Summary

• Dengue fever is spread through the bite of an infected mosquito.
• It is endemic to Asia, Africa and South America but is not established in Australia except for occasional outbreaks in North Queensland.
• If your symptoms and travel history suggest you may have been infected, you will need to have blood tests so that it can be diagnosed.
• Different types of tests will be used depending on how long you have had symptoms.

What is dengue fever?

Dengue fever, also called dengue virus infection, is spread through the bite of an infected mosquito, usually the Aedes aegypti species.

There are four main types of dengue virus— dengue types 1, 2, 3 and 4. Dengue type 5 is mainly found in primates but a rare case in one human has been reported. After infection, people usually develop immunity to the specific type they have had, but they can still become infected again if they are exposed to one of the other types of dengue virus.

Being infected by a different type of dengue virus, even years later, increases the risk of developing a more severe form of the illness called dengue haemorrhagic fever (DHF).

This can cause damage to small blood vessels and changes in blood clotting which can lead to uncontrolled bleeding and shock. Although there have been deaths from severe dengue, most people make a full recovery.

To be infected with dengue you must be bitten by a mosquito carrying the virus. Dengue fever cannot be passed directly from person to person, although rare case have been reported through blood transfusions.

Australian mosquitoes do not normally carry the dengue virus, and most dengue cases occur in travellers returning from countries where dengue is endemic. This is called an imported case.

However, outbreaks do occur in North Queensland every year. This happens when someone is infected overseas and then a local Aedes aegypti mosquito bites this person, becomes infected with the virus, and passes it on to other people by biting them. These are known as locally acquired cases.

Dengue fever is a notifiable disease in Australia. This means that if your doctor finds you have dengue fever, they are legally required to report it to the Australian health authorities. If you have a notifiable disease, all your information is confidential, and your privacy is protected by law. It is used for planning purposes and for research. Personal identifying information is removed before sharing the information.

Why get tested

Some people who have been infected by dengue virus do not develop symptoms or have only a mild illness. In children especially, dengue fever can go unnoticed.

Those people who develop symptoms commonly experience a sudden high fever and extreme tiredness that appear about four to seven days after being bitten (the incubation period). These can be accompanied by a range of symptoms that include:

• intense headache (especially behind the eyes)
• muscle and joint pain (ankles, knees and elbows)
• loss of appetite, vomiting, diarrhoea, abdominal pain, a metallic taste
• flushed skin on face and neck, fine skin rash as fever subsides
• red rash on arms and legs, itching, peeling of skin and hair loss
• minor bleeding (nose or gums) and heavy menstrual periods.

The illness lasts for up to a week. In some cases, fever and possibly worse symptoms may return for another two to three days.

Dengue haemorrhagic fever (DHF)

Dengue haemorrhagic fever (DHF) occurs most often in children and young adults who have previously been infected with dengue virus.

In DHF, the walls of small blood vessels become leaky, allowing fluid to escape into surrounding tissues. This can lead to:

• Bleeding (such as from the nose or gums, or easy bruising)
• Low platelet levels
• Fluid build-up in the chest or abdomen
• Low blood pressure

In more serious cases, the loss of fluid from the bloodstream can lead to dengue shock syndrome, when circulation becomes unstable, and this can be life-threatening.

Testing

Dengue fever is diagnosed using blood tests. The type of test used depends on how many days it has been since your symptoms started.
Early infection tests (days 1–5 of illness)

NS1 antigen test (direct testing): The NS1 antigen test detects a specific protein made by the virus, allowing it to identify infection early—often before other types of tests become positive. The test can be done with a rapid lateral-flow test - a point-of-care cassette or strip, rather like a pregnancy test - or done in the lab with an Enzyme-Linked Immunosorbent Assay (ELISA).

The rapid lateral-flow test is very fast, often providing results in less than an hour and is useful for early clinical decisions. Because of its speed, it is commonly used in hospitals and clinics. However, the ELISA is more accurate, and this method can also tell the difference between current versus past infection.

PCR tests (direct testing): PCR is a test that looks directly for the genetic material (RNA) of the virus. It is extremely accurate and very specific to dengue. It can show which of the four types of the virus is present. PCR is used in the earliest phase of infection. This is when the virus is circulating in the blood (a process called viraemia). After this period, virus levels fall off and PCR becomes less reliable. This is when antibody tests (IgM/IgG) become more useful.

NS1 and PCR tests are known as direct tests This means they directly detect the virus, rather than looking for the body’s immune response.

From day 5 onward

Serology tests (indirect testing): These are tests that look for the antibodies your immune system makes to fight the virus. It takes time after you are infected before antibodies develop and you show a positive result. Two types of antibodies are tested.

• IgM antibodies are usually detectable at about 3–7 days after symptoms begin.
• IgG antibodies are detectable slightly later — typically about 7–10 days after symptoms begin.

After this, serology is not always definitive on a single testing. A second blood sample may be needed, 10-14 days after the first.

Antibody test limitations

Dengue virus belongs to a group of viruses called flaviviruses and the interpretation of antibody testing is complicated by the fact that antibodies made against one flavivirus can also react with other flaviviruses in tests. Cross-reactivity is common in serology testing which is why direct tests (NS1, PCR) are preferred early in the illness.

Secondary dengue and DHF testing

If you are infected for a second time and with another type of the dengue virus (called secondary dengue) your diagnosis may be more complicated. This is because your immune response behaves very differently compared to the first infection.

In the first infection, IgM rises first and IgG appears later. In a secondary infection, IgG rises very quickly and to high levels while IgM may be low, delayed or even absent. This makes standard test result interpretation tricky. Also, there is a greater chance of cross reaction with other flaviviruses.

This is when your doctors will need to place greater reliance on your symptoms to help interpret your test results. Additional tests are used to help assess severity and complications. Some of these tests will look for blood clotting problems such as Full Blood Count (FBC), White Blood Cell Count (WBC), haematocrit and platelet counts. As DHF can affect the liver, Liver Function Tests will also be used and in particular AST and ALT levels will be measured. This can help show the severity of the illness.

Having the test

Sample

Blood.

Any preparation?

None.

Your results

Any more to know

For more information about Dengue fever, see the Queensland Health website.

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help.

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

More information

Pathology and diagnostic imaging reports can be added to your My Health Record. You and your healthcare provider can now access your results whenever and wherever needed.

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