Rabies booster study shows pets overdue for rabies vaccination are still protected

A research team at Kansas State University recently demonstrated that pets with out-of-date rabies vaccinations are still protected from the rabies virus—provided they receive a booster immediately after exposure.   This study, appearing in the Journal of the American Veterinary Medical Association is the first of its kind to evaluate the rabies antibody levels in pets that are not up-to-date with their rabies boosters and may have significant implications for how such pets are handled after a potential exposure.

When a pet is bitten by a rabid animal, it’s an emotional and distressing time for owner and pet alike, often because owners worry that their pet will have to be euthanized. In the US, for suspected rabies exposures, animals that have never been vaccinated must be euthanized or quarantined long-term at the owner’s expense. Pets that are current with their vaccinations fare much better after a potential exposure: a rabies booster can be administered, and the pet can be released for home-monitoring. 

However, for animals that have received at least one rabies vaccination but are past due for their rabies booster, the options are not as straightforward.  Public health officials typically take a conservative stance and consider non-current animals to be “unvaccinated”, mandating that the animal be euthanized or quarantined after an exposure.  Yet, because these out-of-date animals did receive a least one prior vaccination, a strong possibility still exists that protective antibodies may be circulating in their bloodstreams—raising questions over the necessity of such extreme measures for under-vaccinated pets.

In an effort to supply the missing clinical data to resolve this question, Moore and his colleagues tested the blood antibody levels in 74 dogs and 33 cats that were given a rabies booster either in response to a potential rabies exposure or for booster administration.  Some of the animals were up-to-date on their vaccinations, while others were 1-4 years past-due for a booster.  Using a rapid fluorescent focus inhibition test to monitor antibody levels, the researchers found that non-current pets quickly renewed their antibody production upon administration of the booster, generating levels of protection equal to (or even higher than) antibody levels generated by the up-to-date pets.  Additionally, the rate of antibody production and median increase in level were often higher in the non-current pets than in the up-to-date population, indicating a vigorous immune response regardless of booster status. All pets achieved a recommended antibody level (≥ 0.5 IU/mL) after boosting. Irrespective of their vaccination history, cats responded rapidly to the booster vaccination.

These initial findings are intriguing and clearly show that even when owners do not adhere to the recommended vaccine schedule, their pet still may be protected from exposure to the rabies virus with an immediate booster, as long as the pet received a primary immunization in the past.  After the initial vaccination, a pet’s immune system will generate neutralizing antibodies, which over time, can fall below the accepted, recommended level. 

However, a booster can rapidly recharge the pet’s immune system, even if antibody levels have declined, indicating that active immunity against rabies remains for a lengthier period of time than that indicated by the vaccine’s expiration date. 

This discovery clears the way for veterinarians and public health officials to re-evaluate and clarify the current post-exposure recommendations for treating pets with out-of-date vaccinations.  The study involved a relatively small number of study animals, and just 12 known or suspected to have been exposed to rabid animals. However, the study authors recommend extension of the same treatment for up-to-date and non-current vaccinated animals after rabies exposure, with confirmatory blood tests to show an adequate response to the booster if reassurance is necessary.   It should be noted that rabies vaccinations are usually required by law, and so unless recommendations change they should still be adhered to.

Contributed by Laura Baker, a GARC volunteer.  The paper is entitled “Comparison of anamnestic responses to rabies vaccination in dogs and cats with current and out-of-date vaccination status”  by Moore et al. and was published in JAVMA, Vol 246, No. 2, January 15, 2015.