Fruit bats may be Ebola reservoir

Researchers find antibody and RNA evidence that three fruit bat species harbor Ebola virus

By | December 1, 2005

Three species of fruit bats may be the long-sought reservoirs of Ebola virus in Africa, according to a report in this week's Nature. Researchers have found antibodies specific to Ebola as well as viral RNA sequences in symptom-less bats, which they collected in central Africa during Ebola outbreaks in humans and other great apes.

"It's a very significant finding for all of us who have been working with this virus for a very long time," said Joseph McCormick of the University of Texas School of Public Health at Brownsville, who was not involved in the study.

From 2001 to 2003, Eric Leroy of the International Center for Medical Research in Gabon and his colleagues collected more than 1,000 small vertebrates in areas of Gabon and the Republic of the Congo that were stricken with primate outbreaks of Ebola hemorrhagic fever.

Of these animals, the researchers detected immunoglobulin G (IgG) specific for Ebola virus in 16 fruit bats from three species. They also used nested PCR to fish for Ebola genetic material, and found Ebola RNA sequences in the liver and spleen of 13 other bats from the same three species, but not in any other animals.

Sequencing the viral RNA revealed seven different fragments, all of which cluster in the Ebola Zaire clade – one of four known subtypes of Ebola virus. The fragments differed between and within bat species. This diversity suggests that Ebola virus has been present in bats for some time, Leroy told The Scientist in an Email, indicating they are probably its primary reservoir.

However, Leroy and his colleagues did not isolate the complete virus from any of the bat samples, which would have made their case stronger, Leroy said. "I'm not too surprised at that," said Clarence J. Peters of the University of Texas Medical Branch in Galveston, who was not involved in the research. The need to use nested PCR to extract RNA suggests that the bats carried a very low viral load, Peters said.

Ebola RNA and Ebola-specific IgG were never detected in the same individual bat, which was a bit of a surprise, the authors say. It's possible that bats recently infected with RNA may not have yet developed an immune response and that bats positive for IgG may have cleared the virus from their systems, according to Leroy. This result suggests that Ebola infection in bats may not be chronic, but it's too soon to know for sure, McCormick said.

The bats positive for antibodies or RNA were not only of different species but different genera, which is "a little unusual" for a viral reservoir, Peters told The Scientist. Chronic viral infections in rodents, for example, are usually species-specific, he said. "It makes you wonder if maybe these guys are all three getting it from somewhere else." Many viruses are transmitted between vertebrates by arthropods, but "nobody really thinks that arthopods are involved in maintaining Ebola," Peters said.

A different animal transmitting the virus between bat species "seems somewhat unlikely," according to McCormick. Future studies on the genetic diversity of Ebola viruses in bats and the geographic range of infected bats will likely help to answer the question, he said. If Ebola viruses are "fairly diverse coming out of bats from different regions," that strengthens the case for bats as the source.

Leroy's work gives "the best leading candidate ever" for an Ebola reservoir, Peters said, but to be sure that these bats are the primary reservoir, "you'd have to work out all the details of the natural cycle of the virus," including whether Ebola infection in bats is chronic, becomes latent and is later reactivated, or is completely wiped out by the immune system.

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