Causes, incidence, and risk factors
Malaria is caused by a
parasite that is passed from one human to another by the bite of infected Anopheles mosquitoes.
After infection, the parasites (called sporozoites) travel through the
bloodstream to the liver, where they mature and release another form, the
merozoites. The parasites enter the bloodstream and infect red blood cells.
The parasites multiply
inside the red blood cells, which then break open within 48 to 72 hours,
infecting more red blood cells. The first symptoms usually occur 10 days to 4
weeks after infection, though they can appear as early as 8 days or as long as
a year after infection. The symptoms occur in cycles of 48 to 72 hours.
Most symptoms are caused
by:
·
The release of
merozoites into the bloodstream
·
Anemia resulting from the destruction of the red blood cells
·
Large amounts of free hemoglobin being released into circulation after red blood cells break
open
Malaria can also be
transmitted from a mother to her unborn baby (congenitally) and by blood
transfusions. Malaria can be carried by mosquitoes in temperate climates, but
the parasite disappears over the winter.
The disease is a major
health problem in much of the tropics and subtropics. The WHO estimates that
there are 300-500 million cases of malaria each year, and more than 1 million
people die from it. It presents a major disease hazard for travelers to warm
climates.
In some areas of the world, mosquitoes that carry malaria have
developed resistance to insecticides. In addition, the parasites have developed resistance
to some antibiotics. These conditions have led to difficulty in controlling
both the rate of infection and spread of this disease.
There are four types of
common malaria parasites. Recently, a fifth type, Plasmodium knowlesi,
has been causing malaria in Malaysia and areas of southeast Asia. Another type,
falciparum malaria, affects more red blood cells than the other types and is
much more serious. It can be fatal within a few hours of the first symptoms.
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