At 4:50 a.m. at the Kasanka National Park in northern Zambia, tourists watch from a platform in a tree as the sound of millions of wings accompanies the sunrise.

About 10 million straw-colored fruit bats are returning from a night of feeding, some flying as far as 100 kilometers (62 miles) to feast on berries and figs. The animals may hold a clue to finding the cure for the Ebola disease that’s killed more than 8,000 people in west Africa in the biggest-ever outbreak, according to Aaron Mweene, professor at the University of Zambia’s veterinary medicine school. That outbreak, which is yet to be quelled, has been blamed on bats.

Researchers including scientists from Japan’s Hokkaido University did a study that found a high prevalence of Ebola antibodies in the creatures that undertake the world’s second-largest mammal migration from the Democratic Republic of Congo to roost in Zambia, Mweene said. That indicates that they come into contact with the virus and are able to cure themselves.

“The antibodies have been found in about 10 percent of the animals; it’s a significant part of the total,” Frank Willems, ecologist at Kasanka, said in an interview in the park. “It might well be that specifically this species will form the clue to actually finding the cure for Ebola.”

The bats migrate each year to roost in an evergreen Marsh Fig forest in Kasanka, 390 kilometers northeast of Lusaka, the capital. They arrive from October and stay until December, roosting in an area as small as a hectare (2.47 acres). During the day the average density in the forest is as much as 1,000 bats per square meter (11 square feet) as the bats, which have an 80-centimeter (31-inch) wingspan, seek protection in numbers from the raptors that eat them.

Bats, Mice

Their migration is only superseded by the annual movement of Mexican free-tailed bats from the the U.S. to Mexico.

Prior to the Hokkaido University research, the results of which have yet to be officially released, Ebola antibodies had only been found once in this species in a specimen captured in a roost in the Ghanaian capital of Accra in 2008, Janusz Paweska, who works at the National Institute for Communicable Diseases in Johannesburg as head of its Centre for Zoonotic and Emerging Diseases, said in an e-mailed response to questions this month. Ayato Takada, head of the Hokkaido team, that did the study, declined to comment, saying the research will be available when published.

Still, some scientists are skeptical.

“With viruses like Ebola and Lassa, the virus and the host – such as bats and mice – are very well adapted to each other, having spent thousands of years together,” Ben Neuman, a virologist at the U.K.’s University of Reading, said in an interview. “Antibodies can help, but antibodies aren’t usually a solution by themselves for a virus.”

Defense Mechanism

Lassa fever, like Ebola, causes hemorrhaging. The Marburg virus is also closely related to Ebola.

Already trials of experimental treatments using antibodies found in human survivors of the disease are under way in Guinea.

Mweene has been doing research on the bats since 2005 and working with the researchers from Hokkaido University.

“We cannot categorically say that the cure for the Ebola virus disease lies in the bats, but we could say that surely the bats possess certain characteristics that enable them to survive the infection,” Mweene said in an e-mailed response to questions.

None of the bats, which weigh about 300 grams (10.6 ounces) each, were found to be infected with the actual virus.

“They will pick up the virus; they just have a very effective defense mechanism against it,” Willems said as hippos snorted in the lake behind him.

‘Hunted Treasures’

To understand how the bats react to Ebola, scientists need to carry out experimental infections of the animals. Because the virus is so dangerous, they can only do this in the highest level of biosecurity containment laboratories, Mweene said.

Africa only has one of these, the institute in Johannesburg, he said.

“The mechanisms of Ebola and Marburg virus natural transmission cycles still remain one of the most hunted treasures in modern virology,” Paweska said. “Plans are in place to expand our current bat colonies for experimental infection studies, but also to intensify surveillance studies, including sampling of bats at targeted sites in South Africa and other African countries.”

While bats are suspected to be natural reservoirs for Ebola, this is unproven.

Tracking Equipment

This year, researchers have for the first time fitted some of the animals with global positioning system devices to track their movements back to the DRC, Willems said. The batteries last about three months, which is about the time it takes for them to reach their destinations, he said.

The tracking unit comprises a collar and a transmitter on the bat’s back that’s about 4 centimeters long. Researchers are working on solar-power technology to allow for the year-round tracking of the bats, but “for a forest animal that is nocturnal, that’s tricky,” Willems said.

This may assist in providing clues as to where the animals come into contact with the virus and possibly lead to its origin, he said. Reports of the bats migrating to Kasanka go back to the 1950s, though Willems believes they’ve been returning each year for thousands, if not millions, of years.

Bat Soup

While Zambia has no recorded cases of Ebola, the outbreak in west Africa is denting the reputation of bats across the continent.

The first person to contract Ebola in its most recent outbreak in west Africa may have been infected by insectivorous free-tailed bats in Guinea, according to a study published on Dec. 29 in the Heidelberg, Germany-based EMBO Molecular Medicine journal. In the West African outbreak the virus has mostly killed people in Guinea, Sierra Leone and Liberia, according to the World Health Organization.

Television advertisements screened as far away as South Africa advise people not to eat bats, a delicacy in some parts of west Africa where they are smoked, grilled or made into a soup.

Ebola’s connection with bats has already kept some people from visiting the 390 square kilometer (151 square mile) Kasanka park in Zambia, Willems said. He estimates the number of bat-viewing tourists fell by as much as a fifth this year.

Kasanka Trust Ltd., which manages the park, said in an Oct. 15 statement there is “no risk” in watching the bats.

Back in the tree platform, the shrill sound of the chattering bats returning for the day fills the air. For the second straight morning there are only two tourists witnessing the spectacle.