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Malaria
Dr. Breman is D.T.P.H, Senior Scientific Advisor at Fogarty International Center, National Institutes of Health, Bethesda, Maryland.
Most of us think of the mosquito-borne disease malaria as ancient history, but the truth is it remains a threat in tropical areas that are home to three billion people, killing between one and three million each year. Mosquito control has eliminated malaria in the U.S. and Canada, but it survives and has even made a comeback in many parts of the world. Despite promising new research efforts, malaria continues to be a major world health problem. How Malaria Happens
Malaria is caused by a parasite that is carried by mosquitos who transmit the parasite to people when they bite them to feed on human blood. The parasite that causes malaria belongs to the genus Plasmodium. It has four subtypes that cause nearly all malarial infections in humans. They are called P. falciparum, P. vivax, P. ovale and P. malariae.
Almost all deaths from malaria are caused by the first one on the list, falciparum malaria. Human infection occurs when a female anopheles mosquito bites a person and transmits cells called sporozoites from its salivary gland to a person's bloodstream. The Malaria Transmission Cycle
Sporozoite cells deposited in the human bloodstream after a mosquito bite travel rapidly to the liver, where they invade liver cells; the infected liver cells swell and eventually burst, discharging cells (called merozoites) into the bloodstream. These infected cells then invade red blood cells and multiply 6- to 20-fold every 48 to 72 hours. In certain strains of malaria, P. vivax and P. ovale, infected cells (called hypnozoites) remain dormant for between 3 weeks and a year or more. These dormant infected cells are what are behind the relapses that so often characterize certain types of malaria.
After reentering the bloodstream the infected merozoites rapidly invade red blood cells. By the end of 48 hours, (72 hours for P. malariae), the parasite has grown to occupy most of the affected red blood cells. Multiple nuclear divisions then occur and the red blood cell then ruptures to release 6 to 30 daughter merozoites, each potentially capable of invading a new red blood cell. The disease we call malaria is a result of the direct and indirect effects of red blood cell invasion and destruction by the parasite. After a series of asexual reproduction cycles (P. falciparum) or immediately after release from the liver (P. vivax, P. ovale, P. malariae), some of the parasites develop into distinct, longer-lived sexual forms (gametocytes) that continue the cycle of malaria transmissionafater ingestion of gametocytes by the mosquito. The lifecycle of the the parasite within the mosquito continues in the following manner. After being ingested along with human blood by a biting female anopheles mosquito, male and female gametocytes form a cell called a zygote in the insect's stomach. This develops into something called an oocyst, which expands asexually, by division, until it bursts to liberate many sporozoites, which then migrate to the salivary gland of the mosquito to await transmission to another human at the next feeding. Partial Immunity Is Not Necessarily Protection
After surviving repeated exposures to malaria, the human immune system can develop the capacity to protect the body from many of the effects of the disease, but not from further infection. As a result, a state of infection and partial immunity without illness is common among adults and older children in some areas. Despite this, the complexity of the human immune response to malaria, the sophistication of the parasites' evasion mechanisms and other factors have slowed progress toward an effective vaccine.
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