Photo by Holly Herman

A bat that wandered into an enclosed patio at the home of Holly Herman tangled with Herman’s two indoor cats. Although the cats were behind on their rabies shots, recent research indicates that boosters in overdue dogs and cats are as protective as boosters in dogs and cats that are up-to-date on rabies vaccinations. The Herman felines caught up on their shots within hours of the bat encounter.

The Herman family cats had rabies shots as kittens, but their owners skipped the boosters that came due last year. They figured the vaccinations weren't necessary because the cats stayed indoors and lived in a region where rabies isn't much of a problem.

Then one afternoon in April, a bat came into the Hermans' enclosed patio. Holly Herman found the bat on the floor, fur in its fangs. The cats, Chili and Jasper, stood over it.

Within an hour, Herman had the cats in the veterinarian's office getting boosters.

Elsewhere, the pets might have been euthanized or, at best, quarantined for months away from home. But in Calgary, where the Hermans live, rabies vaccinations aren't required, so the fact that Chili and Jasper might have been exposed while behind on their shots didn't cause a stir.

Recent research published in the Journal of the American Veterinary Medical Association (JAVMA) demonstrates that with any overdue dog or cat possibly exposed to the deadly virus, the same no-fuss approach of administering rabies booster shots may be justified anywhere, regardless of local rabies risk.

The research, led by a veterinarian in the Kansas State Veterinary Diagnostic Lab at Kansas State University, found that rabies boosters in dogs and cats that were late for their vaccinations were just as effective as boosters in dogs and cats that were up-to-date on their vaccinations.

The finding will be reflected in the next version of the Compendium of Animal Rabies Prevention and Control, expected to be published later this year. While public health rules on rabies vary by locality, officials tend to follow the compendium as an authoritative guide.

Changing the recommendation for handling possibly exposed dogs and cats that have been immunized but are behind on boosters has the potential to save the lives of thousands, perhaps tens of thousands, of pets, said Dr. Mike Moore, a project manager in the Rabies Lab of the Kansas State Veterinary Diagnostic Laboratory, and lead author of the study.

His estimate is based on the fact that each of the roughly 6,000 known rabies cases in wildlife in the United States each year is associated with one or more exposed dogs or cats. "There's usually multiple animals that are potentially exposed," Moore said.

Consider the number of pets with bites of unknown origin, often treated as rabies risks, and the estimate rises dramatically. Rolan Davis, a reference diagnostician in the lab and member of the research team, noted, "If you said every positive (wildlife rabies case) in the country has probably a (domestic) animal or two involved with it, then add the bite of unknown origin, the number probably goes up tenfold."

In the current edition of the compendium, last updated in 2011, the guidance for handling pets suspected of rabies exposure and that are overdue for rabies vaccinations is not definitive. It states that they "should be evaluated on a case-by-case basis based upon severity of exposure, time elapsed since last vaccination, number of previous vaccinations, current health status, and local rabies epidemiology to determine need for euthanasia or immediate revaccination and observation with isolation."

In jurisdictions where rabies is prevalent, such pets are likely to be treated like pets that have never been vaccinated — immediately euthanized or quarantined for six months.

The quarantine option can be tantamount to a death sentence because of the expense. "You're looking at 4,000 to 6,000 bucks ... (which) most people can't afford," Moore said.

The study was inspired by requests from pet owners and veterinarians attempting to save animals in that tenuous position. They wanted to know if the laboratory at Kansas State could check the animals' immune status by measuring their rabies titers — the concentration of antibodies in serum, a component of blood.

It was a simple enough request, Moore said. The laboratory already routinely measures rabies titers in humans and in animals bound for export. But for suspected rabies cases, Moore discovered, the measurement wasn't considered acceptable proof that a pet was safe.

"We attempted to do it for some, but the local health departments wouldn't budge," he reported.

So Moore and colleagues decided to approach the subject scientifically and submit their findings to the experts in charge of the rabies compendium.

Working for a laboratory that, according to its website, is "one of the highest-volume rabies serology centers in the world for both humans and animals, handling over 60,000 samples a year," the researchers had ample access to the needed data.

Although the research couldn't help the study subjects, their owners and doctors were amenable to participating all the same. "It was wonderful the response we got from veterinarians and owners who were willing to help us build a database for animals in the future, even knowing that it wasn't going to change the disposition of their animal," Moore said.

In all, researchers measured titers of 74 overdue dogs and 33 overdue cats. The results were unequivocal. Five to 15 days following a booster shot, every animal had antirabies antibodies of at least 0.5 international units per milliliter of serum, which Moore described as a robust response. Some had levels more than 24 times that.

"This says that the animal's immune systems haven't forgotten," Davis said.

He and Moore explained that even if antibodies to the rabies virus diminish, memory cells in the immune system are primed by earlier vaccinations and remain ready to act.

The length of time the study subjects were overdue for their shots ranged up to 46.1 months in the cats and up to 36.1 months in the dogs.

Legally, an animal is considered overdue if it's one day late for its booster, Moore said. "That's the crazy part about this. … Your immune system doesn't look at the calendar and go, ‘OK, we're shutting down.' " But the public-health policy clock is set by the length of time that a vaccine is labeled for, which is based upon the duration that the vaccine-maker is able and willing to test animals.

Not only did the researchers find that overdue dogs and cats had a strong immune response to booster shots, they discovered that overdue dogs had a stronger immune response on average than dogs that received their booster shots on time.

Why? Moore suspects the body reacts like villagers in the story of the boy who cried wolf: "If you see an antigen over and over and over, finally your body goes, ‘Oh right, whatever.' If you have those primed cells and they haven't seen it in a while, ‘Oh ho, let's get geared up, we haven't seen this in a while!' "

Further demonstrating the complex and varied nature of immune systems, the researchers found that some pets that received rabies vaccinations on schedule — and presumably were fully protected — had low rabies titers, below the target 0.5 IU/ml.

The low baseline levels were found in nine out of 55 on-time dogs and in five out of 19 overdue dogs.

The intriguing results suggest that a better way of establishing animals' rabies immune status may be by measuring their titers rather than putting them on a uniform vaccination schedule, the researchers say.

That's already done with humans, Moore pointed out. "In people, we don't worry whether they're out-of-date; we monitor their titers," he said. "With veterinarians who could be exposed, we monitor their titers every two years. Here in the lab, since we're potentially exposed to rabies every day, we check our titers every six months. If our titer falls below 0.5, we boost ourselves."

The researchers are continuing their studies in hopes of establishing titer measurements as a legal means of identifying an animal's rabies risk. Moore said the group is scheduled to present information to the compendium committee this fall and hopes for further changes to the rabies policy recommendations in the future.

Meanwhile, a compendium update that reflects the published study about the efficacy of rabies boosters in overdue dogs and cats will be released this year, Dr. Catherine Brown, chair of the compendium committee, confirmed. She said the document will be published simultaneously in JAVMA and in Mortality and Morbidity Weekly Report, a publication of the U.S. Centers for Disease Control and Prevention.

Dr. Gary Balsamo, president of the National Association of State Public Health Veterinarians, which produces the rabies compendium, said he found the study interesting and not surprising, in light of years of data from Texas showing that even unvaccinated animals can be protected by vaccination immediately following rabies exposure.

While there is no treatment for rabies, the particular nature of the disease makes a counterattack possible post-exposure: When the virus enters the body, it travels through the nervous system toward the brain, where it multiplies. Until the pathogen reaches the brain, the immune system can be primed to ward it off. Rabies incubation can take months, hence the long quarantine periods.

The effectiveness of late vaccinations notwithstanding, Balsamo said he hopes pet owners won't become complacent about immunizing their pets on schedule. Relying on post-exposure shots is dangerous because owners don't always know when their animals have been exposed, he said.

In his experience as the state public health veterinarian of Louisiana, Balsamo said, "That's been the case more than it hasn't been. We had one a couple of years ago where the owner was sure it was a snake bite but it turned out it was a skunk bite, and the dog ended up with rabies."

He said the dog was euthanized but not before two people were exposed by it to rabies.

Moore said elevating public recognition that rabies antibody levels are significant would address the concern that pet owners might become lax in keeping up with rabies vaccinations.

"We don't want people to vaccinate and forget," he said. "We want people to know the immune status of their animal and know whether they're protected or not."