Before mosquitoes were identified as the vectors of malaria, people thought the disease was the result of drinking bad water or breathing bad air (in fact, malaria means bad air). Even after scientists realized mosquitoes carried malaria, many believed infection resulted from drinking water in which mosquitoes had died. Today, scientists know that Plasmodium spp., the parasites that cause malaria, require both the mosquito and the vertebrate host to complete their life cycle.
When someone has symptoms of malaria, the parasites circulate in the blood stream; at this stage in their life cycle they are found inside red blood cells (erythrocytes). The so-called erythrocytic phase of the malaria parasite culminates with the development of gametocytes - male and female malarial parasites.
How Mosquitoes Get Malaria
When a mosquito bites a person who has malaria, it sucks both blood and malarial parasites into its stomach. Most of these parasites die and are digested with the blood, but if the mosquito is an Anopheles sp. mosquito, and there are gametocytes in the ingested blood, the gametocytes develop to gametes.
Once an Anopheles mosquito is infected with malaria, it remains infected for life and can infect a human each time it takes a blood meal. It appears that infected Anopheles mosquitoes feed more often than those that are not infected, increasing the potential for the parasite to be passed on.
The Life Cycle of Malaria Parasites in Mosquitoes
Just one pair of gametocytes in a blood meal results in the production of thousands of infective Plasmodium sp. sporozoites in only ten to twelve days:
- In the female gametocyte, the nuclear material moves to the edge of the cell. In the male, the nucleus divides to produce six to eight microgametes composed of a bit of nuclear material and a flagellum. This transformation takes less than fifteen minutes.
- Microgametes fertilize female gametes (macrogametes) by penetrating the nuclear material at the edge of the cell. The resulting fertilized cell is called an ookinete.
- The ookinete moves through the mosquito’s stomach lining into the hemocoel (the space where the insect’s hemolymph – a fluid analogous to blood is found). Rounding up, the ookinete becomes an oocyst.
- Within the oocyst, the nuclear material divides repeatedly, producing thousands of sporozoites.
- Sporozoites break out of the oocyst and move through the mosquito’s tissue to the salivary glands. Once there, they remain, waiting for the insect to feed again.
How Humans Get Malaria
When a mosquito bites, it injects a small amount of fluid. If the mosquito has Plasmodium sp. sporozoites in its salivary glands, some of the sporozoites enter the prey bloodstream. Sporozoites remain in the blood only minutes to hours before they invade liver cells, where they begin multiplying again.
Although malaria is occasionally passed on with a blood transfusion, or by shared syringes used for intravenous drug injection, the majority of human cases of malaria follow the bite of an infected Anopheles sp. mosquito.
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Sources
Beaver, Paul Chester, Rodney Clifton Jung, and Eddie Wayne Cupp. Clinical Parasitology 9th ed. Philadelphia: Lea and Febiger, 1984.
Esch, Gerald W. Parasites and Infectious Disease. Cambridge: Cambridge University Press, 2007.
Roberts, Larry S. and John Janovy Jr. Foundations of Parasitology 6th ed. Boston: McGraw Hill, 2000.
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
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