Malaria (Italian: "bad
air"; formerly called ague or marsh fever in English) is an infectious
disease which causes about 500 million infections and 2 million deaths
annually, mainly in the tropics and sub-Saharan Africa. The protozoan
cause of malaria was discovered by a French army doctor, Charles Louis
Alphonse Laveran, for which he was awarded the Nobel Prize for Physiology
or Medicine in 1907.
Malaria is caused by the
protozoan parasite, Plasmodium (one of the Apicomplexa). The recognised
species causing disease in man are P. falciparum (which alone accounts
for 80% of the recognised cases and ~90% of the deaths) and P. vivax, but
P. ovale, P. malariae, P. knowesli and P. semiovale are also known to
cause malaria. The vector for human malarial parasite is the Anopheles
Other mammals (bats,
rodents, non-human primates) as well as birds and reptiles also suffer
Symptoms of malaria include
fever, shivering, arthralgia (joint pain), vomiting, and convulsions.
There may be the feeling of tingling in the skin, particularly with
malaria caused by P. falciparum. Complications of malaria include coma
and death if untreated-young children are especially vulnerable.
Mechanism of the disease
Infected female Anopheles
mosquitoes carry Plasmodium sporozoites in their salivary glands. If they
bite a person, which they usually do starting at dusk and during the
night, the sporozoites enter the person's body via the mosquito's saliva,
migrate to the liver where they multiply within hepatic liver cells. They
then turn into merozoites which then enter red blood cells. There they
multiply further, periodically breaking out of the red blood cells. The
classical description of waves of fever coming every three or four days
arises from simultaneous waves of merozoites breaking out of red blood
cells during the same day.
The parasite is relatively
protected from attack by the body's immune system because it stays inside
liver and blood cells. However, circulating infected blood cells are
killed in the spleen. To avoid this fate, the parasite produces certain
surface proteins which infected blood cells express on their cell
surface, causing the blood cells to stick to the walls of blood vessels.
These surface proteins are highly variable and cannot serve as a reliable
target for the immune system. The stickiness of the red blood cells is
particularly pronounced in Plasmodium falciparum malaria and this is the
main factor giving rise to hemorrhagic complications of malaria.
Some merozoites turn into
male and female gametocytes. If a mosquito bites the infected person and
picks up gametocytes with the blood, fertilization occurs in the
mosquito's gut, new sporozoites develop and travel to the mosquito's
salivary gland, completing the cycle.
Treatment and prevention
If diagnosed early, malaria
can be treated, but prevention is always much better, and substances that
inhibit the parasite are widely used by visitors to the tropics. Since
the 17th century quinine has been the prophylactic of choice for malaria.
The development of quinacrine, chloroquine, and primaquine in the 20th
century reduced the reliance on quinine. These anti-malarial medications
can be taken preventively, which is recommended for travellers to
Certain strains of
Plasmodium have recently developed resistance to chloroquine which has
been the first line of treatment in many countries, thus complicating the
treatment. In West Africa, where the local strains of malaria are
particularly virulent, mefloquine is now the recommended prophylactic,
despite causing psychological problems in some vulnerable people.
Doxycycline, an antibiotic, is also prescribed as a prophylactic against
quinine-resistant malaria, though its use is less common than Lariam
because it must be consumed daily. It seems inevitable that resistance to
these drugs will also occur.
In addition to the
antimalarial drugs, the use of mosquito repellents such as DEET, and
mosquito nets and screens can reduce the chance of malaria, as well as
the discomfort of insect bites.
Extracts from the plant
Artemisia (specifically Artemisia annua), containing the compound
artemisinin, a substance unrelated to the quinine derivatives, offer some
Prospects of disease control
Vaccines for malaria are
under development, with no completely effective vaccine yet available (as
of November 2004). A team announced a partially successful field trial in
October 2004, for a vaccine which reduces infection risk by 30% and
severity of infections by over 50%. Further necessary research will delay
this vaccine from commercial release until around 2010. In January 2005
Edinburgh University scientists announced the discovery of an antibody
which protects against the disease. Since most of the deaths today occur
in poor rural areas of Africa which lack health care, the distribution to
children of mosquito nets impregnated with insect repellants has been
suggested as the most cost-effective prevention method. These nets can
often be obtained for less than US$10 or 10 euros when purchased in bulk
from the United Nations or other organizations, and need to be
re-impregnated with the chemical about every six months.
Malaria was eliminated from
the northern parts of the USA in the early twentieth century, and the use
of the pesticide DDT during the 1950s eliminated it from the south.
Spraying interior walls
with DDT, in areas where the mosquitoes are not already DDT-resistant, is
also effective. This public health use of small amounts of DDT is allowed
for in the Stockholm Convention on persistent organic pollutants (POPs)
which prohibits the agricultural use of DDT for large-scale field
There have been widespread
claims that a Western environmental agenda, from Rachel Carson's Silent
Spring onwards, has created perverse restrictions on DDT use.
Sickle cell anemia and other genetic
Carriers of the sickle cell
anaemia gene are protected against malaria because of their particular
hemoglobin mutation; this explains why sickle cell anemia is particularly
common among people of African origin. There is a theory that another
hemoglobin mutation, which causes the genetic disease thalassemia, may
also give its carriers an enhanced immunity to malaria.
Another disease that gives
protection against malaria is glucose-6-phosphate dehydrogenase
deficiency (G6PD). It protects against malaria caused by Plasmodium
falciparum as the presence of this enzyme is critical to survival of
these parasites within red blood cells.
It is thought that humans
have been affected by malaria for about 50,000 years, and several human
genes responsible for blood cell proteins and the immune system have been
shaped by the struggle against the parasite.