The World Bank states that there are 300 to 500 million cases of malaria in humans worldwide each year. Millions die. The cause of this parasitic disease is a tiny blood parasite with the Genus name Plasmodium transmitted by the bite of a mosquito. Most species of Plasmodium are parasites of birds—others infect reptiles and mammals. Only four species infect humans: P. falciparum, P. vivax, P malariae, and P. ovale.
Like many parasites Plasmodium spp. have an interesting and complex lifecycle:
- An infected Anopheles sp. mosquito bites a vertebrate host, drawing a blood meal and injecting the infectious form of the parasite—“sporozoites”—into the host’s bloodstream.
- The microscopic sporozoites disappear from the blood quickly. In a human host, they invade liver cells within one or two days.
- Inside liver cells, sporozoites transform into “trophozoites,” which multiply asexually, producing many “merozoites.”
- Merozoites burst out of the liver into the blood and invade red blood cells, where they once again multiply asexually, destroying the cell and bursting out into the blood. Invading new red blood cells, they repeat the process over and over again.
- Eventually, some merozoites entering red blood cells develop into “gametocytes”—male and female forms that require a mosquito host to develop further.
- A mosquito taking a blood meal from the infected host draws out gametocytes. In the stomach of the mosquito, male gametes fertilize female gametes resulting in forms called “ookinetes.”
- Ookinetes penetrate the stomach wall of the mosquito and mature in the stomach lining, producing “oocysts.”
- Oocysts undergo a process called “sporogony,” producing the infective sporozoites, which move to the salivary glands of the insect.
- The mosquito, infected for life, will inject sporozoites from its salivary glands into every potential host that it feeds on. When an appropriate host is bitten, a new malaria infection results.
In humans, symptoms of malaria coincide with the multiplication of parasites in red blood cells, and the subsequent destruction of huge numbers of red cells. A cyclical fever pattern is typical, with a fever spike each time merozoites burst out of red cells and invade new ones. The first fever spike is often preceded by flu-like symptoms: aching muscles, loss of appetite, headache, and a general feeing of being unwell. With the fever come nausea, vomiting, and sometimes delirium. Destruction of red cells can result in profound anemia.
Different species of Plasmodium typically cause varying degrees of illness, with P. falciparum being the most devastating in humans, and P. ovale being the mildest. Death of the host is common in severe cases.
Related articles:
Mosquito Borne Disease Prevention
Sources:
Garcia, Lynn S. and David A. Bruckner. Diagnostic Medical Parasitology 3rd ed. Washington: ASM Press, 1997.
Roberts, Larry S. and John Janovy Jr. Foundations of Parasitology 6th Ed. Boston: McGraw Hill, 2000.
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