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Causes
By Mayo Clinic staff
Evolving strains of drug-resistant parasites and insecticide-resistant mosquitoes continue to make malaria a global health problem.
A one-celled parasite, plasmodium, causes malaria. About 170 species of plasmodium exist, but only four cause malaria in humans:
- P. falciparum. This species, predominant in Africa, produces the most severe symptoms and is responsible for most malaria deaths.
- P. vivax. This species, found mostly in tropical areas of Asia, produces less severe symptoms but can remain in your liver and cause relapses for up to four years.
- P. malariae. This species, found in Africa, can cause typical malaria symptoms, but on rare occasions it can remain in your bloodstream for years without producing symptoms. In these cases, you may pass on the parasite to a mosquito or to another person through a blood transfusion.
- P. ovale. This species is found mostly in West Africa. Although rare, it can also remain in your liver and cause relapses for up to four years.
The process of transmission
The transmitter (vector) of the plasmodium parasite to humans is a female anopheles mosquito. When a mosquito bites a person infected with malaria, it ingests a form of the parasite called gametocytes. The plasmodium completes part of its life cycle inside the mosquito, eventually making its way to the mosquito's salivary glands. Then, when the mosquito bites you, it injects the parasite into your bloodstream.
The parasite migrates rapidly to your liver, where it infects certain liver cells and develops for a week or so. The liver cells eventually burst, releasing a multiplied form of the infection into your bloodstream. Once in your red blood cells, the parasites reproduce further and some develop into the form that's available to be ingested by a mosquito (gametocytes), thus renewing the transmission cycle. In some cases of P. vivax or P. ovale infection, a form of the parasite can remain inactive in the liver for extended periods of time. Later, reactivation of the parasite's life cycle causes a relapse.
Other means of transmission
A pregnant woman can transmit the infection to her unborn baby. Malaria also can be transmitted through blood transfusions. In the United States, steps have been taken to prevent this type of transmission. People who have been in a malaria-endemic area are prohibited from donating blood for a year after returning from such an area, or three years if they've been a resident of a malaria-endemic area or have been treated for malaria.
