France reported the first human case of the Shamonda virus usually found in livestock on Thursday in a hospital patient who died, though authorities said there was no risk to the general population.
It is suspected that the victim was bitten by a midge carrying the virus in eastern France where it had circulated among animals this spring and summer, the government said in a statement.
The man has been admitted to hospital in Lyon with an unrelated serious disease, a French Health Ministry official said.
The victim was in his 60s and lived in the Lyon area, the newspaper Le Progres reported.
The UN's World Health Organisation said it was in contact with French authorities about the outbreak.
“Based on current information and knowledge of the transmission of closely related viruses, this is an isolated human case and it is unlikely to lead to wider spread at this time,” said the Geneva-based WHO, which co-ordinates responses to international public health issues and emergencies.
Gregory Emery, head of France's health directorate, told reporters: “We are clearly dealing with a case that is unprecedented from a scientific and virological point of view. It is linked to this man's severely weakened immune system. From a medical and public health point of view, there is no alert and no crisis.”
Shamonda is transmitted by biting midges and most commonly found in cattle, though it has also been found in horses, alpaca, sheep and goats, according to the World Organisation for Animal Health.
There has been no report of cattle-to-human or human-to-human transmission, Mr Emery said.
The virus is not classified as a disease under the EU Animal Health Law and not subject to official surveillance or control measures.
Identified in Nigeria in the mid-1960s, Shamonda was first confirmed in Europe in late July after genomic tests on sick cattle in southern Germany, Switzerland and eastern France.
France's weekly animal health surveillance bulletin said it had also been detected in Austria, Belgium, Ireland, Italy, Liechtenstein, Luxembourg, the Netherlands, Portugal, Slovenia, Spain and Jersey.
The virus has several effects on animals, with infected cattle showing diarrhoea, weakness, loss of appetite and, less often, fever, French animal health authorities said.
A study of nine French dairy farms found milk production fell by up to 75 per cent.