Anopheles quadrimaculatus Say is historically the most important vector of malaria in the United States. Malaria was a serious plague in the United States until its eradication in the 1950s (Rutledge et al. 2005). However there are still occasional cases of local transmission of malaria in the United States vectored by A. quadrimaculatus in the east and Anopheles freeborni in the west (CDC 2005 in Rios and Connelly, 2008).
This mosquito is susceptible to infection with malaria causing Plasmodium falciparum, Plasmodium vivax and Plasmodium malariae(Carpenter and LaCasse 1955). The Ohio State University Mosquito Pest Management Bulletin (1998) reports that, A. quadrimaculatus is the most important vector of malaria attacking humans in the eastern United States and can be found frequently in houses and other shelters. Their bites are less painful than many other mosquitoes and often go unnoticed.
A. quadrimaculatus can also transmit Cache Valley virus (CV) (Blackmore et al), West Nile Virus (CDC, 2007) and transmission of St. Louis encephalitis has been obtained with this species in laboratory experiment (Horsfall 1972 in O’Malley, 1992).
A. quadrimaculatus has been found to be an excellent host for dog heartworm (Dirofilaria immitis). According to Lewandowski et al. (1980), this is probably one of the most important species involved in the natural transmission of dog heartworm in Michigan. In central New York, this species was also the most efficient host of dog heartworm out of several species tested, both in the laboratory and the wild (Todaro and Morris 1975).
A. quadrimaculatus can be a vector for the myositic parasite Trachipleistophora hominis. Weidner et al. (1999) found that, Microsporidian spores of T. hominis Hollister, isolated from a human, readily infected larval stages of both A. quadrimaculatus. The authors state that, "Nearly 50% of the infected mosquito larvae survived to the adult stage. Spores recovered from adult mosquitoes were inoculated into mice and resulted in significant muscle infection at the site of injection".