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Essays > Malaria (October 19, 1999 by Pat Heyman)
"Uncle Robb says it’s malaria. What’s the cure for Malaria?"
"There is na one that I know. I’m no doctor. ‘Malaria’ is Latin for ‘bad air.’ That’s all I know—anyone knows. Mother of God! If the air of Happy Valley’s bad, we’re ruined." He prayed that the fever wasn’t malaria. If it was, the plague could touch anyone who slept in Happy Valley. It was common knowledge that the earth in certain areas in the world were malaria poisoned and for some reason gave off lethal gases by night.
According to Robb, the fever had begun mysteriously four weeks ago. First it had struck Chinese laborers. Then it had afflicted others—a European trader here, a child there. But only in Happy Valley. Nowhere else in Hong Kong. Now four or five hundred Chinese were infected, and twenty or thirty Europeans. The Chinese were superstitiously afraid, certain that the gods were punishing them for working on Hong Kong against the emperor’s decree. Only increased wages had persuaded them to return. –Tai-Pan, James Clavell (1966)
Malaria is caused by four species of protozoa of the Plasmodium
genera (P. vivax, P. falciparum, P. ovale, and P.
malariae). With the exception of P. malariae, which affects
some higher primates, humans are the only known hosts of the Plasmodium
parasites. Anopheles mosquitoes are the only mosquitoes capable
of transmitting malaria, but of the 380 Anopheles species, only
64 actually carry the parasites. Alternating between host and vector with
diploid and haploid stages, the Plasmodia lifecycle is extremely complex
and resembles a mixture of the zygotic lifecycle of fungus and the sporic
lifecycle of plants.
Before describing the lifecycle, some definitions of key terms are in order:
- Gametocytes: cells or organisms that give rise to gametes. Macrogametocytes and macrogametes are male, while the micro- cells are female.
- Ookinete: diploid zygotic cell that has developed and is mobile; when it anchors, it becomes an oocyst.
- Sporozoite: cells released from the oocyst.
- Merozoite: cells released from the mature sporozoite; gametocytes arise from the merozoite.
- Hypnozoite: dormant form of sporozoite.
Lifecycle: An infected female mosquito bites a human. Sporozoites from the mosquitoes gut enter the bloodstream. Upon entering the human bloodstream, the sporozoite travels to the liver, perhaps by chemotaxis. (Wernsdorfer, 1980). They infect hepatocytes and begin to divide. (Some may become dormant in the liver cells and are known as hypnozoites.) As the hepatocyte lyses, merozoites are released. This portion of the cycle is known as exoerythrocytic schizogony (division outside erythrocytes). The merozoites enter red blood cells and mature and divide. The fully developed merozoite (called a schizont) breaks into individual merozoites, which again invade erythrocytes. This portion of the lifecycle is known as erythrocytic schizogony (division inside erythrocytes). Upon invading an erythrocyte, the merozoite can repeat the above process or develop into a gametocyte. When gametocytes mature and unite, the result is a zygote that matures into an ookinete, a motile cell.
At this point in the process, another mosquito must feed upon the infected human. Ookinetes that are taken up into the mosquitoes gut mature into oocysts. Oocysts divide and eventually release sporozoites, and the process begins anew.
There is some question as to which division processes are mitotic and which are meiotic. It is generally accepted that sporozoites, merozoites, and gametocytes are haploid cells (Hallowes, 1999).
Etiology
Pathogenesis
Morphology
Clinical Manifestations