In Sudan, a woman walks for hours to get hold of a household item that for most in the West would be cheap to buy: a mosquito net treated with insecticide that could just be the difference between life and death for her family. Distributed for free by the WHO, these nets are an important step in preventing those most at risk from malaria from being bitten – but protecting people from the agents of the disease is a more difficult problem than even the widespread distribution of such aid can solve.
Image: James Gathany, CDC
Mosquito phase two: Mosquito larvae soon to become pupae.
Standing water is of course where the life cycle of the mosquito begins. Marshes, mangrove swamps, rice fields, grassy ditches, puddles and the edges of rivers and streams all provide this blood-sucker with environments in which the adult female can lay her eggs. In the space of just 5–14 days, the eggs hatch to become algae-feeding larvae, then develop into pupae – which like the larvae must come to the water’s surface often to breathe – before the adult mosquito emerges.
Image: Matthew Burrard-Lucas
Phase three becomes four: Mosquito hatching from pupal stage.
Adult mosquitoes typically mate within days after emerging from the pupal stage but females can live for up to two weeks. Though the female can feed on sugar sources for energy, she usually needs a blood meal for the development of her eggs. Once satisfied, she will rest for a few days while the blood is digested and eggs are developed before laying her eggs and seeking a new host. In that short interim period, she may well have become an infective vector for the lethal malaria parasite.
Phase four: Potential harbinger of death in flight.
In that tiny proboscis, death could well await. Malaria – caused not by mosquitoes per se but by parasites carried only by the Anopheles mosquito – affects up to 500 million people every year. Of those, it kills a further one to three million individuals – the vast majority in sub-Saharan Africa – and is a major barrier to economic development. So what’s the story of how this deadly disease comes to infect humans, and why is it such a serious problem in tropical parts of the world?
Image: James Gathany
First blood: Female Anopheles mosquito feeding on the arm of a human host.
In the blood of an infected person, the Plasmodium protozoan parasites that cause malaria are just waiting for a mosquito to bite their human host; in this way, they will be borne to other potential carriers. The young female Anopheles mosquito lands and bites the infected individual, piercing the skin with its pointed proboscis, before sucking up a blood meal. The quantity of blood ingested, though small, contains the microscopic malaria parasites. The mosquito is now infective.
Image: Ute Frevert / Margaret Shear
Inside the mosquito: Malaria Plasmodium migrating through cells in host’s gut.
Roughly a week later, hunger comes knocking at the mosquito’s door once more, and the whining winged insect goes in search of another blood meal. When it gets it, the malaria parasites will have infected a new individual. How? In the mosquito’s gut, the parasites fertilise and sexually recombine before migrating through their host’s body to its salivary glands. There, they mix with saliva ready to be injected into the person bitten through its flying vector’s syringe-like proboscis.
Second bite: Anopheles mosquito with infected saliva taking another blood meal.
In the newly bitten person, the parasites quickly enter the bloodstream and migrate to the liver, beginning to multiply asexually within 30 minutes. Once in the liver, the parasites infect, then rupture their host cells, escaping undetected back into the bloodstream wrapped in a clever disguise: the cell membrane of infected host liver cells. After a week or two, the parasites begin to infect red blood cells, where they multiply again and bust out in waves to invade fresh red blood cells.
Image: CDC/Dr. Mae Melvin
Blood smear: Plasmodium falciparum parasites infecting red blood cells.
The parasites multiplying in red blood cells brings on symptoms of anaemia like light-headedness and shortness of breath, plus other symptoms including chills, vomiting and waves of fever. In severe cases, coma and even death may follow, while children especially are vulnerable to brain damage. The lethal P. falciparum parasite makes infected blood cells stick to blood vessel walls to prevent their being destroyed in the spleen, which can cause fatal blockages to major organs.
Serious consequences: Infant malaria victim in a coma.
Yet the malaria parasite is more or less protected from attack by the body’s immune system due to the fact that it inhabits the liver and blood cells and so is relatively invisible to immune surveillance. Even when gluing red blood cells, P. falciparum does not serve as a good target because, like a thief changing disguises or a spy with multiple passports, the parasite switches between a broad repertoire of surface adhesive proteins, thus staying a step ahead of the body’s pursuing defences.
Human stage: Life cycle of malaria parasites in the human body.
Safe inside the blood of the human carrier, the larger life cycle of the malaria parasite is set to start again. It only takes a mosquito freshly hatched from the pupal stage of its own life cycle to take a blood meal. Most adults in endemic areas have a degree of recurring long-term infection – as well as partial immunity – aiding the spread of the disease. Meanwhile, those affected by severe malaria can suffer consequences ranging from enlarged spleen or liver to kidney failure or worse.
Image: PD-LAYOUT; PD-USGOV
Cycle starts again: P. falciparum parasites ready to be transmitted by a mosquito.
How can this harbinger of death be fought? Preventative drugs are often too costly and impractical for the poor in endemic areas, and the parasites have become resistant to many anti-malarial drugs – meaning in some places only a few drugs remain effective medicines. Malaria transmission is reduced by mosquito nets and insect repellents as well as mosquito control measures like spraying insecticides in houses and draining standing water, but again such steps are often unaffordable.
Where is the solution? Mosquito in place to carry the parasite.
Until one of the many malaria vaccines in development in proven effective and become available, increased bite protection and mosquito control will be needed to keep the major world problem that is malaria in check.