German researchers have identified three deaths associated with the bornavirus, which had previously been thought to only be fatal in animals.

The bornavirus is spread by field mice and has long been a danger for animals like horses. Humans were thought to be immune, said Martin Beer, head of the Institute for Diagnostic Virology at the Friedrich Loeffler Institute in Greifswald, Germany.

Two of the cases were people who received organ transplants from the same donor. Details are still not clear about the third case.

"We assume that in these cases we're dealing with a very unusual, unique case," said Beer. Researchers from Berlin's Robert Koch Institute are also looking into the matter.

Researchers became interested in the case in 2016 after doctors could not clarify why two patients had died of brain inflammation or encephalitis.

They called in researchers who had been trying since 2015 to explain three unexplained cases of fatal encephalitis among people who raised squirrels in eastern Germany. A new analytic method used in the research into the squirrel breeder deaths led to the discovery of a new kind of bornavirus, VSBV-1.

Using that same method on the unexplained deaths, they discovered that the new deaths were due to classic bornavirus, BoDV-1.