Ebola transmission in Liberia tracked to one source

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.

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.

Genome sequencing reveals paths of transmission

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.