The largest-ever outbreak of Ebola was triggered by a toddler's chance contact with a single infected bat, a team of international researchers will reveal, after a major investigation of the origins of the deadly disease now ravaging Guinea, Liberia, Ivory Coast and Nigeria.

A group of 17 European and African tropical disease researchers, ecologists and anthropologists spent three weeks talking to people and capturing bats and other animals near the village of Meliandoua in remote eastern Guinea, where the present epidemic appeared in December 2013. They have concluded that the disease was spread by colonies of migratory fruit bats. Their research is expected to be published in a major journal in the next few weeks.

News of the research came as the first confirmed case of a Briton contracting the disease emerged on Saturday night. Professor John Watson, deputy chief medical officer, said the overall risk to the UK public remains "very low".

Early studies suggested that a new strain of Ebola had emerged in west Africa but, according to epidemiologist Fabian Leendertz, a disease ecologist at the Robert Koch Institute in Berlin, who led the large team of scientists to Guinea, it is likely the virus in Guinea is closely related to the one known as Zaire ebolavirus, identified more than 10 years ago in the Democratic Republic of the Congo. Leendertz said the virus had probably arrived in west Africa via an infected straw-coloured fruit bat. These bats migrate across long distances and are commonly found in giant colonies near cities and in forests.

The outbreak has killed more than 1,300 people in west Africa so far, many of the deaths occurring in Liberia. Within a week of the two-year-old boy catching the disease in Meliandoua, both he and his mother had died and it was spread to nearby communities and urban areas by mourners at a funeral.

Scientists have suspected for several years that bats are the wild "reservoirs" of Ebola, but direct transmission to humans is extremely rare, despite communities regularlyhunting the bats for food. Nearly all previous epidemics had been linked to the bushmeat trade, with hunters picking up dead infected animals in the forestand selling them on. Previous outbreaks saw catastrophic death rates in gorilla and chimpanzee populations, which led some scientists to think they may be responsible for the disease spreading.

Chimps, gorillas, some antelopes and even pigs – which possibly eat fruit dropped to the ground by infected bats – have all been linked by the World Health Organisation to the spread of the disease, but the researchers now say no evidence has been found of other animals apart from bats being infected.

Primatologists and conservationists in the region who were contacted by the Observer all confirmed that vulnerable chimpanzee colonies had so far escaped.

Rebecca Kormos, a primatologist who spent two years conducting a chimpanzee survey in Guinea, said: "The other aspect that worries me is … the human population density is high in Guinea and people forage and travel in nearly all of the forests where chimpanzees live. This means that the risk from transmission between apes and humans would be high."

Fruit bats, however, are widely eaten in rural west Africa – either smoked, grilled or in a spicy soup. Leendertz said: "We spent eight days in Meliandoua. They told us they regularly catch bats, like every other village in Guinea, Sierra Leone and Liberia. The evidence is not 100% and we can only say that it is possible. I would not say that the virus has emerged from central Africa. There are huge colonies of bats which regularly migrate. They can travel far in one night. I don't think an individual bat or colony migrated all the way from Congo or Gabon to west Africa. These big colonies are connected. There is a possibility for the virus to mix between colonies. They [the bats] share the same fruit. It is likely not to have even been one species of bat. The virus may jump from one species to another."

If bats are confirmed as the reservoir of the disease, forest communities could try to destroy their vast colonies. "That would be an ecological disaster," said Leendertz, "because bats pollinate plants and devour insects. And bat hunts would also only increase human contact with potentially infected animals."