Ugandan authorities said on August 9 they had contained the outbreak, three weeks after a 29-year-old miner died on July 14. Another case, believed to be a secondary infection, was discharged from hospital.
Around 40 people working where the disease appeared at Kitanga gold mine, near the border with the Democratic Republic of Congo, have been quarantined.
A WHO official said that 300 fruit and insectivorous bats had been captured in the mine so far in an attempt to find out if they are the source of the deadly virus.
The natural reservoirs for Marburg, and its notorious cousin Ebola, are thought to be in the African tropical forest, but the precise animal source remains unknown.
"Today we are focusing on the bats because they are the main mammals in the mine," WHO scientist Pierre Formenty said by telephone from Uganda. "The victim was probably infected in the mine."
Wearing respiratory masks and protective clothing, an international team from the WHO, the US Centers for Disease Control and South Africa trapped the live bats in nets in the early evening as the mammals flew out of the mine, he explained.
The animals are then dissected in a laboratory, and tissue and blood samples are sent to CDC and the National Institute for Communicable Diseases (NICD) in South Africa.
The research is expected to take weeks before yielding results, said Bob Swanepoel from the NICD.
"Just simply finding the virus doesn't incriminate the bat," he underlined.
The disease takes its name from the German town of Marburg, where it was first detected in 1967 among lab workers who were infected by monkeys from Uganda.
It spreads through contact with blood, excrement, vomit, saliva, sweat and tears, and has no vaccine or specific treatment.
The world's biggest outbreak of Marburg occurred Angola from October 2004 to July 2005, infecting 374 people, 329 of whom died, according to an official toll.