Deadly Ebola virus seen to thrive in bats.

Most pet owners know the hazards of vaccinating an irate cat or dog. But just imagine wielding the hypodermic to inoculate a variety of wild jungle critters, including snakes, lizards, and bats. Imagine also that an accidental needle stick could be fatal because the syringe teems with a virus that is spectacularly lethal to humans.

Robert Swanepoel of South Africa's National Institute for Virology in Sandringham and his coworkers took such risks and have something to show for their efforts. They have demonstrated for the first time that healthy bats may harbor the fearsome Ebola virus.

Team members caution, however, that the study does not prove bats are the sole reservoir of the virus. "If you did an exhaustive search, you might find other species as well," says Thomas G. Ksiazek of the federal Centers for Disease Control and Prevention in Atlanta, one of Swanepoel's collaborators. Still, Ksiazek notes, bats could be the animal host scientists have sought since Ebola first emerged in Yambuku, Zaire, in 1976.

Ebola has inflamed the public's imagination even though fewer than 5,000 people have been infected worldwide. Sufferers go mad, they endure terrific pain, and 60 to 90 percent of them die of florid hemorrhages, with blood leaking from every orifice. Yet no one knows where the virus comes from, where or when it will strike next, or how to treat those who cross its path.

Peter Jahrling of the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID) at Ft. Detrick in Fredericksburg, Md., says the new study's findings are consistent with what is known about Ebola's epidemiology.

For instance, epidemics in Nzara, Sudan, in 1976 and 1979 were traced to a cotton factory where thousands of bats dangle from the rafters as workers toil beneath them. Other outbreaks have been linked to the Kitum Cave in a mountain in Uganda. "Guess what you've got plenty of in that cave. Bats," Jahrling says. Researchers are now tracking the virus in Ivory Coast's Tai Forest, where chimps and humans have contracted the disease.

Finding Ebola's natural home has proved challenging for several reasons. Ebola outbreaks are rare: Only eight have been recorded in humans. Epidemics occur in widely separated geographic locations, often in difficult-to-reach, jungle-choked regions of Africa.

As a result, field expeditions are extraordinarily difficult. Scientists viewed the 1995 outbreak in Kikwit, Zaire-which afflicted 315 people, killing nearly 300 of them-as an opportunity to find out where the virus lurks.

After the outbreak, scientists from CDC, USAMRIID, and the South African institute combed Kikwit to test local creatures for the virus. In total, the researchers trapped 2,500 mammals (mostly rodents), 36,000 insects, and 500 other animals, including snakes. So far, the results have been negative, but completing the tests will take time.

Swanepoel's team also took another tack. Under laboratory quarantine, the scientists tried to infect birds, bats, snakes, lizards, tortoises, rodents, insects, and plants. Only the bats proved capable of carrying Ebola, the team reports in the October-December Emerging Infectious Diseases, the CDC's online journal.

The bats carried large quantities of replicating virus without becoming ill. Only one bat made antibodies, indicating that the virus does not usually provoke the animals' antiviral defenses. In the absence of any such counterattack, the virus remains virulent in its host. Indeed, the researchers found live Ebola in bat feces, which may represent a source of transmission to other animals.

Although humans have gotten Ebola from infected monkeys, these primates are considered unlikely long-term hosts because the virus kills them. Rather, monkeys are a suspected link in the chain of transmission. Colobus monkeys, for example, can transmit the virus to chimpanzees, which hunt them. In February, 21 people in Gabon got Ebola after eating a dead chimp.