Among mosquito-borne diseases, malaria is king—hogging all the spotlight, including Bill Gates’ money. But in recent years, dengue has emerged as the more formidable killer—becoming more dangerous each passing year on a warming planet.
Remind me, what’s the difference between malaria and dengue?
Apart from the fact they both are spread by mosquitoes, almost everything.
Aedes vs Anopheles: There are three kinds of disease-carrying mosquitoes: Anopheles, Aedes and Culex. All of them are carriers of deadly illnesses.
Anopheles quadrimaculatus is the best known—it carries the plasmodium parasite that causes malaria. Culex pipiens and Culex quinquefasciatus transmit the West Nile virus. Aedes mosquitoes also carry viruses (not parasites) that cause dengue, Zika, chikungunya, and yellow fever. There are two sorts. The most prevalent is the Aedes aegytpi—though Aedes albopictus aka the Asian tiger mosquito, have become more common in recent years.
Dengue vs malaria: While both malaria and dengue cause high fever, the latter’s signature symptom is severe aches in your joints and muscles. Hence its nickname ‘break-bone fever’. And while there are anti-malarial meds, there’s not much you can do with dengue:
While there is no cure for dengue fever, people experiencing symptoms of infection can work with their health care provider to manage them. This may include taking medication to relieve pain, increasing fluid intake to prevent dehydration and getting plenty of rest.
The tricky bit: about dengue is that there are four strains of the virus. Getting infected by one offers no immunity against the others. In fact, it can result in even more severe disease:
If the person gets infected again with a different strain, that can launch a process called antibody-dependent enhancement, where the immune system’s collection of antibodies “acts as a chaperone,” Durbin said, escorting the virus throughout the body. The person can become extremely ill and may require hospitalization.
And here’s the really dismal part: first-time infections tend to be mild and often go undetected. People don’t realise they have been exposed until they are bitten the second time—and fall seriously ill. The disease can result in haemorrhage and, even, death. Data point to note:
[A]bout 5% of people who become sick will progress to what’s called severe dengue. Plasma, the protein-rich fluid component of blood, can start to leak out of blood vessels, causing patients to go into shock or have organ failure.
Most of the victims in the epidemic devastating Brazil right now are children: “Those under 5 have the highest mortality rate of any age group, followed by those aged 5 to 9.”
And the number of dengue cases are going up…
Yes, around the world—from the first to the third. Here are some quick stats:
- Around 400 million people contract dengue each year.
- The number of deaths has doubled from 20,000 in 2000 to 40,000 in 2022. At the same time, the number of malaria deaths declines by 30%.
- The spike in the number of cases is staggering in Latin America. It jumped from an average of 535,000 cases a year in 2000-05 to 4.6 million in 2023. The number so far in 2024: 5.9 million.
The cases are rising exponentially in not just Latin America—but also South Asia. Yes, that includes India. In 2023, we officially recorded nearly 95,000 cases and 91 deaths by September. And that was a good year. In 2022, we witnessed 233,000 cases and 303 deaths.
More importantly: The disease is spreading to new parts of the world. There have been cases reported in Uruguay—which hasn’t seen dengue in a century. The disease is spreading into virgin territory—including Sudan, Afghanistan, parts of Southern Europe, and the Middle East. The disease was reported in 80 countries last year.
Let me guess, it’s all because of climate change…
Well, some of it is improved reporting and more frequent diagnoses. But yes, it’s mostly about the weather. According to the Scientific American:
A March study published in the journal Nature found that mosquito reproduction speed is “strongly influenced” by temperature and rainfall because mosquitoes die off in colder weather and precipitation makes puddles for mosquitoes to lay eggs. “Every heat wave is a push that builds up dengue transmission,” said [co-author] Christovam Barcellos…
Barcellos said heat waves mean not only more mosquitoes: “People change their behaviour when a heat wave comes, they go out on the streets more” which increases their exposure to disease-carrying insects. “It’s a complementary phenomenon,” he said.
The El Niño effect: The weather phenomenon has brought heavier rain to the Americas—which collided with an unusual heatwave in Brazil. It allowed the A. aegypti mosquito to spread farther—and survive longer. It has driven the number of cases through the roof. Last year, long and heavy monsoons doomed Bangladesh for similar reasons.
And of course, climate change: A warmer world is a friendlier world for the A. aegypti. A 2020 Lancet study revealed that dengue cases had jumped by 9% around the world since the 1950s. High altitude locations like Nepal are no longer inhospitable. According to a leading climate scientist:
In the future, the belt of transmission is going to continue expanding northwards and upwards. There’s going to be more months that are going to be suitable for transmission, and because people are likely to travel more, there’s also going to be more opportunities for moving the mosquitoes and moving the disease to different areas.
Predicted areas of expansion: “Southeast Asia — especially parts of Malaysia, Indonesia, the Philippines, Vietnam, and some areas of the Indian subcontinent — and in South America, particularly Brazil.”
Irony alert: Dengue cases will spike when the world gets hotter. But if it gets even hotter after that, dengue cases will fall:
In higher-warming scenarios, however, some areas in India, Bangladesh, and Thailand, among others, could become less suitable for A. aegypti, resulting in a decline in cases in the second half of the century, according to Colón-González’s climate modeling. The optimal temperature range for Aedes mosquitoes is thought to be 77 to 86° F (25 to 30° C).
Also, the D-word: Unplanned development is a huge boost to the dengue virus. Although the disease is associated with cities, the A. aegypti does best in where there’s lots of construction:
[A] recent study by Colón-González and others looking at transmission dynamics in Southeast Asia found that densely populated urban areas may be at lower risk compared to surrounding or developing rural areas, perhaps because highly developed cities are likely to be wealthier and have better infrastructure — from trash collection to water access — than newly urbanising areas.
That said, Indian cities may be the exception—given the continual and high rate of construction.
Key point to note: As we noted before, a second infection from a different strain of the dengue virus carries a high risk of serious disease. This also means a higher death toll—which is what happened in Bangladesh last year. There were three strains circulating in the population—which creates a domino effect: “The evolution of the virus happens faster when there is massive spread and new serotypes [emerge], which in turn leads to massive endemic outbreaks.” And as we learned from the coronavirus, extended epidemics result in new mutations.
So what’s the solution—other than cooling the planet?
The problem with dengue is that it can’t be fixed with sensible, low-cost solutions like bed nets treated with poison—which works for malaria. That’s because the anopheles mosquito hunts at night—while the aedes is active during the day—when people are not in bed. So solutions are harder—and more expensive.
Throw money at it: There’s good civic hygiene—which is easier in rich countries like Singapore:
It helps that it is rich enough to pay armies of public-health workers to traipse through its streets, searching out the standing water in which mosquitoes breed, pipetting puddles and issuing fines. The city-state models outbreaks and then deploys platoons of insecticide sprayers in hazmat suits to the predicted epicentres.
And places in the US can retreat behind screened windows and air-conditioning. Poorer countries have smaller budgets and greater distances to cover. Also: slums:
Millions of Brazilians live in densely populated, irregular communities — either called favelas or “the periphery” — often beyond the reach of government services and basic utilities. With unreliable plumbing, people often resort to storing water outside, creating countless mosquito breeding sites.
Money offers a solution for some—but not for many others.
Spread a bacteria: Singapore has also tried one other trick—that may work as well—irrespective of the GDP. It’s called the wolbachia bacteria. It prevents mosquitoes and their offspring from transmitting the dengue virus. It isn’t cheap—around $6 per person. But it works:
Trials of wolbachia infection in Colombia have seen a 94% drop in dengue incidents in the area where the mosquitoes are released. The world’s largest wolbachia-mosquito factory is due to start operations in the Brazilian city of Curitiba this year.
Besides, dealing with dengue epidemics isn’t cheap. Total medical expenses for dengue patients was estimated to have jumped to $91 million in Bangladesh last year—up from $41 million in 2019.
Get a vaccine: Right now, there are two vaccines for dengue—but one is a pricey two-shot jab—and the other only works if you’ve already been infected once. Earlier this month, Brazil announced encouraging trial results of a new one-shot vaccine. But it won’t be ready any time soon—and will only be produced for Brazil. Merck has a similar vaccine in the works—but it hasn’t been tested in a late-stage trial.
One Indian prospect: A cheaper version of that super-expensive vaccine. It’s called Qdenga and costs $115 per dose in Europe. But Brazil is paying $19 per dose—because it’s bought all available global stock. The good news:
Takeda Pharmaceuticals, which makes Qdenga, announced a deal last month with Biological E, a large Indian generic drug maker, to license and produce up to 50 million doses a year, part of a race to accelerate production. The Indian vaccine should cost considerably less.
The not-so-good news: Rolling out a cheaper version will take years.
The bottomline: Dengue may be easier to prevent in richer countries—but not in poorer countries that just have some very rich people. Something to think about.
Reading list
This Yale e360 deep dive is the best, most comprehensive deep dive on the rise of dengue. Washington Post looks at the devastation in Brazil. Japan Times has more on Nepal—and the role of climate change. The Economist (login) has the latest numbers. New York Times looks at the rush to develop a cheap vaccine. PBS offers a good overview of dengue. India Today looks at the costs of battling dengue in India and Sri Lanka.
