Ebola’s introduction and spread in Liberia, the second worst-affected country in the recent outbreak, has been explained in a study published in Cell Host & Microbe. It is likely that most cases in the severe, second wave of the epidemic originated from just one source.

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The recent Ebola epidemic claimed at least 11,000 lives.

Late in 2013, the largest ever epidemic of the human Ebola virus disease (EVD) was triggered in West Africa, after a new form of the disease emerged.

EVD was first discovered in 1976. It is transmitted from a host to humans and then spreads through human-to-human transmission. Around 50% of infections are fatal.

Previous outbreaks have been limited due to EVD mainly affecting remote villages in Africa.

However, the recent outbreak reached major urban areas in Guinea, Liberia, Sierra Leone and nearby countries, causing more than 28,000 infections and over 11,000 deaths. The highest number of deaths was in Liberia.

Senior study author Gustavo Palacios and first author Jason Ladner, of the US Army Medical Research Institute of Infectious Diseases (USAMRIID), used genome sequencing to provide a detailed view into the ongoing spread and diversification of EVD.

The team sequenced Ebola virus genomes from 139 EVD patients affected in the second, largest wave of the Liberian outbreak, and they also analyzed 782 previously published sequences from throughout Western Africa.

The samples provide data for nearly a year of the epidemic, including the time during which 99% of the confirmed and probable cases were reported in Liberia.

Although EVD spread to Liberia several times from neighboring countries early during the outbreak, it now appears that most Liberian cases stemmed from a single introduction of the virus, in late May or early June 2014, around the start of the second wave of Liberian cases. EVD then spread rapidly through the country, refueling the ongoing outbreak in Guinea.

Contact tracing has revealed at least three potential introductions of the Ebola virus to Liberia from Sierra Leone around the start of the second wave of Liberian cases.

One of these introductions probably led to the largest wave of cases in Liberia. The same source has been linked to several cases in Monrovia, including health care workers at Redemption Hospital.

From here, EVD spread rapidly within Liberia, refueled the outbreak in Guinea and traveled to Mali.

Transmission chains from other infected individuals who were entering Liberia from neighboring countries do not appear to have substantially contributed to the outbreak in Liberia.

Ladner says:

“The widespread movement of the Ebola virus within Liberia, due to a high rate of migration in the country, is likely to have played an important role in the magnitude and longevity of the Liberian portion of the Ebola outbreak. Regular migration of infected individuals complicates surveillance and isolation efforts, which are critical for controlling Ebola outbreaks.”

On September 3, 2015, the World Health Organization (WHO) declared that the transmission of EVD had stopped in Liberia, and on November 7, Sierra Leone was declared clear. By November 22, the last patient in Guinea had twice tested negative for EVD.

However, in November, three new cases were reported in Liberia, in the light of which, Palacios emphasizes the need for “robust surveillance measures to ensure the rapid detection of any reintroduction or re-emergence of the virus.”

Genome sequencing plays an important role in identifying and confirming chains of transmission, and the current study supports ongoing surveillance and isolation efforts and provides critical information for developing effective control strategies.

The authors stress the need for more research to understand how the virus originally spread to humans in this outbreak. They also recommend investigating the public health implications of the findings, including a detailed investigation of EVD control measures throughout Western Africa, to ensure the effectiveness of various management approaches.

Medical News Today recently reported on a major study about the handling of the EVD epidemic.