parasites of the genus Plasmodium. There are approximately 156
named species of Plasmodium which infect various species of
vertebrates. Four are known to infect humans:
P. falciparum, P. vivax, P. ovale and P.
parasite life cycle involves two hosts. During a blood meal, a
malaria-infected female Anopheles mosquito inoculates sporozoites
into the human host
Sporozoites infect liver cells
mature into schizonts
which rupture and release merozoites
(Of note, in P. vivax and P. ovale a dormant stage [hypnozoites]
can persist in the liver and cause relapses by invading the bloodstream
weeks, or even years later.) After this initial replication in the
liver (exo-erythrocytic schizogony
the parasites undergo asexual multiplication in the erythrocytes
Merozoites infect red blood cells
The ring stage trophozoites mature into schizonts, which rupture
Some parasites differentiate into sexual erythrocytic stages
Blood stage parasites are responsible for the clinical manifestations of
The gametocytes, male (microgametocytes) and female (macrogametocytes),
are ingested by an Anopheles mosquito during a blood meal
The parasites’ multiplication in the mosquito is known as the sporogonic
While in the mosquito's stomach, the microgametes penetrate the
macrogametes generating zygotes
The zygotes in turn become motile and elongated (ookinetes)
invade the midgut wall of the mosquito where they develop into oocysts
The oocysts grow, rupture, and release sporozoites
which make their way to the mosquito's salivary glands. Inoculation of
the sporozoites into a new human host perpetuates the malaria life cycle
occurs in areas where environmental conditions allow parasite
multiplication in the vector. Thus, malaria is usually restricted to
tropical and subtropical areas (see map) and altitudes below 1,500 m.
However, this distribution might be affected by climatic changes,
especially global warming, and population movements. Both Plasmodium
falciparum and P. malariae are encountered in all shaded
areas of the map (with P. falciparum by far the most prevalent).
Plasmodium vivax and P. ovale are traditionally thought to
occupy complementary niches, with P. ovale predominating in
Sub-Saharan Africa and P. vivax in the other areas; however these
two species are not always distinguishable on the basis of morphologic
characteristics alone; the use of molecular tools will help clarify
their exact distribution.
symptoms of uncomplicated malaria can be rather non-specific and the
diagnosis can be missed if health providers are not alert to the
possibility of this disease. Since untreated malaria can progress to
severe forms that may be rapidly (<24 hours) fatal, malaria should
always be considered in patients who have a history of exposure (mostly:
past travel or residence in disease-endemic areas). The most frequent
symptoms include fever and chills, which can be accompanied by headache,
myalgias, arthralgias, weakness, vomiting, and diarrhea. Other clinical
features include splenomegaly, anemia, thrombocytopenia, hypoglycemia,
pulmonary or renal dysfunction, and neurologic changes. The clinical
presentation can vary substantially depending on the infecting species,
the level of parasitemia, and the immune status of the patient.
Infections caused by P. falciparum can progress to severe,
potentially fatal forms with central nervous system involvement
(cerebral malaria), acute renal failure, severe anemia, or adult
respiratory distress syndrome. Complications of P. vivax malaria
include splenomegaly (with, rarely, splenic rupture), and those of P.
malariae include nephrotic syndrome.
first step toward diagnosis (and treatment) of malaria is to consider
malaria in the differential
Microscopic identification is the method most frequently used to
demonstrate an active infection.
of Plasmodium species
diagnosis techniques can complement microscopy, especially in
Detection can detect past (not necessarily active) infections.
Immunologic/Biochemical detection of malaria parasite products are
available and under evaluation.
Bench aids for
according to the infecting species, the geographic area where the
infection was acquired, and the severity of the disease.