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       This is one of the Alphavirus spp. that occurs in North and South America.  Service (2008) notes that it is the most severe encephalitis virus in humans and horses.  The causative agent was first found in infected horses in 1933.  By 1938 human cases were identified when 30 children died from the infection in the eastern United States.  These deaths were directly associated with outbreaks in horses of the area.  In humans the fatality rate is about 34 percent and there is no treatment for infections.  Antigenic variants of the virus are the highly pathogenic North American type and the less potent South American kind, which some authorities suggest might be two distinct viruses.


       Infection is found in many different mammals, reptiles amphibians and birds.  A bird-mosquito cycle sustains the virus in nature where two mosquito species are primarily involved:  Culiseta melanura and Culiseta morsitans.  The mosquitoes draw blood from birds and the virus increases throughout the summer as more mosquitoes and birds become infected.


       Other mosquito species that feed both upon birds and mammals are involved in the transmission to humans.  In the eastern United States the virus is found mostly in urban areas where according to Service (2008) it is vectored by Culex quinquefasciatus and Culex pipiens.  However, later studies have focused attention rather on Aedes vexans, Coquillettidia perturbans, Ochlerotatus canadensis and Ochlerotatus sollicitans.  Domestic chickens and wild birds are the amplifying hosts.  In the western United Sates it is found mostly in rural areas associated with the rice field-breeding mosquito Culex tarsalis.


       The virus titre in in the blood of horses, humans and other mammals is never high enough to infect additional mosquitoes.  However, the virus has been contracted from exposure of different parts of the body to parts of dead infected animals.


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Key References:     <medvet.ref.htm>    <Hexapoda>


     Matheson, R. 1950.  Medical Entomology.  Comstock Publ. Co, Inc.  610 p.

      Service, M.  2008.  Medical Entomology For Students.  Cambridge Univ. Press.  289 p

      Legner, E. F.  1995.  Biological control of Diptera of medical and veterinary importance.  J. Vector Ecology 20(1): 59_120.

      Legner, E. F.  2000.  Biological control of aquatic Diptera.  p. 847_870.  Contributions to a Manual of Palaearctic Diptera,

          Vol. 1, Science  Herald, Budapest.  978 p.