Officials at the Patuxent Wildlife Research Center in Laurel are designing a vaccine to protect the refuge's 33 remaining captive whooping cranes after a virus killed seven of the rare birds last year.

The virus has been identified as eastern equine encephalitis, a sleeping sickness that can infect people and horses and is spread to birds by swamp-breeding mosquitoes, according to Dr. Jim Carpenter, a research veterinarian for the U.S. Fish and Wildlife Service's captive propagation program at the center.

"It's a new threat to our birds. The virus came to this area before but never caused death. We're taking it very seriously because we can't afford another loss," Carpenter said recently.

With fewer than 150 of the big-winged, delicate birds surviving in North America, researchers are scrambling to perfect a vaccine to protect the cranes at the 4,700-acre refuge at Patuxent, the world's largest wildlife research center.

There are only two flocks of wild whooping cranes -- one consisting of 83 birds that migrate between Canada and Texas and another flock of 28 birds that migrate between Idaho and a refuge in New Mexico.

Carpenter said three pairs of breeding cranes at the refuge have produced eight eggs this spring. By the end of the breeding season, officials hope, the cranes will lay 18 eggs that should produce 12 to 14 live whooping crane chicks, he said.

The population of whooping cranes in the United States, which has never been large, has dwindled as a result of the draining of prairie potholes -- the birds' habitat -- to provide agricultural land in the West.