What is Malaria?
by Prof. J. Gordon Edwards
is a genus of protozoaos that includes four species that cause malana in human
bewgs. The most common of thése are Plasmodium
mild) and Plasmodium falciparum
(frequently fatal). Their life cycle is outlined
here in a very simplified form.
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When the female Anopheles
mosquito sucks blood (the males never bite)
from a person with malaria, she also swallows male and female plasmodia. The
plasmodia mate in her stornach, producing zygotes.
Each zygote penetrates the stomach wall, and grows into a large cyst
on the outside of that wall. Hundreds, or even thousands of young plasmodia
a (called sporozoites),
burst out of each cyst and travel throughout the mosquito's body.
Some reach the salivary glands. When the mosquito
feeds again, many sporozoites are injected into the host with its saliva. They
travel to the human liver, where they reproduce asexually. Six to 25 days later,
depending to some extent on the species of Plasrnodium, large numbers
of a new form of blood cell (erythrocyte) and reproduces asexually. In 48 hours,
each one has produced between 6 and 26 more merozoites They all burst out of
the blood cells at the same time, and each one enters a new blood cell, where
a new generation appears 48 hours later This process is repeated every 48 hours.
After the number of merozoites exceeds about 50 per cubic milliliter of blood
(that is, more than 150 million merozoites in a 140-pound person), the victim
suffers a typical malana attack every 48 hours In a heavily infested person,
as many as 2 million plasmodia can occcur in each drop of blood about
a cubic milliliter.
When millions of red btood cells are simultaneously destroyed, the victim suffers
a chill. As the cells are ruptured, toxins are released, resulting in alternating
chills and fevers. If a large number of plasmodia invades the brain, death
ln victims infected with Plasmodium
vivax, a few
plasmodia may remain in the liver, reproducing every 48 hours, but not increasing
in overali numbers. Years later, some of these may enter the blood and infest
red blood cells, bringing on a relapse. Plasmodium
falciparurn evidently does not linger in
Plasmodia can reproduce only in Anopheles
mosquitoes. All other mosquitoes fail to develop the sporozoites in their body,
and consequently they cannot be vectors of malaria. The plasmodia of human malaria
can live only in human beings and in a very few kinds of monkeys. Other species
of the genus Plasmodium cause malaria in birds and many other kinds of
mammals, but cannot survive in humans. They have been useful in the study of
malaria life cycles, and in attempts to develop vaccines.
If the average annual temperature is higher than 100° Fahrenheit or lower
than 60° Fahrenheit, the plasmodia do not survive long in the mosquitoes
and no malaria epidemics occur. The complicated life cycle and the complex nature
of the various life stages of malaria make the possibility
of producing a vaccine for human malaria very slim.
Immunity to malaria
Malaria is very common in the tropics and subtropics, and
kills millions of children under
fwe years of age. Adults may have it more
or less permanently, along with other diseases that are fostered by their chronicalty
weakened condition. Vivax malaria is somewhat seff-limiting, and people who
live with it appear to develop a degree of immunity, although they are periodically
ill. One scientists thought that the lack of severe attacks in an African village
indicated that a mild form of the disease had developed there. He infected himself,
to find out, and suffered a very severe attack as a result. The plasmodia were
still destructwe, but the villagers had become better adapted to them.
Falciparum malaria (malignant
tertian malaria),.is much more severe, and in typical outbreak may kill up to
40 percent of its victims. Now that strains that are rnuch more resistant to
antimalarial drugs have developed in so many areas the mortality rate will continue
to increase. That may well include the southern half of the United States and
rnuch of Cahfornia.
In Africa, a different kind of imrnunity to malaria has developed as a result
of another severe disease, sickie cell anemia.
Persons with sickle cell anemia have unhealthy red blood cells that are hard
and somewhat curved (hence the name). They do not flow freeIy tbrough the small
capillaries of the body, thus causing pain and frequently death. Malarial plasmodia
do not fare well in these erythrocytes, so persons with sickle cell anemia are
somewhat immune to malaria.
Children who inhent the gene for sickle cell from both parents usually die
early of the anemia. Those who do not get
the gene from either parent usually die early
of malaria. Those who are "heterozygous"
for the sickle cell gene getting the gene from only one parent
do not suffer much from sickle cell anemia, but their erythrocytes are not conductive
to the development of malaria plasmodia; thus they are not severely affected
A "balance" has therefore become established between homozygous sickie
cell sufferers (who may die as a result),
homozygous non-sickle-cell bearers (who may die from malaria as a result), and
heterozygous individuals who suffer only slightly from both the anemia and the
malaria. A large proportion of people living in malarious regions of Africa
are heterozygous and pass the genes to their children.
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