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Received for publication February 25, 2002; accepted February 25, 2002 (02/033).
of the 20th century are discussed, as is the need for rebuilding the public health infrastructure to deal with epidemic vector-borne diseases in the 21st century. © 2002 IMSS. Published by Elsevier Science Inc.
fever, Venezuelan equine encephalitis, West Nile, Yellow fever.
Centers for Disease Control and Prevention, Public Health Service, U.S.
the arthropod to become infected while taking a blood meal.
genera. Most arboviruses of public health importance belong to three families: Flaviviridae; Togaviridae, and Bunyaviridae (8) (Table 1).
0188-4409/02 $–see front matter. Copyright © 2002 IMSS. Published by Elsevier Science Inc.
ME SFI.S. Emergence/Resurgence of Arboviral Diseases In the past 20 years. primates Birds. underline designates the most important ecology.R R. Europe. e. most caused by viruses once thought to be controlled or by viruses that were known but that had never been considered public health problems. rodents Sandflies Mosquitoes Mosquitoes Mosquitoes Ticks Midges ? ? Rodents Rodents Rodents ? Geographic distribution Epidemics SFI SFI. In general. accompanied by more frequent and larger epidemics. Saudi Arabia Asia Europe. Louis encephalitis West Nilea? Kyasanar Forest diseasea Omsk hemorrhagic fever Tick-borne encephalitis Bunyaviridae Sandfly fevera Rift Valley fevera La Crosse encephalitis California encephalitis Congo-Crimean hemorrhagic fevera Oropouchea Vector Vertebrate host Ecologyb Disease in humansa Mosquitoes Mosquitoes Mosquitoes Mosquitoes Mosquitoes Humans. marsupials Birds ? Birds U. pigs Birds Birds Birds Birds Primates. ME SFI. which in turn influence vegetation patterns and other ecologic parameters that determine the geographic distribution of arthropod vectors and vertebrate hosts. Asia. although these viruses are also still maintained in zoonotic cycles in the rain forests of Asia and Africa in a mosquito-monkey-mosquito cycle (10). the viruses have been introduced into new geographic regions and have taken advantage of susceptible vertebrate and arthropod hosts to become established and cause major epidemics/epizootics. the important limiting factors include temperature. ME SFI.ME SFI. ME  meningoencephalitis. but the majority are found in tropical areas where climate conditions permit year-round transmission by cold-blooded arthropods (5.Global Resurgence of Arboviral Diseases 331 Table 1. HF SFI. with few exceptions. ME SFI. .R R. ME SFI. The more important arboviruses causing human disease Family/Virus Togaviridae Chikungunyaa Ross Rivera Mayaroa O’nyong-nyonga Sinbis Barmah Foresta Eastern equine encephalitis Western equine encephalitis Venezuelan equine encephalitisa Flaviviridae Dengue 14a Yellow fevera Japanese encephalitisa? Murray Valley encephalitis Rocio St.7. HF  hemorrhagic fever.. primates Humans. Europe. global epidemic arboviral activity has dramatically increased (5).S.S.U R R SFI SFI SFI SFI SFI Mosquitoes Mosquitoes Mosquitoes Mosquitoes ? Birds Birds. R  rural.. zoonoses that depend on animal species other than humans for maintenance in nature (7). Asia Europe. the most important arboviruses are those that cause major mosquito-borne epidemics because they produce viremia in humans (9) (Table 1). The most important reservoir hosts for arboviruses of public health importance are birds or rodents. Arboviruses as a group have a global distribution. In still other cases.U R. North America Yes Yes Yes Yes Yes Yes Yes Yes No No R R R. HF. ME Worldwide in tropics Africa. S  suburban. viruses have expanded their geographic distribution regionally. ME SFI. rabbits Rodents R R R R Mosquitoes Mosquitoes Mosquitoes Mosquitoes Mosquitoes Mosquitoes Mosquitoes Ticks Ticks Ticks Humans. In other cases. ME Africa. U.9). Africa.S. a The arboviruses are. West Nile virus in the U. rodents. Americas Australia Americas Americas Americas Yes Yes Yes Yes Yes Yes Yes Yes Yes U. Africa Central and South America Yes Yes No Yes Yes Yes Arboviruses that produce significant human viremia levels. ME SFI.S. primates Humans.S R R R. ME SFI. Asia. HF SFI. South America Asia. At least one arbovirus (dengue).g. Asia. and humidity. Africa. Europe. Asia Australia. India.S. Figure 1 highlights the most important epidemics occurring since 1990. Australia.U R. HF SFI. Few regions of the world escaped epidemic arboviral disease during this time. has adapted completely to humans and is maintained in large urban centers of the tropics in a mosquito-human-mosquito transmission cycle that no longer depends on other animal reservoirs. dengue fever. Individual arboviruses may have a focal geographic distribution that is limited by the ecologic parameters governing their transmission cycle.S. and the most important arthropod vectors are mosquitoes and ticks (Table 1). however. South Pacific South America Africa Asia. The geographic distribution of some mosquito vectors and some viruses has expanded globally. In terms of public health.HF. Asia Africa.S SFI. rainfall patterns.U SFI SFI. Pacific Australia South America Americas Africa.U R R R.S.U R. HF SFI Europe. ME SFI.g.S. e.S. Humans generally are dead-end or incidental hosts who usually do not contribute to the transmission cycle by developing viremia and infecting arthropods.U R R R. ME SFI. camels Rodents Birds. bU  urban.S. Middle East North America North America. cSFI  systemic febrile illness.
with several hundred thousand cases of the severe form of disease. Several diseases will be discussed as case studies to illustrate the changes that have occurred and that have allowed these agents to become major public health problems at the beginning of the 21st century. Epidemic arboviral diseases. All four viruses are now maintained in an endemic cycle involving humans and Aedes aegypti mosquitoes in most urban centers of the tropics. They are likely a combination of the increased movement of viruses in people among countries and regions. Co-circulation of various virus serotypes in a community (hyperendemicity) is the single most common risk factor associated with the emergence of the severe form of disease—dengue hemorrhagic fever (DHF)—in an area (10. and genetic changes in circulating or introduced viruses that give them greater epidemic potential. With the exception of Southeast Asia.5–3 billion people are at risk of infection (10–12) (Figure 2). epidemic dengue was effectively controlled in most of these countries in the 1950s and 1960s as a side benefit of malaria and yellow fever control programs. Dengue fever is an old disease that was spread around the world as commercial shipping expanded in the eighteenth and nineteenth centuries (10). at least one virus. has the potential to cause a global public health emergency because most urban centers of the tropics are permissive for transmission. 1990–2000. depending on epidemic activity. Rift Valley fever. and thousands of deaths. DEN-2. Each year an estimated 50–100 million dengue infections occur. DEN-3. Ross River. One to four DEN viruses may be endemic in the same human population.g. There are four dengue viruses (DEN-1. yellow fever. and DEN-4) that are antigenically distinct but have the same epidemiology and cause similar illness in humans. By the beginning of the 20th century. Dengue/Dengue Hemorrhagic Fever The dengue viruses (family Flaviviridae: genus Flavivirus) are unique among the arboviruses as the only members of this group that have evolved and fully adapted to the human host and his/her environment. DHF. e. and Japanese and Venezuelan equine encephalitis. generally. it was a major public health problem in most tropical countries. e.11). thus rendering them more likely to be transmitted from host to host..g.. The factors responsible for emergence of hyperendemicity associated with periodic epidemics in an area are not well understood. they are associated with only mild illness (silent transmission) during inter-epidemic periods but cause epidemics associated with more severe disease every 3–5 years. essentially eliminating the need for maintenance in the primitive enzootic forest cycle. causing major epidemics. The dengue viruses are the most widespread geographically of the arboviruses and are found in tropical and subtropical areas where 2.332 Gubler/ Archives of Medical Research 33 (2002) 330–342 Figure 1. The changing epidemiology of arboviral diseases is complex and unique to each virus. In many . Finally. the level of herd immunity to specific virus serotypes in human population.
Romania in 1996 was reported to the international health community (18). Italy. Other than sporadic epidemics in France (1962) ) and South Africa (1974 and 1983–1984) and occasional isolations from vertebrate hosts or mosquitoes. therefore. the Czech Republic. 1999. Emergence of dengue hemorrhagic fever in the Americas. thus. 1998. 2001. 2000. 2000.. The reasons for this dramatic emergence of epizootic/epidemic disease caused by a virus that rarely gave rise to severe disease are not well understood. health authorities are often not notified. none is expected in the near future.S. WN virus was rarely seen and was considered of only minor importance to public health. 1999. In 2002. 1997. 2000).. 2001). U. 2001. The public health community was unaware of the out- . but a new more virulent strain of virus with greater epidemic potential likely emerged and was propagated in the Mediterranean region and subsequently spread to new geographic regions in the late 1990s (19). Epizootics and epidemics of severe neurologic disease in horses. 1998. only the epidemic in Bucharest. U. 1998. 2001. such as occurred in North Africa and the Middle East in the 1990s. dengue is the most important arboviral disease of humans. Thus. increased disease incidence and epidemic activity are closely correlated with trends of increased population growth in urban areas of the tropical developing world (Figure 4). when WN virus was shown to be widespread in the Middle East and India and caused outbreaks of human disease in Israel. Epizootics occurred in horses (Morocco. In the American tropics there has been a dramatic resurgence of epidemic dengue fever and the emergence for the first time in history of DHF in the past 20 years (Figure 3). 1997. DHF is a leading cause of hospitalization and death among children. Although considerable progress has been made in recent years to develop a vaccine for DEN/DHF. In the mid-1990s. Congo. birds. It was not observed again until the 1950s. Surveillance for arbovirus diseases is generally poor in most countries. Prevention and control of epidemic disease. West Nile Virus West Nile (WN) virus (family Flaviviridae: genus Flavivirus) was first isolated from a person with a nonspecific febrile illness in Uganda in 1937 (16).13). Israel. 1997. 1994. U. here it will not be discussed further (10–15). World distribution of dengue. Romania. and humans began to occur with increasing frequency (17–25). 2000) and in birds (Israel. Israel. 1997. and lack of effective mosquito control (11..Global Resurgence of Arboviral Diseases 333 Figure 2.S. 1996.S. Also. and epidemics occurred in humans (Algeria. 2000. 2000. 1996. and when epidemics do occur they are often not reported to international health agencies. Figure 3. 1998. 1999. increased movement of people among population centers via modern transportation. 2000. Russia. There have been several recent reviews of this disease. 1999. Tunisia. when epidemics primarily affect animals with only peripheral human involvement. 1999. 1999. Southeast Asian countries. 1998. must rely on mosquito control for the indefinite future. France. the epidemiology of WN virus apparently changed.
S. including an epizootic on Long Island.334 Gubler/ Archives of Medical Research 33 (2002) 330–342 Figure 4. Concurrent with the 1999 U. it is known that WN virus moves north in spring and south in fall with migrating birds (25. and Maryland) and involved severe and fatal neurologic disease in humans. although an epidemic was not reported. Geographic expansion of WN virus to the Western Hemisphere was detected in 1999. two patients died of WN virus-associated neurologic disease during that time. it is likely that the major epizootic among birds in New York City in summer 1999 would have been recognized and associated with WN virus earlier. there is a large legal and illegal traffic of exotic birds and other animals in the U. 40 persons died. virus is identical to the WN virus that caused the epizootic in domestic geese in Israel in 1998 (30) (Figure 5). there were an estimated 480 human cases of overt WN disease.32). horses. from the Middle East could have been infected with WN virus. suggesting more widespread transmission (30). By the end of the 1999 transmission season. Had these epizootics been discussed among public health and veterinary health officials. but there are several possibilities. thus. Israel may have occurred in the fall of 1999. an outbreak among humans in Tel Aviv.S. is not certain. with 25 clinical cases and nine deaths (36% CFR) (26). including seven deaths (11% case-fatality rate [CFR]) were reported (23). Numerous equine cases were documented. Evidence suggests WN virus was introduced into the New York City area in the spring or early summer of 1999 from the Middle East. Finally. unpublished data). outbreak. New Jersey.. Russia. and of neurologic disease in horses in a number of countries. although there is uncertainty concerning when and from where introduction actually occurred. First. A serologic survey among stable mates and other horses in the area showed that 31% had been infected with WN virus by detection of specific neutralizing antibody (CDC 1999. Connecticut. 84 of which were diagnosed as encephalitis. (19. human disease might have been diminished. In that epidemic.S.31. a major epidemic in humans associated with severe and fatal neurologic disease occurred in Volgograd. and one or more of these animals imported into the U. if not prevented. A third possibility is that a WN virus-infected mosquito hitched a ride on an airplane from the Middle East. but no disease in birds was reported (22).S. The epicenter of the outbreak was in the Queens section of New York.27–29). How WN virus was introduced into the U.S. breaks of neurologic disease in domestic geese in Israel that began in 1997 and continued through 2000. Thousands of birds belonging to many native species died of WN virus infection in northeastern U. Genetic sequence studies have shown that the U. but the epizootic spread to at least four states (New York. Relationship between global population growth and DF/DHF incidence. These planes land at JFK International Airport in New . and several other mammalian species (19). Second. New York. A WN virus-infected bird migrating north from Africa may have become disoriented and ended up in New York City.S. 62 human cases of severe neurologic disease. birds.
was the epizootic among birds. and control were drafted and implemented in all 48 lower continental states (33). Fourth. Limited data from the Israel epidemics suggest that some humans have a viremia level high enough to infect mosquitoes (34. Because the most dramatic effect of the introduction of WN virus into the U. the most likely scenario is that the virus was introduced by some of the many thousands of visitors who come to New York City each week from the Middle East.S. unpublished data) (33).. In this case. . the previously mentioned possibilities would likely be rare events. With the exception of those involving humans and birds.S.35). however. especially the American crow (Corvus brachyrhyncos)—thousands of crows. if they occurred at all. however. it was an alert for public health officials in the U. horses. WN virus could have been purposely introduced into New York City. It should be remembered that invasive species rarely become established in a new geographic region as a result of a single introduction. guidelines for surveillance. Finally. and birds have found no evidence of WN virus in the U. Phylogenetic relationships of West Nile viruses.S. prevention. Congress. and there is no evidence to suggest that the epizootic/epidemic began in that area.S. a human traveler infected in the Middle East and incubating the virus could have become ill after arriving in New York City. Serologic surveys of limited archived serum samples from humans.Global Resurgence of Arboviral Diseases 335 Figure 5. With special funding from the U. a large number of people travel back and forth between New York City and the Middle East on a weekly basis. York City. prior to 1999 (CDC. The appearance of WN virus in the Western Hemisphere in 1999 was yet another instance in which an exotic virus was introduced into a new geographic area and became a public health problem. Although we will probably never know for sure. On the other hand. and hundreds of exotic birds and other animals are imported to the New York area each year. There is no evidence to support this latter possibility as all data collected to date are compatible with a natural introduction in the spring of 1999. where the public health infrastructure for vector-borne diseases had deteriorated badly over the past 30 years and became virtually nonexistent in many state and local health departments (36).
but infection can lead to illness and death (39).28. have died of WN virus infection over the past 3 years—dead bird surveillance provides a highly sensitive method of monitoring the geographic spread of WN virus in North America and was used to monitor geographic and temporal spread of WN virus in the U.S. chipmunks. and as many as 25% of survivors may have serious neurologic sequelae (40). prevention.33). It is likely that WN virus will become established in the western part of the U. the virus has been isolated from a variety of mammals. It is primarily a disease of children. In 2001. rabbits. the state farthest south being North Carolina. WN virus was detected in all states (27 states and the District of Columbia) east of the Mississippi River.000 cases reported annually in Asia (40). World distribution of West Nile virus. Prevention and control of epidemic/ epizootic disease must rely on active surveillance and effective mosquito control. as well as in the Caribbean and in Central and South America. all states of the continental U. squirrels. 21 cases and two deaths (CFR  9. unpublished data). unpublished data) (37). including two species of bats. In addition. The virus had most likely been introduced into more southern states. mosquito surveillance. It is difficult to estimate the total economic impact of this disease. A single case of WN virus encephalitis in a person with no travel history was documented in the Cayman Islands. with 733 confirmed cases and numerous deaths. There is no human vaccine for WN virus. All data suggest that the epizootic in birds is being driven by Culex pipiens-complex mosquito species. and cats.28. The introduction of WN virus into the Western Hemisphere already has had a great public health and economic impact on the areas affected. Humans and horses are both incidental hosts for JE virus. but New York has estimated that its costs alone have been in excess of $100 million U. suggesting that WN virus was probably introduced into that area in the fall of 1999 or 2000. Emphasis was placed on dead bird surveillance. most likely because of intensive mosquito control efforts in northeastern states (19). In 2001. In 2000 and 2001. Overall. with 66 cases and nine deaths (CFR  14%) reported from 10 states (CDC. In 2000. except for West Virginia and South Carolina (CDC. the safety and efficacy of this vaccine have not yet been determined. The epidemiology and transmission patterns of JE virus have changed over the past 20 years (41). second. both belonging to the JE serocomplex (family Flaviviradae: genus Flavivirus).33). surveillance for WN virus was intensified and expanded to include the first states to which the virus was expected to spread south via migratory birds and.S. states. Other states have spent millions to tens of millions of USD rebuilding and improving the pub- Figure 6. JE virus is maintained in an enzootic cycle involving aquatic birds and primarily Culex species mosquitoes (39). JE is enzootic in Asia (Figure 7) and is most commonly found in rural rice-growing areas where flooded fields and irrigation systems provide ideal larval habitats for Culex vector mosquitoes.336 Gubler/ Archives of Medical Research 33 (2002) 330–342 as well as other bird species. however. JE has existed in two transmission patterns: 1) an enzootic/ endemic pattern in tropical areas with year-round trans- . and on surveillance for neurologic disease in equines and humans. dollars (USD). WN virus was detected in 12 states and the District of Columbia. because in 2001 WN virus was found in Florida in early June.S. The epizootic in equines intensified dramatically in 2001. JE virus is the leading cause of viral encephalitis in the world with more than 40. Historically. 111 species of North American birds have been affected by WN virus in 27 U. the District of Columbia. and Connecticut) (19. the virus was much more widespread. but an experimental killed vaccine for equines has recently been developed. Japanese Encephalitis Japanese encephalitis (JE) virus is closely related to WN virus. CFR in humans ranges from 10 to 35%.S. however. New Jersey. Countries that have endemic dengue and yellow fever will need to redesign their surveillance systems to distinguish between these flaviviruses. but has not been detected because of lack of effective surveillance. but the number of opportunistic and mammalphilic species found infected greatly increases the risk to humans and horses (38). lic health infrastructure needed to implement surveillance. However. The virus is no doubt present in the latter two states as well. and from 29 species of mosquitoes belonging to seven genera.5%) were reported from three states (New York. and control programs for WN virus and other arboviral diseases. Pigs act as efficient amplification hosts. fewer human cases of WN virus infection occurred than in 1999. unpublished data) (37). skunks. suggesting that the virus has also been introduced into the Caribbean and Central and South America (CDC. during 2000 and 2001(19. (19.28). and their presence in the peridomestic environment in JE-enzootic areas greatly increases the risk of human and equine infections. Canada (Figure 6).23. and Ontario. In 2000.S.
JE virus has caused major epidemics in parts of India where the virus had never been detected previously. JE has become one of the most important public health problems in Nepal. parts of Southeast Asia. The role that the virus strain plays in determining disease transmission patterns is not yet fully understood. A killed vaccine prepared in suckling mouse brain (Biken vaccine) has been instrumental in helping prevent epidemic JE in Japan. changes in agricultural and animal husbandry practices have also contributed to decreasing the risk of this infection. In Nepal. Major epidemics have occurred in West Africa. aegypti.44). is the type species of the family Flaviviridae: genus Flavivirus. two human cases were detected in 1998 (45). including large cities located in the Amazon basin where YF virus is maintained in an enzootic cycle (Figure 8). 2001. and in the past 20 years most have been reinfested with the principal urban vector mosquito.52). well-known disease that caused major epidemics in the Americas and in Africa from the seventeenth to the twentieth centuries. suggesting more intense transmission (51. and immune status of host populations. safe. epidemic JE was first reported in 1978 but has since spread throughout Nepalese lowland plains and currently occurs in all 25 Terai districts. the urban centers of the American tropics have grown dramatically. these were primarily urban epidemics transmitted by Ae. aegypti. 1930s. and 2) an epidemic pattern in subtropical and temperate regions with seasonal epidemics occurring during summer months (42). geography. It is maintained in an enzootic cycle involving monkeys and canopydwelling Aedes species mosquitoes in sub-Saharan tropical rain forests. The last urban YF epidemic in Latin America occurred in 1942 (48). and epizootic YF has occurred in Kenya. a similar rain forest enzootic cycle became established in the Amazon region. and in the Pacific (41). YF is an old. the epidemic form of disease has been controlled through vaccination and changes in agricultural and animal husbandry practices. are living in crowded urban centers of the American tropics in intimate association with equally large popula- Figure 8. In 2002. most of whom are susceptible to YF virus. In addition. . JE virus had not been detected in the Western Pacific since 1947 and had never been reported in New Guinea or Australia. a native virus of Africa. where at least seven mosquito species and native birds are susceptible to and capable of transmitting JE virus (46). In the intervening 60 years. In subtropical and temperate countries. aegypti mosquitoes were introduced into the Western Hemisphere during the slave trade in the early 1600s. mission. Yellow fever (YF). Since 1995. Taiwan. attenuated vaccine prepared in primary hamster kidney cells has been effective in China (47). involving New World monkeys and mosquitoes of Haemagogus species. East Africa for the first time in history in 1992–1993 (50).. The epidemiologic pattern of virus activity is most likely a function of the climate.Global Resurgence of Arboviral Diseases 337 Yellow Fever Figure 7. There are two effective vaccines for JE virus. At the same time. Moreover. Epidemic YF was controlled in Africa at the same time by immunization with a highly effective. Korea. and economical vaccine. Like DEN/DHF. Elimination of this mosquito from most countries of Central and South America in the 1950s and 1960s effectively controlled urban YF epidemics as well as epidemic DEN in the region (Figure 8). however. continued JE virus activity has been documented. Both YF virus and Ae. Australia in 1995 (43. and Thailand and a live. at least two fatal cases of YF were imported to Europe from West Africa in recent years. aegypti (48). JE has become an emerging disease in the Indian subcontinent. an estimated 150– 300 million people. it is periodically introduced into urban areas where it causes epidemics transmitted by the domesticated form of Ae. 1970. In the American tropics. JE virus moved into the Pacific region. causing an epidemic in Saipan in 1990 and in Torres Strait. It is possible that JE virus could become established in northern Australia and perhaps in other regions such as the U. Geographic distribution of Japanese encephalitis virus.S. Ae. In the 1990s. Aedes aegypti distribution in the Americas. In the past 15 years there has been a resurgence of epidemic YF in Africa (49).
aegypti. Because of this observation as well as some experimental evidence. and the probability of a person incubating YF virus and arriving in an area in which there were adequate Ae. Although some experimental evidence suggests that Ae. thus increasing the risk of epidemic transmission in a new geographic region in which approximately two billion people are susceptible to YF infection. Sabin. a large epidemic occurred in Egypt for the first time (65). A number of Aedes as well as Culex species of mosquitoes have shown to be efficient vectors of this virus (62–68). and most residents of Asian countries have detectable flavivirus antibodies. however. Ae. explosive epizootics occur when there is heavy rainfall in areas where there are herds of sheep or cattle and other livestock (60. aegypti infestation of the area. is currently at the highest risk for epidemics of urban YF in 50 years (53). The natural history of RVF virus is not fully understood. the virus will be repeatedly introduced into Asia and Pacific countries on a regular basis. like DEN viruses. Bolivia in 1998 (54). a number of other flaviviruses are endemic to Asia. When it rains. It is thought that the virus is maintained via transovarial transmission in these floodwater mosquitoes (61–64). It is not known which of these factors were the most important in preventing epidemic YF in Asia in the past. RVF caused a large epidemic in Somalia and Kenya associated with the heavy rains that followed the 1997–1998 El Niño season (69). possibly all three contributed. There is limited experimental evidence that heterotypic flavivirus (DEN) antibody modulates YF infection in monkeys causing milder illness and lower levels of viremia. infected mosquito eggs are deposited in the mud of damboes (ground depressions). It has subsequently been shown to have a natural geographic distribution in most countries of sub-Saharan Africa (60). where the virus is protected until the eggs hatch. global demographics and population movement are very different in that literally tens of millions of persons travel to and from Asia from the Americas via air each year.57). The reasons for this are not well understood because YF was likely introduced into the area in the past. will spread rapidly throughout the American region. aegypti from different geographic areas vary in their susceptibility to YF. greatly increasing the probability that if urban transmission begins in the Americas. In 2000. During 1977–1978.61). RVF virus again moved into a new geo- . First. belonging to the subgenera Aedimorphus and Neomelaniconion. A major concern at the beginning of the 21st century is that urban YF epidemics will once again occur in the American tropics. and a proportion of the adult mosquitoes are already infected with RVF virus when they emerge. an epidemic occurred in Brazil and although urban transmission was not documented. Third. Periodically. This disease could be effectively controlled and would no longer be a potential global public health threat if YF vaccine were incorporated into the WHO Expanded Program on Immunization in all African and American countries at risk. In 1998. past urban YF epidemics occurred in the Americas and West Africa before modern transportation. Although epizootics/epidemics of RVF have occurred over the majority of sub-Saharan Africa. that heterotypic flavivirus antibody does not convey protection against YF infection. It should be noted. it is hypothesized that floodwater Aedes mosquitoes. therefore. most of which are heavily infested with Ae. Rift Valley Fever Rift Valley fever (RVF) virus (family Bunyaviridae: genus Phlebovirus) was first isolated in 1930 during an outbreak in the Rift Valley in Kenya (59). data to support this hypothesis are not definitive (56. If this occurs in today’s world of modern transportation and crowded urban populations. The virus disappeared after a few years but returned again in the early 1990s. aegypti mosquitoes in Asia may not be as susceptible to YF virus as those in the Americas and Africa. There is an effective live-attenuated virus vaccine for YF (48). the eggs hatch. the virus has expanded its geographic distribution in the past 25 years. unpublished data). A small outbreak of urban YF has already been documented in Santa Cruz. A. YF. Secondly. aegypti (10. it was most likely a rare occurrence. YF has never been documented in Asia. The whole region.53). At least three plausible reasons may explain why epidemic YF never occurred in Asian and Pacific countries (55). Epizootics of RVF are unique because they often do not begin in one place and spread to other areas. They thus transmit the virus when they partake of their first blood meal. however. these damboes are flooded. It is in the best interest of the global public health community to initiate this program without delay rather than waiting until the crisis occurs and then attempting to respond after it is too late (36). The natural vertebrate reservoir host is not known. 1946. are the actual reservoirs for RVF virus. Domestic livestock serve as amplifying hosts for the virus.338 Gubler/ Archives of Medical Research 33 (2002) 330–342 tions of Ae. they erupt almost simultaneously over wide geographic areas in association with increased rainfall. In 2001. and the simple logistics of introducing YF virus into Asia were much more difficult than at present. but it is clear that the virus is maintained enzootically over much of sub-Saharan Africa. which serve in turn to infect more mosquitoes. In 2002. This scenario would result in a major global public health emergency. there was a high risk because of Ae. While YF virus was probably introduced. thus allowing the monkeys to survive YF infection (58. Rather. initiating amplification and possibly epidemic transmission if there are herds of cattle or sheep in the area. aegypti mosquitoes to initiate secondary transmission was very low. mainly the four dengue serotypes and JE. and with increasingly rapid movement of larger numbers of people among population centers. and from there most likely to Asian and Pacific countries.
Death is rare but can occur following infection with either enzootic or epizootic variants of VEE virus. It is not known how they are maintained in nature. or ocular disease. but modern molecular technology is helping answer some of the questions (75. Clinically. and F viruses are considered to be enzootic and cycle in tropical and subtropical swamps and forests in a natural rodent reservoir host—Culex (Melanoconion) species mosquito vector cycle. and thus human. and is capable of transovarial transmission. It could also be used to prevent the spread of RVF virus to new locations. Other Arboviruses The diseases discussed above are only a few of the more important arbovirus diseases that are a threat to human and domestic animal health at the beginning of the 21st century. the chances of RVF virus becoming established in new geographic areas into which it is introduced are great. The introduction of RVF virus into new geographic regions is of particular concern because it can cause devastating epizootics among domestic animals. In humans. In the Americas. The majority of infected humans have a nonspecific viral syndrome. only VEE virus (subtype I) will be discussed. A veterinary vaccine is available and could be used more effectively to prevent animal. Prevention and control of epizootic/epidemic VEE depends on effective use of veterinary vaccines for these viruses. Epizootic VEE then disappeared for 19 years until 1992. E. has a very broad mosquito host range. whereas many infections with enzootic variants may be inapparent. caused by IAB VEE. For this paper. In addition. but they have been responsible for major epizootics that have caused thousands of equine and human infections. suggesting that the 1995 epizootic may have been caused by a virus that escaped from the laboratory (78). RVF virus infection causes abortion and a high CFR (72). Many others have the potential to cause epidemic disease .S. Another option. It is not known how the virus was introduced into Egypt or the Middle East.75). These viruses are classified on the basis of serology into six subtypes. began in Guatemala/ El Salvador and spread throughout Central America and Mexico. encephalitis. causing a large epidemic/epizootic in Saudi Arabia and Yemen (70). RVF virus infection causes several different disease syndromes. data suggest that the majority of infections with epizootic-variant viruses lead to disease. Efforts should be focused on the floodwater Aedes species that may be involved in the maintenance cycle. known damboes in areas where humans and domestic animals are at risk could be kept from flooding by plowing ditches to drain the water after it rains. and Peru) from the 1930s through the 1960s (73. and an unnamed virus (AG80-663-VI). when a small outbreak of IC virus occurred in Venezuela (75.Global Resurgence of Arboviral Diseases graphic region. beginning in Venezuela and moving around the La Guajira Peninsula into Colombia (75.76). but a small percentage of patients may progress to develop hemorrhagic fever. U. but more difficult to implement effectively. VEE (subtype I) viruses are further subdivided into five variants or serotypes (AB–F).77). In 1969. control strategies must be developed for Culex and other species that may be involved in epidemic/epizootic transmission. equine vaccination in many countries is not widespread. Colombia. Either way. Mucambo (III). several species of mosquitoes are highly susceptible to RVF virus (71). Pixuna (IV). Epizootic virus infection can lead to encephalitis in a small proportion of cases. For example. This latter epizootic was caused by an IC VEE virus different from the 1992 outbreak virus but similar to a virus not isolated since 1962–1964 (76. Cabasson (V). Another large epidemic/epizootic of IC VEE occurred in 1995.78). These viruses have an American geographic distribution ranging from Argentina to the U. Although it is not known for sure. During epizootic/epidemic transmission. In cattle and sheep. but it may have been transported in infected domestic animals. Others think that RVF virus may have been introduced by infected floodwater Aedes mosquitoes flying from Africa. for example.. rodents.78). occurring more frequently in children than in adults. VEE D. Sequences of the 1964 and 1995 viruses were identical. as well as major epidemics of severe and fatal disease among humans. It is not known for sure where the viruses that caused these epidemics/epizootics originated.75).000 human infections in Colombia alone. Case-fatality rate in humans is 1% (72). the potential for future movement of this virus is great because the virus infects a wide variety of domestic animals.S. attenuated vaccine (TC-83). a killed vaccine (C-84) and a live. especially sheep and cattle. disease. Venezuelan Equine Encephalitis Venezuelan equine encephalitis (VEE) virus (family Togaviridae: genus Alphavirus) is the prototype for an antigenic complex of very closely related viruses isolated from a variety of animals including horses. This epidemic/epizootic involved thousands of horses and an estimated 92.76. in 1971–1972 (73. These latter viruses are not virulent for equines and are not known to cause epizootics (73–75). Periodic epizootics/epidemics of VEE occurred in northern South America (Venezuela. Everglades (II). Prevention and control of RVF in humans rely on preventing disease in domestic animals in the peridomestic environment. and causes a high level of viremia in many. Ecuador. Unfortunately. VEE AB and C viruses are considered epizootic variants and are pathogenic for horses 339 (73–75).75). is mosquito control. a major epizootic. Thus. There are two vaccines available. and mosquitoes (73– 75). mosquito control is an important adjunct to vaccination. ultimately reaching Texas. which include VEE (subtype I). patients infected with both epizootic or enzootic variants of VEE virus develop a nonspecific viral syndrome (74.
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