Risk, Human Health, and Toxicology - Environmental Science - page 332

many unsuspecting tourists—because there is no vac-
cine for this disease.
Malaria is caused by a parasite that is spread by
the bites of certain mosquito species. It infects and
destroys red blood cells, causing fever, chills, drench-
ing sweats, anemia, severe abdominal pain, headaches,
vomiting, extreme weakness, and greater susceptibil-
ity to other diseases. It kills about 3 million people each
year—on a par with AIDS (Figure 14-3)—and causes
an average of 8,200 deaths per day. Approximately
90% of those dying are children younger than age 5.
Many children who survive bouts of severe malaria
have brain damage or impaired learning ability.
Four species of protozoan parasites in the genus
Plasmodium cause malaria. Most cases of the disease
occur when an uninfected female of any of about 60
Anopheles mosquito species bites a person infected
with Plasmodium, ingests blood that contains the para-
site, and later bites an uninfected person (Figure 14-6).
Plasmodium parasites then move out of the mosquito
and into the human's bloodstream, multiply in the
liver, and enter blood cells to continue multiplying.
Malaria can also be transmitted by blood transfusions
or by sharing needles.
The malaria cycle repeats itself until immunity de-
velops, treatment is given, or the victim dies. Over the
course of human history, malarial protozoa probably have
killed more people than all the wars ever fought.
During the 1950s and 1960s, the spread of malaria
was sharply curtailed by draining swamplands and
marshes, spraying breeding areas with insecticides,
and using drugs to kill the parasites in the blood-
stream. Since 1970, however, malaria has come roaring
back. Most species of the Anopheles mosquito have be-
come genetically resistant to most insecticides. Even

worse, the Plasmodium parasites have become geneti-
cally resistant to common antimalarial drugs.
Researchers are working to develop new anti-
malarial drugs, vaccines, and biological controls for
Anopheles mosquitoes. They are also using artemisinins
derived from qinghaosu, an ancient Chinese herbal
remedy that cures 90% of malaria patients in three
days. Unfortunately, such approaches receive too little
funding and have proved more difficult to implement
than originally thought.
For now, prevention is the best approach to slow-
ing the spread of malaria. Methods include increasing
water flow in irrigation systems to prevent mosquito
larvae from developing (an expensive and wasteful use
of water), fixing leaking water pipes, and providing
poor people in malarial regions with window screens
for their dwellings and insecticide-treated bed nets.
Other approaches include cultivating fish that feed
on mosquito larvae (biological control), clearing vege-
tation around houses, planting trees that soak up water
in low-lying marsh areas where mosquitoes thrive (a
method that can degrade or destroy ecologically impor-
tant wetlands), and using zinc and vitamin A supple-
ments to boost resistance to malaria in children.
Spraying the insides of homes with low concentra-
tions of the pesticide DDT twice a year greatly reduces
the number of malaria cases. Under an international
treaty enacted in 2002, DDT and five of its chlorinated-
hydrocarbon cousins are being phased out in develop-
ing countries. However, the treaty allows 25 countries
to continue using DDT for malaria control until other
alternatives become available. Health officials in de-
veloping countries call for much greater funding of re-
search geared toward finding ways to prevent and
treat malaria and of schemes to provide poor people in
malaria-prone countries with window screens and in-
secticide-treated bed nets.

Watch through a microscope what happens when a
mosquito infects a human with malaria at Environmental
ScienceNow.

Female mosquito bites
infected human, ingesting
blood that contains
Plasmodium gametocytes

Merozoites enter
bloodstream
and develop into
gametocytes
causing malaria
and making
infected person
a new reservoir

Solutions: Reducing the Incidence of
Infectious Diseases
There are a number of ways to reduce the incidence of
infectious diseases if the world is willing to provide
the necessary funds and assistance.
Good news. According to the WHO, the global death rate
from infectious diseases decreased by about two-thirds
between 1970 and 2000 and is projected to continue
dropping. Also, between 1971 and 2000, the percentage
of children in developing countries immunized with
vaccines to prevent tetanus, measles, diphtheria, ty-
phoid fever, and polio increased from 10% to 84%—
saving about 10 million lives each year.

Plasmodium
develop in
mosquito

Sporozoites
penetrate liver
and develop
into merozoites

Female mosquito injects
Plasmodium sporozoites
into human host.

Figure 14-6 Science: the life cycle of malaria. Plasmodium
circulates from mosquito to human and back to mosquito.