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Ebola virus genome provides clues to repeated disease ‘flare-ups’ in Western Africa

Ebola virus samples taken from patients in Liberia in June 2015 are strikingly similar in their genetic makeup to other Ebola virus sequences from Western Africa, according to research published online in the journal Science Advances. The study sheds light on several aspects of the “flare-ups” that have occurred in Liberia since the country was initially declared free of Ebola virus disease.

As described in the paper, genomic analysis and the epidemiological investigation indicate that the June 2015 flare-up was a re-emergence of a Liberian transmission chain originating from a persistently infected source. This was also the case with a March 2015 Liberian flare-up. Neither event was caused by re-introduction of the virus from an animal reservoir or from a neighboring country with active person-to-person transmission, according to the research team. While the March flare-up was traced to sexual contact, no definitive link has been found for the June event.

“When the June cluster of cases was first detected, the initial expectation of investigators was that it likely originated from a re-introduction of the virus from Sierra Leone or Guinea, where human-to-human transmission was active,” explained study co-first author Jason T. Ladner, Ph.D., of the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID). “This explanation was favored because the typical route of Ebola virus transmission is through infectious body fluids from individuals in the acute phase of the disease, when they are most symptomatic.”

However, genomic sequencing, combined with epidemiological investigation, indicated that the cases did not represent a re-introduction from a neighboring country, but instead were the result of virus transmission from a “persistently infected” source within Liberia–meaning, for example, a disease survivor who continued to carry the virus for several months.

“Although this finding did not necessarily play a major role in the control of this particular cluster, it certainly emphasized the risk for additional flare-ups, even within areas where active spread of the virus has been stopped,” Ladner said. “This understanding has led to heightened vigilance, which has allowed for rapid response to the additional flare-ups that have occurred.”

Ebola virus causes severe hemorrhagic fever in humans and nonhuman primates with high mortality rates and continues to emerge in new geographic locations, including Western Africa, the site of the largest recorded outbreak to date. More than 28,000 confirmed, probable and suspected cases have been reported in Guinea, Liberia and Sierra Leone, with more than 11,000 reported deaths, according to the World Health Organization.

In the paper, the research team–which includes scientists from the U.S. Centers for Disease Control and Prevention, the Liberian Ministry of Health, and USAMRIID–points out the risk of Ebola virus disease flare-ups even after an outbreak is declared to be over. Since the June 2015 flare-up, at least four more documented flare-ups have occurred (2 in Liberia, 1 in Sierra Leone and 1 in Guinea), according to the authors.