Dengue Virus: Properties, Pathogenesis, and Lab Diagnosis

Dengue is a mosquito-borne viral disease caused by the dengue virus, an Arbovirus (arthropod-borne virus). Also known as break bone fever, a severe form of dengue is called dengue hemorrhagic fever (DHF). The disease is transmitted via the bite of an infected female Aedes aegypti mosquito.

Globally, 2.5 billion people live in areas where dengue viruses can be transmitted. Cases of this disease are seen in tropical and sub-tropical climates worldwide, mostly in urban and semi-urban areas.

Properties

• It is composed of single-stranded RNA (SSRNA).
• It has four distinct but closely related serotypes DEN-1, DEN-2, DEN-3, and DEN-4.  Subsequent infections by other serotypes increase the risk of developing severe dengue.

Transmission

This disease is the most common mosquito-borne viral disease in humans. The virus is transmitted to humans through the bites of infected female mosquitoes. Aedes aegypti mosquito, a daytime feeder mosquito, is the primary vector.

Aedes albopictus is a secondary dengue vector. These species are active for approximately two hours after sunrise and several hours before sunset but can bite at night in well-lit areas. This mosquito can bite people without being noticed because it approaches from behind and bites on the ankles and elbows. 

Clinical Findings

The clinical disease begins 4-7 days (3-14 days) after an infective mosquito bite.

Clinical signs/ symptoms:

1. Fever, malaise, chills, headache, pain behind the eyes
2. Characteristic deep bone pain and myalgia
3. Enlargement of lymph nodes.
4. Classic dengue fever is a self-limited disease
5. Secondary infection with dengue type 2 following a type 1 infection is a risk factor for severe disease.

Classical dengue fever is a mild illness, but DHF and dengue shock syndrome (DSS) are severe forms of the disease. Severe dengue is a leading cause of hospitalization and death among children. Immunity after recovery is lifelong for that serotype but partial and temporary for others.

• Serious life-threatening condition
• Children mostly affected
• In those who have previously been infected by one serotype and subsequently infected by another serotype
• Secondary immune response leads to the formation of virus-antibody complex, which activates complement, causing vascular damage and DIC with spontaneous bleeding.
• In some patients, DHF progresses to circulatory collapse (Dengue shock syndrome)
• Mortality rate: 10-40%

Host Immune Response

Primary response

• IgM antibodies appear about 5 days after symptoms onset and continue to rise to 21 days and decrease gradually.
• IgG antibodies appear about 14 days after symptoms onset and persist at a low level for life.

Secondary response

• Weaker and shorter IgM response
• Rapid IgG response (usually 2 days after reinfection)
• High IgG level persists for 30-40 days

Laboratory Diagnosis

• Platelet count: significantly reduced in DHF (100X10⁹/l)
• Haematocrit (PCV): Rise (>20% in some cases)
• White cell count and differential count: Variable; Leucopenia common with blood film may show reactive lymphocytes.
• Coagulation (in DHF)
  • Bleeding and clotting time is prolonged
  • Prothrombin and thromboplastin time increased
  • Fibrinogen levels decreased
• Serum aminotransferases and blood urea: raised
• Serum sodium and albumin reduced
• Albuminuria and Hematuria present in some patients 

Laboratory diagnosis method for confirming dengue viral infection involves using one or a combination of any of the following four methods. 

1. Microscopy and staining
2. Culture
3. Serology
   • Detection of antigen
   • Detection of antibody
4. Molecular diagnosis

Note: These methods are used for diagnosing any infections (viral, bacterial, parasitic, or fungal). The relative importance of a particular method differs among infections. Students are expected to know the main diagnostic test(s) of the particular infection.

Sample

1. Early stages of the disease: after the onset of illness, the virus can be detected in blood (serum, plasma) or tissues; methods employed are; virus isolation, nucleic acid, or antigen detection.
2. Serology is the method of choice at the end of an acute phase of infection.

Note: For virus culture, it is important to keep blood samples cooled or frozen to preserve the viability of the virus during transport from the patient to the laboratory.

1. Microscopy and staining:  Direct visualization of the virus in the sample (using electron microscopy or via fluorescent staining technique) can be done. This is not the preferred method in diagnostic laboratories.
2. Culture: Virus isolation in cell culture is difficult and is not the commonly used method in diagnostic laboratories because it is a demanding procedure (both in terms of infrastructure and technical expertise). Virus may be recovered from serum, plasma, and peripheral blood mononuclear cells. Inoculation of a mosquito cell line with patient serum, coupled with nucleic acid assays to identify the recovered virus is a commonly used approach in the research lab.

Serological test: Serological tests are the mainstay in the diagnosis of viral infections. 

• Detection of viral antigen:
  • Dengue NS1 antigen detection is useful for the diagnosis of acute dengue infections up to 0-7 days of symptoms but not recommended after 7 days.
  • NS1 antigen has been detected in the serum of DENV infected patients as early as 1-day post-onset of symptoms (DPO), and up to 18 DPO.
  • NS1 ELISA based antigen assay is commercially available
  • NS1 assay may also be useful for differential diagnostics between flaviviruses because of the specificity of the assay.
• Result interpretation 
  • A positive NS1 test result confirms dengue virus infection but does not provide serotype information.
  • A negative NS1 test result does not rule out infection. People with negative NS1 results should be tested for the presence of dengue IgM antibodies to determine possible recent dengue exposure.

Detection of anti-dengue antibodies in serum or other body fluids by ELISA or other rapid tests.  Various methods (IgM/IgG ELISA, Hemagglutination Inhibition Test, or rapid diagnostic kits) are available to detect anti-dengue antibodies; IgM detection:  

• Useful for the diagnosis of primary dengue infection and in distinguishing dengue from other flavivirus infections.
• IgM antibodies are detectable in 99% of patients by day 10 after the onset of illness.
• IgM levels peak about two weeks after the onset of symptoms and then decline to undetectable levels over 2–3 months. 
• Sensitivity: 65-75% sensitive in a single acute serum sample.

IgG detection: Tests that detect IgG are useful in diagnosing secondary disease (IgG is the dominant immunoglobulin type in secondary infection). The test is complicated by cross-reactivity of IgG antibodies to heterologous flavivirus antigens (West Nile virus, tick-borne encephalitis virus, yellow fever virus, zika virus).
Note: To distinguish between primary and secondary dengue infections, IgM/IgG antibody ratios are now more commonly used than the hemagglutination-inhibition test (HI).

1. Molecular diagnosis:   detection of viral RNA in plasma or serum or tissues using nucleic acid amplification test (NAAT).  RT-PCR based methods for rapid identification and serotyping of dengue virus in acute phase serum are available.

Interpretation of Dengue Diagnostic Tests:

Highly Suggestive	Confirmed
One of the following:  IgM + in a single serum sample IgG + in a single serum sample with a HI titre of 1280 or greater	One of the following: PCR + Virus culture + IgM seroconversion in paired sera IgG seroconversion in paired sera or fourfold IgG titer increase in paired sera

References and Further Readings:

1. WHO: Dengue Guidelines for Diagnosis, Treatment, Prevention and Control
2. CDC: http://www.cdc.gov/dengue/clinicalLab/laboratory.html