EEE: The Nature of the Disease & Its Effect on People

Mosquito-caused encephalitis. In the U.S., perhaps the worst danger from mosquitoes is encephalitis. There are certain viruses - carried by mosquitoes - which cause encephalitis in people and sometimes horses. These diseases are collectively called the encephalitides. They are essentially animal diseases, called "zoonoses", which only occasionally get transmitted to people. Most of them circulate among birds or small mammals by way of mosquito bites. Humans (or horses) get infected by accident - and thus are called "accidental hosts". All encephalitides more or less cause similar symptoms in humans, although with great differences in severity. Some of them are extremely mild with a 1% or less mortality rate, while others may kill as many as half of the people infected. And, making matters worse, even for the survivors, there are sometimes long-lasting effects such as memory loss, personality changes, etc. Below is a breakdown of the common mosquito-carried encephalitis viruses in the United States and some characteristics of each one.

West Nile encephalitis. West Nile virus (WNV) was identified for the first time in the Western Hemisphere in New York in 1999. Apparently, it was accidentally brought to the U.S. in an infected mosquito (maybe inside an airplane), an infected animal (perhaps a bird), or an infected person (traveling from an international location). By the end of the year, the virus had caused encephalitis in 62 people and numerous horses in and around New York City, resulting in 7 human and 10 equine deaths.10-11 The virus continued to spread in 2000, and now evidence of WNV has been found in at least 12 states and the District of Columbia. WNV will likely eventually occur throughout the eastern United States. As far as severity of the disease, WNV is no more dangerous than SLE (one of our "native" encephalitis viruses), with a mortality rate of 3-20%. Like SLE, WNV is more dangerous to older patients. A lot is yet unknown about the ecology of WNV in the U. S., but we do know the virus causes a bird disease, and is transmitted by mosquitoes. WNV is believed to be transmitted to humans by Culex pipiens, Cx. restuans, Cx. salinarius, and possibly Aedes japonicus.

Eastern equine encephalitis. Eastern equine encephalitis (EEE) is generally the worst strain, being severe and frequently fatal (mortality rate 30 to 60%). It is especially bad in children. Fortunately, large and widespread outbreaks are not common; between 1961 and 1985 only 99 human cases were reported in the U.S.5 EEE occurs in late summer and early fall in the central and northcentral U.S., parts of Canada, southward along the coastal margins of the eastern U.S. and the Gulf of Mexico, and sparsely throughout Central and South America. The life cycle of EEE is poorly understood. The virus circulates in wild bird populations by bird-feeding mosquitoes, but the exact mechanism of spread to humans is largely speculative. It is believed to be transmitted to humans by the mosquitoes, Aedes. sollicitans, Coquillettidia perturbans, and possibly Ae. vexans and Anopheles crucians.

St. Louis encephalitis. St. Louis encephalitis (SLE) produces lower mortality rates than EEE (3 to 20%), but occurs occasionally in large epidemics over much of the U.S. In contrast to EEE, SLE is worse in older people. But, like EEE, most cases occur in late summer. In 1933 there were 1,095 cases in the St. Louis area with more than 200 deaths.6 In 1975-76 there were over 2,000 cases reported from 30 states, primarily in the Mississippi valley.6 SLE is transmitted by Culex tarsalis (western and southwestern U.S.), Cx. quinquefasciatus (central and southeastern U.S.), and Cx. nigripalpus (southeastern U.S.).

Western equine encephalitis. Western equine encephalitis (WEE), occurring in the western and central U.S., parts of Canada, and parts of South America, has occurred in several large outbreaks. There were large epidemics in the north central U.S. in 1941 and in the central valley of California in 1952. The 1941 outbreak involved 3,000 cases. During 1964-1997, there were 639 human WEE cases reported to the CDC, for a national average of 19 cases per year.7 WEE is generally less severe than EEE and SLE, with a mortality rate of only 2 to 5%. Cases appear in early to midsummer, and are primarily due to bites by infected Culex tarsalis mosquitoes.

LaCrosse encephalitis. LaCrosse encephalitis (LAC) is a California group encephalitis which primarily affects children in the midwestern states of Ohio, Indiana, Minnesota, and Wisconsin. Cases have also occurred in the southern states, but certainly not to the extent that they have in the midwestern U.S. The mortality rate of LAC is less than 1%, but infection often leads to seizures. In fact, that is one of the main symptoms - seizures in infants and children. The national average for LAC cases is 73 per year.7 Most cases occur in July, August, and September. LAC is transmitted to humans by the tree hole mosquito, Aedes triseriatus. Interestingly, the virus may be transferred from adult female Ae. triseriatus to her offspring through the eggs. Some amplification of the virus takes place in nature through an Ae. triseriatus, wild vertebrate cycle.

Other California group encephalitis. Although LAC (above) encephalitis is probably the most notorious, several other California group encephalitis viruses exist. North American forms include California encephalitis (CE), Jamestown Canyon (JC), Jerry Slough (JS), Keystone (KEY), San Angelo (SA), Trivittatus (TVT), and others. Viruses in the California serogroup are primarily pathogens of rodents and rabbits. They are transmitted to people by several species of mosquitoes, but especially the tree-hole, floodwater, and snow pool Aedes spp. California group encephalitis viruses generally produce only mild illness in humans (mortality rates 1% or so).

Venezuelan equine encephalitis. Venezuelan equine encephalitis (VEE) is relatively mild in humans and rarely affects the central nervous system, but it will be included here. VEE is endemic in Mexico and Central and South America; epidemics occasionally reach the southern U.S. Cases generally appear during the rainy season. Although the mortality rate is generally <1%, significant morbidity is produced by this virus. In an outbreak in Venezuela from 1962 to 1964, there were more than 23,000 reported human cases with 156 deaths.8 In 1971, an outbreak of VEE in Mexico extended into Texas resulting in 84 human cases.9 There has been a relatively recent outbreak in Colombia and Venezuela during the summer of 1995 with at least 13,000 human cases.

Cause for Future Concern

It appears that we are in a precarious situation. The entire ecosystem - including plant and animal life on earth - is being affected by humans. People once lived in far-removed, relatively isolated groups. Now we are all essentially one large community. Further, things such as population increases, building cities in/near jungles, and widespread and frequent air travel are providing the opportunity for a great plague. For example, the number of international departures from U.S. airports doubled from 20 million to nearly 40 million between 1983 and 1995.12 A person hiking in the Amazon jungles today might be in New York City tomorrow. Should one or more new "emerging" mosquito-borne diseases begin to spread, control of the epidemic would be difficult. If the disease agent is a virus, specific treatments are unavailable (or, at least, untested against the arboviruses). The only way to stop a viral mosquito-borne illness is to kill the mosquitoes to a low enough level to interrupt virus transmission. Since mosquitoes can fly, control of an epidemic is even harder. Compounding all of this, many mosquito species are resistant to many of the currently available (older class) insecticides used to control them.


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