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Immune response to rabies vaccination

Feb 20, 2015

Moore MC, Davis RD, et al.  Comparison of anamnestic response to rabies vaccination in dogs and cats with current and out-of-date vaccination status.  J Am Vet Med Assoc. 2015;246(2):205-11.

In the United States, the domestic animal most likely to exposed to rabies from wildlife is the cat. There are increasingly frequent stories in news media regarding pets with lapsed rabies vaccinations or no rabies vaccination having been actually or potentially exposed to rabies from contact with sick wildlife or other domestic animals. In many of these cases the wild or domestic animal that may have exposed the pet to rabies is unavailable for testing.  Public health officials and veterinarians in companion animal practice generally use the Compendium of Animal Rabies Prevention and Control for guidance in such situations.

cat_kitten_red_cat_226409The guidelines in the most recent version of the Compendium of Animal Rabies Prevention and Control are clear cut in the case of both dogs and cats with a current rabies vaccination, and those who have never been vaccinated against rabies, who are exposed to an animal that is actually or potentially rabid.  These guidelines call for re-vaccination of an exposed and currently vaccinated dog or cat and then observation for 45 days, generally under the owner's supervision with no contact restrictions.  Unvaccinated and exposed cats and dogs must be euthanized or confined for 6 months in a specialized facility; the latter is often a prohibitively expensive option for many owners. When a cat or dog previously vaccinated against rabies is exposed after their most recent vaccination is out-of-date, the situation becomes more complicated and the guidelines become less clear; these animals must be evaluated for risk of development of rabies on a case-by-case basis.  As rabies is considered universally fatal and a serious public health risk, exposed cats and dogs with lapsed rabies vaccinations are often considered unvaccinated and handled accordingly.

The goal of this cross-sectional study of 33 cats and 74 dogs from 13 states was to compare anamnestic antibody responses in animals with current rabies vaccination status versus out-of-date rabies vaccination status.  Animals in the study, all of whom were considered otherwise healthy, had either been exposed to rabies and brought to a veterinarian for proactive serological monitoring or were unexposed and presented to a veterinarian for a booster rabies vaccination.  Serum samples were collected from all patients on the day of initial evaluation (day 0) and again 5 to 15 days later.  The paired serum samples were analyzed at the Rabies Diagnostic Laboratory at Kansas State University (KSU) with a rapid fluorescent focus inhibition test (a serum neutralization test).  All patients also received a rabies vaccine of the attending veterinarian's choice on day 0.

Two cats and 10 dogs were enrolled in the first phase of the study. These animals had been exposed to rabies and in all these cases the attending veterinarian or owner had contacted the KSU Rabies Diagnostic Laboratory for help in assessing the immune status of the animal.  The 2 cats had previously received 3 year rabies vaccinations that were out-of-date.  One cat had been bitten by a raccoon; this cat's rabies vaccination was 2.7 months out-of-date. The other cat had been exposed to a bat and its rabies vaccination was 8.9 months out-of-date.  Both cats were quarantined for 6 months after rabies exposure and neither developed signs of rabies; both survived.  A rabies neutralizing antibody titer of > 0.5 IU/mL is considered by the World Health Organization to be protective.  The cat who sustained the raccoon bite had a baseline titer (day 0) of 0.3 IU/mL, while the one exposed to a rabid bat had a baseline titer of 12 IU/mL.  After booster vaccination, both cats had titers of 12 IU/mL.

The researchers found that the anamnestic responses of cats and dogs with an out-of-date rabies vaccination were similar to the responses in animals whose rabies vaccination was current. In 24 cats with an out-of-date rabies vaccination, only one had a baseline rabies antibody titer of < 0.5 IU/mL; all achieved a titer of > 0.5 IU/mL after booster vaccination, with only 3/24 attaining a titer less than 9 IU/mL.  Overdue time period for these cats' rabies vaccinations varied from a few days to 46.1 months.  The number of previous rabies vaccines received by these cats was not significant.  Three of the 24 cats had had only one previous rabies vaccination, while 21/24 had two or more earlier rabies vaccinations. Because of the small number of cats enrolled in the study and the fact that almost all the cats had a titer of > 12 IU/mL 5 to 15 days after booster vaccination, a proportional hazards analysis designed to analyze the response to booster vaccination in cats with current versus out-of-date rabies vaccination status was not performed, although it was for the data on the 74 dogs enrolled in the study. Based on the results of this study, the investigators recommend that any previously vaccinated cat or dog actually or potentially exposed to rabies be managed in the same way, regardless of whether its rabies vaccination is current or out-of-date.  Cats and dogs with lapsed rabies vaccinations should, like those with current rabies vaccinations, receive an immediate booster vaccination followed by a 45-day observation period, rather than euthanasia or 6 months' quarantine in a specialized facility.  As an additional precaution, rabies antibody titers could be measured prior to and again 5 to 7 days post-booster vaccination to identify an anamnestic response. [PJS]

See also:
Mansfield KL, Burr PD, et al.  Factors affecting the serological response of dogs and cats to rabies vaccination.  Vet Rec 2004;154(14):423-6.

 

Brown CM, Conti L, et al.  Compendium of animal rabies prevention and control, 2011.  J Am Vet Med Assoc 2011;239:609-17.


rabies vaccination antibody titer

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