Why dengue cases have surged this year in across the world

What is dengue?

Dengue is a viral infection which is transmitted by the Aedes aegypti mosquitoe. Most people with the infection report mild symptoms, but the disease is known to cause fever, severe headache, muscle and joint pains, nausea and vomiting, pain behind the eyes, and rashes. In severe cases, however, the infection can lead to internal bleeding, and if not managed properly, even death.

Over the past two decades, there has been a “tenfold increase in reported cases” an editorial by The Lancet said, adding that “even this figure is likely an underestimate”. This makes dengue the “only infectious disease for which annual mortality is rising”.

How many people have been infected with dengue this year?

More than 12 million cases and 6,991 deaths have been recorded globally till August this year, according to data from the WHO’s global dengue surveillance. This is more than double the 5.27 million cases recorded in all of last year, which itself was a record. Prior to last year, over the past decade, around two to three million annual cases of dengue had been reported at best.

Rise in dengue cases from 2015 to present.

Experts say that even 2024’s record numbers are likely to be an undercount. This is because not all countries — including India — report their data to the global surveillance network. Even among countries which report the data, not every dengue patient might have been tested and reported to the health authorities.

What is the situation on dengue cases in India?

Several cities have reported a surge in dengue cases over the past two months. More than 32,000 cases and 32 deaths attributable to dengue were reported till June end, according to the latest available data from the National Vector Borne Disease Control Programme.

This number is likely to have shot up in the last two months. Union Health Secretary Apurva Chandra, in early August, had said that India had seen an almost 50% rise in the number of dengue cases reported this year, as compared to the same period in 2023.

Notably, India is also witnessing an increase in the geography of the infection. The disease spread from only eight states and Union Territories in 2001 to every single state and UT in 2022, with Ladakh reporting the first two cases that year.

What is behind the surge in dengue cases?

The Lancet editorial identifies “the triad of urbanisation, climate change, and the movement of people and goods” as facilitating the spread of dengue, and its mosquito vector.

URBANISATION: The disease can spread more quickly in urban areas with a dense population. This is because urban spaces provide ample breeding grounds to the Aedes aegypti mosquito which breeds in clean, stagnant water. An increase in cases is generally reported during and just after the monsoon.
“If you just look at Delhi, we have been experiencing rains with warm spells in between, which are the best conditions for the mosquito to proliferate,” said Dr Sujatha Sunil from the Vector Borne Diseases Group in New Delhi’s International Centre for Genetic Engineering and Biotechnology.

CLIMATE CHANGE: An increase in temperatures allows mosquitoes to breed in places where they previously could not — in higher altitudes for instance. “Global warming has definitely led to an increase in the prevalence of the vector in geographies it might previously not have been found,” Sunil said.

Moreover, climate change has also led to the virus becoming more robust and transmitting better. On the current outbreak, the US Centres for Disease Control and Prevention said: “Higher temperatures can expand the range of the mosquitoes that spread dengue, as well as affect other factors that facilitate virus transmission like faster viral amplification in the mosquito, increased vector survival, and changes in reproduction and biting rates.”

MOVEMENT OF PEOPLE: The global movement of people and goods has, in general, led to a greater spread of infections that these people carry with them. However, better testing and reporting might also add to the “increase”, Sunil said.

Apart from dengue, other infections such as chikungunya and zika, which are also transmitted by the same vector, have also been on the rise. Zika was first reported in India in 2016 but there have been several outbreaks since then.

“There is a need to study whether getting one of the infections reduces the immunity of the vector mosquitoes and makes them more susceptible to get the other two. If this were to be the case, there could be an increase in the transmission of all three infections,” Sunil said.

How can dengue be prevented?

First, individuals have to ensure that mosquitoes do not breed at their homes, or in their neighbourhoods. Collection of water in pots, planters, bird baths, etc. need to be prevented.

Second, people need to protect themselves against mosquito bites. The Aedes aegypti mosquitoes bite during the day — wearing clothes which cover the entire body, especially during the monsoons, can prevent bites.

Third, public health systems have to focus on surveillance, and prediction of outbreaks. This will ultimately help reduce the number of cases, and consequently deaths due to the infection.

Are there any vaccines against dengue?

Yes. The WHO recommends two vaccines: Sanofi’s Dengvaxia and Takeda’s QDenga. However, these have not received approval in India. That said, India is also working on several vaccines of its own, some in collaboration with foreign institutes. The two at the most advanced stages are Serum Institute of India’s vaccine candidate developed using a genetically engineered weakened virus from the National Institute of Allergy and Infectious Diseases in the United States. Another candidate using the same virus is being developed by Panacea Biotec.