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FIGURE 12.24 Geographic distribution of dengue and dengue hemorrhagic fever. (Reconstructed from World Health Organization, Vector Biology and Control Division, 1989, and other sources.)

that the virus has been spreading into more susceptible populations. Still another hypothesis is that, as human populations in tropical cities in Asia and the Americas increase and as dengue epidemics in general increase in frequency, there are simply more cases with the noticeable manifestations of DHF and DSS. The fourth hypothesis, and the one currently viewed as most likely, is that prior exposure to one serotype, followed by exposure to another serotype within a critical period of 5 years, leads to the development of hemorrhagic fever and, in some cases, shock syndrome. The more recent epidemics taking place in various parts of the world, especially in the Americas, indicate that DEN has replaced YF as the major urban, epidemic flavivirus of importance worldwide.

The main epidemic vector of DEN viruses, Ae. aegypti (Fig. 12.23), is ideal, because it commonly rests indoors, feeds preferentially on humans, has a tendency to take supplementary blood meals, and often moves from one residence to another as it oviposits. The larvae develop in such vessels as water barrels and jars, potted-plant containers, cemetery urns, and discarded tires. The close proximity of these larval habitats to human dwellings further facilitates Ae. aegypti— human contact and allows large mosquito populations to develop. To the extent that larval sites are hand-filled, DEN transmission is independent of rainfall patterns. Indeed, in some places where water stores are replenished independently of rainfall, epidemics may occur in the hot, dry season when temperatures are higher and the extrinsic incubation period of the viruses in the mosquitoes is shortened. However, in areas where breeding containers depend primarily on rainwater, dengue epidemics occur during rainy seasons.

Other vectors of DEN viruses are Ae. albopictus (Fig. 12.25) in rural areas of Southeast Asia and Ae. polynesiensis, Ae. scutellaris, Ae. pseudoscutellaris, and

FIGURE 12.25 Asian tiger mosquito (Aedes albopictus), female feeding on human. (Photo by W. A. Foster.)

Ae. rotumae in the Pacific region. The role of the newly introduced Ae. albopictus in the New World as a vector of DEN viruses remains to be determined. In peninsular Malaysia, a series of studies showed that DEN viruses circulated between monkeys and mosquitoes of the Ochlerotatus niveus group, suggesting the possibility of an enzootic sylvatic cycle. However, humans generally are considered to be the only vertebrate host in situations where monkeys do not occur, such as congested urban slums in huge tropical cities, including Bangkok, Manila, Jakarta, Caracas, and Guayaquil. Thus, a mosquito-human—mosquito cycle of DEN transmission is the usual means of virus maintenance and epidemic spread. Transovarial transmission of some of the DEN viruses has been demonstrated in Ae. aegypti and Ae. albopictus, and therefore mosquitoes may be reservoirs, particularly in periods of low-level transmission among humans.

Japanese Encephalitis Virus Complex

Another important group of mosquito-borne flaviviruses is the Japanese encephalitis virus antigenic complex, including West Nile, Japanese encephalitis, St. Louis encephalitis, and Murray Valley encephalitis viruses. Some authorities refer to this complex as the West Nile antigenic complex. These viruses occur in widely separated geographic regions but show similarities in the nature of their enzootic cycles. Each has Culex vectors (Table V) and birds as vertebrate reservoirs. In the case of JE virus, pigs often serve as amplifying hosts. Figure 12.26 shows the worldwide distribution of this complex ofviruses. The most important of these in terms of human morbidity and mortality is JE virus. However, all cause human illness of varying severity, depending upon the virus and age and health of the person.

West Nile (WN) virus This virus is widely distributed in Africa, the Middle East, Europe, parts of the former Soviet Union, India, and Indonesia (Fig. 12.26) (Hayes, 1988). It has been the cause of endemic and epidemic fever, myalgia, and rash, especially in children in the Middle East, and has caused encephalitis in some instances. Seroprevalence of antibody to WN virus in Egypt can exceed 60% in adults. Recent epidemics have occurred in Israel (1950s), southern France (1962), South Africa (1974 and 1983-1984), Romania (1996), and other eastern European countries. WN virus was first isolated from the blood of a febrile man in Uganda in 1937. Like the other members of this complex, the primary mosquito vectors of WN virus are in the genus Culex, particularly Cx. pipiens and Cx. univittatus. Other species also may be important. As with the other viruses in this complex, birds are vertebrate reservoirs and amplifying hosts during epidemics. In 1999, WN virus was introduced through unknown means into New York City and was linked to human encephalitis in 61 confirmed cases (mostly in the

FIGURE 12.25 Asian tiger mosquito (Aedes albopictus), female feeding on human. (Photo by W. A. Foster.)

St. Louis encephalitis (SLE) virus This virus was identified as the causative agent of disease during an outbreak of encephalitis-like illness in the United States, in Paris, Illinois, in 1932 and St, Louis, Missouri, in 1933. It was isolated from a patient with encephalitis in the Yakima Valley of Washington in the early 1940s and found to be a frequent cause of human illness in the Central Valley of California in the 1930s and 1940s. This virus is distributed widely in North America and also occurs in parts of Mexico, Central America, the Caribbean, and South America to Argentina (Fig. 12,26). The encephalitic illness caused by SLE virus shows a bimodal age distribution, with children and elderly people most frequently affected. Attack rates during epidemics range from 5 to 800 per 100,000 population, depending upon location, year, strain virulence, and population immunity due to earlier epidemics. In the eastern United States, mortality rates have ranged from about 3 to 20% of laboratory-diagnosed cases, but in the western United States mortality rates are lower.

In North America, three enzootic cycles of SLE virus have been described. In the eastern United States, north of Florida but including Texas, the primary vectors are Culexpipiens and Cx. quinquefascmtus (Fig. 12.27). The former mosquito occurs in a more northerly distribution, whereas the latter is more southerly, with a hybrid zone at about the latitude of Memphis, Tennessee. Females of both species feed on birds. In addition, Cx. quinquefasciatus females frequently feed on mammals as the summer progresses. Whether these mosquitoes alone or other vectors function in transmission to humans depends upon the abundance of these two species and other competent vectors. House sparrows are important vertebrate amplifier hosts in peridomestic settings. In the

FIGURE 12.27 Culex sjuinquefasciatusfemales laying eggs in form of floating rafts; these are vectors of filariasis and St. Louis encephalitis, among other diseases. (Photo by W. A. Foster.)

western United States, a mosquito—bird—mosquito cycle similar to that of WEE virus, involving Cx. tarsalis, has been elucidated. In addition, in California both Cx. pip-iens and Cx. quinquefasciatus, and possibly Cx. stigmatosoma, function secondarily as either enzootic or epidemic vectors. In Florida, Cx. nigripalpus apparently is the enzootic, epizootic, and epidemic vector of SLE virus. In Latin America and in the Caribbean basin, SLE virus has been isolated from many different species of Culex, Sa-bethes, Mansonia¡ Wyeomyia, and other genera and from a wide variety of birds and mammals. In these areas, human St. Louis encephalitis generally is rare. The mechanism of virus overwintering in North America is not well known. There is some evidence of virus persistence in overwintering, diapausing female mosquitoes.

The history of St. Louis encephalitis in North America has been that of epidemics, either local or widespread, with intervening years when there was apparently no virus activity or were no epidemics, and either no case or a few isolated human cases. The first epidemic, in the early 1930s in St. Louis, Missouri, USA, was accompanied by hot and dry weather, which favored the development of populations of mosquitoes of the Cx. pipiens complex, the larvae of which develop in water rich in sewage. The epidemic involved about 1100 human cases and 200 deaths. Since that time, there have been some 50 outbreaks of St. Louis encephalitis in the United States. Cases during three recent decades are shown by state in Fig. 12.28. Human cases also have occurred in Manitoba and Ontario, Canada. These epidemics have been both rural and urban. A very large outbreak occurred in 1975, involving 30 states and the District of Columbia, with over 1800 cases reported. More recently, epidemics have occurred in such disparate locations of the United States as Pine Bluff (Arkansas), Florida, Los Angeles (California), Houston (Texas), New Orleans (Louisiana), and Grand Junction (Colorado). A total of 4437 human cases of SLE infection were reported to the United States Centers for Disease Control and Prevention from 1964 to 1995, with an average of 139 cases per year (range, 4-1967). Monath (1980) and Tsai and Mitchell (1988) have reviewed the ecology and public health significance of St. Louis encephalitis.

Murray Valley encephalitis (MVE) virus This virus has been associated with encephalitis-type illness in humans in eastern and western parts of Australia and in New Guinea (Marshall, 1988) (Fig. 12.26). Mortality rates vary greatly during outbreaks, ranging from 18 to 80%. Epidemics of encephalitis in Australia in 1917, 1918, 1922, and 1925 were probably caused by MVE virus. It was isolated from the brain of a human in 1951 and from a pool of Cx. annulirostris in 1960. Later epidemics occurred in Australia in 1956, 1971, 1974, 1978, 1981,

FIGURE 12.27 Culex sjuinquefasciatusfemales laying eggs in form of floating rafts; these are vectors of filariasis and St. Louis encephalitis, among other diseases. (Photo by W. A. Foster.)

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