Viral Encephalitis In the Americas


Encephalitis (encephalomyelitis) is inflammation of the brain, usually caused by a virus.  In the Americas, Eastern equine encephalitis, Western equine encephalitis, Venezuelan equine encephalitis, St. Louis encephalitis and La Crosse encephalitis are all mosquito-borne, viral infections that can cause severe disease in humans.
 
Eastern equine encephalitis (EEE) is a very severe disease, causing 33% mortality and resulting in significant brain damage in many survivors.  It is present throughout the Americas and the Caribbean.  The virus is maintained in nature through a bird-mosquito-bird cycle, but can also infect a wide range of animals, including mammals, reptiles, amphibians, and humans.   EEE is transmitted to humans by mosquito species such as Coquiletidia perturbans, Aedes vexans, Ochlerotatus sollicitans and Ochlerotatus canadensis; so-called ‘bridge vectors’ that feed on both birds and mammals.  Evidence also suggests that the rapidly spreading Asia tiger mosquito, Aedes albopictus, might be a particularly efficient vector of EEE.
 
Western equine encephalitis (WEE) has been isolated from Argentina to western Canada.  Like EEE, Western equine encephalitis also cycles between birds and mosquitoes, with a variety of mammals as incidental hosts.  However, it is less deadly than EEE.  The majority of people infected show no symptoms at all.  The mortality rate among victims is approximately 3-4% and is associated mostly with infection in the elderly.  In North America, important WEE mosquito vectors include Culex tarsalis, Aedes melanimon, Aedes dorsalis and Aedes campestris.  In South America, Aedes albifasciatus, a neotropical flood mosquito, is the primary vector.
 
Venezuelan equine encephalitis (VEE) normally cycles between mosquitoes and rodents or marsupials, with humans and horses as occasional incidental (so-called ‘dead end’) hosts.  However, during epidemic outbreaks in animals, the virus can extend to human and horse populations, often causing a high proportion of horse deaths.  Up to 90% of exposed horses and humans become infected, and the majority display symptoms.  In horses, the disease can be very serious with a case fatality rate ranging from 38-90%. In humans, most infections are mild, and fewer than 1% of adults and 4% of children develop encephalitis.  Of those people who develop encephalitis, however, nearly 20% may succumb.  VEE epidemics typically begin in horses, with human cases developing weeks later. Many mosquito species can transmit epizootic VEE, including those in the genera Aedes, Anopheles, Culex, Mansonia, Psorophora and possibly Ochlerotatus.  Serious VEE outbreaks have occurred in several Central and South American countries in recent years.
 
St. Louis encephalitis (SLE) is found throughout North, Central, and South America, and the Caribbean, but is a public health problem mainly in the United States. The virus normally cycles between common birds (sparrows, finches, blue jays, robins, doves) and specific types of Culex mosquitoes (Culex tarsalis, Culex quinquefasciatus, Culex pipiens) that feed mainly on birds.  Humans are occasional incidental hosts.  The virus is associated with a mortality rate of 5-30%.  Older individuals, especially those 60 years or older, have a much greater risk of developing serious illness from SLE.
 
La Crosse encephalitis is found mainly in the upper midwestern United States and in the Appalachian region.  The virus normally cycles between chipmunks and squirrels and a specific woodland mosquito species (Aedes triseriatus) that breeds in tree holes and manmade containers and bites during the day.  Children and those who spend time in the outdoors are at most risk from the virus.  Most disease cases occur in children under 16 years of age.  However, the mortality rate among victims is low (<1% of clinical cases).  The Asian tiger mosquito (Aedes albopictus) is an efficient vector of the virus and may be helping the disease to spread into other parts of the US.