Why is Dengue a Global Threat?, Op-ed

Dengue (break-bone fever) is a viral infection that spreads from mosquitoes to people. It is more common in tropical and subtropical climates. About half of the world's population is now at risk of dengue with an estimated 100–400 million infections occurring each year.

There are four dengue virus (DENV) types (DENV-1, DENV-2, DENV-3, and DENV-4), all of which are capable of inducing severe disease (dengue hemorrhagic fever /dengue shock syndrome).

Infection with DENV provides long-term protection against disease caused by reinfection with that particular type. However, infection provides only short-lived cross-protection to the other three DENV types.

Symptoms

Most people with dengue have mild or no symptoms and will get better in 1–2 weeks. Rarely, dengue can be severe and lead to death. If symptoms occur, they usually begin 4–10 days after infection and last for 2–7 days. Symptoms may include high fever (40°C), severe headache, pain behind the eyes, muscle and joint pains, nausea, vomiting, swollen glands and rash.

Individuals who are infected for the second time are at greater risk of severe dengue. Severe dengue symptoms often come after the fever has gone away as severe abdominal pain, persistent vomiting, rapid breathing, bleeding gums or nose, fatigue, restlessness, blood in vomit or stool, being very thirsty, pale, and cold skin, and feeling weak. People with these severe symptoms should get care right away. After recovery, people who have had dengue may feel tired for several weeks.

Global Burden

The incidence of dengue has grown dramatically around the world in recent decades, with cases reported to WHO increased from 505 430 cases in 2000 to 5.2 million in 2019. A vast majority of cases are asymptomatic or mild and self-managed, and hence the actual numbers of dengue cases are under-reported. Many cases are also misdiagnosed as other febrile illnesses.

The disease is now endemic in more than 100 countries. The Americas, South-East Asia and Western Pacific regions are the most seriously affected, with Asia representing around 70% of the global disease burden. Dengue is spreading to new areas including Europe, and explosive outbreaks are occurring.

Risk Factors

Being in tropical and subtropical areas increases your risk of exposure to the virus that causes dengue fever. Previous infection with DENV increases the risk of the individual developing severe dengue.

Urbanization (especially unplanned), is associated with dengue transmission through multiple social and environmental factors: population density, human mobility, access to reliable water source, water storage practice etc.

Community’s risks of dengue also depend on the population's knowledge, attitude, and practice towards dengue, as well as the implementation of routine sustainable vector control activities in the community. Consequently, disease risks may change and shift with climate change in tropical and subtropical areas, and vectors might adapt to the new environment and climate.

Transmission

The Dengue virus is transmitted to humans through the bites of infected female mosquitoes, primarily the Aedes aegypti mosquito. Other species within the Aedes genus can also act as vectors, but their contribution is secondary to Aedes aegypti. After feeding on a DENV-infected person, the virus replicates in the mosquito midgut before disseminating to secondary tissues, including the salivary glands. The time it takes from ingesting the virus to actual transmission to a new host is termed the extrinsic incubation period (EIP). The EIP takes about 8–12 days when the ambient temperature is between 25–28°C.

Once infectious, the mosquito can transmit the virus for the rest of its life. Female mosquitoes with an adequate food supply can live up to 5 months or longer, with the average female life span being about 6 weeks.

Human-to-Mosquito Transmission

The mosquitoes typically lay eggs near standing water in containers that hold water, like buckets, bowls, animal dishes, flowerpots, and vases. These mosquitoes prefer to bite people, and live both indoors and outdoors near people.

Mosquitoes can become infected by people who are viremic with DENV. This can be someone who has a symptomatic dengue infection, someone who is yet to have a symptomatic infection (they are pre-symptomatic), but also people who show no signs of illness as well (they are asymptomatic).

Human-to-mosquito transmission can occur up to 2 days before someone shows symptoms of the illness, and up to 2 days after the fever has resolved.

The risk of mosquito infection is positively associated with high viremia and high fever in the patient; conversely, high levels of DENV-specific antibodies are associated with a decreased risk of mosquito infection. Most people are viremic for about 4–5 days, but viremia can last as long as 12 days.

Maternal Transmission

The primary mode of transmission of DENV between humans involves mosquito vectors. There is evidence however, of the possibility of maternal transmission (from a pregnant mother to her baby). At the same time, vertical transmission rates appear low, with the risk of vertical transmission seemingly linked to the timing of the dengue infection during the pregnancy. When a mother does have a DENV infection when she is pregnant, babies may suffer from preterm birth, low birthweight, and fetal distress.

Complications

Severe dengue fever can cause internal bleeding and organ damage. Blood pressure can drop to dangerous levels, causing shock. In some cases, severe dengue fever can lead to death. Dengue in pregnancy adversely affects maternal and fetal outcomes with high maternal mortality of 16%. Prematurity and postpartum hemorrhage are significant risks to mother and baby.

Treatment

Most cases of dengue fever can be treated at home with pain medicine. Preventing mosquito bites is the best way to avoid getting dengue. There is no specific treatment for dengue. The focus is on treating pain symptoms.

Acetaminophen (paracetamol) is often used to control pain. Non-steroidal anti-inflammatory drugs like ibuprofen and aspirin are avoided as they can increase the risk of bleeding.

Prevention

The mosquitoes that spread dengue are active during the day. Lower the risk of getting dengue by protecting yourself from mosquito bites by using clothes that cover as much of your body as possible, mosquito nets if sleeping during the day, ideally nets sprayed with insect repellent, window screens, and mosquito repellents.

If you get dengue, it’s important to rest, drink plenty of liquids, use acetaminophen (paracetamol) for pain, avoid non-steroidal anti-inflammatory drugs, like ibuprofen and aspirin, and watch for severe symptoms and contact your doctor as soon as possible if you notice any.

So far one vaccine has been approved and licensed in some countries. However, only persons with evidence of past dengue infection can be protected by this vaccine.

Reduce mosquito habitat. Reducing the mosquito population involves getting rid of mosquito breeding areas. These areas include any place where still water can collect, such as birdbaths, pet dishes, empty planters, flowerpots, cans, and any empty vessel. These areas should be checked, emptied, or changed regularly. You can help lower mosquito populations by eliminating habitats where they lay their eggs. At least once a week, empty and clean containers that hold standing water, such as planting containers, animal dishes and flower vases. Keep standing water containers covered between cleanings.