– The Ebola virus in western Africa is a novel strain that probably evolved locally and circulated for months before the outbreak became apparent, researchers said. The index case is probably a 2-year-old child from Guinea’s Guéckédou prefecture who died Dec. 6, 2013 — several months before the outbreak was recognized in March, according to Stephan Günther, MD, of the Bernhard Nocht Institute for Tropical Medicine in Hamburg, Germany, and colleagues. The findings come from an early epidemiological “look-back” and genetic examination of virus samples from 15 patients, Günther and colleagues reported online in the New England Journal of Medicine. The report comes as the World Health Organization is reporting that the outbreak now includes 202 suspected or confirmed cases — 168 in Guinea, 28 in Liberia, and six in Mali. There have been 121 deaths. Meanwhile, the health ministry in Guinea was optimistic that the worst is over. A spokesman told reporters the rate of new cases has slowed dramatically in Guinea and the outbreak is nearly under control. Reuters news agency quoted spokesman Rafi Diallo as saying: “The number of new cases has fallen rapidly” and the most recent cases are people who are not sick but are being monitored because they had been in contact with those who had fallen ill. “Once we no longer have any new cases … we can say that it is totally under control,” Diallo was quoted as saying. Günther and colleagues studied samples from 15 patients and concluded the virus affecting them is a novel version of ebolavirus, which has five species: Zaire ebolavirus (or EBOV), Sudan ebolavirus, Bundibugyo ebolavirus, Reston ebolavirus, and Tai Forest ebolavirus. The first three have caused major outbreaks in Africa, while the Tai Forest species has been responsible for a single human case, and the Reston species, which circulates in the Philippines, affects nonhuman primates but not people.

The version in the current outbreak is 97% identical to strains from the Democratic Republic of Congo and Gabon, but is a separate grouping with the EBOV clade, Günther and colleagues found. It probably evolved recently in parallel with the strains from other countries and was not introduced into Guinea from them, they concluded. “It is possible that EBOV has circulated undetected in this region for some time,” they wrote, and its emergence “highlights the risk of EBOV outbreaks in the whole West African subregion.” To try to get a handle on that emergence, the researchers reviewed hospital documentation and interviewed affected families, patients, and inhabitants of villages in which cases occurred. What appears to be the first case — at the “current state of the epidemiologic investigation” — was the 2-year-old, who lived in Meliandou in Guéckédou prefecture, the researchers wrote. Several members of her family also became ill and died, as did several contacts from other villages. Importantly, a healthcare worker who treated family members appears to have been the key player in spreading the virus beyond the local region. The worker became ill, went to hospital in the neighboring Macenta prefecture, and died there. From there, family members carried the virus back to other parts of Guéckédou and other contacts spread the virus to Nzérékoré and Kissidougou prefectures. The virus was apparently transmitted for months before the outbreak became evident, the researchers argued — a length of exposure that “allowed many transmission chains and thus increased the number of cases. Extinction Protocol