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Life Cycle of Malaria | Human
Hosts | Malaria Parasites | Anopheles Mosquitoes
Life
Cycle of Malaria
In
nature, malaria parasites spread by infecting successively two types of
hosts: humans and female Anopheles mosquitoes. In humans, the parasites
grow and multiply first in the liver cells and then in the red cells of
the blood. In the blood, successive broods of parasites grow inside the
red cells and destroy them, releasing daughter parasites ("merozoites")
that continue the cycle by invading other red cells.
The
blood stage parasites are those that cause the symptoms of malaria. When
certain forms of blood stage parasites ("gametocytes") are picked
up by a female Anopheles mosquito during a blood meal, they start
another, different cycle of growth and multiplication in the mosquito.
After
10-18 days, the parasites are found (as "sporozoites") in
the mosquito's salivary glands. When the Anopheles mosquito
takes a blood meal on another human, the sporozoites are injected with
the mosquito's saliva and start another human infection when they parasitize
the liver cells.
Thus
the mosquito carries the disease from one human to another (acting as
a "vector"). Differently from the human host, the mosquito vector
does not suffer from the presence of the parasites.
View Schema
of the life cycle of malaria
Human
Hosts
Humans
infected with malaria parasites can develop a wide range of symptoms.
These vary from asymptomatic infections (no apparent illness), to the
classic symptoms of malaria (fever, chills, sweating, headaches, muscle
pains), to severe complications (cerebral malaria, anemia, kidney failure)
that can result in death. The severity of the symptoms depends on several
factors, such as the species (type) of infecting parasite and the human's
acquired immunity and genetic background.
more:
Human Hosts
Malaria
Parasites
Four species
of malaria parasites can infect humans under natural conditions: Plasmodium
falciparum, P. vivax, P. ovale and P. malariae.
The first two species cause the most infections worldwide. Plasmodium
falciparum is the agent of severe, potentially fatal malaria,
causing an estimated 700,000 - 2.7 million deaths annually, most
of them in young children in Africa. Plasmodium vivax and P.
ovale have dormant
liver stage parasites ("hypnozoites") which can reactivate
("relapse")
and cause malaria several months or years after the infecting mosquito
bite. Plasmodium malariae produces long-lasting infections
and if left untreated can persist
asymptomatically
in the human host for
years, even a lifetime.
More:
Malaria Parasites
Anopheles Mosquitoes
Malaria is
transmitted among humans by female mosquitoes of the genus Anopheles.
Female mosquitoes take blood meals to carry out egg production, and such
blood meals are the link between the human and the mosquito hosts in
the parasite life cycle. Of the approximately 430 known species of Anopheles,
only 30-50 transmit malaria in nature. The successful development of
the malaria parasite in the mosquito (from the "gametocyte" stage
to the "sporozoite" stage) depends on several factors. The
most important is ambient temperature and humidity (higher temperatures
accelerate the parasite growth in the mosquito) and whether the
Anopheles survives long enough to allow the parasite to complete
its cycle in the mosquito host ("sporogonic" or "extrinsic"
cycle, duration 10 to 18 days). Differently from the human host, the
mosquito host does not suffer noticeably from the presence of the parasites.
More: Anopheles
Mosquitoes
Page last modified : April 23, 2004
Content source: Division of Parasitic Diseases
National Center for Zoonotic, Vector-Borne, and Enteric Diseases (ZVED)
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