Does reservoir host mortality enhance transmission of West Nile virus?
BACKGROUND
Since its 1999 emergence in New York City, West Nile virus (WNV) has become the most important and widespread cause of mosquito-transmitted disease in North America. Its sweeping spread from the Atlantic to the Pacific coast was accompanied by widespread mortality among wild birds, especially corvids. Only sporadic avian mortality had previously been associated with this infection in the Old World. Here, we examine the possibility that reservoir host mortality may intensify transmission, both by concentrating vector mosquitoes on remaining hosts and by preventing the accumulation of "herd immunity".


RESULTS
Inspection of the Ross-Macdonald expression of the basic reproductive number (R0) suggests that this quantity may increase with reservoir host mortality. Computer simulation confirms this finding and indicates that the level of virulence is positively associated with the numbers of infectious mosquitoes by the end of the epizootic. The presence of reservoir incompetent hosts in even moderate numbers largely eliminated the transmission-enhancing effect of host mortality. Local host die-off may prevent mosquitoes to "waste" infectious blood meals on immune host and may thus facilitate perpetuation and spread of transmission.


CONCLUSIONS
Under certain conditions, host mortality may enhance transmission of WNV and similarly maintained arboviruses and thus facilitate their emergence and spread. The validity of the assumptions upon which this argument is built need to be empirically examined.
Foppa
Ivo M
IM
Department of Epidemiology, Arnold School of Public Health, University of South Carolina, Columbia, SC 29208, USA. ifoppa@sc.edu
Spielman
Andrew
A
D000818:Animals; D003198:Computer Simulation; D046368:Crows / Q000821:virology; D009033:Culicidae / Q000821:virology; D004197:Disease Reservoirs / Q000821:virology*; D014901:West Nile Fever / Q000453:epidemiology / Q000401:mortality* / Q000635:transmission* / Q000821:virology; D014902:West Nile virus / Q000472:pathogenicity*