Malaria is a vector-borne infectious disease caused by
protozoan parasites.
It is widespread in tropical and subtropical regions, including
parts of the Americas, Asia, and Africa. Each year, there are
approximately 350–500 million cases of malaria, killing
between one and three million people, the majority of whom
are young children in Sub-Saharan Africa. Ninety percent of
malaria-related deaths occur in Sub-Saharan Africa.
Malaria is one of the most common infectious diseases and an enormous
public health problem. The disease is caused by protozoan parasites of the
genus Plasmodium.
4 species of the plasmodium parasite can infect humans; the most serious
forms of the disease are caused by Plasmodium falciparum. Malaria caused
by Plasmodium vivax, Plasmodium ovale and Plasmodium malariae causes
milder disease in humans that is not generally fatal.
People get malaria by being bitten by an infective female Anopheles
mosquito. Only Anopheles mosquitoes can transmit malaria, and they
must have been infected through a previous blood meal taken on an infected
person.
Malaria is transmitted primarily by the bite of infected anopheline
mosquitoes. It can also be transmitted by inoculation of infected
blood.
Anophelines feed at night and their breeding sites are primarily
in rural areas. The greatest risk of malaria is therefore from dusk
to dawn in rural areas. In many malaria-endemic areas, there is
little or no risk in urban areas.
In the late 1950s and early 1960s, it was thought that malaria could be
eradicated through the widespread use of insecticides such as DDT
and by treatment of cases with chloroquine.
Eradication is no longer thought possible, however, because of the
development of drug resistance by both the mosquito and the parasite,
and because of deteriorating social and economic conditions in many
malaria-endemic countries.
These changes have resulted in a dramatic increase in the incidence of
malaria in many parts of the world, and an increase in malaria-related
mortality in some of these areas.
The classic symptom of malaria is cyclical
occurrence of sudden coldness followed by
fever and sweating lasting four to six hours,
occurring every two days. It causes widespread
anemia and also direct brain damage.
Severe malaria is almost exclusively caused by P. falciparum
infection
and
usually
arises
6–14
days
after
infection.
Consequences of severe malaria include coma and death if
untreated. Splenomegaly (enlarged spleen), severe headache,
hepatomegaly
(enlarged
liver),
hypoglycemia,
and
hemoglobinuria with renal failure may occur.
Severe malaria can progress extremely rapidly and cause death
within hours or days.
Special Condition:
Pregnant women are at higher risk of developing
severe
and
fatal
malaria.
Hyperparasitemia,
hypoglycemia and pulmonary edema are more
common in pregnant women with P falciparum
infections. Pregnant women should be treated
promptly with appropriate doses of antimalarials.
The parasite's primary hosts and transmission
vectors
are
female
mosquitoes
of
the
Anopheles genus. Young mosquitoes first
ingest the malaria parasite by feeding on an
infected
human
Anopheles
carrier
mosquitoes
and
carry
the
infected
Plasmodium
sporozoites in their salivary glands.
Only female mosquitoes feed on blood, thus males
do not transmit the disease. The females of the
Anopheles genus of mosquito prefer to feed at night.
They usually start searching for a meal at dusk, and
will continue throughout the night until taking a meal.
Malaria parasites can also be transmitted by blood
transfusions, although this is rare.
The malaria parasite life cycle involves two hosts. During a blood meal, a
malaria infected female Anopheles mosquito inoculates sporozoites into the
human host (1). Sporozoites infect liver cells (2) and mature into schizonts (3),
which rupture and release merozoites (4). After this initial replication in the
liver (exo-erythrocytic schizogony [A]), the parasites undergo asexual
multiplication in the erythrocytes (erythrocytic schizogony [B]). Merozoites infect
red blood cells (5). The ring stage trophozoites mature into schizonts, which
rupture releasing merozoites (6). Some parasites differentiate into sexual
erythrocytic stages (gametocytes) (7). Blood stage parasites are responsible for
the clinical manifestations of the disease.
The
gametocytes,
male
(microgametocytes)
and
female
(macrogametocytes), are ingested by an Anopheles mosquito during a blood
meal (8). The parasites‘ multiplication in the mosquito is known as the
sporogonic cycle [C]. While in the mosquito's stomach, the microgametes
penetrate the macrogametes generating zygotes (9). The zygotes in turn
become motile and elongated (ookinetes) (10) which invade the midgut wall of
the mosquito where they develop into oocysts (11). The oocysts grow, rupture,
and release sporozoites (12), which make their way to the mosquito's salivary
glands. Inoculation of the sporozoites into a new human host perpetuates the
malaria life cycle (1).
Since Charles Laveran first visualised the malaria parasite in blood in
1880, the mainstay of malaria diagnosis has been the microscopic
examination of blood.
Areas that cannot afford even simple laboratory diagnostic tests often
use only a history of subjective fever as the indication to treat for
malaria.
The most economic, preferred, and reliable diagnosis of malaria is
microscopic examination of blood films because each of the four major
parasite species has distinguishing characteristics.
Two sorts of blood film are traditionally used:
Thin films are similar to usual blood films and allow species identification
because the parasite's appearance is best preserved in this preparation.
Thick films allow the microscopist to screen a larger volume of blood and are
about eleven times more sensitive than the thin film, so picking up low levels of
infection is easier on the thick film, but the appearance of the parasite is much
more distorted and therefore distinguishing between the different species can
be much more difficult.
Diagnosis of species can be difficult because the early trophozoites ("ring
form") of all four species look identical and it is never possible to diagnose
species on the basis of a single ring form; species identification is always
based on several trophozoites
Rapid tests:
In areas where microscopy is not available, or where laboratory staff
are not experienced at malaria diagnosis, there are antigen
detection
tests
that
require
only
a
drop
of
blood.
Immunochromatographic tests (also called: Malaria Rapid
Diagnostic Tests, Antigen-Capture Assay or "Dipsticks") have been
developed, distributed and fieldtested. These tests use finger-stick
or venous blood, the completed test takes a total of 15–20 minutes,
and a laboratory is not needed.
Methods used to prevent the spread of disease, or to protect individuals in
areas where malaria is endemic, include:
prophylactic drugs, mosquito eradication, and the prevention of mosquito
bites.
The continued existence of malaria in an area requires a combination of high
human population density, high mosquito population density, and high rates of
transmission from humans to mosquitoes and from mosquitoes to humans. If
any of these is lowered sufficiently, the parasite will sooner or later disappear
from that area, as happened in North America, Europe and much of Middle
East. There is currently no vaccine that will prevent malaria, but this is an active
field of research.

Atovaquone-proguanil, trade name Malarone
(Therapy and prophylaxis)

Quinine (Therapy only)

Chloroquine (Therapy and prophylaxis; usefulness
now reduced due to resistance)

Mefloquine, trade name Lariam (Therapy and
prophylaxis)