Dangerous, Deadly, but Interesting: Malaria and the Blood-Sucking Insect that Everyone Loathe

Malaria is a vector-borne infection transmitted by the bite of female Anopheline mosquitoes. Malaria has coexisted with the human population since the beginning of written history: documentations of malaria infections are found in records as old as 4700 years. Most know that malaria is a deadly disease resulting in the death of more than 400,000 people every year, but few know about the transmitters of the disease, except that they are annoying, tiny insects that wake everyone up from a good night’s sleep every summer night. This article will take a peek at the life of the infamous insect, mosquitoes.

What is malaria?
Malaria is a vector-borne infection that is common in tropical and some subtropical regions of Africa, Central and South America, Asia, and Oceania. However, the area with the highest risk is sub-Saharan Africa, accounting for over 90 percent of deaths from malaria. Conditions in the region create a perfect environment for malaria to thrive: tropical climate accelerates the growth and reproduction of the vectors, and unplanned population growth makes infection more likely. Parasites that cause malaria vary based on geographical regions. In sub-Saharan Africa, Plasmodium falciparum is the primary parasite that causes malaria infections, causing fatal malaria as it invades both mature and immature red blood cells at a rapid rate of reproduction. At the same time, malaria caused by P. vivax is quite uncommon in Africa because most of the population possess an adaptive trait of nonexistent Duffy antigens, making it impossible for the specific species of parasite to swarm in red blood cells.

What exactly causes malaria?
To put it simply, malaria is caused by a bite of a female Anopheles mosquito infected with parasites. When a person is bitten by an infected mosquito, parasite sporozoites enter the bloodstream, which moves to liver cells. There, sporozoites reproduce asexually and produce schizonts. Merozoites, which invade red blood cells and replicate to produce blood-stage schizonts, are produced as liver-stage schizonts rupture. The incubation period of 9 to 30 days is the result of the fact that it takes about a week or two after the infection for schizonts to rupture and release merozoites into the bloodstream. The blood stage schizonts then rupture to produce more merozoites, which invade and destroy red blood cells. The reproduction cycle of parasites are responsible for cyclical fevers, which is the characteristic symptom of malaria. Clinical symptoms of malaria, including fever, diarrhea, and headache, are the result of cytokines due to merozoites in the bloodstream and cellular debris from red blood cell rupture.
However, schizonts and merozoites are not the only form of the parasite produced during the reproduction cycle. Gametocytes, the sexual form of the parasite, can also be produced. When mosquitoes bite the infected individuals, they would ingest the gametocytes, which would reproduce in them to produce sporozoites. Sporozoites then settle in the salivary glands of mosquitoes, making its bite infectious.

Anopheline gambiae: Know the enemy
There are approximately 60 species of mosquitoes that belong to the Anopheles that can become vectors of malaria. However, because of the differences in behavior and physical characteristics of different mosquito species, the distribution of mosquitoes depends on geographical regions. In sub-Saharan Africa, An. gambiae is the most common malaria vector.

Life cycle
An. gambiae mosquitoes follow the common life cycle of semi-aquatic insects. Larvae of An. gambiae mosquitoes hatch from eggs two days after they are deposited in water. It spends its entire larval period in water, going through four weddings and a pupa stage to develop into a mature mosquito. It takes about a week for an egg to develop into an adult mosquito, although it depends on the climate of the region. The typical lifespan of adult mosquitoes range from 10 days to 14 days. The mosquito cannot be ineffective if the life cycle of parasites–the time it takes for the parasite to produce sporozoites–exceeds the lifespan of the adult mosquito. An. gambiae mosquitoes usually spend their entire life within 2 to 3 kilometers from their breeding sites where they spent their larval stages.

Mating
An. gambiae mating usually occurs as female mosquitoes enter male swarms. The swarm markers that aggregate male mosquitos include visual cues, namely dark–light contrast points on the ground, and chemical cues, such as octanal, 3-hydroxy-2-butanone (acetoin), 6-methyl-5-hepten-2-one (sulcatone), decanal, and nonanal. Females are mainly driven by visual and auditory cues associated with male swarming when mating. They are attracted to the typical pattern of An. gambiae male swarms that are spherical in which individual male mosquitoes fly in a zigzag pattern. The specific flying tones of males, which are higher than that of females, also attract the females.

Feeding
Male An. gambiae typically feeds on plant nectar. Five plants, Mangifera indica L. (Anacardiaceae), Delonix regia (Fabaceae), Thevetia neriifolia Juss (Apocynaceae), Senna siamea (Fabaceae), and Cassia sieberiana (both Fabaceae) are known to be primary foodsource of mature An. gambiae males.
Females are those responsible for malarial infections. They primarily feed on the blood of animal hosts, and they have a clear preference for human hosts, while they also feed on wild and domesticated animals. Female bites are more common in the dry season, and they primarily feed indoors at night. Thus, residual pesticide spraying and insecticide-treated bed nets are effective for controlling female populations. An. gambiae females rely almost entirely on heat and chemical cues to seek for human hosts. With heat alone, their flights were observed to be short and straight, which are almost similar to their behavior without any stimulus. With odor alone, decreased flight speed when mosquitoes arrived within 30 cm of the source was observed, which seems like a behavior with a purpose to scan their environment. When odor and heat was presented together, prolonged and highly convoluted flight patterns approaching the source of the stimuli were seen. They landed more often on the source than when presented with either of the stimuli alone.

Closing
Eradication of malaria is one of the important global health goals, for it not only causes damage to the affected individual but hampers the growth of the community. Despite climate change and political instability in endemic regions accelerating the spread of the disease, new technologies and developments illuminates the future without the long, unfriendly companion of human history.