Forces of light do battle with malaria
The mosquitos that transmit malaria may have a simple Achilles heel. Tim Wallace explains.
Exposing mosquitoes to just 10 minutes of bright light can significantly suppress their biting behaviour and flight activity for about two hours, according to a study published in the journal Parasites and Vectors by researchers from the University of Notre Dame in Indiana, US.
The use of light exposure and “multiple photic pulses” at intervals from dusk till dawn may prove to be a crucial tool to complement established control methods, the researchers write, particularly as mosquitoes are becoming increasingly resistant to insecticides and changing their late-night feeding habits to beat control strategies such as insecticide-treated bed nets.
The mosquitoes used in the research were of the genus Anopheles gambiae, which feeds preferentially on humans and is the primary transmitter of the microbial parasite causing malaria in most of sub-Saharan Africa, where 90% of deaths due to the disease occur.
Anopheline mosquitoes have been developing resistance to the insecticides used to treat bed net as well as shifting their feeding to earlier in the evening or later into the early morning, when people are not in bed and therefore not protected by a net.
“So what used to be an efficient method is becoming less effective,” says study co-author Giles Duffield, who specialises in the molecular biology of circadian rhythms and photobiology in mammals and mosquitoes. “We need to discover new methods to address mosquito control and prevention. The systems and tools we currently have including global distribution and usage of insecticide-treated bed nets and spraying are not enough.”
The research involved exposing non-infected mosquitoes to “pulses” of light with a brightness of 800 to 1,000 lux – equivalent to the intensity of lights in a television studio – and then giving the insects the opportunity the opportunity to feed on volunteer human arms. Biting propensity was recorded immediately after light exposure and then at two-hour intervals, and then compared to a control group not exposed to light.
While the researchers hypothesised that a light pulse would inhibit or reduce biting propensity, due to the predominately nocturnal Anopheline mosquitoes concentrating feeding and flight activity to the night, they express some satisfaction at how effective the exposure worked. “This inhibition remained high at 32% for two hours after delivery of the light pulse; and somewhat surprisingly the photic effect upon biting behaviour in some trials was sustained for up to four hours after the pulse.”
The researchers hypothesise a dose dependency of photons per unit time, “such that a higher light intensity would require a lower exposure duration to reach the desired level of suppression of feeding”. For example, they suggest, one minute at 5,000 lux might provide the equivalent inhibition as 10 minutes at 300 lux.
“We predict that there will be a specific wavelength(s) of light that will have maximal efficacy in the immediate and sustained suppression of biting behaviour and related effects of suppression or elevation of flight activity,” they conclude. “Selecting a monochromatic light source in this maximal range would allow for modulation of behaviour at lower intensities of light. Additionally, selecting a monochromatic light source that is outside of the blue colour of the spectrum, even if sub-optimal, might allow for manipulation of mosquito behaviour without disruption of human behaviour/physiology.”