* Outbreak blamed for 122 deaths in Guinea, 13 in Liberia
* Researchers find virus not brought in from outside region
* Raises risk of future outbreaks in West Africa (Recasts and writes through)
By Saliou Samb
CONAKRY, April 17 (Reuters) - An Ebola outbreak blamed for 135 deaths in West Africa in the past month was not imported from Central Africa but caused by a new strain of the disease, a study in a U.S. medical journal said, raising the spectre of further regional epidemics.
The spread of Ebola from a remote corner of Guinea to the capital and into neighbouring Liberia, the first deadly outbreak reported in West Africa, has caused panic across a region struggling with weak healthcare systems and porous borders.
Ebola is endemic to Democratic Republic of Congo, Uganda, South Sudan and Gabon, and scientists initially believed that Central Africa's Zaire strain of the virus was responsible for the outbreak.
Using analysis of blood samples from infected patients, however, researchers determined that while the Guinean form of the Ebola virus (EBOV) showed a 97 percent similarity to the Zaire strain, the disease was not introduced from Central Africa.
"This study demonstrates the emergence of a new EBOV strain in Guinea," wrote the group of more than 30 doctors and scientists, who published their preliminary findings on the website of the New England Journal of Medicine.
There is no vaccine or cure for Ebola - a haemorrhagic fever with a fatality rate of up to 90 percent that causes symptoms ranging from flu-like pains to internal and external bleeding caused by kidney and liver failure. Its suspected origin is forest bats and it can be transmitted between humans by touching victims or through bodily fluids.
"It is possible that EBOV has circulated undetected in this region for some time. The emergence of the virus in Guinea highlights the risk of EBOV outbreaks in the whole West African subregion," the report continued.
Of the 197 clinical cases of Ebola declared in Guinea, 122 have died including several health workers, according to the World Health Organisation's latest update, which cited Guinean health ministry figures. Sixteen of those died in the capital Conakry.
Guinea's government had previously placed the death toll at 106. The health ministry said on Tuesday that the number of new cases had fallen rapidly and the outbreak was nearly under control.
A senior health ministry official told Reuters on Thursday the government planned to stop publicly releasing the death toll to avoid causing unnecessary panic.
Liberia's health ministry has recorded at least 13 deaths from 26 confirmed and suspected cases of Ebola.
The WHO said earlier this month it would take two to four months to contain the outbreak, which it said had been one of the most challenging it had ever faced.
"What is clear to us from the study is that the virus wasn't brought in from the outside, that it is indigenous," said Tarik Jasarevic, a spokesman for the WHO, which was not involved in authoring the study.
"It means there were possibly outbreaks in the past that were just not detected," he said.
Samples tested in Ghana and Sierra Leone have been negative so far. And the WHO tweeted on Thursday that the clinical samples of six suspected cases in Mali also tested negative for Ebola.
In an effort to contain the epidemic, countries in the region have imposed restrictions ranging from basic health checks at airports to Senegal's decision to completely shut its land border with Guinea.
Gambia earlier this month banned Banjul-bound aircraft from picking up passengers in Guinea, Liberia and Sierra Leone. (Writing by Joe Bavier; Editing by Emma Farge and Andrew Roche)