An analysis of Ebola virus genomes from patients in Liberia reveal that most of the country’s cases during this recent West African outbreak can be traced back to a single introduction in 2014. The findings were published in Cell Host & Microbe this week.
Since its discovery in Africa in 1976, the Ebola virus has mainly affected remote villages – which limited the number of Ebola virus disease deaths, despite the average 50 percent fatality rate. But near the end of 2013, a novel variant of the virus emerged in Western Africa and spread throughout major urban areas, triggering the largest ever recorded human epidemic – with more than 28,000 cases and 11,000 deaths reported so far. And while viral genome sequencing has helped researchers identify and confirm various routes of transmission throughout the outbreak, there’s been relatively few sequences from patients in Liberia, the second worst affected country.
Now, a large international team led by Jason Ladner from the U.S. Army Medical Research Institute of Infectious Diseases sequenced 140 Ebola virus genomes from patients in 13 counties throughout 11 months during the second (and largest) wave of the Liberian outbreak. These sequences were analyzed with another 782 previously published sequences from throughout West Africa.
The team found multiple early introductions of the Ebola virus to Liberia from neighboring countries, but importantly, most of Liberia’s Ebola virus disease cases have been tied to a single introduction of the virus in late May or early June of 2014. While there were at least three introductions from Sierra Leone, the viral sequences link a single lineage called SL2 to the majority of cases. It likely came from a woman who traveled from Sierra Leone to Monrovia, the capital of Liberia, and then spread to health care workers at Redemption Hospital.
This was followed by the rapid transmission of the virus – as well as diversification into eight sub-lineages – in different cities throughout the country. Although infected individuals likely continued to enter Liberia from other West African countries, these transmission chains didn’t substantially contribute to the Liberian outbreak. However, multiple reintroductions from Liberia into Guinea fueled ongoing outbreaks in both Guinea and Mali.
According to the U.S. Centers for Disease Control and Prevention, Liberia was declared free of Ebola transmission in early September of this year, but then a new cluster of cases in a suburb of Monrovia was confirmed in late November.
Image in the text: Ladner, Wiley, Mate et al., Cell Host & Microbe 2015
