More than 200 million people develop malaria every year. And about half a million die – mostly in Africa, many of them children. And these amazing numbers are an improvement. The number of malaria deaths has halved since 2000. In many places, a remarkably simple tool has led the battle: bed nets, treated with a mild insecticide, prevent mosquitoes from biting people while they sleep.
Both humans and mosquitoes are farmers in the malaria transmission cycle. When an infected person is bitten by a mosquito, the parasite is picked up in the blood meal. This mosquito can then transmit the parasite to the next person, who bites it. Bed nets prevent mosquitoes from easily attacking motionless sleepers. But now some mosquitoes seem to be giving up the night shift.
“Malaria mosquitoes in Africa tend to change their biting behavior.”
Entomologist Eunho Suh from the Center for Infectious Disease Dynamics at Penn State University.
“Usually they tend to bite people at night, but due to the frequent use of bed nets, these mosquitoes started biting in the early evening or morning.”
Suh and his team wanted to know whether the observed change in bite time had an impact on malaria transmission. Back at the laboratory, they gave Anopheles mosquitoes the opportunity to feed on blood at 6 p.m., midnight, and 6 a.m. When the laboratory was kept at 80 degrees Fahrenheit, it was no more or less likely that evening and morning walkers were more contagious than midnight walkers.
In the real world of warm and humid tropics, the night is a little cooler than the time of day. And when the researchers introduced this temperature swing, the evening bites were much more likely to have strong malaria parasites. The results are in the journal Nature Ecology & Evolution. [Eunho Suh et al., The influence of feeding behaviour and temperature on the capacity of mosquitoes to transmit malaria]
“Not all mosquito bites are created equal. Mosquitos that bite in the evening can therefore have the highest transmission potential compared to mosquitoes that bite at midnight or in the morning. “
Suh believes the difference in the likelihood of mosquitoes becoming infectious is related to how the malaria parasite matures. The parasites have a harder time developing when the mosquitoes are too warm. But when a mosquito ingests the parasites from the blood towards evening, these parasites have more hours of cooler nighttime temperatures to complete their development.
Next, Suh plans to conduct a similar study of wild mosquitoes and wild malaria parasites in Africa to see if the results from his laboratory mosquitoes are correct.
In any case, bed nets remain an important tool. Understanding the behavior of the enemy, however, is crucial information in any battle.
– Jason G. Goldman
[The above text is a transcript of this podcast.]