Test for dengue fever

Sh Nilica Devi *

Dengue fever is a viral infection transmitted to humans by the bite of an infected, day-biting, Aedes mosquito (Aedesaegypti and Aedesalbopictus). Dengue infection is caused by any one of the four related serotypes of Dengue virus (DEN-1, DEN-2, DEN-3 or DEN-4). Blood testing detects the dengue virus or antibodies produced in response to dengue infection.

Dengue fever is a fast-emerging infectious disease with an increasing number of cases and countries affected throughout the world.

Symptoms of dengue fever: Many individuals do not develop any symptoms, or have only a mild illness when exposed to one of the four mentioned serotypes of dengue fever. For those who develop symptoms, prognosis is still very good for full recovery within a few weeks.

The most common initial symptoms are a sudden high fever (104øF or 40øC) and flu-like symptoms that appear roughly 4 to 7 days after being bitten by an infected mosquito. This period, called the incubation period, can last anything between 3 to 14 days.

Additional symptoms may include headache, retro orbital pain in the back and limbs (break-bone fever), skin rash, cold clammy skin, nausea, vomiting, etc.

Some people who progresses to develop severe dengue fever called the dengue haemorrhagic fever or dengue shock syndromehave symptoms like nose bleeds, vomiting blood, difficulty in breathing, etc. This new wave of symptoms appear 3 to 7 days after the initial symptoms appear and as the fever subsides.

During this stage, the virus may attack blood vessels, causing capillaries to leak fluid into the space around the lungs (pleural effusion) or into the abdominal cavity (ascites).

The loss of blood and fluid during the second phase, if left untreated, can progressively worsen and can be fatal.

Diagnosis of dengue fever: Dengue fever is usually diagnosed througha combination of blood tests because the body’s immune response to the virus is varied and complex. Laboratory tests include:

o Antibody tests – These tests are primarily used to help diagnose a current or recent infection. They detect two different classes of antibodies (IgG and IgM) produced by the body in response to a dengue fever infection. Diagnosis may require a combination of the two tests because the body’s immune system produces varying levels of antibodies over the course of the illness.

o Molecular test for dengue virus by Polymerase Chain Reaction (PCR) – The dengue test by PCR can detect the presence of the virus itself. PCR test can diagnose dengue fever up to 7 days after the onset of symptoms and can be used to determine which of the 4 different serotypes of dengue virus is causing the infection. However, molecular tests of blood are not likely to detect the virus after 7 days of illness.

o Other tests – Complete blood count (CBC) is done to check for low platelet count typically associated with the later stages of the illness and to detect the decrease in haematocrit, haemoglobin and red blood cells (RBC) count. The test is also to monitor the depleting haemoglobin (causing anaemia) that occurs with severe dengue fever.Kidney function tests (KFT)is to monitor the kidney function and to look for evidence of dehydration that happens with severe illness.

The PCR test for dengue that detects the presence of the virus is generally considered the most reliable means of diagnosis. A positive result from a PCR is considered conclusive.

A negative result on a PCR test may indicate that no infection is present or that the level of virus is too low to detect, as may happen if the test was performed after the 7-day window period during which the virus is present in the sample collected for this test.

A negative PCR result is to be followed by antibody testing if the symptoms are found indicative of the infection.

Molecular testing (PCR) is to be done within one week of the onset of symptoms to detect acute infection, while antibody testing may be done more than 4 days after symptoms appear.

Sample for the test: A blood sample drawn from a vein in the arm.

Preparation for the test: No prior preparation is required for the test.

Things to know: Currently, there is no immunisation that will prevent a person from contracting dengue fever if exposed to the virus. Limiting exposure to the virus depends upon protecting against mosquito bites.

The virus is not spread by person-to-person contact or exposure to respiratory secretions.

Physical symptoms like rash or aching joints are not a reliable means for diagnosing dengue fever because the symptoms are not likely to appear until after the initial fever has passed.

No laboratory test can predict whether or not the infection will progress to the more severe form, but those who have been previously infected with dengue are at an increased risk for developing severe dengue during the second infection.