Dengue Fever - An Update
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Published: June 2014Print Record of Viewing
2.5 billion people are at risk of contracting dengue fever. There are an estimated 50–100 million infections per year, and 500,000 hospitalizations due to severe disease. Dengue virus is a member of the flaviviridae family, and transmittable to human hosts by the mosquito species Aedes aegypti or Aedes albopictus. Initial infections may be mild, or even asymptomatic, but secondary infections lead to more severe disease. Dr. Hata provides an overview of dengue fever and describes viral transmission, epidemiology, and pathogenesis of the infection. She also explains laboratory testing for diagnostic markers of the illness as well as recent changes in Dengue virus epidemiology.
Presenter: D. Jane Hata, PhD, D(ABMM)
Director of the Clinical Microbiology Laboratory in the Department of Laboratory Medicine and Pathology at Mayo Clinic Florida
Questions and Feedback
Welcome to Mayo Medical Laboratories Hot Topics. These presentations provide short discussion of current topics and may be helpful to you in your practice.
Our speaker for this program is Dr. Jane Hata, Director of the Clinical Microbiology Laboratory in the Department of Laboratory Medicine and Pathology Mayo Clinic Florida. Dr. Hata presents an updated review of dengue fever, including transmission, presentation, and diagnostic methods.
The subject for today’s Hot Topic is a review of dengue fever, including transmission, presentation, and diagnostic methods. I have no significant commercial affiliations or other disclosures to make in connection with this presentation.
The following points may be helpful as you consider dengue fever in your differential diagnosis: Dengue virus is the most common mosquito-borne infection in the world.
Awareness of the clinical stage of disease will aid selection of the most appropriate laboratory test for diagnosis of dengue virus infection. Dengue should be considered as part of differential diagnosis in an acute febrile illness in travelers to tropical regions, or residence in certain parts of the United States.
In this presentation we will discuss: The virus which results in dengue fever and history of the disease, its epidemiology and pathogenesis, the symptoms of dengue, treatment modalities, as well as diagnostic methods and future considerations such as emerging epidemiology, vector control, and vaccine development.
Dengue virus infection is the most common mosquito-borne infection worldwide – even surpassing malaria. This is a member of the Flaviviridae. Other members of the Flaviviridae family include: yellow fever, West Nile virus, St. Louis and Japanese encephalitis.
The virus as seen in this electron micrograph consists of a 40-50 nm sphere, surrounded by a lipopolysaccharide envelope. There are 4 serotypes (1 – 4), all of which exhibit the same clinical manifestations. We have seen a shift in the expression of viral serotypes; in the 1970s, dengue 1-4 was limited to Asia, whereas types 1 and 2 were seen in South America and Africa. Today we generally see all 4 serotypes distributed worldwide throughout the “dengue belt” which has traditionally extended in a zone where winter temperatures average 50 degrees Fahrenheit. Generally speaking, this covers the area between 30 degrees north and south latitude.
Structure of Dengue Virus
Dengue virus is a positive-sense, single stranded RNA virus which is approximately 11 kilobases in length. The genome depicted here contains regions encoding the viral capsid, the viral membrane, and envelope as well as 7 nonstructural proteins; labeled NS in the figure. The capsid, membrane, and envelope are all associated with viral structure and subsequent uptake of virus by host cells. Envelope glycoproteins are associated with binding to host receptors, agglutination of erythrocytes, development of neutralizing antibody, and host immune response. Each subtype of dengue is 65% genetically related to one another. This figure is taken from an excellent review from Guzman et al, which is cited in the references at the end of this presentation. I strongly recommend this paper if you are interested in additional information on dengue.
Disease due to what we believe was dengue, was first described in a Chinese medical encyclopedia dating somewhere between AD 265 to 420. This document included a disease description, and reported an association between illness in the community and the presence of “poison water”. Association with flying insects was also described.
According to various historical documents, multiple epidemics attributed to dengue occurred between 1635 and 1699. Epidemics were reported in the Caribbean, Asia, Africa, as well as North America. During this period, sailing vessels were the primary means of transporting mosquito vectors from endemic regions to new areas.
This is an engraving is from the National Library of Medicine of Dr. Benjamin Rush. Not only was he a signatory of the Declaration of Independence, but also a professor of chemistry and medical theory at the University of Pennsylvania. During the Philadelphia epidemic of 1779 to 1780, Dr. Rush first described the symptoms of dengue as “Breakbone Fever”. Jump forward to the late 1930s, where World War II facilitated spread of dengue through Asia/Pacific region.
Epidemiology of Dengue
2.5 billion people are at risk of dengue due to living in an endemic area, in addition to actual living conditions. There are an estimated 50 to 100 million infections per year, and 500,000 hospitalizations due to severe disease. Case fatality rate from severe dengue approaches 5% and sometimes higher, however the risk of fatal disease drops to less than 1% with appropriate supportive therapy. There is a seasonal component to dengue. Peak frequency depends on where you are in the world; generally speaking, warm and wet conditions favor development of mosquitoes which transmit the virus. Climate conditions will vary between countries and it’s essential to remember in certain areas, risk of dengue exists year round. According to the World Health Organization, there has been a 30-fold increase in dengue incidence over the last 50 years; it is considered to represent a pandemic threat.
Vectors of Dengue
Let us now turn our attention to the very important discussion of the mosquito vectors for the dengue virus. Aedes aegypti is the primary vector associated with transmission of dengue. You can see the lyre-shaped dorsal pattern on the thorax of the mosquito on the slide, and silver bands on the legs. This is a day biter, and preferentially feeds on humans, which makes it more likely to transmit dengue. It likes to both rest and feed indoors. This species survives in the 50 degree Farenheit band; basically, Mexico to southern Brazil. But it can also survive where human conditions allow, thus it is not solely dependent on climate. Aedes aegypti has historically persisted in Memphis, Tennessee, which incidentally was also the site of a major epidemic of Yellow Fever in the late 1800s and also transmitted by Aedes aegypti.
Note that I mentioned vectors - plural-we have an additional mosquito species now associated with transmission of dengue.
Aedes albopictus is also known as the Asian tiger mosquito – you can see why by the striking white bands on the legs. A single dorsal stripe on the thorax differentiates this species from Aedes aegypti. This species moves in when Aedes aegypti declines in population. It has markedly different habits in that it is an aggressive biter, both day and night, and prefers to live outside. Humans are not its sole source of food, in that it bites a variety of other wild and domestic vertebrates as well. Aedes albopictus is also a vector for eastern equine encephalitis, in addition to dengue. This species first arrived in United States at the port of Houston 1980s via a shipment of old tires. This mosquito migrates further from breeding sites than Aedes aegypti, and thus can extend the geographic range in which Dengue exposure may occur.
Life Cycle of Dengue Virus
The life cycle of dengue virus is strictly dependent on only mosquitoes and humans, which gives us potential points at which to impede viral transmission, which I will discuss later in this presentation. Once the mosquito feeds on a viremic human, there is a 10-day extrinsic incubation period in which the virus develops and passes from the mosquito intestinal tract to its salivary glands.
Human infection then results from the bite of the infected mosquito. The mosquito remains infectious for its entire 1-month life, and can transmit infection with as few as 100 viral particles. Mosquito eggs can survive desiccation for several months, and repopulation will occur once they are rehydrated, thus ensuring repopulation of this vector.
Pathogenesis of Dengue
In the human host, after a bite, the dengue virus infects immature dendritic cells of the host immune system. The infected dendritic cells then migrate to lymph nodes with activation of the cellular and humoral immune response. Viral replication occurs in macrophages of lymph nodes, liver, spleen, parenchymal cells, and peripheral blood monocytes. The host will then become viremic and symptomatic within 3 to 6 days.
Classification of Dengue Disease
Disease due to dengue virus was originally classified as dengue fever and dengue hemorrhagic fever. This placed an excessive emphasis on actual hemorrhage, whereas the most important clinical manifestation was actually that of vascular permeability. Consequently, the World Health Organization revised the disease case classification in 2009; replacing earlier recommendations released in 1997.
Dengue is currently divided into 3 syndromes; dengue without warning signs, formerly known as dengue fever and the most common presentation of primary infection, dengue with warning signs, previously known as dengue hemorrhagic fever; and severe dengue, formerly known as dengue shock syndrome (DSS), which may involve both plasma leakage and hemorrhage.
Initial infections may be mild, or even asymptomatic, but secondary infections lead to more severe disease particularly if an infection with DENV 1 is followed by DENV 2 or 3 or an infection with DENV 3 is followed by DENV 2. Age and nutritional status also affects severity of disease.
Antibody-Dependent Enhancement (ADE)
It is well established that the primary risk factor of development of the shock and hemorrhagic manifestations of dengue is evidence of a previous infection with dengue virus. The question is, why are secondary infections more severe? Antibodies formed in response to a dengue infection are not cross-protective against other subtypes of the virus. In fact they may result in more severe disease due to a phenomenon known as Antibody-Dependent Enhancement or ADE. This occurs when there is a formation of immune complexes between dengue virus and pre-existing non-neutralizing antibodies. Non-neutralizing antibodies result from previous dengue infections or low level of maternal antibodies in the cases of infant sera.
Mononuclear phagocytes are infected through their Fc receptors by immune complexes that form between dengue virus and non-neutralizing antibodies.This results in suppression of the host’s innate immune response and release of a storm of inflammatory cytokines and chemokines including gamma interferon, tumor necrosis factor and IL-10, all of which result in enhanced disease.
Symptoms of Dengue
There are 3 phases of disease due to dengue virus infection, the febrile phase, critical phase, and recovery phase.
The febrile phase is characterized by: high fevers, headache, retro-orbital pain, and generalized arthralgia and myalgia; thus the “breakbone” description. Progressive leukopenia is noted. These symptoms are characteristic of dengue without warning signs. Petechiae, and bleeding from mucous membranes may occur, a characteristic of dengue with warning signs.
In the critical phase, we see defervescence. However, leukopenia, and thrombocytopenia may progress. Importantly, we see an increase of capillary permeability leading to plasma leakage. This is a primary indicator of the severity of disease. Abdominal pain is often a sign of plasma leakage, if uncorrected by careful IV rehydration; plasma leakage can lead to metabolic acidosis and disseminated intravascular coagulation, resulting in a high risk of death.
During the recovery phase, we see a resorption of leaked plasma. The hemodynamic status stabilizes, and an increase in urinary output is noted. There is an overall clinical improvement. It is very important to note that the prevention of fluid overload during the recovery phase is critical, as this is a primary preventable cause of death in severe dengue.
There are warning signs that indicate that the patient has progressed during the critical phase to what is classified as “Severe Dengue” disease which may be fatal. This is characterized by: hypovolemic shock, bleeding from the GI tract, liver failure, neurologic manifestations such as encephalopathy and weakness, bradycardia and multi-organ failure. Volume overload and respiratory distress may also be noted.
Treatment of Dengue
The treatment of both dengue with and without warning signs and Severe Dengue consists of supportive care only. No antiviral agents are currently available for treatment. Acetaminophen is recommended for pain and fever; corticosteroids, aspirin or nonsteroidal anti-inflammatory agents should be avoided. The patient should also be carefully monitored for hydration status and for development of plasma leak, as demonstrated by a rise in hematocrit. There is no vaccine currently available for dengue, although some are in development. The best prevention to avoid dengue is to avoid mosquito bites!
Diagnosis of Dengue
Although we will discuss the laboratory diagnosis of dengue, this may not always be utilized. In 75% of cases, the disease may be asymptomatic. In many cases, dengue may be diagnosed by a combination of clinical signs and travel history. Bear in mind that a variety of other differential diagnoses need to be ruled out. Dengue presentation may be similar to: influenza, malaria, typhoid fever, leptospirosis, and other acute febrile syndromes.
Diagnostic Markers for Dengue
As with many other infectious diseases, an understanding of the sequence in which viral markers are expressed may help guide us in selection of the best test to use for diagnosis. You can see by the illustration on the slide, that the period of actual viremia is fairly short – 5 to 6 days after onset of illness. IgM starts to appear as viremia declines and peaks approximately 14 days after onset of symptoms. IgM may persist up to 3 months.
IgG appears at the end of the first week of illness, and slowly increases. IgG may be detectable over the lifetime of the individual. In secondary infections, high levels of IgG are detectable even in the acute phase of illness, whereas IgM levels are significantly lower.
The nonstructural protein, NS1 is expressed during the first 10 days of illness, but disappears after serconversion occurs. As we will discuss, all of these markers have proven useful for diagnostic methods.
Laboratory Confirmation of Dengue
A time-honored method for the confirmation of the presence of dengue virus is that of viral replication in cell culture. Viral cell culture for the dengue virus is performed using the C6/36 cell line of Aedes albopictus for growth – however, this cell line is unlikely to be available in clinical laboratories. Viral cell culture is confirmatory, and Identifies serotypes via reactivity with fluorescent monoclonal antibodies. An acute serum sample must be collected within the first 5 days of symptoms for best yield of virus. Growth of dengue virus in culture cannot differentiate between primary and secondary infections in the host. This test is generally performed only at research laboratories, or certain state department of health laboratories. Cell culture facilities are required, and testing will usually take at least a week before results are available.
There are several molecular formats that have proven useful for the confirmation of dengue virus in patient samples. Nucleic acid amplification tests or NAAT tests can utilize a nested PCR format, which initially detects a highly conserved region of the virus, followed by a serotype-specific secondary PCR reaction.
Reverse transcriptase PCR and nucleic-acid sequence-based amplification or NASBA formats can also be used. Real-time PCR methodologies have important advantages due to their rapid turnaround time. Like other molecular tests they are highly sensitive and specific, and useful for serum or plasma collected during the first 5 to 6 days of symptoms. Real-time PCR has been reported to have from 80 to 90% sensitivity and greater than 95% specificity.
It is recommended that negative results from a molecular test are followed-up with a serologic method. Some real-time PCR assays are commercially available, but only as research use only reagents. An FDA-approved test protocol is available from CDC, which can be used to subtype all 4 dengue types.
The IgM capture ELISA method, also known as MAC-ELISA, can be used for confirmation of infection, as well as serotyping of infection. In this method an anti-human IgM bound to a microtiter plate captures IgM in the patient sample. A serotype-specific dengue antigen (protein E envelope glycoprotein) is added and detected with an enzyme-conjugated, dengue specific monoclonal antibody.
It’s important to note that false positive results due to cross-reactivity with other flaviviruses such as Yellow fever, West Nile virus, and St. Louis encephalitis can occur with this test. The reported sensitivity of MAC-ELISA varies between 61.5 to 99%; specificity varies between 79.9 to 97.8%. IgM can persist up to 3 months. An FDA-approved test using this format became available in April of 2011.
The IgG ELISA is also available as a confirmatory test, if multiple specimens are collected during the course of disease. This test uses the same antigens as MAC-ELISA, the protein E membrane glycoprotein.
The IgG ELISA can differentiate primary or secondary infections, however, you must test both acute and convalescent specimens. A negative IgG followed by a positive IgG is indicative of a primary infection. A 4-fold rise between acute and convalescent specimens is indicative of a secondary dengue infection. This assay cannot be used to serotype dengue infections. Commercial tests are available; but they are not FDA approved at this time.
An additional method that has been used for determination of dengue infection is the detection of antigen and antibody to NS1. Remember, this is a nonstructural protein which is expressed by all flaviviruses, and is detectable up to 10 days after onset of illness. The NS1 antigen will disappear once seroconversion has occurred in the host.
Both antigen-capture ELISAs and lateral flow-antigen detection methods are used, as well as NS1-specific IgM and IgG assays. Commercial assays are available, however none are currently FDA approved. As a whole, NS1 tests have excellent specificity; although sensitivity may vary between 60 and 90%. Some test formats are reportedly useful for serotyping of dengue virus.
As I alluded to in the previous slide, a number of immunochromatographic lateral flow tests (cassette tests) are available for the detection of dengue infections. These tests are based on detection of IgM,or IgG antibody, or detection of the NS1 antigen.
Recall that the NS1 nonstructural protein antigen is expressed within the first 10 days of illness. This provides a good marker for early detection of infections. However since NS1 is detected in response to infections due to other flaviviruses, cross-reactivity can be an issue. These rapid tests are simple to perform. They take 15 minutes to result, and have assumed an important role in outpatient screening or other field work. Studies in Southeast Asia have indicated a combined approach using rapid IgM or IgG antibody and antigen tests may increase sensitivity for acute dengue, and allow earlier detection of infection.
Finally, the plaque reduction and neutralization (PRNT) assay is the most specific serologic tool for the detection of dengue. This test will subtype dengue infections, and measures titer of neutralizing antibodies. It is labor-intensive and requires maintenance of very specific cell lines, thus it is primarily limited to research laboratories and certain state health departments.
The CDC website listed on the slide gives an up-to-date overview of available dengue tests, which you may find helpful. As of 2009, dengue became a nationally reportable disease.
Clinical Diagnosis of Dengue: Best Approaches
Before selecting the most appropriate dengue virus test from the options we have discussed, it is essential to take several factors into consideration. Remember in many cases, diagnosis of dengue may be based on clinical signs. Obtaining a comprehensive history is essential, given the ramifications of development of severe dengue disease secondary to antibody-dependent enhancement in individuals who have had a previous dengue infection. Knowing the onset of illness is very important, as it will dictate the most appropriate test for confirmation of dengue infection, as we will discuss momentarily.
Finally, are you trying to obtain a clinical diagnosis of an acute infection, are you following up on possible infections several weeks or months out, or are you performing epidemiologic studies in a geographic area? Focusing on the specific need for testing will help you in the selection of the most appropriate diagnostic test. This can save time, effort, and potential expense to the patient.
Test Combinations for Diagnosis of Acute DENV Infections
We have discussed a variety of test options in this presentation; however, what is the best test or combination of tests to use for diagnosis of acute dengue virus infections? A test algorithm based on the natural course of infection is useful. The need for timely diagnostics may limit the utility of labor-intensive techniques such as cell culture or PRNT. If you have a high suspicion of dengue virus infection based on clinical presentation, travel history or epidemiology, real-time PCR may be useful if specimens are collected within the first 6 days of symptoms. Remember that the duration of dengue viremia is actually quite short.
Detection of the NS-1 antigen presents a useful option providing a greater window of time for detection of dengue infection. NS-1 can be detected up to 10 days after the onset of symptoms. A recent study by Anderson and colleagues demonstrated excellent positive agreement (96%) in retrospective serum specimens when compared to an RT-PCR method. If serum specimens are submitted later in the course of infection, detection of IgM is of clinical value, although the possibility of test cross-reactivity with other flaviviruses may be a concern.
For detection of acute dengue virus infections, a combination of NS-1 antigen testing and IgM serology can provide a good alternative to molecular testing with an increased time period of detection, or if timely access to a RT-PCR method is limited. Depending on the specific test used, the ability to perform viral subtyping may vary. If this is an important criteria, a conversation with the performing laboratory to confirm dengue virus typing capabilities and availability of additional testing options may be in order.
Let’s now shift gears and discuss what we may expect from dengue in the future.
Why Are We Concerned About dengue?
Why are we so concerned about the occurrence of dengue in the United States?
For 1 thing, range of Aedes species has expanded, moving out of the southern states and persisting over multiple seasons. We have a greater frequency of travel to endemic areas, and we have the ability to travel long distances in a relatively short period of time. We have had local outbreaks of dengue in the United States: In Hawaii in 2001 and 2002 and in Texas in 2005
And notably, outbreaks have occurred in relief workers returning from Haitian earthquake in Jan. 2010. In 2010, an epidemic in Puerto Rico, accounting for 21,000 cases.
A recent MMWR report of a death in a Texas resident due to dengue type 3 was due to onset of acquired hemophagocytic lymphohistiocytosis triggered by dengue virus infection. HLH is a hyperinflammatory syndrome characterized by pancytopenia, hepatosplenomegaly and increased serum ferritin. This case was also complicated by severe liver injury leading to death. A complicating factor in this case was the initial diagnosis of West Nile virus infection based on a weakly positive serologic test result. After subsequent PCR testing for both West Nile virus (which was negative) and dengue (positive for dengue virus 3), the initial serologic result was determined to be most likely due to a cross-reactive anti-dengue virus antibody.
Further investigation in this case determined the patient had acquired dengue infection either in Texas, or during travel to New Mexico. This was the third reported dengue death in the United States in the past 10 years. Clearly, a level of concern for dengue infection is in order. In the last several years, there has been an interesting turn of events in that we have had several self-sustaining epidemics of dengue fever in the continental United States.
In the fall of 2009, a 34 year old female from Rochester, NY was diagnosed with dengue and had returned from 1 week in Key West. Shortly thereafter, a 48 year old male from Key West was also diagnosed with dengue, however, he had no history of travel. Key West is 396 miles from Jacksonville, as the mosquito flies.
In the follow-up investigation, the Florida department of health, working with the CDC, conducted a serosurvey of Key West, which resulted in identification of 26 additional cases. In addition mosquito pools tested positive for dengue type-1. Separate study in 2009 indicated seroprevalence in the Key West population of 3 to 5% among residents. This was indicative that dengue had become established in the population.
An additional 31 cases were confirmed in 2010 in the area surrounding Key West, in individuals with no associated travel history. Phylogenetic analysis of these strains indicated a similarity to the 2009 Key West dengue type 1 strains. These strains were also unique from Central American and Caribbean lineages of dengue type 1, and also distinct from other travel-associated dengue type 1 viruses isolated in other areas of Florida. This provided evidence that unique subtype of dengue can now be considered endemic in south Florida.
Why Re-Emergence of Dengue in Florida?
We have re-emergence of dengue in Florida. Many of the epidemiologic characteristics apply to other areas of the United States as well. We have lots of travelers going to and from endemic areas. We have a prevalence of Aedes mosquito species.; both Aedes aegypti, and Aedes albopictus. Both species can easily overwinter here in Florida and in other states. Climate variations may make overwintering of mosquito populations in many areas of the US a distinct probability. We have a largely dengue non-immune population. We certainly have the opportunity for mosquito exposure; very few visitors come to Florida and stay inside for an entire week. The take home message is that dengue should now be included in differential diagnosis of any acute febrile illness. Especially among those who Live or travel in southern US or tropics.
Prevention of Dengue in Susceptible Polulations
Let’s briefly discuss a few efforts in the prevention of dengue infection. Most work in this area focuses on preventing interactions between humans and mosquitoes as well as development of an effective dengue vaccine.
Prevention of Mosquito Exposure
When we talk about prevention of mosquito exposure, clearly this is easier said than done! Historically we have targeted populations of mosquito larvae or immature mosquitoes through cleanup of the environment; removal of water retaining mosquito breeding sites like old tires, discarded containers, or covering of rain barrels. We have also relied on use of larvicidal chemicals or biological controls (like larvae-eating fish) to use in water supplies.
We have addressed adult mosquitoes by spraying or fogging of insecticides. This is clearly only a temporary solution to a dengue outbreak, and not sustainable as insecticide coverage can be difficult to truly control and resistance to the chemicals may develop in the mosquito.
Certain strains of mosquitoes have been genetically altered in order to suppress their natural reproduction, or inhibit dengue virus replication within the host. This is currently an experimental approach, and not yet proven effective for widespread use.
Multiple methods to prevent mosquito exposure within homes have been attempted. Given the 2.5 billion people at risk of dengue, these controls must be easy to use, have low toxicity, and most importantly, affordable. Several studies have used household barriers such as screens or curtains treated with insecticides such as deltamethrin with varying degrees of success.
Studies have determined that in order for these types of household controls to be effective, homes need to be constructed such that insects come into physical contact with an insecticide-treated curtain or screen. It is not sufficient nor effective to simply hang a curtain over an opening in the structure.
Treated clothing may help to reduce mosquito exposure, although not a solution for infants or young children. Treated clothing and application of a permethrin-based repellent are often recommended if travelling in a known endemic area, and may also reduce exposure to other mosquito-borne diseases such as malaria.
Improvements in socio-economic conditions resulting in improved home construction, elimination of standing water sources, or the implementation of air conditioning are proven methods leading to reductions in dengue transmission.
I will briefly discuss the challenges and progress in development of a dengue vaccine. One of the main challenges is the need to develop a tetravalent vaccine, effective against all 4 dengue serotypes, as the role of antibody-dependent enhancement in a subsequent infection is contributory to increased severity of dengue disease.
A suitable vaccine must be able to generate an immune response rapidly, either to reduce infections in outbreak situations, or generate immunity in travelers, or the military leaving for endemic areas. Immunity must be durable, reducing or eliminating the need for repeated booster vaccinations.
Given the large population at risk of dengue, a vaccine candidate would have a long shelf life and no need for refrigeration, so that it can be transported to remote areas. Finally, a truly ideal vaccine would meet all of these requirements and have a favorable safety profile in populations including pregnant women and children, with few side effects. This is indeed a tall order.
Diverse Approaches to Dengue Vaccines
Due to these challenges and needs for a dengue virus vaccine, several approaches have been used for the development of a vaccine candidate. Use of vaccines composed of attenuated dengue viruses, recombinant vaccines consisting of truncated dengue envelope proteins in a Drosophila cell expression system, dengue virus membrane and envelope genes inserted into the cDNA backbone of the yellow fever 17D vaccine, formalin-inactivated whole dengue virus, and a plasmid DNA vaccine containing dengue virus sequences with a eukaryotic promoter to drive presentation to the immune system are all in various stages of clinical trials.
Although preliminary results are encouraging, no clear-cut candidate has emerged. Multiple trials are ongoing and hopefully we will see some positive results within the next few years.
So, to summarize today’s presentation. Remember that dengue virus is a single-strand RNA flavivirus, with 4 serotypes which is the most common mosquito-borne viral infection worldwide. Dengue is transmitted by Aedes mosquitoes; Aedes aegypti and Aedes albopictus, and the incubation period after a mosquito bite is approximately 3 to 6 days.
According to the World Health Organization, there are 3 case definitions of disease due to dengue virus: dengue without warning signs, dengue with warning signs, and severe dengue. Shock, plasma leak and hemorrhage can be ominous clinical signs. There are 3 phases of disease; febrile, critical, and recovery, and supportive care is the only recommended treatment.
Dengue can be diagnosed using the methods you see listed on the slide. Availability of these tests may be limited. Certain PCR and IgM ELISA tests are now FDA approved. All of these methods have their strengths and weaknesses, and you need to be aware of the phase of disease the patient is in, in order to make the best use of these various diagnostic tests.
Finally, we discussed US outbreaks of dengue, and evidence of sustained endemic transmission. An overall increase of dengue prevalence, and locally-sustained infections have also been attributed to travel, prevalence of mosquito populations, and a largely dengue non-immune population. Future control of dengue may rely on control of mosquito vectors and development of a multi-strain vaccine. The most important takeaway point is that dengue should be considered as part of the differential diagnosis in an acute febrile illness in travelers to tropical regions, or residence in certain parts of the United States.
For additional information on the dengue virus consult: http://www.cdc.gov/Dengue/
This is an excellent resource that provides up to date information on all things dengue, and also contains an interactive map which provides locations of recent outbreaks.
Thank you for your attention.