A red squirrel project on Anglesey has suffered a setback after an animal died, for the first time in Wales, from a virus carried by grey squirrels.
The wood will be retrapped to find how many squirrels are left
The adult red squirrel died of the squirrel pox virus, despite being caught and given antibiotics.
No further infected squirrels have been found, said Dr Craig Shuttleworth from the project.
But it will be next month before it is known exactly how many squirrels are left following a woodland survey.
"Squirrel pox virus has led to the deaths of hundreds of red squirrels in England but until now it has never been recorded in Wales," said Dr Shuttleworth.
"The virus is carried by grey squirrels which show no symptoms, but if a red squirrel gets the virus it is dead within two to three weeks."
The Anglesey squirrel died despite being discovered whilst still alive and taken to the Welsh Mountain Zoo at Colwyn Bay where it was treated with antibiotics.
"Unfortunately the animal died five days later and a post mortem revealed it had died from squirrel pox virus."
As a result of the death, Dr Shuttleworth said, all the woodland nest boxes were emptied and cleaned with an anti-viral solution.
Fresh bedding was placed inside each, and then it was treated with a powder insecticide in order to kill any flea eggs that remained, he added.
Also, all feeders in the immediate locality were taken down and then destroyed.
"However, those further away were left in place so that we could monitor at least part of the red squirrel population."
The Newborough forest was also trapped for four weeks, with four grey squirrels killed.
"Despite this setback we should end on a positive note," added Dr Shuttleworth.
"Wild red squirrels are still visiting all the feeders that are present away from the area where the infected red was found."
He said the 700 hectare woodland would be retrapped to quantify how many animals were lost to 'squirrel pox' and also to ensure that no further grey squirrels were present.