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This review, we mainly focus on zoonotic encephalitides caused by arthropod-borne viruses (arboviruses) of the families Flaviviridae (genus Flavivirus) and Togaviridae (genus Alphavirus) that are important in both humans and domestic animals. Specifically, we will focus on alphaviruses (Eastern equine encephalitis virus, Western equine encephalitis virus, Venezuelan equine encephalitis virus) and flaviviruses (Japanese encephalitis virus and West Nile virus). Most of these viruses were originally found in tropical regions such as Africa and South America or in some regions in Asia.
However, they have dispersed widely and currently cause diseases around the world. Global warming, increasing urbanization and population size in tropical regions, faster transportation and rapid spread of arthropod vectors contribute to the continuous spreading of arboviruses into new geographic areas causing reemerging or resurging diseases. Most of the reemerging arboviruses also have emerged as zoonotic disease agents and created major public health issues and disease epidemics.
“Zoonosis” is defined as a disease or infection that is naturally transmitted from vertebrate animals to humans. Zoonotic diseases can be either transmitted via direct or indirect contact. Transmission of an infectious agent from a vertebrate animal to a human by an arthropod vector is an example of indirect transmission of zoonotic disease. Viruses that maintain transmission cycles between vertebrate animal reservoirs as main amplifying hosts and insects as primary vectors are known as arboviruses (arthropod-borne viruses). Arboviruses must replicate in the arthropod vectors, such as mosquitoes, ticks, midges or sandflies, prior to transmission. Female mosquitoes acquire virus during blood feeding of an infected animal and the virus replicates in the mesenteron epithelial cells.
The virus released from the mesenteronal epithelial cells infects salivary glands after secondary amplification in other cells and tissues. Some arboviruses can infect the salivary glands without secondary amplification in other cells and tissues. Subsequently, the virus released from the salivary gland epithelium is transmitted during blood feeding of the vertebrate host. Arboviruses are included in different taxonomic families, including Flaviviridae (genus Flavivirus), Bunyaviridae (genus Nairovirus, Orthobunyavirus, Phlebovirus, and Tospovirus), Togaviridae (genus Alphavirus), Rhabdoviridae (genus Vesiculovirus), Orthomyxoviridae (genus Thogotovirus), and Reoviridae (genus Orbivirus and Coltivirus) Many of the important zoonotic arboviruses belong to the families Togaviridae and Flaviviridae .
However, there are many other clinically important human and animal arboviruses belonging to the Bunyaviridae family, such as Crimean-Congo hemorrhagic fever virus (tick-borne) in the genus Nairovirus and Toscana virus (sandfly-borne) and Rift Valley fever virus (mosquito-borne) in the genus Phlebovirus. Colorado tick fever virus in the family Reoviridae (genus Coltivirus) is also an important human arbovirus. Arboviruses are maintained in complex life cycles involving nonhuman primate/vertebrate hosts and primary arthropod vectors. Mosquitoes are the most important vectors that transmit zoonotic viruses. Different mosquito species (Culex spp., Aedes spp., etc.) may act as vectors for the same virus in different vertebrate hosts depending on different geographical and ecological locations. Ticks, sandflies (Phlebotomus spp.) and gnats (Culicoides spp.) are also important vectors of some arboviruses.
Vertical transmission (transovarial and transstadial) occurs in some arthropod vectors as they transmit some arboviruses from parent arthropod to offspring arthropods. This type of transmission mainly occurs in tick-borne encephalitis viruses (TBEVs) but it has been also reported in some mosquito-borne viruses. For example, La Crosse virus, one of the most important viruses among agents causing California encephalitis, is transmitted by its main vector, Aedes triseriatus, not only by transovarial and transstadial routes but also sexually. Most known arboviruses were first isolated in tropical regions such as Africa and South America and in some Asian countries. However, the geographic distribution and frequency of epidemic outbreaks of arboviral diseases have expanded dramatically across the world in the past several decades. Several factors such as changes in viral genetics, host and/or vector population, and climate changes facilitated expansion and transmission of arboviruses resulting in emergence gence/reemergence of arboviral disease outbreaks in new regions in the world.
Extensive tropical urbanization and faster and increased movement of humans and animals with modern transportation helped vectors to be in closer contact with vertebrate reservoir hosts raising transmission potential. Introduction of West Nile virus into the New World and the emergence of Japanese encephalitis virus (JEV) in Australia are a few prominent examples of recent unexpected emerging/reemerging zoonotic diseases. Epidemics/epizootics of humans and domestic animals usually occur when the enzootic virus is introduced into rural environments or comes to close contact with humans by a bridge vector. Usually, humans and domestic animals develop the clinical disease but do not develop a sufficient level of viremia to infect arthropods, thus, they are considered dead-end hosts and do not contribute to the transmission cycle. However, some arboviruses such as dengue fever (DF), yellow fever, and chikungunya (CHIKV) viruses cause high levels of viremia in humans and can be transmitted from person to person by mosquitoes (urban cycle).In this review, we will mainly focus on the transmission and epidemiology of mosquito-borne arboviruses, especially alphaviruses and flaviviruses that are pathogenic to humans as well as domestic animals, thus, increasing the public health and economic significance. Although these viruses, with the exception of JEV, are not currently circulating in the Korean peninsula, there is a great chance for other viruses to emerge when a competent vector and vertebrate host populations happen to be temporally and spatially together in a permissive environment.AlphavirusesAlphaviruses (formerly, group A arboviruses) are enveloped, positive-sense, single-stranded RNA viruses that belong to the genus Alphavirus in the family Togaviridae. The alphavirus genome varies between 11 and 12 kb in length and is composed of a non-segmented, single-strand RNA with a 7-methylguanosine and a poly-A tail at 5´- and 3´-terminus, respectively. It encodes four non-structural proteins responsible for genome replication and protein processing and generates a subgenomic mRNA (26S).
The five structural proteins (C, E3, E2, 6K, and E1) are translated from the 26S subgenomic mRNA.The alphaviruses are widely distributed throughout the world. They have been classified as belonging to either New and Old World alphaviruses: New World alphaviruses (e.g., Eastern equine encephalitis virus [EEEV], Western equine encephalitis virus [WEEV], and Venezuelan equine encephalitis virus [VEEV]) are distributed across the Americas and cause encephalitis in humans, whereas Old World alphaviruses (e.g., Sindbis virus [SINV], CHIKV, O’nyong-nyong virus [ONNV], Ross River virus [RRV], Barmah Forest virus [BFV], and Semliki Forest virus [SFV]), characterized by fever, rash, and arthritis, are found in Europe, Asia, Australia, and parts of Africa. However, RRV, SINV, and CHIKV have been occasionally associated with encephalitis. The alphavirus serogroups can be divided into seven antigenically related complexes: Barmah Forest, Eastern equine encephalitis (EEE), Middleburg, Ndumu, Semliki Forest, Venezuelan equine encephalitis (VEE), and Western equine encephalitis (WEE). All clinically relevant alphaviruses are transmitted by mosquitoes. More than one mosquito species is usually involved in the alphavirus transmission cycle. The survival of alphaviruses in a certain geographic region depends on the presence of competent vectors (mosquitoes) and of vertebrate hosts that develop a viremic infection with low pathogenicity. Important amplification hosts are birds (for SINV, SFV, EEEV, and WEEV), rodents (for RRV, VEEV, BFV), and monkeys (for CHIKV, ONNV, and Mayaro fever virus). EEEV is a zoonotic virus transmitted by mosquitoes and originating in birds. In North America, EEEV is an important cause of disease in domestic animals and humans. The disease is severe in horses, pigs, dogs, and some species of birds. EEEV was first isolated in 1933 from the brains of affected horses during a widespread outbreak in the northeastern US, in New Jersey and Virginia. However, horse deaths have recently been reported further north along the eastern coast of the US (New Hampshire and Maine) and Canada.
In 1936, South American EEEV was first isolated from a horse in Argentina. The EEEV strains present in South and North America are antigenically and genetically different from each other and also differ in human pathogenicity. There are four lineages (I, II, III, and IV) of EEEV based on their antigenicity and distribution in various geographic regions. The closely related North American EEEV (NA-EEEV) lineage I viruses that occur in the US, Canada, and the Caribbean are the most virulent to horses and humans. In contrast, infection of horses or humans with more genetically and antigenically diverse virus strain, enzootic in Central and South America (lineages II, III, and IV [SA-EEEV]), rarely results in significant clinical disease. The lineage II strains are distributed along the coasts of South and Central America, lineage III in the Amazon Basin, and lineage IV in Brazil.
In North America, EEEV is enzootic from the eastern and gulf coasts as far as to inland sites (Texas) [36,37]. NA-EEEV strains are genetically highly conserved, with only one major lineage (lineage I) from the first isolation in 1933. Most EEE outbreaks in North America occur in the late summer and early fall, often associated with heavy rainfall. Outbreaks in horses are common and often accompanied by high case-fatality rates. Eighty to 90% of the infected horses develop the acute and lethal disease, and about 66% of the survivors develop severe neurologic sequelae. During outbreaks or epidemics of EEE, horses do not serve as amplifying hosts but they tend to be the first to develop clinical signs and often serve as an indicator of the start of an outbreak or epidemic Thus, the rapid detection of EEEV in equine specimens is critical for control of disease outbreaks in humans, horses, and other animal species. The NA-EEEV strains are responsible for most human cases.
Human infections are usually asymptomatic, but some progress to severe encephalitis accompanied by high fatality rate or incapacitating sequelae. The disease is generally more severe in the elderly and infants. Although only a few cases of human EEEV infection have been reported annually since the 1960s, the high mortality rate and severe neurologic sequelae in infected patients make EEEV an important human pathogen. In South America, enzootic EEEV is widely distributed in most areas of tropical forests, in the Amazon Basin in Brazil, and in Northern Argentina. In these regions, EEEV is principally an equine pathogen and equine cases can occur year-round. However, human EEEV infections were rarely detected even during major equine epizootics. In temperate regions of South America (e.g., Argentina), EEEV infections often occur during the summer. EEEV transmission cycleThe EEEV transmission cycle in North America is maintained between the passerine birds as reservoir/amplification hosts and ornithophilic mosquito, Culiseta melanura, as the main enzootic vector in swamp habitats.
In addition, studies have shown that C. melanura, regarded as a bridge vector in human and equine infections so far, may also serve as the main epizootic vector as well. Mosquito species such as C. peccator, C. erraticus, and Uranotaenia sapphirina may also serve as enzootic vectors in some regions of the southeastern US. These mosquitoes are known to feed on reptiles and amphibians. Recently, snakes have been suggested to play a role in the enzootic EEEV transmission cycle as over-wintering hosts. EEEV infections in birds are usually asymptomatic; however, disease with high-titered viremia and high mortality rate has been reported in chukar partridges, pheasants, egrets, glossy ibises (Plegadis falcinellus), rock doves, house sparrows, psittacine birds, ratites (emus, ostriches), African penguins, chicken (<14 days old), pigeons, Pekin ducks, and whooping cranes. Passerine birds develop extremely high levels of viremia, enough to infect both enzootic vectors as well as a variety of bridge vectors (e.g., Aedes and Coquillettidia) that transmit the virus from enzootic cycle to humans and horses. In pheasants, EEEV is transmitted through feather picking and cannibalism. Humans and equids are dead-end hosts since they do not develop sufficient viremia to transmit the virus. In South America, reservoirs and amplification hosts involved in the enzootic EEEV transmission cycle are not known yet.
However, seroprevalence and experimental studies suggest that the Culex (Melanoconion) subgenus and rodents/marsupials may serve as principal enzootic vectors and reservoirs, respectively, and they may play a more important role in enzootic EEEV transmission in South America. The virus mainly causes disease in horses and occasional cases of encephalitis have also been reported in sheep, cattle, deer, South American camelids (llamas and alpacas), and pigs. In addition, infections have been seen in dogs, goats, bats, and small mammals including rodents.
There is a formalin-inactivated vaccine based on an NA-EEEV strain (PE-6) used in horses and emus, however, it does not induce significant neutralizing anti-E2 antibody to SA-EEEV. The vaccine is used in laboratory workers to protect from accidental exposure. A similar formalin-inactivated vaccine is available for horses. There is no specific therapy for EEE at the moment. Western equine encephalitis virusWEEV is genetically diverse and both epizootic and enzootic strains have been identified. Epizootic North American strains are more virulent than strains that are enzootic in South AmerFig a (sporadic cases of WEE). WEE was the first equine encephalitic arbovirus identified in North America. It is closely related to Sindbis and SFV since it emerged from a recombination of viruses in the EEE and the Sindbis lineages. WEEV was first isolated from brains of affected horses during an equine epizootic outbreak in the San Joaquin Valley of California in 1930.
In 1938, the first lethal human infection of WEEV was confirmed and since then it spread to the west of North America and the American Midwest with periodic equine epizootics and epidemics. Epidemiological studies have shown that WEEV occurs throughout most of the Americas from the western half of North to South America, including Guyana, Ecuador, Brazil, Uruguay, and Argentina . In South America, with the exception of Argentina, only small equine epizootics, but no human WEE cases have been reported. WEEV continues to cause equine encephalitis in northern South America and Central America with occasional outbreaks in Florida and the southwestern US, but only a few human cases of WEE have been reported, with low fatality rate, in the past several decades in North America. Most WEEV infections in humans and equines occur in summer, June, and July, and slightly later in temperate regions like Canada. Although most human cases of WEE are asymptomatic, infants and children are highly susceptible to WEEV infection and are most likely to develop severe encephalitis. Clinical manifestations develop after 2 to 10 days of incubation and are characterized by nonspecific febrile viremia, malaise, and headache often in association with meningismus. The case fatality rate in humans is about 3% to 4%. The case fatality rate in horses is 20% to 30% but can be up to 50% in some epidemics. For horses, WEEV is less virulent than EEEV.In addition, Highland J virus (HJV), Fort Morgan virus (FMV), and related Buggy Creek virus (BCRV), distinct but closely related to WEEV, were also isolated in North America . HJV has been identified in the eastern US (Florida) and is transmitted from C. melanura mosquitoes to songbirds in freshwater swamps.
It has a low pathogenicity in mammals and is rarely seen in humans or horses. Exposure to HJV has not been directly associated with human illness. However, HJV can cause sporadic encephalitis in horses and is also pathogenic to turkeys and partridges [53,75-78]. Similar to WEEV, BCRV is a natural recombinant virus derived from Old World SINV and New World EEEV . BCRV (and the closely related FMV) is apparently widely distributed in North America, having been found in Texas, Oklahoma, Nebraska, Colorado, Colorado, South Dakota, and Washington State. It was first isolated in 1980 at Buggy Creek in Grady County, Oklahoma. However, the ecologically very similar FMV was discovered in the 1970s in Colorado. BCRV is commonly associated with the cimicid swallow bug (Oeciacus vicarius). The bug is an ectoparasite of the colonially nesting cliff swallow (Petrochelidon pyrrhonota) and, to a lesser extent, the house sparrow (Passer domesticus), with both birds serving as hosts to BCRV. FMV is also associated with swallow bugs, cliff swallows, and house sparrows. These two viruses are pathogenic to swallows but not to humans or horses. These four viruses in North America (WEEV, BCRV, FMV, and HJV), the Aura virus in South America, and SINV with its four subtypes found in Africa, Asia, Australia, and Europe, are regarded as members of the WEE complex.
WEEV transmission cycleWEEV is maintained in an enzootic cycle between passerine birds as reservoirs and its specific mosquito vector, Culex tarsalis, abundant in agricultural settings in the western US. Domestic and wild birds are considered important reservoir and epizootic amplifying hosts (Fig. 5). It has been also suggEpizootic transmission to horses and humans is mediated by bridge vectors, such as Ochlerotatus melanimon in California, Aedes dorsalis in Utah and New Mexico and A. campestris in New Mexico. The seasonal continuation of the natural WEEV transmission cycle in temperate regions is not clear. However, the annual reintroduction of migratory birds and vertical transmission among A. dorsalis mosquitoes are suspected for the maintenance mechanism in temperate regions.WEEV vaccineFormalin-inactivated vaccines have been developed experimentally for the protection of laboratory workers and other persons at high risk . There is a formalin-inactivated vaccine that is available as a double vaccine in combination with EEEV only for veterinary use (horses.
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