WASHINGTON, July 31 (Reuters) - Thousands of bats in a cave in Uganda are infected with Marburg virus, a cousin of the Ebola virus, researchers said on Friday, strengthening the theory the mammals are natural carriers of the deadly viruses.

A study by Pierre Rollin and colleagues at the Special Pathogens Branch at the U.S. Centers for Disease Control and Prevention found live virus in 5 percent of the bats tested in the cave, where miners were infected with Marburg in 2007.

"Our finding of active virus infection in approximately 5 percent of R. aegyptiacus bats and their population exceeding 100,000 in Kitaka cave in Uganda suggests there are likely over 5,000 Marburg virus-infected bats in this cave, which is only one of many such cave populations throughout Africa," they wrote in their report in the Public Library of Science journal PLoS Pathogens.

"Clearly, these bats could serve as a major source of virus with potential to initiate human epidemics, and the implications for public health are striking."

Researchers have long suspected bats were the natural reservoirs of Ebola and Marburg -- both lethal viruses in the same family. Ebola can kill between 50 percent and 90 percent of patients, while Marburg is a bit less deadly.

A natural reservoir is an animal that carries and transmits an infection without becoming ill.

Rollin's team sampled the blood of bats in the giant cave, where one miner died from Marburg in 2007.

The virus samples from the sick miners and from the bats were close genetic matches, they reported. "These data indicate common Egyptian fruit bats can represent a major natural reservoir and source of Marburg virus with potential for spillover into humans," they wrote. (Editing by Paul Simao)

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