July 01, 2016

 

 Exploring a role for titers in rabies vaccination

Posted June 15, 2016

Most states and many municipalities have laws requiring rabies vaccination for dogs and often cats, and these laws do not allow titer testing for rabies antibodies as an alternative to booster vaccination.

Some laws specify revaccination intervals, such as three years, while others defer to vaccine labeling or the Compendium of Animal Rabies Prevention and Control. Pet owners increasingly have shown interest in titer testing as an alternative to routine booster vaccination, nevertheless, because of concerns about the potential adverse effects of vaccines.

In addition to legal limitations, titer testing has been much more expensive than administering a booster. In August 2015, the Rabies Laboratory at Kansas State University completed modifications to a titer test for rabies antibodies such that the laboratory could start offering a micro test at a reduced cost.

“It seems to make sense to us that there might be room for titer testing for rabies” as a tool for revaccination decisions, said Rolan Davis, a reference diagnostician at the laboratory.

Susan Moore, PhD, laboratory director, said, “Pet disease status, medical treatments, and history of adverse reactions to vaccinations are examples of situations for which local or state authorities have accepted rabies titers as proof of continued immunity to rabies.”

The K-State Rabies Laboratory reduced the price of a rapid fluorescent focus inhibition test for rabies antibodies partly through economy of scale, switching from 96-well plates to 384-well plates. Davis said, “We’ve really changed nothing about what we’re doing. It’s just downsizing the traditional rabies serum neutralization test.”

The laboratory also switched from paper to online submission forms for the micro test to reduce data entry. Plus, the results of the micro test are not an exact titer but simply indicate whether the antibody concentration in a blood sample is higher than 0.5 IU/mL—which is predicted to be protective on the basis of published challenge studies in dogs and cats.

In April, the K-State Rabies Laboratory did 194 of the micro tests. The laboratory also continues to offer other rabies titer tests, including a fluorescent antibody virus neutralization test for dogs and cats going to rabies-free regions. 

The Rabies Laboratory at Kansas State University offers titer tests for rabies antibodies. Sami Pralle, research assistant, uses an automated, multichannel pipette to add stain to slides. (Photo by Tommy Theis/K-State Photo Services) 

According to the Canine Vaccination Guidelines from the American Animal Hospital Association, titer tests are useful for monitoring immunity to several viruses, including rabies virus. According to the Feline Vaccination Advisory Panel Report from the American Association of Feline Practitioners, however, “a rabies titer is only an indication of serological response to vaccination. Rabies titers are not recognized as an index of immunity.”

Dr. Richard Ford, an author of the AAHA and AAFP vaccination guidelines, emphasized that a rabies titer is not a legal index of immunity in lieu of revaccination. He said, “While immunologically speaking a titer quite likely does correspond with protective immunity, today veterinarians in practice do not have legal discretion to substitute a titer for vaccination.”

Dr. Ronald Schultz and Dr. Laurie Larson of the University of Wisconsin-Madison School of Veterinary Medicine have found that the presence of rabies antibodies is highly indicative of protective immunity on the basis of challenge studies they have performed in cooperation with Dr. W. Jean Dodds and Kris Christine of the Rabies Challenge Fund and Dr. Zhen Fu of the University of Georgia College of Veterinary Medicine.

In their rabies challenge studies, Drs. Schultz and Larson found that when exposed to the rabies virus, dogs maintain immunity for up to seven years after vaccination. Although rabies vaccines are killed virus vaccines, which generally don’t provide as long of a duration of immunity as modified-live virus vaccines do, Dr. Schultz described rabies virus as an “excellent antigen.” Some rabies vaccines also have adjuvants to increase efficacy and duration of immunity.

“We’ve used virtually every test available to measure rabies antibodies, and as long as the test was antibody-positive, the dogs challenged were protected from the rabies virus challenge for as long as seven years after vaccination,” he said.

Dr. Schultz wouldn’t necessarily push for a change in laws, though. He said, “I think the requirement for a three-year revaccination cycle with rabies hopefully will get more dogs immunized. One of the goals for rabies is herd immunity because you always reduce the likelihood of disease by having the greatest number of animals immune as possible.” 

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