Evolutionary relationships of endemic/epidemic and sylvatic dengue viruses

Fifty years of dengue in India

Dengue is the most important mosquito-borne, human viral disease in many tropical and sub-tropical areas. In India the disease has been essentially described in the form of case series. We reviewed the epidemiology of dengue in India to improve understanding of its evolution in the last 50 years and support the development of effective local prevention and control measures. Early outbreak reports showed a classic epidemic pattern of transmission with sporadic outbreaks, with low to moderate numbers of cases, usually localized to urban centres and neighbouring regions, but occasionally spreading and causing larger epidemics. Trends in recent decades include: larger and more frequent outbreaks; geographic expansion of endemic transmission; spread of the disease from urban to peri-urban and rural areas; an increasing proportion of severe cases and deaths; and progression to hyperendemicity, particularly in large urban areas. The global picture of dengue in India is currently that of a largely endemic country. Understanding demographic differences in infection rates and severity of dengue has important implications for the planning and implementation of effective public health prevention and control measures and targeting of future vaccination campaigns.

Phylogeography and population dynamics of dengue viruses in the Americas

Changes in Dengue virus (DENV) disease patterns in the Americas over recent decades have been attributed, at least in part, to repeated introduction of DENV strains from other regions, resulting in a shift from hypoendemicity to hyperendemicity. Using newly sequenced DENV-1 and DENV-3 envelope (E) gene isolates from 11 Caribbean countries, along with sequences available on GenBank, we sought to document the population genetic and spatiotemporal transmission histories of the four main invading DENV genotypes within the Americas and investigate factors that influence the rate and intensity of DENV transmission. For all genotypes, there was an initial invasion phase characterized by rapid increases in genetic diversity, which coincided with the first confirmed cases of each genotype in the region. Rapid geographic dispersal occurred upon each genotype's introduction, after which individual lineages were locally maintained, and gene flow was primarily observed among neighboring and nearby countries. There were, however, centers of viral diversity (Barbados, Puerto Rico, Colombia, Suriname, Venezuela, and Brazil) that were repeatedly involved in gene flow with more distant locations. For DENV-1 and DENV-2, we found that a "distance-informed" model, which posits that the intensity of virus movement between locations is inversely proportional to the distance between them, provided a better fit than a model assuming equal rates of movement between all pairs of countries. However, for DENV-3 and DENV-4, the more stochastic "equal rates" model was preferred.

Comparative evolutionary epidemiology of dengue virus serotypes

Evolutionary studies on dengue virus have frequently focused on intra-serotype diversity or on specific epidemics. In this study, we compiled a comprehensive data set of the envelope gene of dengue virus serotypes and conducted an extensive comparative study of evolutionary molecular epidemiology. We found that substitution rates are homogeneous among dengue serotypes, although their population dynamics have differed over the past few years as inferred by Bayesian coalescent methods. On a global scale, DENV-2 is the serotype with the highest effective population size. The genealogies also showed geographical structure within the serotypes. Finally, we also explored the causes of dengue virus serotype diversification by investigating the plausibility that it was driven by adaptive changes. Our results suggest that the envelope gene is under significant purifying selection and the hypothesis that dengue virus serotype diversification was the result of stochastic events cannot be ruled out.

Genetic and phenotypic characterization of sylvatic dengue virus type 4 strains

Four serotypes of dengue virus (DENV 1-4) currently circulate between humans and domestic/peridomestic Aedes mosquitoes, resulting in 100 million infections per year. All four serotypes emerged, independently, from sylvatic progenitors transmitted among non-human primates by arboreal Aedes mosquitoes. This study investigated the genetic and phenotypic changes associated with emergence of human DENV-4 from its sylvatic ancestors. Analysis of complete genomes of 3 sylvatic and 4 human strains revealed high conservation of both the 5'- and 3'-untranslated regions but considerable divergence within the open reading frame. Additionally, the two ecotypes did not differ significantly in replication dynamics in cultured human liver (Huh-7), monkey kidney (Vero) or mosquito (C6/36) cells, although significant inter-strain variation within ecotypes was detected. These findings are in partial agreement with previous studies of DENV-2, where human strains produced a larger number of progeny than sylvatic strains in human liver cells but not in monkey or mosquito cells.

"Bird biting" mosquitoes and human disease: a review of the role of Culex pipiens complex mosquitoes in epidemiology

The transmission of vector-borne pathogens is greatly influenced by the ecology of their vector, which is in turn shaped by genetic ancestry, the environment, and the hosts that are fed on. One group of vectors, the mosquitoes in the Culex pipiens complex, play key roles in the transmission of a range of pathogens including several viruses such as West Nile and St. Louis encephalitis viruses, avian malaria (Plasmodium spp.), and filarial worms. The Cx. pipiens complex includes Culex pipiens pipiens with two forms, pipiens and molestus, Culex pipiens pallens, Culex quinquefasciatus, Culex australicus, and Culex globocoxitus. While several members of the complex have limited geographic distributions, Cx. pipienspipiens and Cx. quinquefasciatus are found in all known urban and sub-urban temperate and tropical regions, respectively, across the world, where they are often principal disease vectors. In addition, hybrids are common in areas of overlap. Although gaps in our knowledge still remain, the advent of genetic tools has greatly enhanced our understanding of the history of speciation, domestication, dispersal, and hybridization. We review the taxonomy, genetics, evolution, behavior, and ecology of members of the Cx. pipiens complex and their role in the transmission of medically important pathogens. The adaptation of Cx. pipiens complex mosquitoes to human-altered environments led to their global distribution through dispersal via humans and, combined with their mixed feeding patterns on birds and mammals (including humans), increased the transmission of several avian pathogens to humans. We highlight several unanswered questions that will increase our ability to control diseases transmitted by these mosquitoes.

Phenotypic and genotypic characterization of dengue virus isolates differentiates dengue fever and dengue hemorrhagic fever from dengue shock syndrome

Dengue viruses (DENV) cause 50-100 million cases of acute febrile disease every year, including 500,000 reported cases of dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). Viral factors have been proposed to influence the severity of the disease, but markers of virulence have never been identified on DENV. Three DENV serotype-1 isolates from the 2007 epidemic in Cambodia that are derived from patients experiencing the various clinical forms of dengue were characterized both phenotypically and genetically. Phenotypic characteristics in vitro, based on replication kinetics in different cell lines and apoptosis response, grouped isolates from DF and DHF patients together, whereas the virus isolate from a DSS patient showed unique features: a lower level of replication in mammalian cells and extensive apoptosis in mosquito cells. Genomic comparison of viruses revealed six unique amino acid residues in the membrane, envelope, and in non-structural genes in the virus isolated from the DSS patient.

Contribution of dengue fever to the burden of acute febrile illnesses in Papua New Guinea: an age-specific prospective study

Malaria is a major contributor to the burden of febrile illnesses in Papua New Guinea (PNG). Dengue fever (DF) is likely to contribute; however, its epidemiology in PNG is poorly understood. We performed a prospective age-stratified study in outpatient clinics investigating the prevalence of DF; 578 patients were enrolled, and 317 patients with a negative rapid diagnostic test (RDT) for malaria were tested for dengue. Malaria was confirmed in 52% (301/578, 95% confidence interval [CI] = 48-56%), DF was diagnosed in 8% (46/578, 95% CI = 6-10%), and 40% (95% CI = 36-44%) had neither diagnosis. Among the 317 malaria RDT-negative patients, 14% (45/317, 95% CI = 10-18%) had DF. The seroprevalence of dengue immunoglobulin G (IgG) was 83% (204/247, 95% CI = 78-87%), and no dengue hemorrhagic fever was seen. This study provides good evidence for the first time that DF is common in PNG and is responsible for 8% of fever episodes. The common occurrence of DF in a population with presumed previous exposure to dengue is an important observation.

Contribution of dengue fever to the burden of acute febrile illnesses in Papua New Guinea: an age-specific prospective study

Malaria is a major contributor to the burden of febrile illnesses in Papua New Guinea (PNG). Dengue fever (DF) is likely to contribute; however, its epidemiology in PNG is poorly understood. We performed a prospective age-stratified study in outpatient clinics investigating the prevalence of DF; 578 patients were enrolled, and 317 patients with a negative rapid diagnostic test (RDT) for malaria were tested for dengue. Malaria was confirmed in 52% (301/578, 95% confidence interval [CI] = 48-56%), DF was diagnosed in 8% (46/578, 95% CI = 6-10%), and 40% (95% CI = 36-44%) had neither diagnosis. Among the 317 malaria RDT-negative patients, 14% (45/317, 95% CI = 10-18%) had DF. The seroprevalence of dengue immunoglobulin G (IgG) was 83% (204/247, 95% CI = 78-87%), and no dengue hemorrhagic fever was seen. This study provides good evidence for the first time that DF is common in PNG and is responsible for 8% of fever episodes. The common occurrence of DF in a population with presumed previous exposure to dengue is an important observation.

Dengue--quo tu et quo vadis?

Dengue viruses (DENV) are by far the most important arboviral pathogens in the tropics around the world, putting at risk of infection nearly a third of the global human population. DENV are members of the genus Flavivirus in the Family Flaviviridae and comprise four antigenically distinct serotypes (DENV-1-4). Although they share almost identical epidemiological features, they are genetically distinct. Phylogenetic analyses have revealed valuable insights into the origins, epidemiology and the forces that shape DENV evolution in nature. In this review, we examine the current status of DENV evolution, including but not limited to rates of evolution, selection pressures, population sizes and evolutionary constraints, and we discuss how these factors influence transmission, pathogenesis and emergence.

First report of sylvatic DENV-2-associated dengue hemorrhagic fever in West Africa

Dengue virus (DENV) circulates in human and sylvatic cycles. Sylvatic strains are both ecologically and evolutionarily distinct from endemic viruses. Although sylvatic dengue cycles occur in West African countries and Malaysia, only a few cases of mild human disease caused by sylvatic strains and one single case of dengue hemorrhagic fever in Malaysia have been reported. Here we report a case of dengue hemorrhagic fever (DHF) with thrombocytopenia (13000/µl), a raised hematocrit (32% above baseline) and mucosal bleeding in a 27-year-old male returning to Spain in November 2009 after visiting his home country Guinea Bissau. Sylvatic DENV-2 West African lineage was isolated from blood and sera. This is the first case of DHF associated with sylvatic DENV-2 in Africa and the second case worldwide of DHF caused by a sylvatic strain.

Immunity to dengue virus: a tale of original antigenic sin and tropical cytokine storms

Dengue is a mosquito-borne viral disease of expanding geographical range and incidence. The existence of four viral serotypes and the association of prior dengue virus infection with an increased risk for more severe disease have presented significant obstacles to vaccine development. An increased understanding of the adaptive immune response to natural dengue virus infection and candidate dengue vaccines has helped to define the specific antibody and T cell responses that are associated with either protective or pathological immunity during dengue infection. Further characterization of immunological correlates of disease outcome and the validation of these findings in vaccine trials will be invaluable for developing effective dengue vaccines.

Genetic diversity and lineage dynamic of dengue virus serotype 1 (DENV-1) in Cambodia

In Cambodia, dengue virus (DENV) was first isolated in 1963 and has become endemic with peak epidemic during raining season. Since 2000, the Dengue National Control Program has reported from 10,000 to 40,000 cases per year with fatality rates ranging from 0.7 to 1.7. All four dengue serotypes are found circulating in Cambodia with alternative predominance of serotypes DENV-2 and DENV-3. The DENV-1 represents from 5% to 20% of all circulating viruses, depending upon the year. In this work, 79 clinical strains of DENV-1 were isolated between 2000 and 2009 and their genome fully sequenced. Four distinct lineages with different dynamics were identified. The main evolutionary drive was negative selective pressure but each lineage was characterized by the presence of specific mutations acquired through evolution. Coexistence, extinction and replacement of lineages occurred over the 10-year period. Lineages 1, 2 and 3 were all detected since 2000-2002 and disappeared in 2003, 2004-2005 and 2007, respectively. Lineages 1 and 2 displayed different dynamics. Lineage 1 was very diverse whereas lineage 2 was very homogeneous. Lineage 4 which derived from lineage 3 in 2003 remained the only one at the end of the sampling period in 2008-2009 owing to a selective sweep. The lineages dynamic of DENV-1 viruses and consequences for molecular epidemiology are discussed.

Fever from the forest: prospects for the continued emergence of sylvatic dengue virus and its impact on public health

The four dengue virus (DENV) serotypes that circulate among humans emerged independently from ancestral sylvatic progenitors that were present in non-human primates, following the establishment of human populations that were large and dense enough to support continuous inter-human transmission by mosquitoes. This ancestral sylvatic-DENV transmission cycle still exists and is maintained in non-human primates and Aedes mosquitoes in the forests of Southeast Asia and West Africa. Here, we provide an overview of the ecology and molecular evolution of sylvatic DENV and its potential for adaptation to human transmission. We also emphasize how the study of sylvatic DENV will improve our ability to understand, predict and, ideally, avert further DENV emergence.

Dengue: a continuing global threat

Dengue fever and dengue haemorrhagic fever are important arthropod-borne viral diseases. Each year, there are ∼50 million dengue infections and ∼500,000 individuals are hospitalized with dengue haemorrhagic fever, mainly in Southeast Asia, the Pacific and the Americas. Illness is produced by any of the four dengue virus serotypes. A global strategy aimed at increasing the capacity for surveillance and outbreak response, changing behaviours and reducing the disease burden using integrated vector management in conjunction with early and accurate diagnosis has been advocated. Antiviral drugs and vaccines that are currently under development could also make an important contribution to dengue control in the future.

Isolation of ancestral sylvatic dengue virus type 1, Malaysia

Ancestral sylvatic dengue virus type 1, which was isolated from a monkey in 1972, was isolated from a patient with dengue fever in Malaysia. The virus is neutralized by serum of patients with endemic DENV-1 infection. Rare isolation of this virus suggests a limited spillover infection from an otherwise restricted sylvatic cycle.

Host alternation of chikungunya virus increases fitness while restricting population diversity and adaptability to novel selective pressures

The mechanisms by which RNA arboviruses, including chikungunya virus (CHIKV), evolve and maintain the ability to infect vertebrate and invertebrate hosts are poorly understood. To understand how host specificity shapes arbovirus populations, we studied CHIKV populations passaged alternately between invertebrate and vertebrate cells (invertebrate ↔ vertebrate) to simulate natural alternation and contrasted the results with those for populations that were artificially released from cycling by passage in single cell types. These CHIKV populations were characterized by measuring genetic diversity, changes in fitness, and adaptability to novel selective pressures. The greatest fitness increases were observed in alternately passaged CHIKV, without drastic changes in population diversity. The greatest increases in genetic diversity were observed after serial passage and correlated with greater adaptability. These results suggest an evolutionary trade-off between maintaining fitness for invertebrate ↔ vertebrate cell cycling, where maximum adaptability is possible only via enhanced population diversity and extensive exploration of sequence space.

Meta-analysis of all immune epitope data in the Flavivirus genus: inventory of current immune epitope data status in the context of virus immunity and immunopathology

A meta-analysis was performed in order to inventory the immune epitope data related to viruses in the genus Flavivirus. Nearly 2000 epitopes were captured from over 130 individual Flavivirus-related references identified from PubMed and reported as of September 2009. This report includes all epitope structures and associated immune reactivity from the past and current literature, including: the epitope distribution among pathogens and related strains, the epitope distribution among different pathogen antigens, the number of epitopes defined in human and animal models of disease, the relationship between epitopes identified in different disease states following natural (or experimental) infection, and data from studies focused on candidate vaccines. We found that the majority of epitopes were defined for dengue virus (DENV) and West Nile virus (WNV). The prominence of DENV and WNV data in the epitope literature is likely a reflection of their overall worldwide impact on human disease, and the lack of vaccines. Conversely, the relatively smaller number of epitopes defined for the other viruses within the genus (yellow fever and Japanese encephalitis virus) most likely reflects the presence of established prophylaxis and/or their more modest impact on morbidity and mortality globally. Through this work we hope to provide useful data to those working in the area of Flavivirus research.

Mosquitoes infected with dengue viruses in Brazil

Dengue epidemics have been reported in Brazil since 1985. The scenery has worsened in the last decade because several serotypes are circulating and producing a hyper-endemic situation, with an increase of DHF/DSS cases as well as the number of fatalities. Herein, we report dengue virus surveillance in mosquitoes using a Flavivirus genus-specific RT-Hemi-Nested-PCR assay. The mosquitoes (Culicidae, n = 1700) collected in the Northeast, Southeast and South of Brazil, between 1999 and 2005, were grouped into 154 pools. Putative genomes of DENV-1, -2 and -3 were detected in 6 mosquito pools (3.8%). One amplicon of putative DENV-1 was detected in a pool of Haemagogus leucocelaenus suggesting that this virus could be involved in a sylvatic cycle. DENV-3 was found infecting 3 pools of larvae of Aedes albopictus and the nucleotide sequence of one of these viruses was identified as DENV-3 of genotype III, phylogenetically related to other DENV-3 isolated in Brazil. This is the first report of a nucleotide sequence of DENV-3 from larvae of Aedes albopictus.

Present and future arboviral threats

Arthropod-borne viruses (arboviruses) are important causes of human disease nearly worldwide. All arboviruses circulate among wild animals, and many cause disease after spillover transmission to humans and agriculturally important domestic animals that are incidental or dead-end hosts. Viruses such as dengue (DENV) and chikungunya (CHIKV) that have lost the requirement for enzootic amplification now produce extensive epidemics in tropical urban centers. Many arboviruses recently have increased in importance as human and veterinary pathogens using a variety of mechanisms. Beginning in 1999, West Nile virus (WNV) underwent a dramatic geographic expansion into the Americas. High amplification associated with avian virulence coupled with adaptation for replication at higher temperatures in mosquito vectors, has caused the largest epidemic of arboviral encephalitis ever reported in the Americas. Japanese encephalitis virus (JEV), the most frequent arboviral cause of encephalitis worldwide, has spread throughout most of Asia and as far south as Australia from its putative origin in Indonesia and Malaysia. JEV has caused major epidemics as it invaded new areas, often enabled by rice culture and amplification in domesticated swine. Rift Valley fever virus (RVFV), another arbovirus that infects humans after amplification in domesticated animals, undergoes epizootic transmission during wet years following droughts. Warming of the Indian Ocean, linked to the El Niño-Southern Oscillation in the Pacific, leads to heavy rainfall in east Africa inundating surface pools and vertically infected mosquito eggs laid during previous seasons. Like WNV, JEV and RVFV could become epizootic and epidemic in the Americas if introduced unintentionally via commerce or intentionally for nefarious purposes. Climate warming also could facilitate the expansion of the distributions of many arboviruses, as documented for bluetongue viruses (BTV), major pathogens of ruminants. BTV, especially BTV-8, invaded Europe after climate warming and enabled the major midge vector to expand is distribution northward into southern Europe, extending the transmission season and vectorial capacity of local midge species. Perhaps the greatest health risk of arboviral emergence comes from extensive tropical urbanization and the colonization of this expanding habitat by the highly anthropophilic (attracted to humans) mosquito, Aedes aegypti. These factors led to the emergence of permanent endemic cycles of urban DENV and CHIKV, as well as seasonal interhuman transmission of yellow fever virus. The recent invasion into the Americas, Europe and Africa by Aedes albopictus, an important CHIKV and secondary DENV vector, could enhance urban transmission of these viruses in tropical as well as temperate regions. The minimal requirements for sustained endemic arbovirus transmission, adequate human viremia and vector competence of Ae. aegypti and/or Ae. albopictus, may be met by two other viruses with the potential to become major human pathogens: Venezuelan equine encephalitis virus, already an important cause of neurological disease in humans and equids throughout the Americas, and Mayaro virus, a close relative of CHIKV that produces a comparably debilitating arthralgic disease in South America. Further research is needed to understand the potential of these and other arboviruses to emerge in the future, invade new geographic areas, and become important public and veterinary health problems.

Global spread of epidemic dengue: the influence of environmental change

Dengue/dengue hemorrhagic fever is the most important vector-borne viral disease globally, with over half of the world’s population living in areas at risk of infection. Frequent and cyclical epidemics are reported throughout the tropical world, with regular importation of the virus via viremic travelers into both endemic and nonendemic countries. These events coincide with the recently observed global warming that is associated with climate change. Whether these events are coincidental is examined in this article. The history of dengue emergence is traced to determine the major drivers responsible for the spread of both the viruses and mosquito vectors to new geographic regions. We conclude that demographic- and anthropogenic-driven environmental changes, combined with globalization and inefficient public health measures rather than climate change, are the principal driving forces for the re-emergence and spread of epidemic dengue in the past 40 years. These trends are likely to continue given the global trends projected by the United Nations.

Dengue virus markers of virulence and pathogenicity

The increased spread of dengue fever and its more severe form, dengue hemorrhagic fever, have made the study of the mosquito-borne dengue viruses that cause these diseases a public health priority. Little is known about how or why the four different (serotypes 1-4) dengue viruses cause pathology in humans only, and there have been no animal models of disease to date. Therefore, there are no vaccines or antivirals to prevent or treat infection and mortality rates of dengue hemorrhagic fever patients can reach up to 20%. Cases occur mainly in tropical zones within developing countries worldwide, and control measures have been limited to the elimination of the mosquito vectors. Thus, it is imperative that we develop new methods of studying dengue virus pathogenicity. This article presents new approaches that may help us to understand dengue virus virulence and the specific mechanisms that lead to dengue fever and severe disease.

RNA virus genomics: a world of possibilities

The increasing availability of complete genome sequences of RNA viruses has the potential to shed new light on fundamental aspects of their biology. Here, I use case studies of 3 RNA viruses to explore the impact of genomic sequence data, with particular emphasis on influenza A virus. Notably, the studies of RNA virus genomics undertaken to date largely focused on issues of evolution and epidemiology, and they have given these disciplines new impetus. However, genomic data have so far made fewer inroads into areas of more direct importance for disease, prevention, and control; thus, harnessing their full potential remains an important goal.

Genetic diversity of the E protein of dengue type 3 virus

Background

Dengue is the most important arbovirus disease in tropical and subtropical countries. The viral envelope (E) protein is responsible for cell receptor binding and is the main target of neutralizing antibodies. The aim of this study was to analyze the diversity of the E protein gene of DENV-3. E protein gene sequences of 20 new viruses isolated in Ribeirao Preto, Brazil, and 427 sequences retrieved from GenBank were aligned for diversity and phylogenetic analysis.

Results

Comparison of the E protein gene sequences revealed the presence of 47 variable sites distributed in the protein; most of those amino acids changes are located on the viral surface. The phylogenetic analysis showed the distribution of DENV-3 in four genotypes. Genotypes I, II and III revealed internal groups that we have called lineages and sub-lineages. All amino acids that characterize a group (genotype, lineage, or sub-lineage) are located in the 47 variable sites of the E protein.

Conclusion

Our results provide information about the most frequent amino acid changes and diversity of the E protein of DENV-3.

Molecular evolution of dengue viruses: contributions of phylogenetics to understanding the history and epidemiology of the preeminent arboviral disease

Dengue viruses (DENV) are the most important arboviral pathogens in tropical and subtropical regions throughout the world, putting at risk of infection nearly a third of the global human population. Evidence from the historical record suggests a long association between these viruses and humans. The transmission of DENV includes a sylvatic, enzootic cycle between nonhuman primates and arboreal mosquitoes of the genus Aedes, and an urban, endemic/epidemic cycle between Aedes aegypti, a mosquito with larval development in peridomestic water containers, and human reservoir hosts. DENV are members of the genus Flavivirus in the Family Flaviviridae and comprise of 4 antigenically distinct serotypes (DENV-1-4). Although they are nearly identical epidemiologically, the 4 DENV serotypes are genetically quite distinct. Utilization of phylogenetic analyses based on partial and/or complete genomic sequences has elucidated the origins, epidemiology (genetic diversity, transmission dynamics and epidemic potential), and the forces that shape DENV molecular evolution (rates of evolution, selection pressures, population sizes, putative recombination and evolutionary constraints) in nature. In this review, we examine how phylogenetics have improved understanding of DENV population dynamics and sizes at various stages of infection and transmission, and how this information may influence pathogenesis and improve our ability to understand and predict DENV emergence.

Mosquitoes put the brake on arbovirus evolution: experimental evolution reveals slower mutation accumulation in mosquito than vertebrate cells

Like other arthropod-borne viruses (arboviruses), mosquito-borne dengue virus (DENV) is maintained in an alternating cycle of replication in arthropod and vertebrate hosts. The trade-off hypothesis suggests that this alternation constrains DENV evolution because a fitness increase in one host usually diminishes fitness in the other. Moreover, the hypothesis predicts that releasing DENV from host alternation should facilitate adaptation. To test this prediction, DENV was serially passaged in either a single human cell line (Huh-7), a single mosquito cell line (C6/36), or in alternating passages between Huh-7 and C6/36 cells. After 10 passages, consensus mutations were identified and fitness was assayed by evaluating replication kinetics in both cell types as well as in a novel cell type (Vero) that was not utilized in any of the passage series. Viruses allowed to specialize in single host cell types exhibited fitness gains in the cell type in which they were passaged, but fitness losses in the bypassed cell type, and most alternating passages, exhibited fitness gains in both cell types. Interestingly, fitness gains were observed in the alternately passaged, cloned viruses, an observation that may be attributed to the acquisition of both host cell-specific and amphi-cell-specific adaptations or to recovery from the fitness losses due to the genetic bottleneck of biological cloning. Amino acid changes common to both passage series suggested convergent evolution to replication in cell culture via positive selection. However, intriguingly, mutations accumulated more rapidly in viruses passed in Huh-7 cells than in those passed in C6/36 cells or in alternation. These results support the hypothesis that releasing DENV from host alternation facilitates adaptation, but there is limited support for the hypothesis that such alternation necessitates a fitness trade-off. Moreover, these findings suggest that patterns of genetic evolution may differ between viruses replicating in mammalian and mosquito cells.

Dengue virus serotype 2 from a sylvatic lineage isolated from a patient with dengue hemorrhagic fever

Dengue viruses circulate in both human and sylvatic cycles. Although dengue viruses (DENV) infecting humans can cause major epidemics and severe disease, relatively little is known about the epidemiology and etiology of sylvatic dengue viruses. A 20-year-old male developed dengue hemorrhagic fever (DHF) with thrombocytopenia (12,000/ul) and a raised hematocrit (29.5% above baseline) in January 2008 in Malaysia. Dengue virus serotype 2 was isolated from his blood on day 4 of fever. A phylogenetic analysis of the complete genome sequence revealed that this virus was a member of a sylvatic lineage of DENV-2 and most closely related to a virus isolated from a sentinel monkey in Malaysia in 1970. This is the first identification of a sylvatic DENV circulating in Asia since 1975.

Dengue viruses are mosquito-borne RNA viruses that cause a spectrum of illness from mild disease to life-threatening dengue hemorrhagic fever (DHF). Dengue viruses exist in two separate cycles in nature, circulating in either non-human primates or humans. The viruses that are endemic in humans today most likely evolved from non-human primate dengue viruses a few hundred years ago and have since established themselves as four distinct serotypes in human populations, causing periodic epidemics and severe disease. During the course of routine surveillance for dengue transmission in Malaysia during 2008, a sylvatic dengue virus type 2 was isolated from a young human male with DHF. The complete genome sequence of the virus was determined and found to be closely related to that isolated from a monkey in a similar region of Malaysia in 1970. This is the first sylvatic dengue virus isolated in Asia for more than 30 years.

Density-dependent competitive suppression of sylvatic dengue virus by endemic dengue virus in cultured mosquito cells

Mosquito-borne dengue viruses are maintained in two discrete transmission cycles: a sylvatic cycle between nonhuman primates and sylvatic Aedes mosquitoes, and an endemic cycle between humans and peridomestic Aedes (primarily Ae. aegypti and Ae. albopictus). Most sylvatic strains are genetically distinct from endemic strains, and human infections with sylvatic strains have been detected only rarely. Interestingly, sylvatic strains replicate as well as endemic strains in Ae. aegypti and experimental models of replication in humans, suggesting that adaptive constraints may not explain the limited spillover of sylvatic strains into the endemic cycle. Within-host competition is another mechanism known to decrease emergence of strains into occupied niches. In the current study, we examined the magnitude of competitive suppression between sylvatic and endemic dengue strains of different serotypes in pair-wise mixed infections of cultured Ae. albopictus cells to test whether the ecotype or the initial ratio of the two strains influenced the outcome of competition. Strains isolated from nonhuman primates were competitively inferior to those isolated from humans. Moreover, competition was density-dependent; the magnitude of suppression increased as the starting density of a strain relative to its competitor decreased. These data suggest that competitive inferiority in endemic vectors coupled with a numerical disadvantage relative to resident endemic strains could restrict reemergence of sylvatic strains into the endemic cycle and contribute to the ecologically correlated genetic divergence between sylvatic and endemic strains.

The history and evolution of human dengue emergence

Dengue viruses (DENV) are the most important human arboviral pathogens. Transmission in tropical and subtropical regions of the world includes a sylvatic, enzootic cycle between nonhuman primates and arboreal mosquitoes of the genus Aedes, and an urban, endemic/epidemic cycle principally between Aedes aegypti, a mosquito that exploits peridomestic water containers as its larval habitats, and human reservoir hosts that are preferred for blood feeding. Genetic studies suggest that all four serotypes of endemic/epidemic DENV evolved independently from ancestral, sylvatic viruses and subsequently became both ecologically and evolutionarily distinct. The independent evolution of these four serotypes was accompanied by the expansion of the sylvatic progenitors' host range in Asia to new vectors and hosts, which probably occurred gradually over a period of several hundred years. Although many emerging viral pathogens adapt to human replication and transmission, the available evidence indicates that adaptation to humans is probably not a necessary component of sylvatic DENV emergence. These findings imply that the sylvatic DENV cycles in Asia and West Africa will remain a potential source of re-emergence. Sustained urban vector control programs and/or human vaccination will be required to control DEN because the enzootic vectors and primate reservoir hosts are not amenable to interventions.

Dengue in Southeast Asia: epidemiological characteristics and strategic challenges in disease prevention

Dengue emerged as a public health burden in Southeast Asia during and following the Second World War and has become increasingly important, with progressively longer and more frequent cyclical epidemics of dengue fever/dengue hemorrhagic fever. Despite this trend, surveillance for this vector-borne viral disease remains largely passive in most Southeast Asian countries, without adequate laboratory support. We review here the factors that may have contributed to the changing epidemiology of dengue in Southeast Asia as well as challenges of disease prevention. We also discuss a regional approach to active dengue virus surveillance, focusing on urban areas where the viruses are maintained, which may be a solution to limited financial resources since most of the countries in the region have developing economies. A regional approach would also result in a greater likelihood of success in disease prevention since the large volume of human travel is a major factor contributing to the geographical spread of dengue viruses.

Global spread and persistence of dengue

Dengue is a spectrum of disease caused by four serotypes of the most prevalent arthropod-borne virus affecting humans today, and its incidence has increased dramatically in the past 50 years. Due in part to population growth and uncontrolled urbanization in tropical and subtropical countries, breeding sites for the mosquitoes that transmit dengue virus have proliferated, and successful vector control has proven problematic. Dengue viruses have evolved rapidly as they have spread worldwide, and genotypes associated with increased virulence have expanded from South and Southeast Asia into the Pacific and the Americas. This review explores the human, mosquito, and viral factors that contribute to the global spread and persistence of dengue, as well as the interaction between the three spheres, in the context of ecological and climate changes. What is known, as well as gaps in knowledge, is emphasized in light of future prospects for control and prevention of this pandemic disease.

Geographic expansion of dengue: the impact of international travel

Dengue has emerged as an international public health problem. Reasons for the resurgence of dengue in the tropics and subtropics are complex and include unprecedented urbanization with substandard living conditions, lack of vector control, virus evolution, and international travel. Of all these factors, urbanization has probably had the most impact on the amplification of dengue within a given country, and travel has had the most impact for the spread of dengue from country to country and continent to continent. Epidemics of dengue, their seasonality, and oscillations over time are reflected by the epidemiology of dengue in travelers. Sentinel surveillance of travelers could augment existing national public health surveillance systems.

Influence of hosts on the ecology of arboviral transmission: potential mechanisms influencing dengue, Murray Valley encephalitis, and Ross River virus in Australia

Ecological interactions are fundamental to the transmission of infectious disease. Arboviruses are particularly elegant examples, where rich arrays of mechanisms influence transmission between vectors and hosts. Research on host contributions to the ecology of arboviral diseases has been undertaken within multiple subdisciplines, but significant gaps in knowledge remain and multidisciplinary approaches are needed. Through our multidisciplinary review of the literature we have identified five broad areas where hosts may influence the ecology of arboviral transmission: host immunity; cross-protective immunity and antibody-dependent enhancement; host abundance; host diversity; and pathogen spillover and dispersal. Herein we discuss the known and theoretical roles of hosts within these topics and then apply this knowledge to three epidemiologically important mosquito-borne arboviruses that occur in Australia: dengue virus (DENV), Murray Valley encephalitis virus (MVEV), and Ross River virus (RRV). We argue that the underlying mechanisms by which hosts influence arboviral activity are numerous and attempts to delineate these mechanisms further are needed. Investigations that focus on hosts of vector-borne diseases are likely to be rewarding, particularly where the ecology of vectors is relatively well understood. From an applied perspective, enhanced knowledge of host influences upon vector-borne disease transmission is likely to enable better management of disease burden. Finally, we suggest a framework that may be useful to identify and determine host contributions to the ecology of arboviruses.

Differential modulation of prM cleavage, extracellular particle distribution, and virus infectivity by conserved residues at nonfurin consensus positions of the dengue virus pr-M junction

In the generation of flavivirus particles, an internal cleavage of the envelope glycoprotein prM by furin is required for the acquisition of infectivity. Unlike cleavage of the prM of other flaviviruses, cleavage of dengue virus prM is incomplete in many cell lines; the partial cleavage reflects the influence of residues at furin nonconsensus positions of the pr-M junction, as flaviviruses share basic residues at positions P1, P2, and P4, recognized by furin. In this study, viruses harboring the alanine-scanning and other multiple-point mutations of the pr-M junction were generated, employing a dengue virus background that exhibited 60 to 70% prM cleavage and a preponderance of virion-sized extracellular particles. Analysis of prM and its cleavage products in viable mutants revealed a cleavage-suppressive effect at the conserved P3 Glu residue, as well as the cleavage-augmenting effects at the P5 Arg and P6 His residues, indicating an interplay between opposing modulatory influences mediated by these residues on the cleavage of the pr-M junction. Changes in the prM cleavage level were associated with altered proportions of extracellular virions and subviral particles; mutants with reduced cleavage were enriched with subviral particles and prM-containing virions, whereas the mutant with enhanced cleavage was deprived of these particles. Alterations of virus multiplication were detected in mutants with reduced prM cleavage and were correlated with their low specific infectivities. These findings define the functional roles of charged residues located adjacent to the furin consensus sequence in the cleavage of dengue virus prM and provide plausible mechanisms by which the reduction in the pr-M junction cleavability may affect virus replication.

Genetic and phenotypic characterization of sylvatic dengue virus type 2 strains

The four serotypes of endemic dengue viruses (DENV) circulate between humans and peridomestic Aedes mosquitoes. At present endemic DENV infect 100 million people per year, and a third of the global population is at risk. In contrast, sylvatic DENV strains are maintained in a transmission cycle between nonhuman primates and sylvatic Aedes species, and are evolutionarily and ecologically distinct from endemic DENV strains. Phylogenetic analyses place sylvatic strains basal to each of the endemic serotypes, supporting the hypothesis that each of the endemic DENV serotypes emerged independently from sylvatic ancestors. We utilized complete genome analyses of both sylvatic and endemic DENV serotype 2 (DENV-2) to expand our understanding of their genetic relationships. A high degree of conservation was observed in both the 5'- and 3'-untranslated genome regions, whereas considerable differences at the nucleotide and amino acid levels were observed within the open reading frame. Additionally, replication of the two genotypes was compared in cultured cells, where endemic DENV strains produced a significantly higher output of progeny in human liver cells, but not in monkey kidney or mosquito cells. Understanding the genetic relationships and phenotypic differences between endemic and sylvatic DENV genotypes may provide valuable insight into DENV emergence and guide monitoring of future outbreaks.

Mouse models of dengue virus infection and disease

Dengue virus (DENV) causes the most significant mosquito-borne viral disease in the world in terms of illness, death, and economic cost, due to the lack of an approved vaccine or antiviral. Infections with one of the four serotypes of DENV (DENV1-4) can result in diseases ranging from an acute, self-limiting febrile illness (dengue fever, DF) to life-threatening dengue hemorrhagic fever (DHF) or dengue shock syndrome (DSS), yet exactly how viral and host factors contribute to the severe disease is unknown. Clinical observations have provided information on DENV pathogenesis, but the lack of an adequate animal model has hindered research on this important human pathogen. A mouse model is ideal for investigating host-pathogen interactions due to the immunological tools available, however, wild-type mice are resistant to DENV-induced disease. Therefore, the mouse models for DENV infection developed to date include infection of severely immunocompromised mice, non-physiologic routes of infection, and mouse-human chimeras, which all have their limitations. An inbred mouse model in which mice develop signs of human DENV-induced disease is needed to investigate the contribution of various immune components to protection and pathogenesis of DENV infections, and to test the efficacy of DENV vaccines and antivirals.

The dynamics of dengue virus serotype 3 introduction and dispersion in the state of Bahia, Brazil

By 2002, dengue virus serotype 1 (DENV-1) and DENV-2 had circulated for more than a decade in Brazil. In 2002, the introduction of DENV-3 in the state of Bahia produced a massive epidemic and the first cases of dengue hemorrhagic fever. Based on the standardized frequency, timing and location of viral isolations by the state's Central Laboratory, DENV-3 probably entered Bahia through its capital, Salvador, and then rapidly disseminated to other cities, following the main roads. A linear regression model that included traffic flow, distance from the capital and DENV-1 circulation (r2 = 0.24, p = 0.001) supported this hypothesis. This pattern was not seen for serotypes already in circulation and was not seen for DENV-3 in the following year. Human population density was another important factor in the intensity of viral circulation. Neither DENV-1 nor DENV-2 fit this model for 2001 or 2003. Since the vector has limited flight range and vector densities fail to correlate with intensity of viral circulation, this distribution represents the movement of infected people and to some extent mosquitoes. This pattern may mimic person-to-person spread of a new infection.