Dengue

What is it?

– Dengue has seen spectacular spread through tropical and subtropical countries around the world for three decades, causing massive epidemics. Although fatal in only a small minority of cases, the virus causes awful pains, which is why it’s sometimes called “breakbone fever,” and because so many people become infected, it has a considerable economic impact.

– The virus is transmitted by mosquitoes and is a member of the Flaviviruses, a group that also includes the viruses that cause yellow fever and Zika.

 

What are the symptoms?

– The symptoms of dengue fever usually start within 4 to 10 days after the bite of an infected mosquito. They can include high fever, severe headache, pain behind the eyes, muscle and joint pains, nausea, vomiting, rash and swollen glands. Symptoms usually last for 2 to 7 days. Some people have very mild symptoms or none at all.

– A minority of patients develop ‘severe dengue’, also called dengue hemorrhagic fever, which can be fatal. Its symptoms include severe abdominal pain, vomiting (including blood), rapid breathing, bleeding gums, and fatigue.

– There are four different ‘types’ of dengue viruses. Becoming infected with one type usually means lifelong immunity against that type, but it increases the risk of severe dengue when you’re later infected with another type.

The Aedes aegypti mosquito, seen here feeding on a human, can transmit dengue. Credit: James Gathany – PHIL, CDC

 

How does it spread?

– Like Zika and yellow fever, dengue is primarily spread by Aedes aegypti, also known as the yellow fever mosquito. Another species that transmits the disease is the Asian tiger mosquito (Aedes albopictus).

Aedes aegypti is very common in urban areas in the world’s tropical and subtropical regions.WHO estimates there are now 50 million to 100 million infections annually in over 100 countries in Mexico, Central and South America, Africa, the Middle East, South Asia and Southeast Asia. But some estimates say the number may be several times higher. Almost half of the world’s population is at risk of dengue.

Dengue occurs in over 100 countries, according to WHO. Credit: CDC

 

How is it treated?

– There are no specific drugs for dengue; resting and drinking plenty of fluids are the recommended. Patients can also take paracetamol to fight fever and joint pains; two other widely sold painkillers, aspirin and ibuprofen, are not recommended because they can increase the risk of bleeding. Proper hospital care can reduce the risk of death in case of severe dengue.

 

How can it be prevented?

– Controlling Aedes mosquito populations is one important way to control dengue. There is a range of measures countries can take to fight the mosquito in all of its life stages. They include spraying insecticides against mosquitoes around houses and removing small reservoirs of water (such as pots, buckets, and old tires) where mosquitoes find a place to breed. Insecticide spraying is not very effective however, and the other measures take the involvement of the community; past experience has shown that they can be difficult to keep up.

– People can reduce their individual risk of being bitten by using insect repellents, wearing clothes that cover the skin, sleeping under bed nets (both during the day and the night) and screening off windows to keep mosquitoes out.

– There is a new dengue vaccine, called Dengvaxia, developed by Sanofi Pasteur. It is designed to protects against all four types of dengue virus, and so far, at least ten countries have approved it for use. But it’s a complicated vaccine. It reduces the risk of dengue by only about 60% overall. And in people who have not been infected with dengue before, the vaccine appears to increase the risk of severe dengue. That means that the vaccine’s harm may sometimes outweigh the benefit.

– An expert group at WHO has recommended that the vaccine should only be used in areas where at least 50% of the population has had a prior dengue infection, and should only be given to children at least 9 years of age. (Some also say that the best strategy would be to test recipients first to make sure they have had dengue before.) All in all, experts believe Sanofi Pasteur’s vaccine to have a moderate benefit at best. Several other vaccines are still in development; one, from Japanese company Takeda, entered a large phase III trial in September 2016.

 

What’s the outlook?

– Dengue remains difficult to fight because Aedes mosquitoes are difficult to control. (The same is true for Zika, chikungunya, and yellow fever.) Insecticide spraying is expensive and not effective enough in many areas, and the removal of breeding sites is difficult to sustain.

– New control methods are needed; one option is the mass release of mosquitoes that are unable to transmit dengue, or are genetically engineered to bring their own population down. Whether these approaches work on a large scale, and are affordable enough, is still unclear, however. The other and perhaps best hope to beat dengue is a better vaccine.

 

Resources

WHO information about dengue.

A summary of WHO’s position paper on dengue vaccines.

A story on STAT about Sanofi Pasteur’s dengue vaccine and its problems.

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