"Virulent to equids and humans and involved in epizootics. The other subtypes and varieties are enzootic.

b A variety related to IIIB, Bijou Bridge virus, is not listed here. It is associated with cliff swallow bugs (Oeciacus vicarius; Hemiptera: Cimicidae) in western North America.

"Virulent to equids and humans and involved in epizootics. The other subtypes and varieties are enzootic.

b A variety related to IIIB, Bijou Bridge virus, is not listed here. It is associated with cliff swallow bugs (Oeciacus vicarius; Hemiptera: Cimicidae) in western North America.

shows the classification and vector associations of these viruses.

Many VEE "enzootic" virus subtypes and varieties exist in cycles involving rodents and mosquitoes of the Culex (Melanoconion) group, such as Cx. ocossa, Cx. panocossa, andCx. taeniopus in Central and South America. Rodents in the genera Sigmodon, Oryzomys, Zy-godontomys, Heteromys, Peromyscus, and Proechimys are important vertebrate hosts; birds, opossums, and bats also may be reservoir hosts. The ecology of the epizootic viruses is quite different. A large number of species of mosquitoes in several different genera (see Table IV) have been implicated as vectors of the epizootic/epidemic virus strains. Equids attain sufficient viremia to infect these mosquitoes. VEE epidemics can be maintained by mosquito-equid-mosquito transmission, unlike WEE and EEE epidemics, in which equids are for the most part dead-end hosts. Wading birds, particularly green herons, have been incriminated as vertebrate hosts of epizootic strain IA-B in Panama, with the crab hole mosquito De. pseudes functioning as vector. Persistence of epizootic strains of VEE in interepidemic periods is not well understood; thus their emergence in epidemics among equids and humans is difficult to predict.

The single representative of the VEE viruses in the United States, other than during epizootics in Texas, occurs in the Everglades region of southern Florida. Called Everglades virus, it is associated with Cx. (Melanoconion) cedecei (formerly, Cx. opisthopus) as vector and cotton rats (Sigmodon hispidus) and cotton mice (Peromyscus gossyp-inus) as vertebrate hosts. The zoonotic setting is the hardwood hammocks of the Everglades, where mosquito and rodent habitats overlap. Serosurveys of Seminole and Miccosukee Indians in these regions have shown that many Indians have antibodies to VEE virus, but there have been very few cases of human disease attributable to this virus.

Humans infected with an epizootic or certain enzootic strains of VEE virus may show no symptoms, only mild flulike symptoms, or severe encephalitis with acute onset of vomiting, headache, seizures, and fever. Symptoms tend to be most severe in children. During epidemics, the mortality rate is typically less than 1%, although in some epidemics the mortality rate has been considerably higher.

The VEE viruses in Central America and northern South America have been intensively studied because of the history of epidemics among equids and humans in these regions. Outbreaks of VEE have occurred periodically in South America, Central America, and Mexico since the 1930s. The first VEE virus was isolated in 1938 from a dead horse in Venezuela. In 1969, a large outbreak of VEE involving both equids and humans in Central America spread northward through Mexico in the next 2 years, moving into Texas in 1971. Cases continued in Mexico through 1972. There were thousands of both horse and human cases throughout this region during that time, but epizootic virus activity did not occur again there until an outbreak in Venezuela in 1992—1993 and in Chiapas, Mexico, in 1993. More recently, an outbreak of VEE occurred in northern Colombia in 1995. The rapidity of spread of these outbreaks over large geographic areas undoubtedly is due to the role of horses as competent reservoir hosts.

Chikungunya (CHIK) virus This virus occurs in eastern Africa and parts of India and southeastern Asia (Jupp and Mcintosh, 1988). It generally does not cause the encephalitis-type symptoms characteristic of EEE, WEE, and VEE viral infections, but rather causes a DEN-like arthralgic illness of fever, rash, and severe pain in the joints. In Africa, CHIK virus infects nonhuman primates such as vervet monkeys (Cercopithecus aethiops) and baboons (Papio ursinus), with Ae. africanus, Ae. luteo-cephalus, Ae. opok, Ae. furcifer, Ae. taylori, and Ae. cordel-lieri as enzootic vectors in savannah and forest cycles. In humans it produces viremia sufficient to infect Ae. ae-gypti, the vector in urban areas.

Ae. aegypti is the vector of CHIK virus in urban India and Asia, where it causes epidemics of arthralgic disease during rainy seasons. A 1994 epidemic in Vellore, southern India, showed that human cases increased during August and September, reaching a peak in October, as the human-biting frequency and viral infection rate of Ae. aegypti increased. The epidemic lasted about 5 months and affected 44% of the city's population. This epidemic and others, along with experimental studies, indicate that interrupted feeding resulting in partial blood meals may facilitate both mechanical and biological transmission of CHIK virus, thus rapidly amplifying the virus in human populations.

Other Alphaviruses

There are other important alphaviruses that occur endem-ically and epidemically and cause fever, arthralgia, and other symptoms in humans.

O'nyong-nyong (ONN) virus This is an antigenic subtype of CHIK virus that is transmitted among humans by Anopheles species in widespread parts of Africa. The vectors are the same ones that transmit human malaria parasites (i.e., An. gambiae and An. funestus). A large epidemic occurred from 1959 through the 1960s, infecting about 2 million people in Uganda, Kenya, Tanzania, Malawi, Zambia, and Mozambique. The virus, whose name comes from an Acholi African word meaning "weakening of the joints," was first isolated from humans during this time and was later isolated from An. funestus in 1974. Only isolated cases occurred from that time until an epidemic in Uganda in 1997. Neither a vertebrate animal host nor the mechanism of persistence of ONN virus between these epidemics is known. The reservoir host is probably humans. A closely related virus, called Igbo Orel, also is transmitted by Anopheles mosquitoes and occurs in parts of West Africa, where it was associated with an outbreak of CHIK-like disease in the Ivory Coast in 1984. ONN and Igbo Ora are the only arboviruses causing human disease that have Anopheles mosquitoes as the primary vectors.

Sindbis (SIN) virus This virus is distributed widely in Eastern Europe, Scandinavia, the former Soviet Union, Asia, Africa, the Middle East, and Australia. The virus is a member of the WEE virus complex. It was originally isolated from Cx. univittatusm Egypt in 1952 and has been associated with a human disease of rash, fever, and muscle and joint pain in Uganda, South Africa, and Australia. Birds are vertebrate hosts. A subtype of SIN virus is Ockelbo, which is distributed in Sweden, Finland, and northern Russia. Ockelbo is transmitted by Cs. ochroptera, and possibly Culex and Aedes species, and has been associated with human disease similar to that caused by SIN virus.

Ross River (RR) virus This virus occurs in Australia, Fiji, and the Cook Islands, where it causes an illness known as epidemic polyarthritis, consisting of fever, rash, and arthralgia (Kay and Aaskov, 1988). In Australia, RR virus is transmitted by Ochlerotatus camptorhynchus, Oc. vigilax, and Cx. annulirostris, whereas in the islands the vectors are Ae. aegypti and Ae. polynesiensis. The virus was first isolated from Oc. vigilax in 1963. The vertebrate reservoir hosts are unknown, but in Australia they may be marsupials. Barmah Forest virus is another mosquito-borne alphavirus in Australia that causes symptoms similar to RR virus.

Mayaro virus This virus occurs in the Caribbean and parts of South America. In humans the illness is similar to CHIK. The virus was first isolated from febrile patients in Trinidad in 1954. Marmosets are the reservoir hosts of the virus, and Haemagogus mosquitoes are vectors.

The Flaviviridae, genus Flavivirus, contains eight antigenic complexes plus many unassigned viruses involving 70 types, subtypes, and varieties distributed worldwide. Some of these have mosquitoes as vectors, while others are associated with ticks or with rodents or bats. The flavivirus diseases include some of the most dangerous and historically significant infections of humans. Table V

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