The Ebola virus has been circulating in West Africa since at least 2006, scientists have confirmed.
Experts from the US Army Medical Research Institute of Infectious Diseases (USAMRIID) have been working in the region since 2006 looking to develop tests for a different deadly virus, the Lassa fever virus, which affects the internal organs and kills about 5,000 people every year.
Over the years, USAMRIID scientists have started to optimise additional tests for emerging diseases and, when the Ebola outbreak began, they were well-positioned to provide aid.
"We had people on hand who were already evaluating samples and volunteered to start testing right away when the current Ebola outbreak started," said lead author Randal J Schoepp.
Between 500 and 700 samples are submitted each year to the Kenema Government Hospital in Sierra Leone, where the scientists are based. Of these, between 30% and 40% test positive for Lassa fever.
Researchers were looking to find out what other viruses had been causing serious illnesses in the region.
Of the samples submitted between 2006 and 2008, they found evidence of dengue fever, West Nile, yellow fever, Rift Valley fever, chikungunya, Marburg and Ebola.
Samples that tested positive for Ebola also reacted to the Zaire strain, the most deadly of all the Ebola viruses and the strain responsible for most outbreaks.
Schoepp said: "Prior to the current outbreak, only one case of Ebola had ever been officially reported in this region, and it was from the Ivory Coast strain. We were surprised to see that Zaire –or a variant of Zaire – was causing infection in West Africa several years ago."
Their findings come as the Ebola death toll reaches 600, with 68 deaths recorded in just four days.