Rapid detection and typing of dengue viruses from clinical samples by using reverse transcriptase-polymerase chain reaction

We report on the development and application of a rapid assay for detecting and typing dengue viruses. Oligonucleotide consensus primers were designed to anneal to any of the four dengue virus types and amplify a 511-bp product in a reverse transcriptase-polymerase chain reaction (PCR). First, we produced a cDNA copy of a portion of the viral genome in a reverse transcriptase reaction in the presence of primer D2 and then carried out a standard PCR (35 cycles of heat denaturation, annealing, and primer extension) with the addition of primer D1. The resulting double-stranded DNA product of the RT-PCR was typed by two methods: dot blot hybridization of the 511-bp amplified product to dengue virus type-specific probes or a second round of PCR amplification (nested PCR) with type-specific primers, yielding DNA products the unique sizes of which were diagnostic for each dengue virus serotype. The accumulated data demonstrated that dengue viruses can be accurately detected and typed from viremic human serum samples.

Dengue viremia titer, antibody response pattern, and virus serotype correlate with disease severity

Viremia titers in serial plasma samples from 168 children with acute dengue virus infection who were enrolled in a prospective study at 2 hospitals in Thailand were examined to determine the role of virus load in the pathogenesis of dengue hemorrhagic fever (DHF). The infecting virus serotype was identified for 165 patients (DEN-1, 46 patients; DEN-2, 47 patients; DEN-3, 47 patients, DEN-4, 25 patients). Patients with DEN-2 infections experienced more severe disease than those infected with other serotypes. Eighty-one percent of patients experienced a secondary dengue virus infection that was associated with more severe disease. Viremia titers were determined for 41 DEN-1 and 46 DEN-2 patients. Higher peak titers were associated with increased disease severity for the 31 patients with a peak titer identified (mean titer of 107.6 for those with dengue fever vs. 108.5 for patients with DHF, P=.01). Increased dengue disease severity correlated with high viremia titer, secondary dengue virus infection, and DEN-2 virus type.

Dengue

The four dengue viruses are transmitted in tropical countries that circle the globe. All can cause syndromes that are self-limited or severe. The common severe syndrome--dengue haemorrhagic fever/dengue shock syndrome (DHF/DSS)--is characterised by sudden vascular permeability generated by cytokines released when T cells attack dengue-infected cells. Dengue 1 virus became prevalent in Hawaii where it was transmitted by Aedes albopictus, producing a classic virgin soil epidemic, with clinical disease seen largely in adults. In Cuba and Singapore, sequential dengue infections at long intervals produced unusually severe disease in adults. Evidence suggests that enhancing and cross-reactive neutralising antibodies regulate dengue epidemics and disease severity. Classic DHF/DSS arises during initial dengue infections in infants with low circulating amounts of maternal dengue antibodies, an observation that precludes an exclusive causal role for secondary T-cell responses. Here, I review and discuss data on clinical diagnosis and pathophysiology of vascular permeability and coagulopathy, parenteral treatment of DHF/DSS, and new laboratory tests.

Pathogenesis of dengue: challenges to molecular biology

Dengue viruses occur as four antigenically related but distinct serotypes transmitted to humans by Aedes aegypti mosquitoes. These viruses generally cause a benign syndrome, dengue fever, in the American and African tropics, and a severe syndrome, dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS), in Southeast Asian children. This severe syndrome, which recently has also been identified in children infected with the virus in Puerto Rico, is characterized by increased vascular permeability and abnormal hemostasis. It occurs in infants less than 1 year of age born to dengue-immune mothers and in children 1 year and older who are immune to one serotype of dengue virus and are experiencing infection with a second serotype. Dengue viruses replicate in cells of mononuclear phagocyte lineage, and subneutralizing concentrations of dengue antibody enhance dengue virus infection in these cells. This antibody-dependent enhancement of infection regulates dengue disease in human beings, although disease severity may also be controlled genetically, possibly by permitting and restricting the growth of virus in monocytes. Monoclonal antibodies show heterogeneous distribution of antigenic epitopes on dengue viruses. These epitopes serve to regulate disease: when antibodies to shared antigens partially neutralize heterotypic virus, infection and disease are dampened; enhancing antibodies alone result in heightened disease response. Further knowledge of the structure of dengue genomes should permit rapid advances in understanding the pathogenetic mechanisms of dengue.

Efficacy and Long-Term Safety of a Dengue Vaccine in Regions of Endemic Disease

BACKGROUND

A candidate tetravalent dengue vaccine is being assessed in three clinical trials involving more than 35,000 children between the ages of 2 and 16 years in Asian-Pacific and Latin American countries. We report the results of long-term follow-up interim analyses and integrated efficacy analyses.

METHODS

We are assessing the incidence of hospitalization for virologically confirmed dengue as a surrogate safety end point during follow-up in years 3 to 6 of two phase 3 trials, CYD14 and CYD15, and a phase 2b trial, CYD23/57. We estimated vaccine efficacy using pooled data from the first 25 months of CYD14 and CYD15.

RESULTS

Follow-up data were available for 10,165 of 10,275 participants (99%) in CYD14 and 19,898 of 20,869 participants (95%) in CYD15. Data were available for 3203 of the 4002 participants (80%) in the CYD23 trial included in CYD57. During year 3 in the CYD14, CYD15, and CYD57 trials combined, hospitalization for virologically confirmed dengue occurred in 65 of 22,177 participants in the vaccine group and 39 of 11,089 participants in the control group. Pooled relative risks of hospitalization for dengue were 0.84 (95% confidence interval [CI], 0.56 to 1.24) among all participants, 1.58 (95% CI, 0.83 to 3.02) among those under the age of 9 years, and 0.50 (95% CI, 0.29 to 0.86) among those 9 years of age or older. During year 3, hospitalization for severe dengue, as defined by the independent data monitoring committee criteria, occurred in 18 of 22,177 participants in the vaccine group and 6 of 11,089 participants in the control group. Pooled rates of efficacy for symptomatic dengue during the first 25 months were 60.3% (95% CI, 55.7 to 64.5) for all participants, 65.6% (95% CI, 60.7 to 69.9) for those 9 years of age or older, and 44.6% (95% CI, 31.6 to 55.0) for those younger than 9 years of age.

CONCLUSIONS

Although the unexplained higher incidence of hospitalization for dengue in year 3 among children younger than 9 years of age needs to be carefully monitored during long-term follow-up, the risk among children 2 to 16 years of age was lower in the vaccine group than in the control group. (Funded by Sanofi Pasteur; ClinicalTrials.gov numbers, NCT00842530, NCT01983553, NCT01373281, and NCT01374516.).

Cross-reacting antibodies enhance dengue virus infection in humans

Dengue virus co-circulates as four serotypes, and sequential infections with more than one serotype are common. One hypothesis for the increased severity seen in secondary infections is antibody-dependent enhancement (ADE) leading to increased replication in Fc receptor-bearing cells. In this study, we have generated a panel of human monoclonal antibodies to dengue virus. Antibodies to the structural precursor-membrane protein (prM) form a major component of the response. These antibodies are highly cross-reactive among the dengue virus serotypes and, even at high concentrations, do not neutralize infection but potently promote ADE. We propose that the partial cleavage of prM from the viral surface reduces the density of antigen available for viral neutralization, leaving dengue viruses susceptible to ADE by antibody to prM, a finding that has implications for future vaccine design.

DC-SIGN (CD209) mediates dengue virus infection of human dendritic cells

Dengue virus is a single-stranded, enveloped RNA virus that productively infects human dendritic cells (DCs) primarily at the immature stage of their differentiation. We now find that all four serotypes of dengue use DC-SIGN (CD209), a C-type lectin, to infect dendritic cells. THP-1 cells become susceptible to dengue infection after transfection of DC-specific ICAM-3 grabbing nonintegrin (DC-SIGN), or its homologue L-SIGN, whereas the infection of dendritic cells is blocked by anti-DC-SIGN antibodies and not by antibodies to other molecules on these cells. Viruses produced by dendritic cells are infectious for DC-SIGN- and L-SIGN-bearing THP-1 cells and other permissive cell lines. Therefore, DC-SIGN may be considered as a new target for designing therapies that block dengue infection.

Original antigenic sin and apoptosis in the pathogenesis of dengue hemorrhagic fever

Dengue virus presents a growing threat to public health in the developing world. Four major serotypes of dengue virus have been characterized, and epidemiological evidence shows that dengue hemorrhagic fever (DHF), the more serious manifestation of the disease, occurs more frequently upon reinfection with a second serotype. We have studied dengue virus-specific T-cell responses in Thai children. During acute infection, few dengue-responsive CD8+ T cells were recovered; most of those present showed an activated phenotype and were undergoing programmed cell death. Many dengue-specific T cells were of low affinity for the infecting virus and showed higher affinity for other, probably previously encountered strains. Profound T-cell activation and death may contribute to the systemic disturbances leading to DHF, and original antigenic sin in the T-cell responses may suppress or delay viral elimination, leading to higher viral loads and increased immunopathology.

The origin, emergence and evolutionary genetics of dengue virus

Dengue is one of the most important emerging viruses, posing a threat to one-third of the global human population. Herein we show how the comparative analysis of gene sequence data has shed light on the origin and spread of dengue virus, as well as on the evolutionary processes that structure its genetic diversity. This reveals that dengue virus has a relatively recent evolutionary history, with the four serotypes originating approximately 1000 years ago and only establishing endemic transmission in humans in the last few hundred years. However, its place of origin remains uncertain as does the extent of genetic and phenotypic diversity present in the sylvatic (primate) transmission cycle. Although there is some evidence that viral strains differ in key phenotypic features such as virulence, and for positive selection at immunologically important sites, it seems likely that stochastic processes also play a major role in shaping viral genetic diversity, with lineage extinction a common occurrence. A more complete understanding of the evolution and epidemiology of dengue virus, particularly with respect to the aetiology of severe disease, will require large-scale prospective studies and the comparative analysis of complete genome sequences.

Prospects for a dengue virus vaccine

The number of cases of severe dengue disease continues to grow in endemic areas of southeast Asia, Central and South America, and other subtropical regions. Children bear the greatest burden of disease, and the development of an effective vaccine remains a global public health priority. A tetravalent vaccine is urgently needed and must be effective against all four dengue virus serotypes, be cost-effective and provide long-term protection. In this Review we discuss the unique immunological concerns in dengue virus vaccine development and the current prospects for the development of an acceptable vaccine, a goal that is likely to be reached in the near future.

Origins of dengue type 2 viruses associated with increased pathogenicity in the Americas

The recent emergence and spread of dengue hemorrhagic fever in the Americas have been a major source of concern. Efforts to control this disease are dependent on understanding the pathogenicity of dengue viruses and their transmission dynamics. Pathogenicity studies have been hampered by the lack of in vitro or in vivo models of severe dengue disease. Alternatively, molecular epidemiologic studies which associate certain dengue virus genetic types with severe dengue outbreaks may point to strains with increased pathogenicity. The comparison of nucleotide sequences (240 bp) from the E/NS1 gene region of the dengue virus genome has been shown to reflect evolutionary relationships and geographic origins of dengue virus strains. This approach was used to demonstrate an association between the introduction of two distinct genotypes of dengue type 2 virus and the appearance of dengue hemorrhagic fever in the Americas. Phylogenetic analyses suggest that these genotypes originated in Southeast Asia and that they displaced the native, American genotype in at least four countries. Vaccination and other control efforts should therefore be directed at decreasing the transmission of these "virulent" genotypes.

Dengue virus structural differences that correlate with pathogenesis

The understanding of dengue virus pathogenesis has been hampered by the lack of in vitro and in vivo models of disease. The study of viral factors involved in the production of severe dengue, dengue hemorrhagic fever (DHF), versus the more common dengue fever (DF), have been limited to indirect clinical and epidemiologic associations. In an effort to identify viral determinants of DHF, we have developed a method for comparing dengue type 2 genomes (reverse transcriptase PCR in six fragments) directly from patient plasma. Samples for comparison were selected from two previously described dengue type 2 genotypes which had been shown to be the cause of DF or DHF. When full genome sequences of 11 dengue viruses were analyzed, several structural differences were seen consistently between those associated with DF only and those with the potential to cause DHF: a total of six encoded amino acid charge differences were seen in the prM, E, NS4b, and NS5 genes, while sequence differences observed within the 5' nontranslated region (NTR) and 3' NTR were predicted to change RNA secondary structures. We hypothesize that the primary determinants of DHF reside in (i) amino acid 390 of the E protein, which purportedly alters virion binding to host cells; (ii) in the downstream loop (nucleotides 68 to 80) of the 5' NTR, which may be involved in translation initiation; and (iii) in the upstream 300 nucleotides of the 3' NTR, which may regulate viral replication via the formation of replicative intermediates. The significance of four amino acid differences in the nonstructural proteins NS4b and NS5, a presumed transport protein and the viral RNA polymerase, respectively, remains unknown. This new approach to the study of dengue virus genome differences should better reflect the true composition of viral RNA populations in the natural host and permit their association with pathogenesis.

Molecular evolution and distribution of dengue viruses type 1 and 2 in nature

During the past several decades, dengue viruses have progressively extended their geographic distribution, and are currently some of the most important mosquito-borne viruses associated with human illness. Determining the genetic variability and transmission patterns of these RNA viruses is crucial in developing effective control strategies for the disease. Primer-extension sequencing of less than 3% of the dengue genome (across the E/NS1 gene junction) provided sufficient information for estimating genetic relationships among 40 dengue type 1 and 40 type 2 virus isolates from diverse geographic areas and hosts. A quantitative comparison of these 240-nucleotide-long sequences revealed previously unrecognized evolutionary relationships between disease outbreaks. Five distinct virus genotypic groups were detected for each of the two serotypes. The evolutionary rates of epidemic dengue viruses of types 1 and 2 were similar, although the transmission pathways of these viruses around the world are different. For dengue type 2, one genotypic group represents an isolated, forest virus cycle which seems to have evolved independently in West Africa. This is the first genetic evidence of the existence of a sylvatic cycle of dengue virus, which is clearly distinct from outbreak viruses.

The dengue viruses

Dengue, a major public health problem throughout subtropical and tropical regions, is an acute infectious disease characterized by biphasic fever, headache, pain in various parts of the body, prostration, rash, lymphadenopathy, and leukopenia. In more severe or complicated dengue, patients present with a severe febrile illness characterized by abnormalities of hemostasis and increased vascular permeability, which in some instances results in a hypovolemic shock. Four distinct serotypes of the dengue virus (dengue-1, dengue-2, dengue-3, and dengue-4) exist, with numerous virus strains found worldwide. Molecular cloning methods have led to a greater understanding of the structure of the RNA genome and definition of virus-specific structural and nonstructural proteins. Progress towards producing safe, effective dengue virus vaccines, a goal for over 45 years, has been made.

Serotype-specific detection of dengue viruses in a fourplex real-time reverse transcriptase PCR assay

The dengue (DEN) viruses are positive-strand RNA viruses in the genus Flavivirus. Dengue fever and dengue hemorrhagic fever/dengue shock syndrome are important human arboviral diseases caused by infection with one of four closely related but serologically distinct DEN viruses, designated DEN-1, DEN-2, DEN-3, and DEN-4 viruses. All four DEN serotypes are currently co-circulating throughout the subtropics and tropics, and genotypic variation occurs among isolates within a serotype. A real-time quantitative nucleic acid amplification assay has been developed to detect viral RNA of a single DEN virus serotype. Each primer-probe set is DEN serotype specific, yet detects all genotypes in a panel of 7 to 10 representative isolates of a serotype. In single reactions and in fourplex reactions (containing four primer-probe sets in a single reaction mixture), standard dilutions of virus equivalent to 0.002 PFU of DEN-2, DEN-3, and DEN-4 viruses were detected; the limit of detection of DEN-1 virus was 0.5 equivalent PFU. Singleplex and fourplex reactions were evaluated in a panel of 40 viremic serum specimens with 10 specimens per serotype, containing 0.002 to 6,000 equivalent PFU/reaction (0.4 to 1.2 x 10(6) PFU/ml). Viral RNA was detected in all viremic serum specimens in singleplex and fourplex reactions. Thus, this serotype-specific, fourplex real-time reverse transcriptase PCR nucleic acid detection assay can be used as a method for differential diagnosis of a specific DEN serotype in viremic dengue patients and as a tool for rapid identification and serotyping of DEN virus isolates.

Variable surface epitopes in the crystal structure of dengue virus type 3 envelope glycoprotein

Dengue virus is an emerging global health threat. The major envelope glycoprotein, E, mediates viral attachment and entry by membrane fusion. Antibodies that bind but fail to neutralize noncognate serotypes enhance infection. We have determined the crystal structure of a soluble fragment of the envelope glycoprotein E from dengue virus type 3. The structure closely resembles those of E proteins from dengue type 2 and tick-borne encephalitis viruses. Serotype-specific neutralization escape mutants in dengue virus E proteins are all located on a surface of domain III, which has been implicated in receptor binding. While antibodies against epitopes in domain I are nonneutralizing in dengue virus, there are neutralizing antibodies that recognize serotype-conserved epitopes in domain II. The mechanism of neutralization for these antibodies is probably inhibition of membrane fusion. Our structure shows that neighboring glycans on the viral surface are spaced widely enough (at least 32 A) that they can interact with multiple carbohydrate recognition domains on oligomeric lectins such as DC-SIGN, ensuring maximum affinity for these putative receptors.

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.

Emergence and global spread of a dengue serotype 3, subtype III virus

Over the past two decades, dengue virus serotype 3 (DENV-3) has caused unexpected epidemics of dengue hemorrhagic fever (DHF) in Sri Lanka, East Africa, and Latin America. We used a phylogenetic approach to evaluate the roles of virus evolution and transport in the emergence of these outbreaks. Isolates from these geographically distant epidemics are closely related and belong to DENV-3, subtype III, which originated in the Indian subcontinent. The emergence of DHF in Sri Lanka in 1989 correlated with the appearance there of a new DENV-3, subtype III variant. This variant likely spread from the Indian subcontinent into Africa in the 1980s and from Africa into Latin America in the mid-1990s. DENV-3, subtype III isolates from mild and severe disease outbreaks formed genetically distinct groups, which suggests a role for viral genetics in DHF.

Dengue virus life cycle: viral and host factors modulating infectivity

Dengue virus (DENV 1-4) represents a major emerging arthropod-borne pathogen. All four DENV serotypes are prevalent in the (sub) tropical regions of the world and infect 50-100 million individuals annually. Whereas the majority of DENV infections proceed asymptomatically or result in self-limited dengue fever, an increasing number of patients present more severe manifestations, such as dengue hemorrhagic fever and dengue shock syndrome. In this review we will give an overview of the infectious life cycle of DENV and will discuss the viral and host factors that are important in controlling DENV infection.

Phylogenetic evidence for recombination in dengue virus

A split decomposition analysis of dengue (DEN) virus gene sequences revealed extensive networked evolution, indicative of recombination, among DEN-1 strains but not within serotypes DEN-2, DEN-3, or DEN-4. Within DEN-1, two viruses sampled from South America in the last 10 years were identified as recombinants. To map the breakpoints and test their statistical support, we developed a novel maximum likelihood method. In both recombinants, the breakpoints were found to be in similar positions, within the fusion peptide of the envelope protein, demonstrating that a single recombination event occurred prior to the divergence of these two strains. This is the first report of recombination in natural populations of dengue virus.

Microevolution and virulence of dengue viruses

The evolution of dengue viruses has had a major impact on their virulence for humans and on the epidemiology of dengue disease around the world. Although antigenic and genetic differences in virus strains had become evident, it is mainly due to the lack of animal models of disease that has made it difficult to detect differences in virulence of dengue viruses. However, phylogenetic studies of many different dengue virus samples have led to the association between specific genotypes (within serotypes) and the presentation of more or less severe disease. Currently, dengue viruses can be classified as being of epidemiologically low, medium, or high impact; i.e., some viruses may remain in sylvatic cycles of little or low transmissibility to humans, others produce dengue fever (DF) only, and some genotypes have been associated with the potential to cause the more severe dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) in addition to DF. Although the factors that contribute to dengue virus epidemiology are complex, studies have suggested that specific viral structures may contribute to increased replication in human target cells and to increased transmission by the mosquito vector; however, the immune status and possibly the genetic background of the host are also determinants of virulence or disease presentation. As to the question of whether dengue viruses are evolving toward virulence as they continue to spread throughout the world, phylogenetic and epidemiological analyses suggest that the more virulent genotypes are now displacing those that have lower epidemiological impact; there is no evidence for the transmission of antigenically aberrant, new strains.

Analytical and clinical performance of the CDC real time RT-PCR assay for detection and typing of dengue virus

Dengue is an acute illness caused by the positive-strand RNA dengue virus (DENV). There are four genetically distinct DENVs (DENV-1–4) that cause disease in tropical and subtropical countries. Most patients are viremic when they present with symptoms; therefore, RT-PCR has been increasingly used in dengue diagnosis. The CDC DENV-1–4 RT-PCR Assay has been developed as an in-vitro diagnostic platform and was recently approved by the US Food and Drug Administration (FDA) for detection of dengue in patients with signs or symptoms of mild or severe dengue. The primers and probes of this test have been designed to detect currently circulating strains of DENV-1–4 from around the world at comparable sensitivity. In a retrospective study with 102 dengue cases confirmed by IgM anti-DENV seroconversion in the convalescent sample, the RT-PCR Assay detected DENV RNA in 98.04% of the paired acute samples. Using sequencing as a positive indicator, the RT-PCR Assay had a 97.92% positive agreement in 86 suspected dengue patients with a single acute serum sample. After extensive validations, the RT-PCR Assay performance was highly reproducible when evaluated across three independent testing sites, did not produce false positive results for etiologic agents of other febrile illnesses, and was not affected by pathological levels of potentially interfering biomolecules. These results indicate that the CDC DENV-1–4 RT-PCR Assay provides a reliable diagnostic platform capable for confirming dengue in suspected cases.

Significant expansion of the four DENV serotypes (DENV-1, -2, -3 and -4) has been reported throughout tropical and sub-tropical regions of the world, with estimates of 390 million cases annually. The need has arisen for expanded diagnostic testing for DENV infections in the United States, as dengue infection has been added to the list of national notifiable diseases. Timely and accurate diagnosis of dengue is important for clinical care, disease surveillance, disease prevention, and control activities. However, current testing is performed with laboratory-developed research-based assays available only in a limited number of laboratories that have not been validated or approved for diagnostic testing in the United States. Here we report the development and evaluation of the CDC DENV-1–4 Real Time RT-PCR Assay, the first molecular test approved by the US Food and Drug Administration for the diagnosis and serotyping of DENV in human serum or plasma samples. This test was designed and validated for the detection of contemporary, clinically relevant DENV strains transmitted globally, facilitating the global deployment of the test and increase detection of traveler-associated dengue cases.

Dengue in the early febrile phase: viremia and antibody responses

A multicenter effort was begun in 1994 to characterize the pathophysiology of dengue using a study design that minimized patient selection bias by offering enrollment to all children with undifferentiated fever for <72 h. In the first year, 189 children were enrolled (age range, 8 months to 14 years). Thirty-two percent of these children had dengue infections (60 volunteers). The percentage of children with a secondary dengue infection was 93%, with only 4 (7%) having a primary dengue infection. The virus isolation rate from the plasma of children with dengue was 98%. Viremia correlated highly with temperature. All four dengue virus serotypes were isolated at both study sites. This study demonstrates that all four serotypes of dengue virus can cause dengue hemorrhagic fever, that all dengue patients as defined by serology experience viremia during the febrile phase, and that as fever subsides, so does viremia.

Evolutionary relationships of endemic/epidemic and sylvatic dengue viruses

Endemic/epidemic dengue viruses (DEN) that are transmitted among humans by the mosquito vectors Aedes aegypti and Aedes albopictus are hypothesized to have evolved from sylvatic DEN strains that are transmitted among nonhuman primates in West Africa and Malaysia by other Aedes mosquitoes. We tested this hypothesis with phylogenetic studies using envelope protein gene sequences of both endemic/epidemic and sylvatic strains. The basal position of sylvatic lineages of DEN-1, -2, and -4 suggested that the endemic/epidemic lineages of these three DEN serotypes evolved independently from sylvatic progenitors. Time estimates for evolution of the endemic/epidemic forms ranged from 100 to 1,500 years ago, and the evolution of endemic/epidemic forms represents relatively recent events in the history of DEN evolution. Analysis of envelope protein amino acid changes predicted to have accompanied endemic/epidemic emergence suggested a role for domain III in adaptation to new mosquito and/or human hosts.