the vaccine product used. Titres are not typically considered equivalent to vaccine status, and usually have no influence on what is done in response to rabies exposure in a pet (e.g. if a dog is overdue for its rabies vaccine, it doesn’t matter what its rabies titre is, it is managed the same as any other exposed overdue pet, which often means a long quarantine period (months) at a minimum).
Why do we continually revaccinated dogs/cats against rabies, instead of periodically measuring rabies titres like we do in people?
Part of the reason for this seemingly very cautious approach to protecting pets from rabies prior to exposure is that we don’t have proven post-exposure prophylaxis for dogs and cats like we do in people. If a person reports exposure to rabies, we know how to administer post-exposure prophylaxis such that we can provide virtually 100% protection from infection, whether the person is vaccinated (easier) or unvaccinated (harder, but still extremely effective). We expect that vaccinated individuals would be able to report most potential exposures and get post-exposure prophylaxis. Pre-exposure vaccination helps reduce the risk associated with any unknown or inadequately addressed rabies exposures that may occur because these individuals are in higher-risk positions, but even a person with an adequate rabies titre should still receive post-exposure prophylaxis to be confident that infections will be prevented.
For dogs and cats, revaccination after an exposure is still highly recommended / required, whether the animal is currently vaccinated or not, just as post-exposure vaccination is recommended in exposed people. We have more confidence in being able to protect a vaccinated vs unvaccinated animal this way, so vaccinated animals usually have lower or sometimes no requirements for quarantine as long as they’re revaccinated in time. Unfortunately though, we have no proven post-exposure treatment in pets, so we can’t be as confident in the protection provided as we can be in people.
We also have less confidence in identifying rabies exposures in animals (because pets can get into all sorts of trouble when we’re not looking!). Therefore, we’re therefore more reliant on vaccination as the main line of defense against rabies in pets. If we had proven post-exposure regimens for dogs and cats, and we had a way to be very confident that we’d identify all exposures, using an identical approach to that for humans would make sense. Since we don’t, and since rabies is almost invariably fatal (and since one infected pet can potentially expose lots of other animals and people), the default is to err on the side of vaccination.
Rabies vaccines are relatively cheap (much cheaper than titres), very safe and effective. The downsides associated with regular revaccination are outweighed by the potential downsides of taking a titre-based approach when we have little data, no proven post-exposure regimens and limited confidence in identifying all potential exposures in pets.