Role of Corvids in Epidemiology of West Nile Virus in Southern California

Abstract

The invasion of different southern California landscapes by West Nile virus (WNV) and its subsequent ampliÞcation to epidemic levels during 2004 enabled us to study the impact of differing corvid populations in three biomes: the hot Colorado desert with few corvids (Coachella Valley), the southern San Joaquin Valley (Kern County) with large western scrub-jay but small American crow populations, and the cool maritime coast (Los Angeles) with a large clustered American crow population. Similar surveillance programs in all three areas monitored infection rates in mosquitoes, seroconversion rates in sentinel chickens, seroprevalence in wild birds, numbers of dead birds reported by the public, and the occurrence of human cases. Infection rates in Culex tarsalis Coquillett and sentinel chicken serocon-version rates were statistically similar among all three areas, indicating that highly competent mosquito hosts were capable of maintaining enzootic WNV transmission among less competent and widely distributed avian hosts, most likely house sparrows and house Þnches. In contrast, infection rates in Culex pipiens quinquefasciatus Say were statistically higher in Kern and Los Angeles counties with elevated corvid populations than in Coachella Valley with few corvids. Spatial analyses of dead corvids showed signiÞcant clusters near known American crow roosts in Los Angeles that were congruent with clusters of human cases. In this area, the incidence of human and Cx. p. quinquefasciatus infection was signiÞcantly greater within corvid clusters than without, indicating their importance in virus ampliÞcation and as a risk factor for human infection. In contrast the uniform dispersion by territorial western scrub-jays resulted in a high, but evenly distributed, incidence of human disease in Kern County. KEY WORDS West Nile virus, American crow, western scrub-jay, Culex tarsalis, Culex pipiens quinquefasciatus THE FAMILY CORVIDAE OF the order Passeriformes contains six species in California, of which American crows, Corvus brachyrhynchus; western scrub-jays, Aphelocoma californica; and common ravens, Corvus corax, are abundant in southern California at lower elevations historically affected by mosquito-borne en-cephalitis viruses (Reeves 1990). These species are highly susceptible to infection with West Nile virus (family Flaviviridae, genus Flavivirus, WNV) and produce elevated viremias during acute infection before death (Komar et al. 2003, Brault et al. 2004, Weingartl et al. 2004, Reisen et al. 2005). Rapid onset and frequent death of corvids because of WNV infection coupled with their large size has facilitated their use as a surveillance tool to track WNV during epidemics (Eidson et al.