America & Rabies

Posted June 1, 2012 12:22 PM


Published:

Fewer than five cases of human rabies are reported annually in the United States, thanks to animal control and vaccination programs, according to the president of the National Association of State Public Health Veterinarians.

But despite hard-won control of the virus in people and domestic animals, authorities continue to battle rabies in wild animal populations, said Kristy K. Bradley, DVM, MPH, who is Oklahoma’s state epidemiologist. During the past century, human deaths from rabies in the U.S. have declined from 100 or more each year to an average of two or three, federal statistics show.

“But there’s a vast difference in North America compared to Asia and Africa,” noted Dr. Bradley, adding that 30,000 to 50,000 people around the globe die from rabies infection each year.

Who to Look For

While domestic dogs are no longer considered a rabies reservoir in the United States, exposure to rabid dogs accounts for more than 90 percent of human exposures to rabies and more than 99 percent of human deaths worldwide, according to the U.S. Centers for Disease Control and Prevention in Atlanta. Bats have caused rabies in at least 35 humans in the United States during the past 16 years.

Rabies control in American people and domestic animals comes at a steep price: More than $300 million annually is spent on disease detection and prevention, including vaccination of companion animals, animal control programs, rabies laboratories maintenance and medical costs, CDC records show.

Rabies and Vaccinations

Yet anecdotal reports of people being bitten by rabid dogs and cats seem contradict government statistics.

“The domestic animals that are acquiring rabies are those that are not vaccinated—or way past due on their vaccinations when an encounter with a rabid animal occurs,” Bradley said. “We simply need to do a better job of achieving a high rate of vaccination coverage among our pet population.”

Reports of rabid pets biting humans make great headlines, and a little media sensationalism isn’t all bad in terms of public education, noted Charles Rupprecht, DVM, Ph.D., and chief of the CDC’s rabies program.

“The local community news does a service by reminding community members that rabies is endemic in nature and that the United States has enzootic rabies in skunks, raccoons, bats and foxes,” Dr. Rupprecht said.

“The rate of human rabies cases is down. One has to separate the scientific, evidence-based epidemiological studies that are conducted to try and generate objective data so one can make conclusions about rabies data in this space and time,” he said, adding that surveillance, recording and diagnostics differ between the CDC and local news outlets.

The number of states that do not require pets to be vaccinated for rabies is  a “moving target” because those laws often are enacted at the municipal or local level under some form of political pressure, Rupprecht said.

For more information on the rabies virus in humans, click here.

Wildlife Factors

With human cases in check, the problem in this country is keeping the virus at bay in certain wildlife populations. Bradley said distribution of viral animal reservoirs depends on region of the country.

Bats: Most Common Rabies Carriers in U.S.
Of 15 human rabies cases recorded from 2006 to 2011 in the United States by the Centers for Disease Control and Prevention, nine were caused by bat virus variants.

Bat rabies variants derive their names from the bat species in which they circulate. Five cited in the CDC report are the silver-haired bat, Eastern pipistrelle, Brazilian (Mexican) free-tailed bat, big brown bat and vampire bat.

Speaking in April at the One Health Conference in Tulsa, Okla., rabies expert Cathleen Hanlon, VMD, Ph.D., Dipl. ACVPM, said most U.S. cases have been associated with insectivorous bats, including the Eastern pipistrelle and silver-haired bats.

Bats in this category weigh less than 20 grams and have tiny, needle-like teeth, noted Hanlon, the director of the Kansas State University Rabies Laboratory in Manhattan, Kan.

“Trauma alone, from a bat bite, is unlikely to send anyone to emergency,” Hanlon said. “For example, an adult male dismissed a bite from a bat and decided the risk of rabies in bats was small enough to not seek post-exposure prophylaxis. He died of rabies several weeks later.”

The rabies cases by year and variant:

  • 2006: Three cases (two bat and one dog)
  • 2007: One case (bat)
  • 2008: Two cases (one fox and one bat)
  • 2009: Four cases (three bat—one person survived—and one dog)
  • 2010: One case (bat)
  • 2011: Four cases (one bat, one unknown—the victim survived— and two dog)

Victims in the dog-related rabies cases were bitten either in another country or by a dog from another country, even though the victims were diagnosed and died in the United States.

  • In 2006, a California resident was infected by a dog from the Philippines.
  • In 2009, a Virginia resident was infected by a dog from India.
  • In 2011, a New Jersey resident was infected by a dog from Haiti and a New York resident was infected by a dog from Afghanistan.

While several bat species pose a threat in all 48 contiguous states, raccoons are a major reservoir on the East Coast and skunks predominate in the Central Plains, Bradley said. Wild-animal rabies has been relatively uncommon in the Western United States, though it is beginning to show up in skunks in Arizona and California.

Feral pigs, and pigs in general, are seldom mentioned in connection with rabies virus, and susceptibility levels to rabies vary among species.

“Some of it relates to anatomy—pigs have relatively thick skin,” Bradley said. “It also can have to do with behavior, [such as] the way the animal reacts to a skunk, coyote or fox.”

For example, it takes 10,000 times the dose of rabies virus to infect an opossum as it does a fox, she said. Moreover, cattle and horses are often kept in a fenced area where a rabid skunk can wander in.

“Cattle will face a skunk head-on, and their vulnerable areas are the face and extremities,” she said.

Oklahoma raccoons don’t carry their species’ strain of rabies virus, but those located on the Eastern Seaboard do, Bradley said. U.S. Department of Agriculture efforts to orally vaccinate foxes, coyotes and raccoons using a fish-meal-based bait have been successful. Skunks, which require a higher dose of vaccine and are pickier eaters, remain a challenge, she said.

“We still employ the same basic tenets of rabies prevention and control,” she said, including striving for high-vaccination coverage in pets and livestock, working to control stray animals and encouraging people to adopt practices to repel wild animals from houses and outbuildings. Public health officials also work to educate people on steps to take when bitten by animals, Bradley said.

How Fast?

How quickly does the rabies vaccine work? CDC studies show that an animal can be considered immunized within 28 days of initial vaccination, when a peak rabies virus antibody titer is reached. Because a rapid anamnestic response is expected, an animal is considered currently vaccinated immediately after a booster vaccination, according to the CDC website.

For cats and dogs, Bradley said, current recommendations are for a single initial rabies vaccination at 12 to 16 weeks, followed by annual boosters.

“The three-year rabies vaccine has met the studies required by USDA to confer protection against a rabies exposure for three years following an adult animal’s second or subsequent rabies vaccination,” she said.

Many states and animal-control jurisdictions recognize the three-year vaccine, she added.

“The requirement for one-year boosters is more a product of licensing and tagging, not valid concerns for the effectiveness of three-year vaccines,” Bradley said.

Other key CDC tidbits: No parenteral rabies vaccines are licensed for use in wild animals or hybrids (offspring of wild animals crossbred to domestic animals). And animal serologic titers can’t be relied on as a measure of immunization status.

“Other immunologic factors also play a role in preventing rabies, and the ability to measure and interpret those other factors are not well developed,” studies show. “Therefore, evidence of circulating rabies virus antibodies should not be used as a substitute for current vaccination in managing rabies exposures or determining the need for booster vaccinations in animals.”

A table of rabies vaccines licensed and marketed in the U.S., last updated in 2008, is available on the CDC website. 

EDITORS' NOTE: This story originally stated that rabbits are a major animal reservoir of rabies on the East Coast. However, raccoons, not rabbits, are a major reservoir in that region. The story was updated on June 1, 2012, with the correct information. The editors regret the error.

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