Dengue fever (pronounced den-gee) is a mosquito-borne tropical disease caused by the dengue virus. Since the Second World War, dengue has been a global problem. It is common in more than 110 countries.
Dengue affects 50-528 million people every year and results in 10,000-20,000 deaths. It is more prevalent in tropical and sub-tropical places, with most cases being reported from south-east Asia, Africa and South America.
It is classified as a “neglected tropical disease” by the Centre for Disease Control and Prevention.

Dengue Virus
A person can contract dengue after being infected by any of the four types of dengue viruses. Dengue Fever Virus is closely related to viruses that cause the West Nile infection and yellow fever.
While a tropical disease, dengue cases have been recorded in Europe and the USA. The victims in these cases are mostly tourists who have travelled to tropical regions.

Dengue spreads through the bite of the Aedes mosquito which is infected by the virus. The most common carrier is the Aedes aegypti mosquito. A female mosquito taking blood from an infected person can become a carrier of the disease.
The virus to incubates for 8-10 days inside the body of the mosquito before it is released in the saliva. This mosquito breeds in stagnant or collected water.
When the virus enters the human bloodstream, it enters the white blood cells and reproduces rapidly. It spreads rapidly with blood circulation. Hence, the white blood cells release a number of proteins, which leads to muscle and joint pain and fever.
In severe cases, the virus may affect organs like the liver and even bone marrow. The spread of the infection results in low blood pressure. Hence, vital organs do not get sufficient blood supply.
When the bone marrow is infected, storm cells get infected. This reduces platelet count. Platelets enable blood clotting. Thus, with decreased platelet count, bleeding may occur from nose or gums.

Transmission of the Disease
Dengue spreads through mosquito bites.
Dengue fever cannot spread directly from one person to another. It is non-contagious. However, it can spread through blood transfusion, if the blood is taken from a dengue patient.
There are cases reported where the disease passed from mother to child during pregnancy or birth, though such instances are extremely rare.

Dengue Fever Symptoms
Visible symptoms of dengue fever include

High fever
Severe headaches
Backache
Joint pain & Fatigue
Nausea and vomiting
Eye pain
Rash (3-4 days after the fever begins)
Usually, these become apparent after 4-7 days of being infected, and the illness lasts 3-14 days, although full recovery can take a month.

Many dengue cases are often mistaken for signs of flu or influenza. In adults, nausea and vomiting may occur during the course of the disease. Sometimes, swelling occurs in lymph nodes in the neck and groin region.
Dengue patients often suffer from rashes. Dengue rashes often resemble rashes from measles. In other cases, the rashes look like “islands of white in a sea of red”.
In about 2% of cases, dengue fever turns into a more severe form called dengue hemorrhagic fever (DHF). This form is characterized by a fever that lasts from 2 to 7 days and in addition to the above common dengue symptoms, is accompanied by:

Hemorrhagic manifestations
Bruising easily or other skin haemorrhages
Bleeding nose or gums
Possible internal bleeding
Fainting
Less frequent urination
The ability to differentiate between dengue and influenza is crucial to stop the progress of dengue to its deadly hemorrhagic form.
A simple blood test is all that is needed to confirm a dengue diagnosis. With good medical management, mortality due to Dengue hemorrhagic fever is as low as 1-5 per cent. Most fatalities occur among children and young adults.

The course of Dengue Fever
Dengue fever usually has three phases- febrile, critical and recovery. High fever occurs during the initial stage. The fever itself breaks and returns for a couple of days.
In some cases, dengue enters a more critical phase following the resolving of the fever. During this period, plasma leaks from blood vessels. This stage lasts for a couple of days. As a result, fluid depletes from the circulation system and blood supply reduces to vital organs. Instead, fluid accumulates in the chest and abdomen. People suffering from dengue have reduced platelet count, often below 20,000. People who otherwise have low platelet count are at a greater risk of developing complications. Children are more likely to develop severe complications and are at greater risk of contracting dengue hemorrhagic fever.
During recovery, bloodstream re-absorbs the fluid. This usually takes two or three days, and often the improvement in the condition of the patient is striking.
This stage shows itching and a slower heart rate. Rashes may also occur, which lead to peeling of dry skin. Many patients report feeling fatigued.
Pregnant women suffering from dengue may suffer miscarriages. It can also lead to low birth weight and premature birth.

Dengue Haemorrhagic Fever: How Is It Different?
The World Health Organisation classifies dengue into two categories: uncomplicated and severe. Dengue hemorrhagic fever is a severe form of the disease. This is again classified into four categories based on severity.
Dengue hemorrhagic fever is a rare condition. It has symptoms like high fever, damage to lymph nodes, bleeding from nose or gums, diarrhoea and vomiting.
This may further progress to shock and death. This is called Dengue Shock Syndrome.
People who have previously suffered from dengue are more vulnerable to the disease.

Dengue Tests and Diagnosis
Doctors can diagnose dengue from blood tests. The most common method is the detection of dengue-specific IgM and IgG-class antibodies.
In many cases, diagnosis requires both, since the human body produces a varied number of antibodies during the course of the disease.
IgM antibodies are developed in the initial stages, and tests for these are more effective when conducted 7-10 days after exposure. after a few months, IgM antibodies decrease and fall below detection levels.

On the other hand, IgG antibodies develop more slowly. They develop during the acute stage and persist for a long time.
NS1 antigen tests are done in the earlier stages of the disease for detecting IgM antibodies. The antibodies-based test IgM-Elisa is done after five days and it is a confirmatory test. Doctors insist on doing the IgM-Elisa even though the fever resolves.
It is difficult to distinguish symptoms of dengue from chikungunya and other mosquito-borne diseases like malaria. Often, confirmation requires multiple tests and prolonged investigation to rule out diseases like malaria, measles and influenza.
The recently discovered Zika virus also shows symptoms similar to dengue.

Dengue Treatment
There is no specific anti-viral treatment for dengue illness.
Since dengue is a virus, antibacterial medicines do not have any effect. Treatment includes giving the patient fluids orally or intravenously. Fever and muscle pain is managed with paracetamol and anti-inflammatory drugs.
Doctors recommend watching patients for signs of dehydration. Dengue weakens patients, and they often feel fatigued and nauseous. Doctors usually recommend ample rest and bed rest in more severe cases.
In India, many recommend taking crushed papaya leaves with milk or other fluids to increase platelet count, though there is no evidence to support this claim.

Dengue Vaccine
In 2016, a partially effective vaccine for dengue fever (Dengvaxia) was approved. It is commercially available in Mexico, the Philippines, Indonesia, Brazil, El Salvador, Costa Rica, Paraguay, Guatemala, Peru, Thailand, and Singapore. However, WHO recommends that it be used only in areas the disease is prevalent.