This virus is common throughout the western hemisphere, and until West Nile Virus was introduced to New York in 1999 it was the mosquito - transmitted disease agent of greatest medical importance in North America. St. Louis Encephalitis is transmitted by Culex mosquitoes in North America. Birds are generally considered to be the most important hosts for amplification of the virus, with the disease transmission cycle typically described as mosquito - bird - mosquito. Nestling birds are considered to be particularly susceptible to infection, possibly because they are confined to the nest and less able to defend themselves from biting mosquitoes. They also display less immunity and display higher levels of viremia than do adults of the same species. Among the birds known to be suitable hosts of St. Louis Encephalitis are northern cardinal, Cardinalis cardinalis ; American robin, Turdus migratorius ; northern bobwhite, Colinus virginianus ; blue jay, Cyanocitta cristata ; northern mockingbird, Mimus polyglottos ; and mourning dove, Zenaidura macroura . In urban environments birds such as the house sparrow, Passer domesticus ; house fi nch, Carpodacus mexicanus ; and European starling, Sturnus vulgaris , are important in virus amplifi cation. Birds inoculated with the virus by mosquitoes quickly develop viremia, allowing other mosquitoes to contract the disease by feeding on the blood of the infected bird, although the virus disappears equally quickly as the birds recover from infection. Thus, the ‘ window of opportunity ’ for mosquitoes is quite short, and disease spread occurs mostly when mosquitoes are very abundant. Outbreak of the disease dissipates as the birds become resistant to the virus. Unlike some other mosquito - borne arboviruses such as Eastern Equine Encephalitis and West Nile Virus, St. Louis Encephalitis does not adversely affect the bird hosts.
Other vertebrates are suspected to be hosts in the southeastern USA, where antibodies were found in Virginia opossum, Didelphis virginiana ; northern raccoon, Procyon lotor ; cotton mouse, Peromyscus gossypinus ; and nine - banded armadillos, Dasypus novemcinctus. Elsewhere, antibodies were found in big brown bat, Eptisicus fuscus ; little brown bat, Myotis lucifugus ; coyote, Canis latrans ; red fox, Vulpes vulpes ; striped skunk, Mephitis mephitis ; jackrabbits, Lepus spp.; deer mouse, Peromyscus maniculatus ; yellow - bellied marmot, Marmota fl aviventris ; and many other vertebrates. The presence of antibodies, though suggestive, does not by itself prove that an animal is an important host because the concentration of virus may be inadequate for successful infection of mosquitoes, or the host may not be attractive to mosquitoes that are important in the spread of disease to humans. A few mammals have been shown experimentally to develop viremia adequate for spread of the virus, including wood rat, Neotoma mexicana ; Audobon ’ s cottontail rabbit, Sylvilagus audubonii ; young (but not old) cotton rat, Sigmodon hispidus ; and least chipmunk, Eutamias minimus .
In Central and South America, the disease is transmitted by at least seven genera of mosquitoes. As in North America, birds seem to be the most important hosts, but the St. Louis Encephalitis virus has also been found in other vertebrates such as vesper mouse, Calomys musculinus ; grass mouse, Akodon arviculoides ; a rice rat, Oryzomys nigripes ; southern opossum, Didelphis marsupialis ; howler monkey, Alouatta nigerrina ; a spider monkey, Atles panisicus ; a three - toed sloth, Bradypus tridactylus ; various bats, and others. Whether or not these non - bird vertebrates are important in disease cycling is not evident, but at the very least these animals provide mosquitoes with blood meals, enhancing mosquito survival and reproduction.
Many vertebrates have become viremic, some consistently, but disease is not apparent in wildlife. In humans, the virus ranges from unapparent infection to coma and fatal encephalitis. However, only about 1% of infections are clinically apparent and most remain undiagnosed. Different strains of the virus exist, and they differ in their ability to cause disease. The young and elderly are more often affected, but typically it is a more severe ailment in the elderly than in the young. St. Louis Encephalitis inflicts neurological damage such as memory loss, paralysis, and deterioration of fine motor skills in humans. The damage is limited to the brain and spinal cord.
