WASHINGTON (AP) - Blood banks should encourage donors to call them back if they suffer symptoms of West Nile virus soon after giving blood, so the bank can decide if the donated blood should be quarantined, the government said Friday.

West Nile virus is spread mostly by infected mosquitoes, but federal scientists discovered last month it also occasionally is spread through donated blood or organs. The Food and Drug Administration cites 15 people who may have caught West Nile from a blood transfusion, out of more than 3,300 cases of the disease this year.

Once the blood connection became known, the FDA reminded blood banks not to allow donations from people with fever and flu-like symptoms, which can signal West Nile as well as other infections.

Updated FDA guidelines released Friday also encourage donors to call back the blood bank if they develop such symptoms within two weeks of giving blood and live in an area where West Nile is circulating. Blood bank directors then would evaluate the case and decide if that person's blood donation should be withheld.

How to protect the blood supply from West Nile is tricky: There is no test for donated blood, only 20 percent of people infected with West Nile develop symptoms, and the vast majority of mosquito bites are innocent.

But the FDA said anyone diagnosed with West Nile or who has suggestive symptoms and lives in an outbreak area should not give blood until two weeks after symptoms disappear or four weeks after they began, whichever is later.