Is dengue the next threat in flood-hit Pakistan?

Katie Anders is an epidemiologist and director of impact assessment at the World Mosquito Program. Alex Jackson is WMP news editor.

As devastating floods rip through Pakistan impacting more than 33 million people, another major threat lies round the corner. Public health experts are warning of the rising risk of mosquito-borne diseases such as dengue spreading across the country.

Pakistan has already been grappling with increasing dengue cases and the unusually early and heavy monsoon rains this year have provided favourable conditions for mosquitoes to breed in.

In much of the world, extreme weather including droughts, heatwaves, floods, and heavy rainfall are increasing in severity and regularity. And an alarming new study concludes that, among the many infectious diseases likely to worsen with climate change, mosquito-borne diseases like dengue will be most affected.

Also known as “breakbone fever” because of the severe pain it can cause, dengue is the fastest spreading tropical disease in the world. The viral infection is transmitted to humans through the bites of female Aedes mosquitoes, which thrive in tropical and subtropical urban areas.

More than half of the global population is now at risk of dengue, with 390 million people infected every year. In 2019, a record 5.2 million dengue cases were reported worldwide and economists estimate the global cost of dengue to be close $9 billion per year.

Across Asia, countries including Malaysia, Vietnam, Singapore and the Philippines are in the midst of major dengue outbreaks. And experts in Brazil are alarmed that the recent surge in dengue transmission is affecting many parts of the country that haven’t previously experienced dengue outbreaks, with 8.7 million Brazilians newly at risk of dengue compared to five years ago.

Climate impact

The transmission of mosquito-borne diseases like dengue is an inherently climate-sensitive process. A temperature increase of only a few degrees can speed up mosquito development, extend their survival and even increase biting frequency.

At higher temperatures, viruses replicate more quickly meaning a dengue-infected mosquito can transmit the infection to the next person sooner. And an increase in water-holding containers, either filled by rainfall or for water storage during droughts, drives mosquito numbers up by providing egg-laying sites.

The latest report by the UN Intergovernmental Panel on Climate Change concludes that warming temperatures are expanding the geographies suitable for dengue transmission, and contributed to the global chikungunya and Zika epidemics during the past decade.

An additional one billion people are projected to be at risk of dengue fever by 2080. In areas already affected by dengue, global temperature rises could lengthen the annual transmission season by up to four months over the next 50 years, driving up case numbers.

These impacts will be experienced disproportionately by countries and communities who are already more vulnerable, are less likely to have access to quality healthcare — and are least responsible for the upstream causes of climate change.

Sustainable solutions

Solutions to reduce the impact of these climate-induced effects are much needed and achievable. Investments in urban planning, infrastructure and housing design can reduce mosquito breeding sites and opportunities for human-mosquito interactions.

The current and future threat of mosquito-borne viruses must be brought up the priority list of national and global health agencies. Attention needs to focus on equipping health systems to diagnose and manage these diseases, strengthening surveillance to detect incursions of mosquitoes and viruses into new areas, and applying evidence-based and sustainable approaches to mosquito control and disease prevention.

Alongside a redoubling of efforts to reduce mosquito populations through conventional approaches of chemical control and environmental management, new tools are needed to curb the spread of dengue. A major milestone was reached recently with Indonesia becoming the first country to licence a new dengue vaccine indicated for use in both children and adults.

Another new tool is Wolbachia. A symbiotic bacteria found naturally in half of all insect species, Wolbachia blocks the transmission of viruses when introduced into Aedes aegypti mosquitoes. Short-term releases of Wolbachia mosquitoes in dengue-affected communities allow Wolbachia to spread into the local mosquito population, effectively ‘immunising’ it against dengue and other viruses.

This self-sustaining, safe and cost-effective method has been deployed in 11 countries over the past decade, reaching more than 10 million people, and its effectiveness for dengue control has been demonstrated in multiple field trials.

In March 2022, the World Health Organization launched a Global Arbovirus Initiative reflecting the urgent need for coordinated and sustainable strategies to control dengue and other mosquito-borne viruses. With clear evidence that climate change is accelerating the emergence and spread of these viruses, the time for action is now — both for addressing the root causes of climate change and to support communities and governments in implementing evidence-based tools for controlling dengue and other mosquito-borne diseases.