Malaria is a serious, acute and chronic relapsing
infection in humans, characterized by periodic attacks of chills and fever,
anemia, splenomegaly (enlargement of the spleen), and often fatal complications.
Malaria also is found in apes, monkeys, rats, birds, and reptiles. It is caused
by various species of protozoa (one-celled organisms) called sporozoans
(subphylum Sporozoa) that belong to the genus Plasmodium. These parasites are
transmitted to humans by the bite of various species of mosquitoesbelonging to
the genus Anopheles .
Malaria is one of the most ancient
infections known. It was noted in some of the West's earliest medical records in
the 5th century BC, when Hippocrates differentiated malarial fevers into three
types according to their time cycles. It is not known when malaria first made
its appearance in the Americas, but it is highly probable that it was a
post-Columbian importation; some rather severe epidemics were first noted in
1493.
The association between swampy or
marshy areas and the disease has long been recognized, but theroles of the
mosquito and of the malarial parasite were not known until the beginning of the
20th century. In 1880 the French army surgeon Alphonse Laveran became the first
person to describe the malarial parasite and to recognize it as the cause of
malaria. In 1897–98 the British physician Sir Ronald Ross proved that bird
malaria is transmitted by Culex mosquitoes, and he described the entire life
cycle of that parasite in the mosquito. In 1898 the Italian investigators Amico
Bignami, Giovanni Battista Grassi, and Giuseppe Bastianelli first infected
humans with malaria by mosquitoes, described the full development of the
parasite in humans, and noted that human malaria is transmitted only by
anopheline mosquitoes. The disease can also be transmitted unnaturally by common
use of the hypodermic needle, as among drug addicts, or occasionally by blood
transfusion from infected donors.
Malaria occurs throughout the
tropical and subtropical regions of the world and is the most prevalent of all
serious infectious diseases. In the late 20th century, annual cases worldwide
were estimated at 250 million, with 2 million deaths resulting. Incomplete or
faulty reports from Africa make even those rough estimates unreliable, however.
Though malaria can occur in temperate regions, it is most common in the tropics
and subtropics, where climatic conditions are favourable for the mosquitoes that
transmit the disease throughout the year. In many parts of sub-Saharan Africa,
entire human populations are infected more or less constantly. Malaria is also
common in Central America, the northern half of South America, and in South and
Southeast Asia. The disease also occurs in countries bordering on the
Mediterranean, in the Middle East, and in East Asia.
Anopheline mosquitoes are the only
known vectors of malaria in humans, and about 60 different species perform this
function throughout the world. These mosquitoes undergo an aquatic larval stage,
pupate, and then hatch into flying adults.The females require a meal of blood to
produce fertile eggs, and females of some species prefer human to animal blood.
The female mosquito ingests the malarial parasite by biting a human already
infected with the parasite.
The malarial parasite has a
complicated double life cycle, with a sexual reproductive cycle while it lives
in the mosquito and an asexual reproductive cycle while in the human host. While
in its asexual, free-swimming stage, when it is known as asporozoite, the
malarial parasite is injected into the human bloodstream by a mosquito, passing
through the skin along with the latter's saliva. The sporozoite eventually
enters a red blood cell of its human host, where it goes through ring-shaped and
amoeba-like forms before fissioning (dividing) into smaller forms called
merozoites. The red blood cell containing these merozoites then ruptures,
releasing them into the bloodstream (and also causing the chills and fever that
are typical symptoms of the disease). The merozoites can then infect other red
blood cells, and their cycle of development is repeated.
A small proportion of the merozoites,
however, become gametocytes, or germ cells, and can go through a sexual
reproductive cycle once back in a mosquito. After they have been ingested by a
mosquito from an infected human host, the separate male and female gametocytes
pair off while in the mosquito's stomach and unite to form a single-celled
zygote, which grows to become an oocyst. This oocyst eventually divides,
releasing a multitude of (asexual, free-swimming) sporozoites that migrate to
the mosquito's head and salivary glands, where they are ready to pass into the
human bloodstream during the mosquito's next bite. The entire (asexual) cycle is
then repeated.
A remarkable feature of the asexual
cycle is that the parasites grow and divide synchronously, and the resulting
mass fissions (into merozoites) produce the regularly recurring attacks, or
paroxysms, that are typical of malaria. A malarial attack normally lasts 4 to 10
hours and consists successively of a stage of shaking and chills; a stage of
fever, with the temperature reaching 105° F, and severe headache; and then a
stage of profuse sweating during which the temperature drops back to normal.
Between attacks, the temperature may be normal or below normal. In the early
days of the infection, the attacks may occur every day, but they soon begin
appearing at regular intervals of either 48 hours (called tertian malaria) or 72
hours (called quartan malaria). The first attack usually occurs from 8 to 25
days after a person has been bitten by an infected mosquito.
Four species of Plasmodium are known
to cause malaria in humans: P. falciparum, P. vivax, P. malariae, and P. ovale .
The most common of these malarial types, accounting for about 50 percent of all
cases, is falciparum (subtertian, or malignanttertian) malaria, which has the
most severe symptoms and is the most frequently fatal; it accounts for as many
as 95 percent of all deaths from malaria. Falciparum malaria requires higher
temperatures for optimal development and is confined more closely to the
tropical areas. In western Africa, for example, it exists almost to the
exclusion of the other varieties. Once a person has recovered from falciparum
malaria, however, relapses rarely if ever occur. Vivax (tertian) malaria
accounts for about 40 percent of all cases and is widespread mainly because of
its ability to withstand therapy and to recur frequently for a period of several
years, though the initial acute phase lasts only two to three weeks. The two
less common types of malaria are quartan malaria (caused by P. malariae), which
is confined to the Mediterranean area, and ovale tertian malaria (caused by P.
ovale), which is basically confined to an isolated area of eastern Africa.
Infections with one or more species can occur simultaneously, however.
Furthermore, a double brood of tertian parasites can segment on alternate days,
giving a daily or quotidian fever.
Besides attacks, persons with malaria
commonly suffer from anemia (owing to the destruction of red blood cells by the
parasites), enlargement of the spleen (the organ responsible for ridding the
body of degenerate red blood cells), and general weakness and debility. In
falciparum malaria, the parasitized blood cells tend to stick together, and some
of the smaller blood vessels may be blocked as a result. Falciparum malaria may
also cause other complications, such as blackwater fever (q.v.).
Malaria can be reliably diagnosed
upon finding the parasites in stained blood smears examined under a microscope.
An effective treatment for malaria was known long before the cause of the
disease was understood: the bark of the cinchona tree, whose most active
principle, quinine, was used to alleviate malarial fevers from 1700 until World
War II, when more effective, synthetic drugs were developed. Chief among these
newer drugs are chloroquine, pamaquine, pyrimethamine, and amodiaquin, all of
which can destroy the malarial parasites while they are living inside red blood
cells. In their initial decades of use, chloroquine and related drugs could
relieve symptoms of an attack that had already started, prevent attacks
altogether, and even wipe out the plasmodial infection entirely. By the late
20th century, however, some vivax strains as well as most falciparum strains had
become resistant to the drugs, which were thus rendered ineffective. As a
result, the incidence of malaria began to increase after having steadily
declined for decades. Both one's natural resistance, as occurs among those who
are carriers of one gene for the sickle-cell trait, and one's acquired immunity
through previous exposure will reduce susceptibility to malaria.
The basic method of prevention is to
eliminate the breeding places of Anopheles mosquitoes by draining and filling
marshes, swamps, stagnant pools, and other large or small bodies of standing
fresh water. DDT, dieldrin, and other, less toxic insecticides have proved
potent in controlling mosquito populations in affected areas. Window screens and
mosquito netting are widely used to secure interior spaces from the mosquitoes,
which are mainly active at night.
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