Fears of Anglesey and Gwynedd red squirrel virus outbreak
People are being asked to keep a look out for sick squirrels on Anglesey and in Gwynedd.
It comes after a red squirrel was found to have died from squirrel pox virus in Treborth Botanic Garden, Bangor.
Conservationists fear that if the virus spreads, it could wipe out large numbers of red squirrels in an area where they have recently been thriving.
Dr Craig Shuttleworth, from Bangor University, said it was "concerning" as it spreads "extremely rapidly".
He said the virus is transferred from grey squirrels, which are not affected by the disease, to red squirrels.
Speaking to BBC Radio Wales' Good Morning Wales programme, he said it could wipe out "90% of red squirrels in the area within a matter of months".
"We're concerned about the squirrels in Gwynedd but we have a huge population on Anglesey very close to the bridges. If it was to get there it would be terrible," he said.
"It's an isolated case but how do you find a dead squirrel in a huge vast area of woodland? There could be others, we don't know.
"We're reliant on the public to send us in sightings and tell us if they've seen anything."
There have been moves to boost red squirrel numbers in Gwynedd following the success of the population on Anglesey, which increased from about 40 squirrels in 1998 to more than 700.