Spurred by consumer concerns about over-vaccination of pets, authorities in Delaware and Minnesota separately are considering rules governing the frequency with which companion animals are immunized against rabies.

In Delaware, state Sen. Karen Peterson has introduced legislation that would subject veterinarians to disciplinary action if they vaccinate animals more often than is recommended by vaccine manufacturers.

In Minnesota, advocates for rules on rabies vaccination intervals are scheduled to address the state Veterinary Medical Board at its next meeting on Sept. 27.

In its present form, Delaware’s SB 147 covers all veterinary vaccinations. However, Peterson told the VIN News Service in an interview last week that she is likely to narrow its scope to rabies vaccination, as that is the only veterinary inoculation mandated by law.

Proposals in both states have caused a stir among practitioners, many of whom believe the issue is a medical matter to be decided between doctors and their clients.

“The veterinarian is a professional,” said Dr. Heather Hirst, Delaware State Veterinarian. “A veterinarian is a source of medical expertise and they need to be allowed to use clinical judgment to make decisions about how they use vaccines.”

Hirst and the Delaware Veterinary Medical Association (DVMA) oppose the legislation. “Manufacturer recommendations are just that — recommendations,” said DVMA Executive Director Lynn Appel. “We’re not in favor of taking the ability to practice veterinary medicine out of the hands of the practitioner.”

Peterson acknowledged that such opposition “could be significant in the Legislature.” She said she would take the time while lawmakers are in recess to retool the bill so it garners more support while maintaining the principle that over-vaccinating pets would violate state professional licensing code. The legislative session resumes in January.

Concerns in Delaware and Minnesota about over-vaccination center around the practice by some small-animal veterinarians of administering every one or two years rabies vaccines that have been proven to be effective for three years.

Peterson said her attention was drawn to the issue by Diane Meier, whose 5-year-old apparently healthy beagle, Molly, died in January shortly after a routine checkup at Savannah Animal Hospital in Lewes, Del., where the dog received shots for rabies and distemper. Shortly afterward, Molly became lethargic, unresponsive and glassy-eyed, according to Meier.

“I took Molly back to the vet twice and was told that they could not tell from the exam, blood work or X-rays what was wrong with her,” Meier said. “I asked if it could be the rabies shot and was told ‘no.’” Within about a week, she said, Molly died.

Later, after examining Molly’s medical record with a veterinarian in a different clinic, Meier said she became aware that Molly had been given a three-year rabies vaccine — IMRAB 3 TF made by Merial — each year for the past two years.

“I was shocked,” Meier said.

Molly was vaccinated at Savannah for rabies a total of four times between 2008 and 2011, Meier said; she suspects but doesn’t know for certain that three-year vaccines were used each time.

Although the cause of death was not established, Meier believes overexposure to rabies inoculant is to blame. “You can say we don’t know, but when you put two and two together, it’s four,” she said.

Meier said she’s not mistrustful of vaccines as a rule. “These folks who don’t get their children vaccinated, I find that totally appalling,” she said. “I am totally in favor of any immunization that can prevent disease and protect the health of people and animals.”

What upsets Meier is that her veterinarian didn’t volunteer information about the type of vaccine being used and the reason for giving boosters more frequently than recommended on the label. “I’m not an uneducated person,” she said. “I am very risk-averse and I am very interested in the health of my animals. But I trusted (veterinarians). I trusted them to be doing no harm. It’s agonizing to me. Why do we have to be experts on rabies-vaccine duration? Why can’t we trust our vets?”

Meier said she filed a complaint with the state Division of Professional Regulation, which reviewed the matter and referred it to the Attorney General’s Office, which ultimately dismissed the case.

Savannah Animal Hospital did not return calls from the VIN News Service seeking comment.

Meier’s experience resonated with Peterson. The Delaware lawmaker said she lost a miniature poodle, Rosie, six years ago to an autoimmune disease — immune-mediated thrombocytopenia  — that she suspects was triggered by overexposure to rabies vaccine.

The condition developed in the 2-year-old dog a few months after a rabies booster shot, Peterson said. She said that with the help of good veterinarians and great effort and expense, Rosie lived until age 5.

Experts say scientific evidence that vaccinations may cause health problems in pets is spotty because the research is difficult and costly and funding is hard to come by.

“You have to do studies that look at a lot of animals,” explained Dr. Philip Kass, a professor of epidemiology at the University of California, Davis, School of Veterinary Medicine. “You can’t do experiments because they would be unbelievably expensive and long. I’m not talking about problems like getting an allergic reaction within a half an hour,” he added. “I’m talking about more substantial, chronic diseases that are uncommon and can take a long time to develop.”

Kass said the development of illness following a vaccination may be purely coincidental. “Since (almost) all dogs are vaccinated and they can come down with disease, you can say vaccines cause everything, including death,” he said.

Dr. Richard Ford, a veterinary internal medicine specialist who has co-written national vaccination guidelines for dogs and cats, made a similar point, citing work he’s done for the federal government on the anthrax vaccine for people, which has been implicated as a potential cause of neurological side effects. He recounted: “One individual who refused to get the vaccine, within a week of having refused it, developed Guillain-Barre,” a neurological autoimmune disorder. Had the person accepted the vaccination, the timing of disease onset would have looked suspicious and the vaccine likely would have been implicated.

“I’m always very, very careful when attempting to establish cause and effect,” said Ford, a professor emeritus of medicine at North Carolina State University College of Veterinary Medicine. “The reality is, sometimes underlying health issues lingering in the background may be the actual cause of an apparent vaccine adverse event.”

Ford noted that in the past, modified-live virus rabies vaccine caused a small number of vaccinated pets to develop clinical signs of rabies and peripheral nerve injury. In 1985, all live-virus rabies vaccines were removed from the U.S. market.

If the question of vaccine adverse effects is complicated, the issue of how often to administer rabies shots and the risks of boosting too frequently or not frequently enough is not much simpler.

According to the 2011 Compendium of Animal Rabies Prevention and Control published by the National Association of State Public Health Veterinarians, Inc., 14 rabies vaccine products for dogs and 17 for cats are licensed and marketed in the United States. Almost all come in one- and three-year versions. (The exception is Continuum Rabies, which has been shown to be effective in cats for four years.)

The array of offerings is produced by four companies: Boehringer Ingelheim Vetmedica, Inc.; Pfizer, Inc.; Merial, Inc.; and Merck Animal Health, owner of what used to be the separate companies Intervet and Schering-Plough Animal Health.

The difference between one- and three-year formulations is, in many cases, little to nothing, according to scientists familiar with the manufacture and testing of vaccines.

Manufacturers’ representatives say a key distinction between most one- and three-year rabies vaccines is the testing they undergo to demonstrate the duration for which they confer immunity.

In the case of Defensor 1 and Defensor 3 vaccines made by Pfizer, testing is the only difference between the products. “The formulations are the same, but regulatory requirements for the one- and three-year vaccines are different, requiring distinct and separate studies for each label,” said Pfizer spokesman Richard Chambers.

Duration of immunity studies for the licensing of rabies vaccines and for labels on other types of vaccines are a significant and costly undertaking. Vaccine makers wishing to market products in the United States must maintain two groups of research subjects for the period for which they wish to label the product. In general, one group must include at least 20 vaccinated animals; the second is a control group that should consist of at least 10 unvaccinated animals. When the period has elapsed — one year, three years, or any other period the vaccine maker desires — both sets of subjects must be exposed to the disease. If at least 85 percent of the vaccinated animals are protected and at least 80 percent of controls develop the disease, the vaccine is considered effective for the period and labeled as such.

In reality, protection could continue much longer than the labeled period with certain vaccines and/or in some or many animals. How long immunity truly lasts for the rabies vaccine beyond the period demonstrated is unknown.

Some one- and three-year formulations do differ, but exactly in what ways is considered a trade secret. Dr. James Hall, senior associate director of veterinary technical services for Boehringer Ingelheim Vetmedica, Inc., maker of RABVAC 1, RABVAC 3 and RABVAC 3 TF, said the formulations vary but declined to specify how. “That’s proprietary information,” Hall said.

Likewise, a spokeswoman for Merial, maker of seven rabies vaccines licensed for use in dogs and/or cats, said its formulations are confidential.

Why make one- and three-year versions of vaccines? The reasons are several. Pfizer spokesman Chambers cited customer preference and varying local rules. “Where local laws and regulations mandate annual rabies vaccination, many veterinarians prefer to use a rabies vaccine that is specifically labeled, based on the required studies, for annual vaccination,” he said.

In addition, manufacturers recommend giving the first rabies booster one year after the primary vaccination. Following the recommendation, a puppy or kitten would be vaccinated for the first time at three months of age, then 15 months, then — depending on the product — every one, three or four years from then on.

Dr. Donna Gatewood, section leader for virology at the U.S. Department of Agriculture’s Center for Veterinary Biologics, which oversees and licenses veterinary vaccines, said her opinion is that giving a three-year vaccine more frequently than every three years is not harmful.

“The difference in potency in one- and three-year products — even in those products where there is a difference — is not great enough to create a safety problem,” Gatewood said.

At the same time, she said, “Every time you inject an animal, there’s some risk. One reason I would give the three-year vaccine rather than the one-year would be to avoid those other two injections.”

While many veterinarians, including Hall at Boehringer Ingelheim Vetmedica and Kass at UC Davis, say they personally believe vaccination intervals should be left to the medical judgment of practitioners in consultation with pet owners, some in veterinary circles see it differently.

Ford, the veterinary internal medicine specialist at North Carolina State, said a mandate to follow rabies vaccine label recommendations wouldn’t be onerous. “The whole impact is pretty minor in that it requires veterinarians to pay attention to the labeling and purchase one-year and three-year vaccine,” he said.

Ford deemed the issue minor from an immunological standpoint, as well.

Ronald Schultz, professor and chair of the Department of Pathobiological Sciences at the University of Wisconsin School of Veterinary Medicine and an authority on veterinary vaccines, supports rules controlling the frequency with which veterinarians administer vaccines.

Vaccines “do have components in them that can cause hypersensitivity reactions and so forth,” he said. “So you never give a vaccine more often than is needed.”

But what constitutes need is another open question. Dr. John Baillie, a clinic owner in Lake Elmo, Minn., said he routinely administers three-year rabies vaccines to patients every two years.

The reason, which he said he explains to clients, is that he prefers to alternate the years in which patients receive distemper and rabies shots. “Then we’re not overloading them with the viral vaccinations,” Baillie said.

A biennial schedule also ensures that patients aren’t late for their required rabies shot, he said. “If everyone was always in on time (with their pet) it would be a moot point,” Baillie said. “But that doesn’t happen.”

He recently monitored how many clients visited the clinic on time for their pets’ rabies vaccinations. In two weeks, he counted 11 clients whose pets were behind schedule, one by six months.

However, because he had used a three-year vaccine on the patient, Baillie said, “that animal was still protected against rabies and the client was protected against legal ramifications.”

Baillie, who is vice president of the Minnesota Veterinary Medical Association, noted that the ultimate purpose of animal rabies immunization is to protect people. “For me, it gets down to what’s best for my patients and my clients and what makes it simplest for them as far as keeping track of it and ensuring that their animal is protected,” he said.

For reasons similar to Baillie’s, other veterinarians also issue rabies certificates for shorter periods than indicated on the label of the vaccine they use. Doing so comes with a risk, warns Dr. Deborah A. Sprong, a relief veterinarian in Maryland. In a discussion of the subject on a message board of the Veterinary Information Network, an online community for the profession, Sprong said:

“The problem with issuing a 1-year certification when a 3-year vaccination ... is given is that if the animal has a wound of unknown origin or a high-exposure-risk situation after the first year but prior to the three-year expiration, it may well wind up being euthanized because it is (seen as) overdue for rabies vaccination. ... This would be a shame if the animal really was current and the record doesn’t reflect it.”

In an interview by email, Sprong added that even if the veterinarian who administered the vaccine documents in the medical record that the animal is protected for three years, the pet could still be vulnerable.

“If the animal presents to an emergency clinic after a tangle with a raccoon, the rabies certificate is all that is available to determine whether the animal is legally protected or not,” she said. “What if something happens to the medical records — a computer crash, fire, bankruptcy? Then the certificate is all there is.”

Meier, the dog owner in Delaware whose experience inspired the proposed legislation, said she is glad the bill has attracted attention to the subject. “However it turns out,” she said, “the issue needs public discussion.”