Arbovirus infection increases with group size

Buggy Creek (BCR) virus is an arthropod-borne alphavirus that is naturally transmitted to its vertebrate host the cliff swallow (Petrochelidon pyrrhon ota) by an invertebrate vector, namely the cimicid swallow bug (Oeciacus vi carius). We examined how the prevalence of the virus varied with the group size of both its vector and host. The study was conducted in southwestern N ebraska where cliff swallows breed in colonies ranging from one to 3700 nes ts and the bug populations at a site vary directly with the cliff swallow c olony size. The percentage of cliff swallow nests containing bugs infected with BCR virus increased significantly with colony size at a site in the cu rrent year and at the site in the previous year. This result could not be e xplained by differences in the bug sampling methods, date of sampling, samp le size of the bugs, age structure of the bugs or the presence of an altern ate host, the house sparrow (Passer domesticus). Colony sites that were reu sed by cliff swallows showed a positive autocorrelation in the percentage o f nests with infected bugs between year t and year t + 1, but the spatial a utocorrelation broke down for year t + 2. The increased prevalence of BCR v irus at larger cliff swallow colonies probably reflects the larger bug popu lations there, which are less likely to decline in size and lead to virus e xtinction. To the authors' knowledge this is the first demonstration of arb ovirus infection increasing with group size and one of the few known predic tive ecological relationships between an arbovirus and its vectors/hosts. T he results have implications for both understanding the fitness consequence s of coloniality for cliff swallows and understanding the temporal and spat ial variation in arboviral epidemics.