Trade-offs shaping transmission of sylvatic dengue and Zika viruses in monkey hosts

Mosquito-borne dengue (DENV) and Zika (ZIKV) viruses originated in Old World sylvatic (forest) cycles involving monkeys and canopy-living Aedes mosquitoes. Both viruses spilled over into human transmission and were translocated to the Americas, opening a path for spillback into Neotropical sylvatic cycles. Studies of the trade-offs that shape within-host dynamics and transmission of these viruses are lacking, hampering efforts to predict spillover and spillback. We infected a native, Asian host species (cynomolgus macaque) and a novel, American host species (squirrel monkey) with sylvatic strains of DENV-2 or ZIKV via mosquito bite. We then monitored aspects of viral replication (viremia), innate and adaptive immune response (natural killer (NK) cells and neutralizing antibodies, respectively), and transmission to mosquitoes. In both hosts, ZIKV reached high titers that translated into high transmission to mosquitoes; in contrast DENV-2 replicated to low levels and, unexpectedly, transmission occurred only when serum viremia was below or near the limit of detection. Our data reveal evidence of an immunologically-mediated trade-off between duration and magnitude of virus replication, as higher peak ZIKV titers are associated with shorter durations of viremia, and higher NK cell levels are associated with lower peak ZIKV titers and lower anti-DENV-2 antibody levels. Furthermore, patterns of transmission of each virus from a Neotropical monkey suggest that ZIKV has greater potential than DENV-2 to establish a sylvatic transmission cycle in the Americas.

Contribution of phylogenetics to understanding the evolution and epidemiology of dengue virus

Dengue virus (DENV) is one of the most important arboviral pathogens in the tropics and subtropics, and nearly one‐third of the world's population is at risk of infection. The transmission of DENV involves a sylvatic cycle between nonhuman primates (NHP) and Aedes genus mosquitoes, and an endemic cycle between human hosts and predominantly Aedes aegypti. DENV belongs to the genus Flavivirus of the family Flaviviridae and consists of four antigenically distinct serotypes (DENV‐1‐4). Phylogenetic analyses of DENV have revealed its origin, epidemiology, and the drivers that determine its molecular evolution in nature. This review discusses how phylogenetic research has improved our understanding of DENV evolution and how it affects viral ecology and improved our ability to analyze and predict future DENV emergence.

Genetic Adaptation by Dengue Virus Serotype 2 to Enhance Infection of Aedes aegypti Mosquito Midguts

Dengue viruses (DENVs), serotypes 1–4, are arthropod-borne viruses transmitted to humans by mosquitoes, primarily Aedes aegypti. The transmission cycle begins when Ae. aegypti ingest blood from a viremic human and the virus infects midgut epithelial cells. In studying viruses derived from the DENV2 infectious clone 30P-NBX, we found that when the virus was delivered to female Ae. aegypti in an infectious blood meal, the midgut infection rate (MIR) was very low. To determine if adaptive mutations in the DENV2 envelope (E) glycoprotein could be induced to increase the MIR, we serially passed 30P-NBX in Ae. aegypti midguts. After four passages, a single, non-conservative mutation in E protein domain II (DII) nucleotide position 1300 became dominant, resulting in replacement of positively-charged amino acid lysine (K) at position 122 with negatively-charged glutamic acid (E; K122E) and a significantly-enhanced MIR. Site directed mutagenesis experiments showed that reducing the positive charge of this surface-exposed region of the E protein DII correlated with improved Ae. aegypti midgut infection.

Lack of Evidence of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Spillover in Free-Living Neotropical Non-Human Primates, Brazil

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the agent of coronavirus disease 2019 (COVID-19), is responsible for the worst pandemic of the 21st century. Like all human coronaviruses, SARS-CoV-2 originated in a wildlife reservoir, most likely from bats. As SARS-CoV-2 has spread across the globe in humans, it has spilled over to infect a variety of non-human animal species in domestic, farm, and zoo settings. Additionally, a broad range of species, including one neotropical monkey, have proven to be susceptible to experimental infection with SARS-CoV-2. Together, these findings raise the specter of establishment of novel enzootic cycles of SARS-CoV-2. To assess the potential exposure of free-living non-human primates to SARS-CoV-2, we sampled 60 neotropical monkeys living in proximity to Manaus and São José do Rio Preto, two hotspots for COVID-19 in Brazil. Our molecular and serological tests detected no evidence of SAR-CoV-2 infection among these populations. While this result is reassuring, sustained surveillance efforts of wildlife living in close association with human populations is warranted, given the stochastic nature of spillover events and the enormous implications of SARS-CoV-2 spillover for human health.

Why Did ZIKV Perinatal Outcomes Differ in Distinct Regions of Brazil? An Exploratory Study of Two Cohorts

The Zika virus (ZIKV) epidemic in Brazil occurred in regions where dengue viruses (DENV) are historically endemic. We investigated the differences in adverse pregnancy/infant outcomes in two cohorts comprising 114 pregnant women with PCR-confirmed ZIKV infection in Rio de Janeiro, Southeastern Brazil (n = 50) and Manaus, in the north region of the country (n = 64). Prior exposure to DENV was evaluated through plaque reduction neutralizing antibody assays (PRNT 80) and DENV IgG serologies. Potential associations between pregnancy outcomes and Zika attack rates in the two cities were explored. Overall, 31 women (27%) had adverse pregnancy/infant outcomes, 27 in Rio (54%) and 4 in Manaus (6%), p < 0.001. This included 4 pregnancy losses (13%) and 27 infants with abnormalities at birth (24%). A total of 93 women (82%) had evidence of prior DENV exposure, 45 in Rio (90%) and 48 in Manaus (75%). Zika attack rates differed; the rate in Rio was 10.28 cases/10,000 and in Manaus, 0.6 cases/10,000, p < 0.001. Only Zika attack rates (Odds Ratio: 17.6, 95% Confidence Interval 5.6–55.9, p < 0.001) and infection in the first trimester of pregnancy (OR: 4.26, 95% CI 1.4–12.9, p = 0.011) were associated with adverse pregnancy and infant outcomes. Pre-existing immunity to DENV was not associated with outcomes (normal or abnormal) in patients with ZIKV infection during pregnancy.

Discriminating arboviral species

Mosquito-borne arboviruses, including a diverse array of alphaviruses and flaviviruses, lead to hundreds of millions of human infections each year. Current methods for species-level classification of arboviruses adhere to guidelines prescribed by the International Committee on Taxonomy of Viruses (ICTV), and generally apply a polyphasic approach that might include information about viral vectors, hosts, geographical distribution, antigenicity, levels of DNA similarity, disease association and/or ecological characteristics. However, there is substantial variation in the criteria used to define viral species, which can lead to the establishment of artificial boundaries between species and inconsistencies when inferring their relatedness, variation and evolutionary history. In this study, we apply a single, uniform principle - that underlying the Biological Species Concept (BSC) - to define biological species of arboviruses based on recombination between genomes. Given that few recombination events have been documented in arboviruses, we investigate the incidence of recombination within and among major arboviral groups using an approach based on the ratio of homoplastic sites (recombinant alleles) to non-homoplastic sites (vertically transmitted alleles). This approach supports many ICTV-designations but also recognizes several cases in which a named species comprises multiple biological species. These findings demonstrate that this metric may be applied to all lifeforms, including viruses, and lead to more consistent and accurate delineation of viral species.

Seroprevalence of Dengue, Zika, and Chikungunya Viruses in Wild Monkeys in Thailand

Zoonotic pathogens such as arboviruses have comprised a significant proportion of emerging infectious diseases in humans. The role of wildlife species as reservoirs for arboviruses is poorly understood, especially in endemic areas such as Southeast Asia. This study aims to determine the exposure history of different macaque species from national parks in Thailand to mosquito-borne flaviviruses and alphavirus by testing the serum samples collected from 25 northern pigtailed macaques, 33 stump-tailed macaques, and 4 long-tailed macaques for the presence of antibodies against dengue, Zika, and chikungunya viruses by plaque reduction neutralization assay. Specific neutralizing antibodies against Dengue virus (DENV1-4) and Zika virus (ZIKV) were mainly found in stump-tailed macaques, whereas neutralizing antibody titers were not detected in long-tailed macaques and pigtailed macaques as determined by 90% plaque reduction neutralization assay (PRNT₉₀). One long-tailed macaque captured from the south of Thailand exhibited antibody titers against chikungunya virus (CHIKV), suggesting enzootic of this virus to nonhuman primates (NHPs) in Thailand. Encroachment of human settlements into the forest has increased the interface that exposes humans to zoonotic pathogens such as arboviruses found in monkeys. Nonhuman primates living in different regions of Thailand showed different patterns of arboviral infections. The presence of neutralizing antibodies among wild monkeys in Thailand strongly suggests the existence of sylvatic cycles for DENV, ZIKV, and CHIKV in Thailand. The transmission of dengue, Zika, and chikungunya viruses among wild macaques may have important public health implications.

Evolution of endemic and sylvatic lineages of dengue virus

Recent disease outbreaks have raised awareness of tropical pathogens, especially mosquito-borne viruses. Dengue virus (DENV) is a widely studied mammalian pathogen transmitted by various species of mosquito in the genus Aedes, especially Aedes aegypti and Aedes albopictus. The prevailing view of the research community is that endemic viral lineages that cause epidemics of DENV in humans have emerged over time from sylvatic viral lineages, which persist in wild, non-human primates. These notions have been examined by researchers through phylogenetic analyses of the envelope gene (E) from the four serotypes of DENV (serotypes DENV-1 to DENV-4). In these previous reports, researchers used visual inspection of a phylogeny in order to assert that sylvatic lineages lead to endemic clades. In making this assertion, these researchers also reasserted the model of periodic sylvatic to endemic disease outbreaks. Since that study, there has been a significant increase in data both in terms of metadata (e.g., place and host of isolation) and genetic sequences of DENV. Here, we re-examine the model of sylvatic to endemic shifts in viral lineages through a phylogenetic tree search and character evolution study of metadata on the tree. We built a dataset of nucleotide sequences for 188 isolates of DENV that have metadata on sylvatic or endemic sampling along with three orthologous sequences from West Nile virus as the outgroup for the phylogenetic analysis. In contrast to previous research, we find that there are several shifts from endemic to sylvatic lineages as well as sylvatic to endemic lineages, indicating a much more dynamic model of evolution. We propose that a model that allows oscillation between sylvatic and endemic hosts better captures the dynamics of DENV transmission.

Genetic Variation between Dengue Virus Type 4 Strains Impacts Human Antibody Binding and Neutralization

There are four distinct DENV serotypes, and within DENV4, there are five distinct genotypes. The impact of genotypic diversity is not known, nor is it clear whether infection with one DENV4 genotype results in protective immunity against the other genotypes. To measure the impact of DENV4 genetic diversity, we generated an isogenic panel of viruses containing the envelope protein from the different genotypes. We characterized many properties of these viruses and find that a small number of amino acids changes within the envelope have disproportionate impacts on virus biology. Additionally, we observe large differences in the ability of DENV4 antibodies, immune sera, and vaccine sera to neutralize the panel, suggesting that DENV4 immunity might not be equally protective against all DENV4s. Our results support the monitoring of changing or emerging DENV genotypes and their role in escaping pre-existing neutralizing antibodies in people who have been vaccinated or exposed to natural DENV4 infections.

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There is amino acid variability within the envelope protein across DENV4 genotypes

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DENV4 viruses differ in maturation, glycosylation, and ability to infect cells

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Monoclonal antibodies differentially bind and neutralize DENV4 genotype viruses

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Infection and vaccination elicit antibodies, which neutralize DENV4s differently

Evidence of natural Zika virus infection in neotropical non-human primates in Brazil

In Africa, Old World Primates are involved in the maintenance of sylvatic circulation of ZIKV. However, in Brazil, the hosts for the sylvatic cycle remain unknown. We hypothesized that free-living NHPs might play a role in urban/periurban ZIKV dynamics, thus we undertook an NHP ZIKV investigation in two cities in Brazil. We identified ZIKV-positive NHPs and sequences obtained were phylogenetically related to the American lineage of ZIKV. Additionally, we inoculated four C. penicillata with ZIKV and our results demonstrated that marmosets had a sustained viremia. The natural and experimental infection of NHPs with ZIKV, support the hypothesis that NHPs may be a vertebrate host in the maintainance of ZIKV transmission/circulation in urban tropical settings. Further studies are needed to understand the role they may play in maintaining the urban cycle of the ZIKV and how they may be a conduit in establishing an enzootic transmission cycle in tropical Latin America.

Understanding dengue virus evolution to support epidemic surveillance and counter-measure development

Dengue virus (DENV) causes a profound burden of morbidity and mortality, and its global burden is rising due to the co-circulation of four divergent DENV serotypes in the ecological context of globalization, travel, climate change, urbanization, and expansion of the geographic range of the Ae.aegypti and Ae.albopictus vectors. Understanding DENV evolution offers valuable opportunities to enhance surveillance and response to DENV epidemics via advances in RNA virus sequencing, bioinformatics, phylogenetic and other computational biology methods. Here we provide a scoping overview of the evolution and molecular epidemiology of DENV and the range of ways that evolutionary analyses can be applied as a public health tool against this arboviral pathogen.

Experimental Zika Virus Infection of Neotropical Primates

The establishment of a sylvatic reservoir of Zika virus (ZIKV) in the Americas is dependent on the susceptibility of primates of sufficient population density, the duration and magnitude of viremia, and their exposure to the human mosquito-borne transmission cycle. To assess the susceptibility of squirrel (Saimiri sp.) and owl monkeys (Aotus sp.) to infection, we inoculated four animals of each species with ZIKV from the current epidemic. Viremia in the absence of detectible disease was observed in both species and seroconversion occurred by day 28. ZIKV was detected in the spleen of three owl monkeys: one at 7 days postinoculation (dpi) and two at 14 dpi. This study confirms the susceptibility to ZIKV infection of two Neotropical primate species that live in close proximity to humans in South America, suggesting that they could support a widespread sylvatic ZIKV cycle there. Collectively, establishment of a ZIKV sylvatic transmission cycle in South America would imperil eradication efforts and could provide a mechanism for continued exposure of humans to ZIKV infection and disease.

The emergence of arthropod-borne viral diseases: A global prospective on dengue, chikungunya and zika fevers

Arthropod-borne viruses (arboviruses) present a substantial threat to human and animal health worldwide. Arboviruses can cause a variety of clinical presentations that range from mild to life threatening symptoms. Many arboviruses are present in nature through two distinct cycles, the urban and sylvatic cycle that are maintained in complex biological cycles. In this review we briefly discuss the factors driving the emergence of arboviruses, such as the anthropogenic aspects of unrestrained human population growth, economic expansion and globalization. Also the important aspects of viruses and vectors in the occurrence of arboviruses epidemics. The focus of this review will be on dengue, zika and chikungunya viruses, particularly because these viruses are currently causing a negative impact on public health and economic damage around the world.

Highly divergent dengue virus type 1 genotype sets a new distance record

Dengue viruses (DENVs) are the leading cause of mosquito-borne viral disease of humans. They exist in both endemic and sylvatic ecotypes. In 2014, a viremic patient who had recently visited the rainforests of Brunei returned to Australia displaying symptoms consistent with DENV infection. A unique DENV strain was subsequently isolated from the patient, which we propose belongs to a new genotype within DENV serotype 1 (DENV-1). Bayesian evolutionary phylogenetic analysis suggests that the putative sylvatic DENV-1 Brunei 2014 (Brun2014) is the most divergent DENV-1 yet recorded and increases the time to the most recent common ancestor (MRCA) for DENV-1 from ≈120 years to ≈315 years. DENV-1 classification of the Brun2014 strain was further supported by monoclonal antibody serotyping data. Phenotypic characterization demonstrated that Brun2014 replication rates in mosquito cells and infection rates in Aedes aegypti mosquitoes were not significantly different from an epidemic DENV-1 strain. Given its ability to cause human illness and infect Ae. aegypti, potential urban spillover and clinical disease from further Brun2014 transmission cannot be discounted.

Dengue viruses cluster antigenically but not as discrete serotypes

The four genetically divergent dengue virus (DENV) types are traditionally classified as serotypes. Antigenic and genetic differences among the DENV types influence disease outcome, vaccine-induced protection, epidemic magnitude, and viral evolution. We characterized antigenic diversity in the DENV types by antigenic maps constructed from neutralizing antibody titers obtained from African green monkeys and after human vaccination and natural infections. Genetically, geographically, and temporally, diverse DENV isolates clustered loosely by type, but we found that many are as similar antigenically to a virus of a different type as to some viruses of the same type. Primary infection antisera did not neutralize all viruses of the same DENV type any better than other types did up to 2 years after infection and did not show improved neutralization to homologous type isolates. That the canonical DENV types are not antigenically homogeneous has implications for vaccination and research on the dynamics of immunity, disease, and the evolution of DENV.

Seroprevalence of neutralizing antibodies against dengue virus in two localities in the state of Morelos, Mexico

Humoral immune response against dengue virus (DENV) is an important component in dengue-endemic transmission. We conducted a cross-sectional nested cohort study to determine the seroprevalence and frequency of neutralizing antibodies against DENV serotypes in two endemic localities in the state of Morelos, Mexico. The cohort participants (N = 1,196) were screened to determine previous exposure to DENV. Overall seroprevalence was 76.6% (95% confidence interval [95% CI] = 73.6-79.2), and prevalence of neutralizing antibodies in the 5- to 9-year-old group was 82.5% (95% CI = 67.2-92.7), 45% (95% CI = 29.3-61.5), and 65% (95% CI = 48.3-79.4) for DENV-1, DENV-2, and DENV-3, respectively. For participants older than 10 years, the observed seroprevalence was above 60% for each serotype, except DENV-4 in the 10- to 25-year-old group (42.9%); 81% of humoral responses were multitypic. The outcomes of our study contribute to understanding the immune component of dengue transmission and provide focal information for the evaluation of vaccine candidates under development.

Infection dynamics of sylvatic dengue virus in a natural primate host, the African Green Monkey

The four serotypes of mosquito-borne dengue virus (DENV-1, -2, -3, and -4) that circulate in humans each emerged from an enzootic, sylvatic cycle in non-human primates. Herein, we present the first study of sylvatic DENV infection dynamics in a primate. Three African green monkeys were inoculated with 10(5) plaque-forming units (pfu) DENV-2 strain PM33974 from the sylvatic cycle, and one African green monkey was inoculated with 10(5) pfu DENV-2 strain New Guinea C from the human cycle. All four monkeys seroconverted (more than fourfold rise in 80% plaque reduction neutralization titer [PRNT80]) against the strain of DENV with which they were inoculated; only one (33%) of three monkeys infected with sylvatic DENV showed a neutralizing antibody response against human-endemic DENV. Virus was detected in two of three monkeys inoculated with sylvatic DENV at low titer (≤ 1.3 log10pfu/mL) and brief duration (≤ 2 days). Clinical signs included rash and elevated aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels.

Identification of dengue fever cases in Houston, Texas, with evidence of autochthonous transmission between 2003 and 2005

Houston, Texas, maintains an environment conducive to dengue virus (DENV) emergence; however, surveillance is passive and diagnostic testing is not readily available. To determine if DENV is present in the area, we tested 3768 clinical specimens (2138 cerebrospinal fluid [CSF] and 1630 serum) collected from patients with suspected mosquito-borne viral disease between 2003 and 2005. We identified 47 immunoglobulin M (IgM)-positive dengue cases, including two cases that were positive for viral RNA in serum for dengue serotype 2. The majority of cases did not report any history of travel outside the Houston area prior to symptom onset. The epidemic curve suggests an outbreak occurred in 2003 with continued low-level transmission in 2004 and 2005. Chart abstractions were completed for 42 of the 47 cases; 57% were diagnosed with meningitis and/or encephalitis, and 43% met the case definition for dengue fever. Two of the 47 cases were fatal, including one with illness compatible with dengue shock syndrome. Our results support local transmission of DENV during the study period. These findings heighten the need for dengue surveillance in the southern United States.

Fever versus fever: the role of host and vector susceptibility and interspecific competition in shaping the current and future distributions of the sylvatic cycles of dengue virus and yellow fever virus

Two different species of flaviviruses, dengue virus (DENV) and yellow fever virus (YFV), that originated in sylvatic cycles maintained in non-human primates and forest-dwelling mosquitoes have emerged repeatedly into sustained human-to-human transmission by Aedes aegypti mosquitoes. Sylvatic cycles of both viruses remain active, and where the two viruses overlap in West Africa they utilize similar suites of monkeys and Aedes mosquitoes. These extensive similarities render the differences in the biogeography and epidemiology of the two viruses all the more striking. First, the sylvatic cycle of YFV originated in Africa and was introduced into the New World, probably as a result of the slave trade, but is absent in Asia; in contrast, sylvatic DENV likely originated in Asia and has spread to Africa but not to the New World. Second, while sylvatic YFV can emerge into extensive urban outbreaks in humans, these invariably die out, whereas four different types of DENV have established human transmission cycles that are ecologically and evolutionarily distinct from their sylvatic ancestors. Finally, transmission of YFV among humans has been documented only in Africa and the Americas, whereas DENV is transmitted among humans across most of the range of competent Aedes vectors, which in the last decade has included every continent save Antarctica. This review summarizes current understanding of sylvatic transmission cycles of YFV and DENV, considers possible explanations for their disjunct distributions, and speculates on the potential consequences of future establishment of a sylvatic cycle of DENV in the Americas.

Factors shaping the adaptive landscape for arboviruses: implications for the emergence of disease

Many examples of the emergence or re-emergence of infectious diseases involve the adaptation of zoonotic viruses to new amplification hosts or to humans themselves. These include several instances of simple mutational adaptations, often to hosts closely related to the natural reservoirs. However, based on theoretical grounds, arthropod-borne viruses, or arboviruses, may face several challenges for adaptation to new hosts. Here, we review recent findings regarding adaptive evolution of arboviruses and its impact on disease emergence. We focus on the zoonotic alphaviruses Venezuelan equine encephalitis and chikungunya viruses, which have undergone adaptive evolution that mediated recent outbreaks of disease, as well as the flaviviruses dengue and West Nile viruses, which have emerged via less dramatic adaptive mechanisms.

Emergence potential of sylvatic dengue virus type 4 in the urban transmission cycle is restrained by vaccination and homotypic immunity

Sylvatic dengue viruses (DENV) are both evolutionarily and ecologically distinct from human DENV and are maintained in an enzootic transmission cycle. Evidence of sylvatic human infections from West Africa and Southeast Asia suggests that sylvatic DENV come into regular contact with humans. Thus, this potential of emergence into the human transmission cycle could limit the potential for eradicating this cycle with vaccines currently in late stages of development. We assessed the likelihood of sylvatic DENV-4 emergence in the face of natural immunity to current human strains and vaccination with two DENV-4 vaccine candidates. Our data indicate homotypic neutralization of sylvatic and human DENV-4 strains by human primary convalescent and vaccinee sera but limited heterotypic immunity. These results suggest that emergence of sylvatic strains into the human cycle would be limited by homotypic immunity mediated by virus neutralizing antibodies produced by natural infection or vaccination.

Variation in dengue virus plaque reduction neutralization testing: systematic review and pooled analysis

Background

The plaque reduction neutralization test (PRNT) remains the gold standard for the detection of serologic immune responses to dengue virus (DENV). While the basic concept of the PRNT remains constant, this test has evolved in multiple laboratories, introducing variation in materials and methods. Despite the importance of laboratory-to-laboratory comparability in DENV vaccine development, the effects of differing PRNT techniques on assay results, particularly the use of different dengue strains within a serotype, have not been fully characterized.

Methods

We conducted a systematic review and pooled analysis of published literature reporting individual-level PRNT titers to identify factors associated with heterogeneity in PRNT results and compared variation between strains within DENV serotypes and between articles using hierarchical models.

Results

The literature search and selection criteria identified 8 vaccine trials and 25 natural exposure studies reporting 4,411 titers from 605 individuals using 4 different neutralization percentages, 3 cell lines, 12 virus concentrations and 51 strains. Of 1,057 titers from primary DENV exposure, titers to the exposure serotype were consistently higher than titers to non-exposure serotypes. In contrast, titers from secondary DENV exposures (n = 628) demonstrated high titers to exposure and non-exposure serotypes. Additionally, PRNT titers from different strains within a serotype varied substantially. A pooled analysis of 1,689 titers demonstrated strain choice accounted for 8.04% (90% credible interval [CrI]: 3.05%, 15.7%) of between-titer variation after adjusting for secondary exposure, time since DENV exposure, vaccination and neutralization percentage. Differences between articles (a proxy for inter-laboratory differences) accounted for 50.7% (90% CrI: 30.8%, 71.6%) of between-titer variance.

Conclusions

As promising vaccine candidates arise, the lack of standardized assays among diagnostic and research laboratories make unbiased inferences about vaccine-induced protection difficult. Clearly defined, widely accessible reference reagents, proficiency testing or algorithms to adjust for protocol differences would be a useful first step in improving dengue PRNT comparability and quality assurance.

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.

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.

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.

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.