Now endemic in 100 countries, half the global population is at risk of dengue fever. Its threat has risen dramatically – between 2000 and 2019 there has been a 10-fold increase in dengue cases, with 2023 seeing the highest number of cases on record.
Not only have Bangladesh, Peru and Burkina Faso all witnessed record-breaking outbreaks over the past 12 months, but there are now reported cases of mosquito-transmitted dengue in France, Italy and Spain.
The cause of this? Scientists argue that climate change is fuelling the increase in this mosquito-borne viral disease, with a warming planet creating more habitable spaces for the insects. As mosquitos become more common, we can expect the time between dengue outbreaks to quicken and transmission seasons to lengthen.
It's a worrying situation. But it's one that the team behind The World Mosquito Program have a possible solution to. They propose treating mosquitoes with bacteria that can prevent the virus from developing in their bodies.
Read more:
What are the symptoms of dengue fever?
Dengue has earned the label ‘breakbone fever’ for good reason. While 80 per cent of cases are asymptomatic, symptoms when they so occur include high fever, muscle and joint pains, severe headache, pain behind the eyes, nausea and vomiting.
Symptoms start 4-10 days after infection and can last from two days up to a week. DHF, or severe dengue, presents with severe abdominal pain, persistent vomiting, bleeding gums or nose, blood in stools or vomit, pale and cold skin, and exhaustion. With no antivirals available, doctors can only alleviate these symptoms.
How does dengue fever spread?
Dengue is spread via the bite of an infected female mosquito species, Aedes aegypti, typically found in tropical and subtropical regions. Originating in the forests of West Africa, Aedes aegypti spread across the globe during the African slave trade and has continued to hitchhike on human transportation ever since.
Other Aedes species can also transmit dengue to a lesser extent. The highly invasive Asian tiger mosquito, is the likely culprit for the dengue transmission in Europe. Unlike malaria mosquitoes – which typically bite at night and can be kept away with insecticide-treated bed nets – dengue mosquitoes are day biters, making them extremely difficult to control.
Mosquitos have now adapted superbly to living with humans, their preferred blood source, becoming a highly urbanised creature. In cities, stagnant water is key to their survival, providing an egg-laying site and a habitat for the developing aquatic larvae and pupae. Mosquitos can exploit small pools of water that accumulate in human-made containers, such as rubbish, used tyres, flowerpots and so on, to breed. In this way, humans have been the major driving force for the success of dengue mosquitoes.
How can we fight the outbreak?
Dengue prevention requires a multi-pronged attack on the mosquito, with insecticide spraying at its core. However, insecticide resistance has developed in mosquito populations globally, which threatens its effectiveness.
What else? Control strategies also include eliminating or adapting breeding sites to stop Aedes aegypti from laying eggs in stagnant water (removing rubbish that can accumulate water and placing covers on water storage containers).
Bacteria toxins are also applied to water bodies to kill mosquito larvae. These strategies are labour-intensive, as identifying, treating and eliminating all breeding sites is challenging. As such, there’s a desperate need for new mosquito control methods.
Cue the World Mosquito Program (WMP), which has come up with a non-chemical and non-genetic modification-based approach for dengue control. A bacteria called Wolbachia, which occurs naturally in many insect species, but not Aedes aegypti.
The WMP found that ‘infecting’ Aedes aegypti with Wolbachia prevented the dengue virus from developing in adult females. From a logistic perspective, the method is self-sustaining as Wolbachia is passed on to the eggs via mating and can therefore spread through wild populations.
The WMP has reported significant reductions in dengue cases where Aedes aegypti carrying Wolbachia have been released. Given that Aedes aegypti also transmits the Zika and chikungunya viruses, the WMP has developed a potential ‘three-for-the-price-of-one’ method for disease control.
It’s safe to say mosquitoes are the most hated insects, but, despite their notorious reputation, of the 3,500 mosquito species, only a handful transmit disease. They’re also important to the ecosystem.
Mosquitoes are a food source for fish, frogs, reptiles, bats and birds, and as the male insects feed on nectar (only the females drink blood), they’re also pollinators. The WMP approach is species-specific, targeting only Aedes aegypti, which contrasts with the ‘blunt tool’ approach of insecticides, which can affect non-target insects.
The current climate change trajectory we’re on is bringing increasing temperatures and changes in rainfall patterns, which will benefit this formidable tiny insect and her virus cargo. As such, we need as many weapons in our armoury as possible to combat the increasing global threat of dengue.
Read more: