The boy’s virus did not match strains still circulating in the continuing outbreak in Guinea and Sierra Leone, meaning he was unlikely to have caught the virus through cross-border travel.
“The origin of this virus is Liberian,” said Stuart Nichol of the Centers for Disease Control and Prevention. “Based on the absence of reported cases for several months, this does push us toward thinking about a possible sexual event as an early step in this cluster of cases.”
Instead, the sequence in the new case most closely matches viruses found circulating in Liberia last July and August, said Michael R. Wiley, a research scientist with the Geneva Foundation and a contractor with the United States Army Medical Research Institute of Infectious Diseases, who flew to Liberia last weekend to help sequence the virus at the Liberia Institute for Biomedical Research.
Scientists had previously shown that the Ebola virus can sometimes persist for months in certain areas of the body that are relatively protected from the immune system, including the testes, the placenta and the inner portion of the eye. Studies of survivors in Liberia and Sierra Leone are working to determine how often this occurs. Experts recommend that survivors practice protected sex until more is known.
Liberian officials and international experts are still exploring other possibilities that could explain the new findings, including that the virus existed in an unknown reservoir before infecting at least five villagers. Experts believe it is unlikely that the virus has been spreading silently in recent months, given the strength of Liberia’s surveillance system, including frequent testing of bodies and sick people.
Given that the virus degrades quickly, within hours to days, in the tropical heat, Dr. Nichol said it was implausible that Ebola had been lying dormant in the environment. He said the close similarity with other viruses found in people in Liberia last year argues against the virus having been reintroduced from wildlife. Dr. Nichol said that an initial suspicion among villagers that the illnesses might be connected to a dog “we think is a red herring.”
The developments showed the power and potential of real-time genetic sequencing done in recent weeks by virologists in West Africa, said Fatorma K. Bolay, director of the Liberia Institute for Biomedical Research. The technique is also being used in Sierra Leone and Guinea, with 27 new cases last week.
In one recent example, scientists used sequencing to connect Ebola cases in two areas of Guinea with cases in the capital, which guided epidemiologists and anthropologists. “It helped to go back and untangle exactly how it got from one place to another,” said Dr. Bruce Aylward, who leads the Ebola response for the World Health Organization.
Dr. Aylward said that while secret burials and hidden illnesses were still complicating the response in Guinea and Sierra Leone, responders had been increasingly able to link new illnesses with exposure to known cases and decrease the number of chains of transmission.
The recurrence in Liberia shows the need to be vigilant, Dr. Aylward said, adding that he does not believe the disease will continue indefinitely. “I can’t accept that and I don’t accept that,” he said.
Tolbert Nyenswah, incident commander for the Ebola response units in Liberia, who was in New York on Thursday for a United Nations donors conference, agreed, saying, “We are confident we can stop this from further spreading.”