August 20 is World Mosquito Day, a celebration of the work of British doctor Sir Ronald Ross who discovered that female mosquitoes (of the Anopheles genus) were responsible for transmitting the malarial parasite. His pioneering work is marked every year by a series of events around the world, led by the London School of Hygiene and Tropical Medicine (LSHTM).
Martin Hibberd, Professor Emerging Infectious Disease at the LSHTM, says a century of mosquito research has improved understanding of vector-borne diseases. He explains there are important differences between how malaria and dengue fever are spread.
Night and day
For starters, malaria and dengue are transmitted by different types of mosquito which tend to bite at different times of the day. This has major implications for vector control.
“For malaria, bed nets can be used to protect you from being bitten by mosquitoes at night. For dengue it’s trickier. The mosquito carrying dengue bites during the day when you are outdoors and active,” says Professor Hibberd.
“They can bite you anywhere – your hand, your foot, your face – so avoiding contact is difficult.”
This has led to widespread use of insect repellents. While some are proven to be effective, the evidence for others is questionable.
A story that (literally) made the headlines recently told of a newspaper in Sri Lanka which was partly printed in ink that contained citronella – a natural substance often associated with dengue prevention.
“There is a worldwide industry selling products such as citronella but there is almost no science behind it. When you do controlled experiments, you find that these natural inhibitors work particularly poorly,” says Professor Hibberd.
The question for effective mosquito repellents remains an active area of research at the London school but other, more sophisticated, ways to contain mosquitoes are also in the works.
“There’s no treatment for dengue fever right now so vector control is the only way of controlling it,” says Professor Hibberd, recalling his time in Singapore where mosquito control is quite advanced.
“The only really effective approach is source reduction. This means identifying egg location and removing these. The mosquito that spreads dengue usually lives inside houses so egg-laying is typically in flower pots or other places with a water source.”
Back at the LSHTM’s insectarium, scientists are exploring other ways of controlling the mosquito population. The insectarium has several rooms buzzing with low-risk mosquitoes, which can be used to study biting and reproduction, and other mosquitoes (kept in contained areas) which are infected with dengue, malaria or other diseases.
Careful study of infected mosquitoes could lead to new ways of blocking transmission.
“One hot area of research is looking to infect mosquitoes with parasitic bacteria which make it more resistant to infection by the dengue virus,” explains Professor Hibberd.
The idea is to release thousands of these bacteria-infected mosquitoes, which would then mate with wild mosquitoes. If this makes the general mosquito population less susceptible to picking up dengue fever, it reduces the chances that dengue will be spread to humans.
Despite the leaps forward made since the first World Mosquito Day in 1897, there are plenty of remaining gaps in scientific knowledge. But Professor Hibberd feels the area is gaining momentum.
“Dengue is still classed as a neglected tropical disease which means it probably doesn’t receive the attention it should – and the number of cases worldwide is still huge and growing. But over the past decade or so since I’ve been working in this area, interest in dengue has risen slowly.”
As we mark another World Mosquito Day, there is reason to hope that research on dengue transmission could deliver new answers to a stubborn problem.
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