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world malaria day: The War on the Anophelines

world malaria day: The War on the Anophelines

It is shortly after the Second World War and armies of men with canisters full of DDT invade villages and cities of Europe and the USA. Their mission? To kill the anopheline mosquitoes and once and for all eliminate malaria from these places. The heavily armed offence did its job in eliminating malaria – it is now difficult to imagine malaria pestering European citizens a mere decades ago. The anopheline mosquito however persisted, in some areas even in large numbers.
The Anopheles mosquito is a ruthless serial killer. Responsible for approximately 590,000 annual deaths it by far bypasses other animals more popularly seen as dangerous such as the shark (10 deaths), crocodile (1,000 deaths) and snake (50,000 deaths). However, just as most serial killers are harmless without a weapon, the Anopheles mosquito is harmless without the malaria parasite. Without malaria, it is merely left as a nuisance that keeps you up at night.
Malaria is many millions years old, indeed, it has recently been shown to have even killed dinosaurs. But it took us humans millions of years to figure out that a mosquito was the transmitting agent, in fact, it was only discovered a little over 100 years ago. With this discovery, our attack on the mosquitoes began.
The war on the anophelines is a multi-frontal attack. Our first front is to harness any weaknesses in their biology, e.g. by cutting off their water supply through reducing breeding site availability, using the odor cues that they use to locate us to trap them, and exploiting their natural predators and pathogens, such as fish and Bacillus thuringiensis (Bti), to kill their larvae. The next front is a chemical warfare: insecticides are sprayed inside people’s homes, impregnated on their bednets and added to potential breeding sites. Additionally, endectocides such as ivermectin can be given to potential hosts – human and/or domestic animals – to kill any bloodfeeding mosquito. Our newest potential front is a genetic war: using modern tools we can genetically modify mosquitoes that cannot transmit malaria or produce sterile male mosquitoes that will be released in the field to reduce numbers of malaria-transmitting anophelines.
Anophelines, however, have so far always found ways to outsmart us. Several species in sub-Saharan Africa can lay their eggs in water pools as small as a rain-filled hoof print; breeding sites that are impossible to target on a continent as big as Africa and absent of predatory fish. As a strike back against the chemical warfare, anophelines have evolved resistance against many insecticides and moreover changed their behavior to start biting people outside – where there is no insecticide – plus moving their preferred biting time a little earlier so people are not yet protected by their bednet. The genetic warfare has not yet been implemented on grand scale, but when it will, we should not be surprised to observe the anophelines outsmarting us yet again by changing their biology and behaviors.
Thus, what can we do to outsmart these little serial killers? As a first step, a bigger artillery is needed. Just as what we have learned to outsmart HIV and antibiotic resistant ´superbugs´, we need to combine more insecticides as it is more difficult to become resistant against multiple insecticides than it is against one. And when we do, we need to think about how we employ our artillery; do we use them all at once or rotate them to make it more difficult to adapt? What kind of evidence do we need to base these decisions on? We also need to understand our foe better: who are they, where are they and when do they attack? We seem to have chased many mosquitoes out of the houses, but where are they now? If IRS and LLINs are not affecting them, we need to come up with a novel strategy of attack. And finally, which species are actually transmitting malaria? There are hundreds of anopheline mosquito species, out of which 30 to 40 transmit malaria, yet we are focusing on a handful of the usual suspects. While we are chasing the gang leaders, are the gang members killing us behind our backs?
A war is never won by keeping on doing what has proven not to work. The mosquito war has been a success in the USA and Europe but real opposition has come from the corners of sub-Saharan Africa, Asia and South America. Novel strategies are needed to jump a surprise attack on the anophelines and we should expect them to strike back and be ready to respond. However, remember, the anopheline is innocent without the malaria parasite: this really isn´t a war against the anophelines, it is a war against the malaria parasite. The war against the anophelines is just one pillar that will help collapse the malaria parasite empire
FACTS ON MOSQUITO

  1.   Mosquitoes have been around since the Jurassic period. That makes them about 210 million years old. They’ve been mentioned throughout history, including in the works of Aristotle around 300 B.C. and in writings by Sidonius Apollinaris in 467 B.C.
  2. The bumps from mosquito bites are caused by saliva. While one tube in the proboscis draws blood, a second pumps in saliva containing a mild painkiller and an anti-coagulant. Most people have minor allergic reactions to the saliva, causing the area around the bite to swell and itch.
     
  3. Malaria is caused by a parasite that lives in mosquitoes. The parasite gets into mosquito saliva and is passed on when the insect bites someone. West Nile and other viruses are passed the same way. Mosquitoes can also carry and pass on canine heartworm.
  4. Mosquitoes do not transmit HIV. The virus that causes AIDS does not replicate in mosquitoes and is actually digested in their stomachs, so it’s broken down without being passed on.
  5. Mosquitoes are considered the deadliest “animal” in the world. The Anopheles mosquito, in particular, is dangerous because it transmits malaria, which kills more than one million people every year, primarily in Africa. Alexander the Great is believed to have died of malaria in 323 B.C
  6. Male mosquitoes locate females by the sound of their wings. Females can beat their wings up to 500 times per second, and the males pick out the higher frequency of those beats when seeking a mate.
  7. Mosquitoes can’t fly very far or very fast. Most mosquitoes can fly no more than about one to three miles, and often stay within several hundred feet of where they were hatched. However, a few salt marsh species can travel up to 40 miles. The top speed for a mosquito is about 1.5 miles per hour.
  8. Mosquitoes generally fly below 25 feet. However, some species have also been found at extraordinary heights, including 8,000 feet up in the Himalayas.
  9. Mosquitoes can smell human breath. They have receptors on their antennae that detect the carbon dioxide released when we exhale. Those plumes of CO2 rise into the air, acting as trails that the mosquitoes follow to find the source.
  10. Sweat helps mosquitoes choose their victims. Our skin produces more than 340 chemical odors, and some of them smell like dinner to mosquitoes. They are fond of octenol, a chemical released in sweat, as well as cholesterol, folic acid, certain bacteria, skin lotions, and perfume.
  11. Body heat marks the target. Mosquitoes use heat sensors around their mouthparts to detect the warmth of your body – actually, the blood inside it – then land on you and locate the best capillaries for tapping.
  12. Mosquitoes feed day and night. Some species, like the Aedes are daytime biters, while others, like Culex, start biting at dusk and continue a few hours into dark.

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