Dengue Fever

Dengue fever is a viral infection transmitted to humans by mosquitoes that live in tropical and subtropical climates and carry the virus. Blood testing detects the dengue virus or antibodies produced in response to dengue infection.

If a person develops a fever within two weeks following travel to an area where dengue fever is present, it may be prudent to test for dengue fever. According to the Centers for Disease Control and Prevention (CDC), dengue infections have been reported in more than 100 countries from parts of Africa, the Americas, the Caribbean, the Eastern Mediterranean, Southeast Asia, and the Western Pacific. It is a fast emerging infectious disease, according to the World Health Organization (WHO), with an increasing number of cases and countries affected throughout the world. Approximately 50 to 100 million cases occur annually worldwide.

Many individuals will develop no symptoms at all, or have only a mild illness when exposed to one of the four serotypes of the dengue virus. For those who do develop symptoms, prognosis is still very good for full recovery within a few weeks. The most common initial symptoms are a sudden high fever (104°F or 40°C) and flu-like symptoms that appear roughly 4 to 7 days after being bitten by an infected mosquito (this is called the incubation period and can range from 3 to 14 days). Additional signs and symptoms may include severe headache, especially behind the eyes, muscle and joint pain, skin rash, nausea, vomiting, and swollen glands.

Some people who develop a fever will recover on their own with no lasting ill effects while others may progress to severe dengue fever (sometimes called Dengue Hemorrhagic Fever). If the disease progresses to this form, a new wave of symptoms will appear 3 to 7 days after initial symptoms and as the fever recedes. These may include nose bleeds, vomiting blood, passing blood in the stool, difficulty breathing and cold clammy skin, especially in the extremities. During the second phase, the virus may attack blood vessels (the vascular system), causing capillaries to leak fluid into the space around the lungs (pleural effusion) or into the abdominal cavity (ascites). Currently, there is no explanation as to why dengue fever resolves itself harmlessly in some people while progressing to the severe form in others.

The loss of blood and fluid during the second phase, if untreated, can sometimes develop into a rare condition known as Dengue Shock Syndrome and can be fatal. In order to avoid that complication, a doctor may hospitalize a patient with dengue fever so that falling blood pressure and dehydration caused by the loss of blood and fluids can be managed while the disease runs its course – generally a period of one to two weeks. During the following week of recovery, a person may develop a second rash that lasts a week or more.

Dengue fever is usually diagnosed via some combination of blood tests because the body's immune response to the virus is dynamic and complex. Laboratory tests may include:

• Molecular tests for dengue virus (PCR)—detect the presence of the virus itself; these tests can diagnose dengue fever up to 5 days after the onset of symptoms.
• Antibody tests, IgM and IgG—detect antibodies produced by the immune system when a person has been exposed to the virus; these tests are most effective when performed at least 7-10 days after exposure.
• Complete blood count (CBC)—to look for low platelet count typical of the later stages of the illness and to detect the decrease in hemoglobin, hematocrit, and red blood cell (RBC) count (evidence of anemia) that would occur with blood loss associated with severe dengue fever
• Basic metabolic panel (BMP) – to monitor kidney function and look for evidence of dehydration that can occur with severe illness

How is the sample collected for testing?

A blood sample is collected by inserting a needle into a vein in the arm.

How is it used?

Dengue fever testing is used to determine whether a person with symptoms and recent potential exposure to dengue has been infected. The infection is difficult to diagnose without laboratory tests because symptoms may initially resemble other diseases, such as malaria. Two types of testing are available:

• Antibody tests—these tests are primarily used to help diagnose a current or recent infection. They detect two different classes of antibodies produced by the body in response to a dengue fever infection, IgG and IgM. Diagnosis may require a combination of these tests because the body's immune system produces varying levels of antibodies over the course of the illness. IgM antibodies are produced first and tests for these are most effective when performed at least 7-10 days after exposure. Levels in the blood rise for a few weeks, then gradually decrease. After a few months, IgM antibodies fall below detectable levels. IgG antibodies are produced more slowly in response to an infection. Typically, the level rises with an acute infection, stabilizes, and then persists long-term. Individuals who have been exposed to the virus prior to the current infection maintain a level of IgG antibodies in the blood that can affect the interpretation of diagnostic results.
• Molecular testing (polymerase chain reaction, PCR)—this type of test detects the genetic material of the dengue virus in blood up to 5 days after symptom onset (fever).