In Sierra Leone, West Africa, 60% to 70% of patients with suspected Lassa virus in 2006 were instead infected with diseases of unknown origin, nearly 9% of which were Ebola virus, according to recent study results.
These findings confirm that Ebola virus was present in that region well before the current outbreak, researchers said.
In the study, the researchers tested samples from 253 patients suspected of having Lassa virus that were submitted to the Kenema Government Hospital Lassa Diagnostic Laboratory in eastern Sierra Leone. In general, only 30% to 40% of the roughly 500 to 700 samples submitted annually to the laboratory test positive for Lassa virus antigen or Lassa virus-specific immunoglobulin M antibodies, and the remaining 60% to 70% are acute infections of unknown origin.
The researchers assayed the samples, which were collected between October 2006 and October 2008, to characterize the arthropod-borne and hemorrhagic fever viral diseases that exist in this region and may mimic Lassa fever.
Of the arthropod-borne viruses for which antibodies were identified, dengue antibodies (4.3%) were most prevalent, followed by chikungunya virus (4%). Antibodies to other viruses accounted for less than 3% of cases, with West Nile virus antibodies identified in 2.8% of samples, yellow fever virus in 2.5%, and Rift Valley fever virus in 2% of samples tested. None of the samples tested positive for antibodies to the tick-borne Crimean-Congo hemorrhagic fever virus. The prevalence of antibodies to Ebola virus was 8.6%, and the prevalence of Marburg virus was 3.6%.
Additionally, the researchers found that the majority of the Ebola-positive samples belonged to the Zaire strain, known to be the deadliest form of the disease. According to researcher Randal J. Schoepp, PhD, who is part of a team from the US Army Medical Research Institute of Infectious Diseases assisting with the Ebola outbreak in West Africa, this finding was surprising.
"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," Schoepp said in a press release. "We were surprised to see that Zaire, or a variant of Zaire, was causing infection in West Africa several years ago."
Disclosure: The researchers report no relevant financial disclosures.