Cause of the disease
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Dengue is an infectious disease caused by the dengue virus (DENV). It belongs to the genus Flavivirus of the Flaviviridae family, which in turn, belongs to the group of Arboviruses (arthropod-borne viruses). The viral particles contain single-stranded RNA that are enwrapped and approximately 40-50 nm in diameter.
There are 4 serotypes called DENV-1, DENV-2, DENV-3 and DENV-4. Infection with one serotype produces lifelong immunity against that serotype reinfection. Successive infection with two different serotypes is a risk factor for developing the severe forms of the disease.
All serotypes have been isolated in the Americas. In several countries they circulate simultaneously, creating a serious risk for an epidemic.
How is it transmitted?
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The Aedes aegypti and Aedes albopictus mosquitoes are transmitters of dengue. The Aedes aegypti female mosquitos are the main source of dengue transmission. These species bite during the day, with the most active feeding period 2 hours before and after dawn and dusk. The mosquito bites an infected person and ingests blood with the dengue virus, which incubates in the mosquito for a period of 8 to 12 days, after which the mosquito begins to transmit the virus biting others. The newly infected person may have symptoms after 5-7 days of infection. Immediate mechanical transmission can also occur when the mosquito interrupts feeding on an infected person and immediately feeds on a susceptible host. This form of transmission does not require virus incubation.
Stages of the disease
Patients develop sudden high fever. This acute febrile phase lasts 2-7 days and is usually accompanied by skin flushing, erythema, generalized body pain, myalgia, arthralgia, headache, and retro-orbital pain. Minor hemorrhagic manifestations, such as petechiae and ecchymoses in the skin may occur. Patients, who improve after the initial fever, are considered dengue without warning signs.
Problems that may arise in the febrile phase: dehydration, high fever associated with neurological disorders, and seizures in young children.
After the fever abates, the patient may experience within the first 3-7 days of illness an increase of capillary permeability parallel with increased hematocrit levels. This marks the beginning of the critical phase with the highest risk of developing plasma leakage with or without hemorrhage, known as severe dengue. Patients who clinically worsen after fever subsides should be monitor for warning signs and hemorrhagic manifestations.
Problems that can occur in the critical phase: shock by plasma extravasation, severe bleeding, and serious organ impairment.
Most patients survive the acute phase (which does not exceed 48 to 72 hours), with timely initiation of judicious fluid management and careful monitoring of patient during critical phase, and enter the recovery or convalescent phase. There is a general improvement in the patient: appetite recovers, gastrointestinal symptoms improve, hemodynamic status stabilizes and diuresis increases.
Problems that can occur in the recovery phase are overload (if intravenous fluid therapy has been excessive or has been extended in this period).
Revised Classification of Dengue
See: Diagram of revised dengue classification (Spanish available only). Under the revised classification, the level of severity has been separated into dengue, without warning signs and with warning signs, and severe dengue as a practical use during the clinical decision process as to where and how intensively the patient should be observed and treated.
Upon making a diagnosis, clinicians should be aware that some signs and symptoms might be difficult to differentiate from those of other diseases such as malaria, rubella, measles, typhoid fever, meningitis and influenza. Rhinorrhea (runny nose) or nasal congestion does not occur in dengue cases.
The definitive diagnosis of dengue infection is made in the laboratory and is dependent on the detection of specific antibodies in the patient’s serum, detection of viral antigen or viral RNA in serum or tissue, or viral isolation. A blood sample in the acute phase should be taken as soon as possible after the onset of febrile sign. A blood sample from the convalescent phase ideally should be taken 2-3 weeks later.
1. Serological Test:
This can be done by immunoglobulin M (IgM) capture enzyme linked immunosorbent assay (MAC-ELISA), indirect immunoglobulin (G ELISA), hemagglutination -inhibition (HI), complement fixation (CF), neutralization test (NT), or non-structural protein 1 enzyme linked immunosorbent assay (NS1 ELISA).
2. Viral Isolation
Four viral isolation systems have been used for dengue virus, intracerebral inoculation in mice 1-3 days of age, cultures of mammalian cells (LLC-MK2), intrathoracic inoculation of adult mosquitoes and using mosquito cell cultures.
3. Viral Identification
The method of choice for the detection of the dengue virus is indirect immunofluorescence (IFA): type-specific monoclonal antibodies produced in tissue cultures or ascites fluid of mice and IgG-fluorescein isothiocyanate conjugate.
4. RT-PCR (Reverse Transcription-Polymerase Chain Reaction)
RT-PCR is a rapid, sensitive, simple and reproducible method with appropriate controls. It is used to detect viral RNA in clinical samples from humans and mosquitoes autopsy tissue. RT-PCR is similar to viral isolation with the advantage that problems in the handling, storage and the presence of antibodies do not influence result. However, it should be emphasized that the PCR does not replace viral isolation techniques.
With immunohistochemical, it is possible to detect the virus antigen in a variety of tissues. This method conjugates polyclonal and monoclonal antibodies to an enzyme, such as phosphatases and peroxidases, that then catalyzes a color-producing reaction (stain) that can be analyze.
6. Rapid Tests
The clinical features of dengue are not very specific and therefore require laboratory confirmation.
Accurate and sophisticated methods such as isolation or polymerase chain reaction (RT-PCR) of virus require advanced equipment and infrastructure. Serological tests are widely available and can provide an alternative to support the diagnosis. Primary infections (new) of DENV typically have a stronger and more specific IgM response; subsequent infections (secondary) show a weaker IgM response but a strong IgG response.
These patterns of differentiation of IgM response to infection underscore the need to assess the sensitivity and specificity of tests commercially available, especially for the diagnosis of secondary DENV infections. WHO / TDR and the Pediatric Dengue Vaccine Initiative (PDVI) collaborated to evaluate IgM anti-DENV diagnostic tests commercially available. A network of seven laboratories in Asia and Latin America was established to carry out this collaboration.
The WHO 2009 report, Evaluation of Commercially Available Anti-Dengue Virus Immunoglobulin M Tests, describes the results of an evaluation of nine commercially available IgM anti-DENV tests using a panel of well-characterized specimens with serum from patients with confirmed infections DENV and other infections, and lists potentially confounding conditions.
The NS1 rapid test is a recent introduction on the market that has a high specificity for the diagnosis of dengue (90 to 100%) but low sensitivity (52-62%), indicating a negative result of this test does not rule dengue infection and necessitating an additional test for confirmation of dengue.
Regional Office for the Americas of the World Health Organization