Canada has issued warnings about mosquitoes

The warning comes as the Scottish National Blood Transfusion Service (SNBTS) is appealing for 60,000 donors this summer to meet demand.

Officials fear they face a bigger challenge than ever this year because of the disease.

An epidemic of West Nile virus, a mosquito-borne summer bug, is expected to hit the USA and Canada this year.

Anyone who visits the countries between 1 July and 31 October will have to wait for 28 days after returning to the UK before they give blood.

The SNBTS believes it will lose up to 2,000 donors because of these restrictions, which are aimed at eliminating the risk of transmitting the virus via blood transfusion.

Launching Blood Donor Week in Glasgow, Dr Moira Carter said: "We must recruit 60,000 new or returning donors in 2003 to meet the needs of Scotland's patients.

This year is particularly worrying due to the impact that West Nile disease will have Moira Carter SNBTS

"Their blood will treat the 4,000 premature babies and over 200 new leukaemia cases we expect in Scotland this year.

"It is always challenging to maintain blood supplies during the summer period and this year is particularly worrying due to the impact that West Nile disease will have on blood collection."

Dr Carter urged anyone planning to travel to Canada or the United States this summer to give blood before they go.

The appeal is being launched at the oncology ward at Glasgow's Yorkhill Children's Hospital.

Dr Carter said: "Every child on this ward will receive blood or blood products as part of their treatment."

One mother from Lewis, whose daughter suffers from leukaemia, said she could not stress how important the service was.

Nichola Finlayson is being treated for leukaemia

Christina Finlayson said five-year-old Nichola has already benefited from 30 units of donated blood and will require further treatment on Tuesday.

"They have six units of blood on standby in case she haemorrhages," she said.

"If we didn't have that blood and she haemorrhaged, she would die."

West Nile virus is found in wild birds and is transmitted between birds by mosquitoes.

Occasionally the virus, which causes some people to suffer a flu-like illness and which can on rare occasions prove fatal, is passed from mosquitoes to humans.

Because of the involvement of mosquitoes, human cases are only found in the summer and autumn which is why the SNBTS is operating its deferral process between July and the end of October.