Bat Rabies Threat Rises With Summer Temperatures

Ben Harder
for National Geographic News
August 15, 2006
The so-called dog days of summer—a muggy stretch from early July to early September—might also be called the season's bat days in the United States.

This is the period when the flitting critters most frequently turn up in attics, bedrooms, and camp cabins across the country, sometimes carrying rabies.

A recent study from the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, says that 46 percent of U.S. rabies cases in humans are caused by bites or scratches from infected bats—more than the 31 percent attributed to dogs.

Five of every six infections from bats occur between July and September, and the frequency peaks in August.

While the threat of rabies provokes fear, transmission from bats is actually rare, according to the Austin, Texas-based nonprofit Bat Conservation International.

Only 48 confirmed cases of rabies from bats have occurred in the United States in the past 55 years, the group reports.

But recent bat activity at a Girl Scouts camp in Virginia's Loudoun County caused health authorities to offer rabies shots to nearly a thousand campers—and brought the bat-rabies connection strongly to the public eye.

More than two dozen campers will reportedly receive the preventive treatment, a series of six to nine shots given over a month.

Such shots are often administered even when people have not been conclusively bitten, says biologist Barbara French of the bat conservation group.

Doctors and health officials tend to err on the side of caution, because the rabies virus becomes incurable once a victim displays symptoms and can kill in as little as ten days after that.

August's Bat Days

Bats disappear from much of the U.S. during colder months, either hibernating or migrating south for the winter.

It's the summer months that provide plenty of good reasons for bats to be out and about, experts say.

"The majority of the bats in the U.S. are insect eating," French said. "Their food supply is most abundant in the late spring and early summer."

Pregnant females give birth around that time. By July or August most of the babies finish nursing and begin to leave the roost.

But the young fliers aren't yet skilled at navigating.

"They're much more likely than other bats to end up in the wrong place"—like someone's bedroom—"this time of year," French said.

Furthermore, people's summertime behavior, such as leaving windows open at night to cool their homes, can invite unintentional encounters.

"An ill bat could find its way into a house that would otherwise be shut," said Charles E. Rupprecht, who heads the rabies program at CDC.

Some kids and other vacationers spend the summer in rustic cabins that have structural holes large enough for bats to squeeze through, he adds.

"Camps are ideal roosts," Rupprecht noted. In effect, "people are inviting themselves into the bats' house. You're invading the bats' domain."

He urges people to exercise common sense and, for instance, not pick up a sick or wounded bat.

"Some people make the mistake of wanting to take care of it," he said.

But simply seeing a bat in one's home isn't usually cause for alarm.

"People sometimes mistakenly believe you couldn't feel a bat bite," French said.

"I've had lots of bat bites, and they hurt. They feel like sharp needle jabs. Any conscious adult is going to know if they're bitten."

Bat bites don't always leave a mark on the skin, so experts worry about possible rabies transmission when a person awakens to find a bat in his or her room.

Unattended children, mentally disabled people, or intoxicated adults might also not notice or properly report a bat bite.

Heat Waves and Hot Zones

Although some people have speculated that intense heat drives more bats indoors, there's probably no link between the recent incident in Virginia and this summer's heat wave, experts say.

But if summers continue to become consistently warmer, it could conceivably increase the frequency of human contact with bats.

That's in part because vampire bats, which now occupy tropical parts of Mexico, might move north under the right conditions, the CDC's Rupprecht says.

"You have to have the right kind of habitat—rain forest-type habitat," French, of Bat Conservation International, noted. "[But] there are just too many factors to say if that could happen."

(See a multimedia feature on vampire bats, with photos, video, and more.)

If blood-feeding bats were to spread north, French's hometown of Austin might be among the places they'd end up.

"This is the place, of course, where we have 1.5 million bats right in the heart of the city," she said. "We have the biggest urban bat colony in the world."

And, she added, "we've never had a rabies case here."

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