A field study on the effects of Fort Morgan virus, an arbovirus transmitted by swallow bugs, on the reproductive success of cliff swallows and symbiotic house sparrows in Morgan County, Colorado, 1976.

We studied the transmission of Fort Morgan (FM) virus within colonies of nesting Cliff Swallows and House Sparrows under three bridges in Morgan County, Colorado during 1976. Nests were examined, and blood or brain specimens were collected from nestlings once or twice a week. Flying birds and small mammals were also studied. We analyzed nesting activity, virus isolations from nestlings of both species, fledging success, multiple infections within a brood of nestlings, infection frequency by age of nestlings, nestling mortality, and infection frequencies by avian species and bridge site. Fort Morgan virus was isolated from 7% (80/1, 156) of the blood and brain samples collected from nestlings. The duration of viremia for nestling House Sparrows was at least 3-4 days based on virus isolation from sequential blood samples. Viremia of nestling Cliff Swallows and House Sparrows did not reduce fledging success, nor were young nestling sparrows viremic more frequently than older nestling sparrows. Nest destruction (by falling down) was a more important cause of nestling mortality than FM virus infection. All age groups of nestling sparrows were viremic at equal rates, but younger nestlings (less than or equal to 7 days old) were more likely than older nestlings (greater than 7 days old) to develop an encephalitic infection. Among nestling House Sparrows, FM virus infections were clustered in time and space. Nestling House Sparrows with FM virus-infected nest-mates were infected more often than conspecifics whose nest-mates were not infected. We concluded that nestling Cliff Swallows and symbiotic House Sparrows that reside in swallow nesting colonies are the principal vertebrate hosts for the maintenance and amplification of FM virus.