What is Dengue Fever?

Dengue fever (DF) is a viral disease transmitted by infected mosquitoes. DF causes severe, flu-like symptoms with high fever and extreme muscle and joint pain. Dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) are less common but more severe forms of the disease. DHF/DSS initially presents with very similar symptoms to DF. The disease then progresses to a stage where the blood vessels become permeable, or “leaky,” causing a breakdown of the circulatory system, fluid loss, and possibly death.

Global Burden

Dengue Fever

Dengue is common in the tropical and sub-tropical countries of Southeast Asia, the Pacific, and the Americas and is also found in Africa and the Eastern Mediterranean. In 2002, a record high of 69 countries reported dengue cases.1 The World Health Organization (WHO) estimates that 2.5 billion people—over 40% of the world’s population-- are at risk for dengue infection. Approximately 50-100 million infections (1 million confirmed) occur each year resulting in 500,000 hospitalizations and 20,000 deaths.2,5

The economic burden of dengue in India alone is estimated to be US$29.3 million.2 Based on a study of eight endemic countries, the estimated total economic burden of dengue is on the order of US$587 million annually. However, known underreporting of dengue infection could increase this estimate to nearly US$1.8 billion.

Causative Agent

Aedes aegypti

Dengue is a positive strand RNA virus and a member of the genus Flavivirus within family Flaviviridae. This genus also includes the viruses that cause West Nile and yellow fever. Dengue virus enters the human body through the saliva of an infected mosquito, invades immature dendritic cells in the skin, and is transported to lymph nodes where viral antigens activate the host immune response. In addition to dendritic cells, the virus replicates in the cytosol of macrophages, lymphocytes, and liver cells.

Dengue is transmitted through the bite of the female mosquito of the genus Aedes (usually A. aegypti, but occasionally by other species such as A. albopictus). A mosquito becomes infected with dengue virus upon taking a blood meal from an infected person. The virus can replicate in the mosquito and is transmitted to the next human host when the mosquito takes another blood meal. Dengue is unusual among arthropod-borne viruses in that it does not require an animal reservoir, and is instead maintained through human-mosquito-human transmission.3

Unlike many other disease vectors, A. aegypti is primarily associated with urban areas. This is attributed to its tendency to reproduce in small water pools, such as collect in tires, buckets, or uncovered water storage containers, which are common in populated settings. The eggs of the A. aegypti mosquito are extremely resilient, increasing the difficulty of vector control.

Pathogenesis

Dengue infection leads to multiple non-specific symptoms such as high fever, rash, and joint pain. It is unclear why, but a small percentage of dengue patients progress to DHF, which is further classified as DSS in its most advanced stages. The majority of DHF/DSS cases occur in children under the age of 15. DHF and DSS are both classified by increased permeability of the blood vessels resulting in a loss of plasma. Symptoms such as bleeding, profuse sweating, and shock are common; death due to dehydration from plasma loss can follow. As syndromes, dengue fever, DHF, and DSS express considerable symptom overlap and are often difficult to distinguish.4 Plans are underway to standardize case descriptions and, ideally, uncover ways of predicting which patients will progress to severe disease. The fact that the virus has generally been cleared from the body by the time DHF/DSS appear complicates the application of potential antivirals to patients presenting with the most serious symptoms.3

There are four distinct serotypes of dengue, DEN-1, DEN-2, DEN-3, and DEN-4. The prevalence of these strains traditionally varied geographically, but now all four serotypes are circulating in most endemic countries. If a patient is infected with one dengue serotype, they will be immune against subsequent infection by that same serotype. However, secondary infection with a new serotype greatly increases the risk of severe disease.

There are at least two immunological phenomena that may contribute to the more severe outcome of secondary infection:

  1. Antibody dependent enhancement (ADE): ADE is a phenomenon whereby sub-optimal levels of antibodies enhance viral spread rather than clearing the infection. Antibodies specific for a particular dengue serotype are produced during primary infection or acquired by infants from an immune mother. During a second infection with a distinct serotype, these antibodies bind to the new virus but are not sufficient to neutralize the particles. Cell types that naturally engulf immune complexes through “Fcg” receptors take up the bound virus. Increased viral replication and exacerbated inflammatory responses resulting from access to Fcg receptor-bearing cell types are thought to lead to severe disease.
  2. “Original antigenic sin”: Original antigenic sin was a term first coined in the 1960s to describe a phenomenon observed for influenza virus whereby the human immune system works against itself when trying to mount a response. When the body has been infected before, it can undergo a memory immune response upon seeing the agent again. In the case of original antigenic sin, the body mistakenly mounts a memory response to a similar, previously seen pathogen rather than to the current infection –- for example against the primary, rather than secondary, dengue serotype. Because the memory response is dominant, the body cannot properly fight the new infection. Original antigenic sin can impact both humoral and cellular immune responses.

ADE and original antigenic sin have implications for public health and for dengue vaccine design. If an immune response to one dengue serotype can worsen the outcome of infection with a distinct serotype, a vaccine that does not provide robust, long-lasting protection against all four serotypes could potentially increase disease severity for the non-vaccine strain(s) or as immunity wanes.

Current Control Strategy

As there are no drugs or vaccines for the treatment or prevention of dengue, control programs focus on:

  1. Integrated vector control (i.e., combined use of environmental modification and chemical or biology interventions to interrupt mosquito reproduction)
  2. Active disease surveillance
  3. Outbreak response preparedness
  4. Training to improve diagnosis and patient management

Existing Products

Drugs

There are currently no drugs approved for the treatment of dengue. Treatment instead focuses on palliative care to manage fever and prevent dehydration, especially for patients with severe disease. Treatment of dehydration associated with plasma loss in DHF can significantly improve patient outcomes, reducing case fatality from 20% down to <1% for DHF.5

Drugs in development for dengue are discussed in the next section.

Vaccines

There is currently no vaccine approved for the prevention of dengue. Vaccines in development are discussed in the next section.

Diagnostics

There are numerous diagnostic assays available for dengue based on a wide range of technologies:

  1. Virus isolation
  2. Serological testing (including IgM antibody capture ELISA, IgG ELISA, neutralization assays, and lateral flow device for NS1-specific antibodies)
  3. Nucleic acid amplification (RT-PCR, real time RT-PCR, and nucleic acid-sequence based amplification assays)
  4. Antigen detection (NS1 antigen detected by antigen capture ELISA)

The gold standard for dengue diagnosis is viral isolation as this allows for the most specific characterization of the dengue virus. However, advanced laboratory facilities are required for this technique, limiting its application in the developing world.

Serological assays are the most commonly used diagnostics for dengue. There are over 50 commercial kits available based on the IgM antibody-capture enzyme-linked immunosorbent assay (MAC-ELISA) developed by the Armed Forces Research Institute of Medical Sciences. A rapid diagnostic test (RDT) for detection of antibodies specific to the dengue NS1 protein is also available. However, the current RDT cannot distinguish between dengue virus serotypes. The WHO's Special Programme for Training and Research in Tropical Diseases (TDR) conducts laboratory-based evaluations of existing dengue virus diagnostics to ensure quality-control.

References

  1. Guzman MG et al. (2010) “Dengue: a continuing global health threat.” Nature Reviews Microbiology 8: S7-S16.
  2. WHO (2010) First WHO report on neglected tropical diseases 2010: working to overcome the global impact of neglected tropical diseases.
  3. Whitehead SS et al. (2007) “Prospects for a dengue virus vaccine.” Nature Reviews Microbiology 5: 518-28.
  4. Deen JL et al. (2006) “The WHO dengue classification and case definitions: time for a reassessment.” The Lancet 368: 170–173.
  5. WHO (2012) Dengue and Dengue Hemorrhagic Fever Fact Sheet.

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Drugs

Analysis

The nucleoside inhibitor (Balapiravir) from Roche has completed Phase I clinical trial. The result has not been officially released. The increasing prevalence of dengue and the more recent re-emergence of dengue in Puerto Rico and the southern United States are raising interest in the development of new dengue treatments.

There are currently multiple pre-clinical or discovery stage projects including RNAi therapeutics and natural products (seeTargets/Technologies). These programs are too preliminary to provide detailed analysis at this time.

Vaccines

Analysis

The vaccine development pipeline for dengue is primarily focused on the development of tetravalent live attenuated vaccines. The most advanced vaccine is the ChimeriVax Dengue vaccine that is being developed by Sanofi Pasteur. The ChimeriVax system is a live, attenuated recombinant virus constructed from an attenuated yellow fever virus in which the envelope protein genes of yellow fever are replaced with those of dengue virus. Sanofi Pasteur is using a similar approach to build vaccines for West Nile virus and Japanese encephalitis. The vaccine is designed to protect against all four dengue serotypes and is currently in phase III clinical trials. There are additional tetravalent live attenuated vaccines in phase I and II trials.

Additional vaccines in development include recombinant protein, DNA, and inactivated whole cell vaccines. Of these other technologies, only one recombinant protein-based vaccine is in clinical development.

 StrengthsWeaknessesOpportunitiesRisks
Live attenuated
Serogroup targeted: DENV1-4 Most advanced program: ChimeriVax, Phase IIIMost advanced clinical productResult of incomplete or short-term seroconversion is unknown (potential to worsen disease)Combination vaccine with other ChimeriVax-based flavivirus vaccinesLong term follow-up of vaccinated patients will be needed to confirm ADE/original antigenic sin are not induced
Recombinant protein
Serotypes targeted: DENV1 (DENV2 and tetravalent have been evaluated in pre-clinical studies) Most advanced program: DEN1-80E, Phase I Published results for pre-clinical studies of non-human primates showed good protection using DEN2-80E and tetravalent vaccine Phase I using only 1 serotype rather than tetravalentDevelopment of tetravalent vaccine May be cheaper to produce and/or deliver than live attenuated vaccine   Long term follow-up of vaccinated patients will be needed to confirm ADE/original antigenic sin are not induced  Phase III product likely to be available many years in advance of this product  

Diagnostics

Analysis

There are numerous diagnostic assays already available for dengue virus. An RDT that would diagnose dengue using saliva is in development as a means of expediting dengue diagnosis and treatment.  However, while new tests, such as RDTs that can distinguish viral serotypes, would be useful for epidemiological purposes, vaccines and treatments are a higher priority for investment in dengue research and development.

References

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The following series of tables describe the availability of tools for research, discovery, and development of novel drugs, vaccines, and diagnostics for dengue. The tools listed in the following tables are not intended to be an all-inclusive list but rather capture the most common tools used for drug, vaccine, and diagnostic development. The tools for dengue are generally well developed.

Drugs Development Tools

Basic Research: Target IdentificationTarget ValidationScreening: Hit/Lead Identification OptimizationPre-clinical ValidationClinical Validation

Genome: Sequenced (all four serotypes) 

Key databases: EBI 2can Support Portal 

In vitro culture:Clinical isolates propagate poorly in vitro. Several infectious laboratory strains are available that grow in mosquito and mammalian cell cultures. Cell culture-infectious molecular clones available for all four serotypes.

Gene knock-outs: Yes, using infectious clones.  

Conditional gene knock-outs: Yes, temperature-sensitive mutants 

Transposon mutagenesis: Possible 

RNAi: Yes, by utilizing machinery of host cell 

Other antisense technology: Yes, antisense oligonucleotides and ribozymes; DNAzymes possible for related flavivirus. 

Viability assays: Yes  

Transcription microarrays: Yes, of infected cells 

Proteomics: Yes 

Crystal structures: Yes

Whole-cell screening assays: Yes, replicons, complete virus infection, luciferase reporter virus infection, virus-like particle infection

Enzymatic screening assays: Yes, for NTPase (NS3), protease (NS3), helicase (NS3), polymerase (NS5), methyltransferase (NS5)

Additional assays: The whole-cell screening assay (described above) can also be used to screen for inhibitors of viral non-structural proteins that lack enzymatic activity.

Virus entry and viral fusion assays are also available for viral envelope protein.

Animal models: Yes Wild-type mice relatively resistant so severely immune-compromized models using non-physiologic infection routes, adapted dengue strains, or engraftment of human cells are used Rhesus macaques and chimpanzees most consistently reproduce symptoms of dengue fever or DHF/DSS Miniature swine model is in development

Monitoring treatment efficacy:Yes 

Availability of endpoints: Yes, viral clearance 

Availability of surrogate endpoints: No 

Access to clinical trial patients / sites: Yes

Vaccines Development Tools

Basic Research: Antigen IdentificationImmune Response CharacterizationClinical Validation

See drug development tools above

Predictive animal models: Animal models available but limited predictive value due to immunodeficiency and/or inability to mimic human disease states 

Detection of endogenous antigen specific response in clinical samples:Yes, neutralizing antibodies against E protein provide lifelong protection against homologous strains 

Natural immunity well characterized:  Natural immunity is well understood for repeated infection with a single viral strain, but still subject of study for heterotypic infections

Surrogate markers of protection:  Yes, neutralizing antibodies can be detected by plaque reduction neutralization technique (PRNT) 

Challenge studies possible:  No

Diagnostics Development Tools

Basic Research: Biomarker IdentificationBiomarker ValidationClinical Validation

See drug development tools above

Biomarkers known:  Yes, viral RNA, proteins, and antibodies detectable 

Access to clinical samples:  Yes 

Possible sample types:  Serum, plasma, blood, saliva, urine

Access to clinical trial patients/sites:  Yes 

Treatment available if diagnosed:  No specific antivirals, supportive and rehydration therapy is standard

References

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To learn how you can get involved in neglected disease drug, vaccine or diagnostic research and development, or to provide updates, changes, or corrections to the Global Health Primer website, please view our FAQs.