The life cycle of the malaria parasite is complex. The process -three phases in the mosquito and two in the human host- has been divided by scientists into nearly a dozen separate steps. The parasite is transmitted to humans by the sporozoites forms in the saliva of infected female mosquitoes of the genus anopheles. Soon after entering the human host, the sporozoites invade liver cells, where during the next 5-15 days they develop into schizonts.
Each schizonts contains 10,000-30,000 ‘daughter’ parasites called merozoite, which are released and invade the red blood cells. Once inside the red blood cells, each merozoite matures into schizonts containing 8-32 new merozoites. The red blood cell eventually ruptures and releases the merozoites, which are then free to invade additional red blood cells. The rupturing of red blood cells is associated with fever and signals the clinical onset of malaria.
Figure 1 the life cycle of the malaria parasite
Some merozoites differentiate into sexual forms, gametocytes, which hare ingested by a mosquito during its next blood meal. Once in the mosquito, the sexual forms leave the blood cells, and male and female gametes fuse to forma a zygote. Over the next 12 to 48 hours, the zygote elongates to form an ookinete. The ookinete penetrate the wall of the insect’s stomach and becomes an oocyst. Over the next week or more, depending on the parasite species and ambient temperature, the oocyst enlarges,, forming more than 10,000 sporozoites. When the oocyst rupture, the sporozoites migrate to the mosquito salivary glands, from where they may be injected back into the human host, thus completing the cycle.
Malarial illness may recur months to years after apparently successful treatment. In patients infected with P. vivax and P. ovale, this phenomenon is known as relapse. Relapse is caused by dormant liver-stage forms of the parasite that resume their developmental cycle and release merozoites into the bloodstream. The recurrence of malaria by P. falciparum and P. malariae is due to recrudescence, which is caused by surviving blood-stage parasites from an earlier infection.
The mosquito
Of the more than 2,500 species know of mosquitoes worldwide, only a sub-groups of 50 to 60 species belonging to the genus Anopheles are capable of transmitting malaria. Female anopheline require blood meals to reproduce. Some anopheline species are indiscriminate feeders; others prefer to fed on animals (zoophilic) or human (anthropophagic).
Anopheline mosquitoes breed in relatively clean water, with certain species having very specific preferences as to their aquatic environment. Understanding these preferences is crucial for targeting effective malaria control interventions. For example, An. stephensi can breed in tin cans and water cisterns, while Anopheles gambiae, the most important malaria vector in Africa south of the Sahara, prefers small, sunlit pools.
The mosquito undergoes four stages of growth: egg, larva, pupa and adult ( imago). Adult females mate once and store the sperm. The female may deposit a total of 200 to 1,000 egg in three or more batches. Actual egg production is dependent on blood consumption. After hatching, anopheline larvae lie along the water-air interface, where it is thought that they feed on organisms along the surface film. Adult mosquitoes develop from the pupil stage within 2 to 4 days. An adult mosquito will emerge from the egg stage in 7 to 20 days, depending on the species of mosquito and environmental conditions.
Female anopheline mosquitoes can survive at least a month under favourable conditions of high humidity and moderate temperatures. That is sufficient time for them to take a blood meal, for the parasite to develop, and the mosquito to take another blood meal and thus transmit the parasite to second human host.
Mosquitoes are rarely found more than a few miles from their larva development site. They are readily blown short distance by the wind and have been transported internationally as unintended stowaways on air-planes. Mosquitoes seek their host in response to a combination of chemical and physical stimuli, including carbon dioxide plumes, certain body odors, warmth, and movement.
Anopheline mosquitoes feed most frequently at night and occasionally in the evening, or in heavily shaded or dark areas during the early morning. During feeding, the mosquito injects a minute amount of salivary fluid into the host to increase blood flow to the area. Sporozoites are transmitted to the host in the salivary fluid.
Anopheles mosquitoes are readily distinguished from other genera by their characteristic stance, in which they appear to be standing on their heads (most mosquitoes hold their bodies relatively parallel to the surface on which they are resting). After feeding, some engorged females seek out cool and humid areas of a house, such as walls and the underside of furniture, while others find dark, secluded spots outdoors near the ground. Some mosquitoes have modified their resting behaviour so as to avoid surfaces treated with pesticides.
The life cycle of the parasite is complicated and involves two hosts, human and anopheles mosquitoes. The disease is transmitted to humans when an infected Anopheles mosquitoes bites a person and injects the malaria parasites (sporozoites) into the blood. Sporozoites travel trough the blood-stream to the liver, mature, and eventually infect the human red blood cells. While in red blood cells, the parasites again develop until a mosquito takes a blood meal from an infected human and ingest human red blood cells containing the parasites. Then the parasites reach the Anopheles mosquito’s stomach and eventually invade the mosquito salivary glands. When an Anopheles mosquito bites a human, these sporozoites complete and repeat the complex Plasmodium life cycle. P. ovale and P. vivax can further complicate the cycle by producing dormant stages (hypnozoites) that may not develop for weeks to years ( see schema of the life cycle of malaria in Chart ).
Malaria as a Disease
The symptoms characteristic of malaria include flu-like illness with fever, chills, muscle aches, and headache. Some patients develop nausea, vomiting, cough, and diarrhea. Cycles of chills, fever, and sweating that repeat every one, two, or three days are typical. There can sometimes be vomiting, diarrhea, coughing, and yellowing (jaundice) of the skin and whites of the eyes due to destruction of red blood cells and liver cells.
People with severe P. falciparum malaria can develop bleeding problems, shock, liver or kidney failure, central nervous system problems, coma and can die from the infection or its complications. Cerebral malaria (coma, or altered mental status or seizures) can occur with severe P. falciparaum infection. It is lethal if not treated quickly; even with treatment, about 15-20% of patients die.
The distinction between infection and disease is particularly important in malaria, since infection with the parasite does not necessarily result in disease. Many infected people in areas where malaria in endemic are asymptomatic: they may harbor large numbers of parasites yet exhibit no outward signs and symptoms of the disease. However, symptomatic individuals are major contributors to the transmission of malaria parasites.
For reasons not fully understood, the epidemiology of malaria transmission and the severity of the disease vary greatly from region to region, village to village, and even from person to person within a village or town. Some of this differences are due to the particular species of parasite.
The degree of compliance with an anti-malarial drug regimen, local patterns of drug resistance, and an individual’s age, genetic makeup, and duration of exposure to infective mosquitoes may also influence the severity of the disease.
Assuming they survive childhood, people in areas of endemic malaria often acquire a moderate level of immunity to malaria by being infected repeatedly. Although they may experience mild symptoms of the disease, including recurrent fevers, they rarely suffer the more severe and potentially fatal consequences. Without repeated exposure, however, this immunity is relatively short-lived, and almost always protects against life-threatening malaria, it does not prevent occasional episodes of fever and chills.
Some population groups have genetic characteristics that render them resistant to certain forms of malaria. For example, person of African descent who lack the so-called Duffy blood group surface antigens cannot be infected with P. vivax. The heterozygous sickle cell trait, often present in the people of African descent, partially protects against the infection with P. falciparum. Other hereditary abnormalities such as glucse-6-phosphate dehydrogenase deficiency, are partially protective against malaria. Those with Sickle cell anemia also have less risks of malaria.
Medical personnel should suspect malaria in anyone with a fever who has recently been in a malaria-endemic region. A definitive diagnosis of malaria infection is made by microscopic examination of stained blood smears for the presence of parasites. In the early stages of infection and at all stages of infection with P. falciparum, when parasites disappear from the peripheral blood during schizogony, few or no parasites may be present in blood smear; examination of blood smears taken at frequent intervals may be necessary to establish a diagnosis. A positive blood smear taken from a feverish patient living in an endemic region does not conclusively implicate malaria a the case of illness, because may asymptomatic individuals have circulating parasites in their blood and the fever may be due to other infections agents.
Despite being spread over large regions of the world, malaria is a “focal” disease. That is, because of the complex interactions among the human host, mosquito vector, malaria parasite, and the local environment, malaria affects discrete population groups in different ways. This lack of uniformity is a major reason why it is so difficult to design effective an all-encompassing control strategies.
Malariya and evolution of human
Why do mosquitoes bite me and not my friend
How moquitoes evolved to crave human blood
Why mosquito chosen human only
How they got the human blood taste
Anthropology of mosquitoes
Mostoitoes taste for human blood may grow as African cities expand.
E. O. Wilson observed: "If all mankind were to disappear, the world would regenerate back to the rich state of equilibrium that existed ten thousand years ago. If insects were to vanish, the environment would collapse into chaos."[13] A Nova segment on the American Public Broadcasting Service framed the relationship with insects in an urban context: "We humans like to think that we run the world. But even in the heart of our great cities, a rival superpower thrives ... These tiny creatures live all around us in vast numbers, though we hardly even notice them. But in many ways, it is they who really run the show.[14
Malaria, transmitted by Anopheles mosquito vectors, belongs among the most dangerous diseases in the world, killing half a million people every year. According to the WHO, nearly half of the world´s population is at the risk of malaria. It is widely suspected that malaria could be linked with the decline of city-state populations in ancient Greece and may have contributed to the fall of the Roman Empire during the 5th century AD. Indeed, malaria was so prevalent in Rome that it became known as "Roman fever". However, estimating the historical distribution of malaria and its vectors has proven to be a challenge.
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