The emergence of COVID-19 has dramatically altered the way we live, with many people spending more time at home. However, even though this may have helped to reduce the transmission of COVID-19 and airborne diseases like influenza, these measures may have had a different impact on diseases that are transmitted by blood-sucking insects like mosquitoes that tend can enter well ventilated homes more easily than air-conditioned workplaces.
Globally, there are an estimated 105 million dengue infections each year, with most cases concentrated in Southeast Asia and the Western Pacific region. The virus can cause severe fever, headache, muscle and joint pain, fatigue, nausea and vomiting, and in rare cases, death. It is transmitted by the female Aedes aegypti mosquito, which are also responsible for transmitting chikungunya, yellow fever and Zika viruses.
So how have COVID-19 restrictions impacted the transmission of dengue virus?
Thailand, Malaysia and Singapore implemented near complete lockdowns to stem the spread of COVID-19, including workplace closures and bans on mass gatherings. To investigate the impact of these measures on dengue transmission, Jue Tao Lim of the National University of Singapore, and colleagues analysed dengue case counts for Thailand, Singapore and Malaysia using national surveillance data.
After controlling for seasonal variations in climate, they calculated that physical distancing has increased the number of dengue infections in Thailand, resulting in an additional 4.32 cases per 100,000 individuals per month – approximately 2,000 additional infections each month, in total – with the greatest increase in Bangkok. Initially, in the paper published in PLOS Neglected Tropical Diseases it looked as though there was no significant impact on dengue transmission in Singapore or Malaysia. However, as the researchers used only total reported case counts per week for Singapore and Malaysia, an increase detected in Singapore did not have sufficient statistical power to identify the rise in case counts attributable to lockdown policies. In a follow-up paper, the researchers found a 37.2% rise in dengue cases among working adults in Singapore due to lockdown policy.
“One key pathway that may explain this result is the propensity for dengue infections to surface at home rather than at work,” the researchers write. They suggest that the increase in cases in Thailand is probably due to office workers who would usually be travelling elsewhere to work. The explanation is similar for Singapore, where office workers are normally in air-conditioned offices that the mosquitoes can’t enter.
Previous studies have associated spending more time in urban residential neighbourhoods with an increased risk of dengue infection – particularly neighbourhoods dominated by low-rise buildings with dense drainage networks. Aedes mosquitoes tend to breed in stagnant water trapped in artificial containers or holes in trees, which such neighbourhoods have in abundance. So, people are more likely to get bitten if they spend more time there. Areas predominated by high-rise buildings tend to have a lower incidence of the disease.
Differences in residential structure between the three countries may explain why Thailand is experiencing a greater increase in dengue infections compared to Malaysia and Singapore, the researchers suggest.