What is Malaria?

by Prof. J. Gordon Edwards

Plasmodium is a genus of protozoaos that includes four species that cause malana in human bewgs. The most common of thése are Plasmodium vivax (usually mild) and Plasmodium falciparum (frequently fatal). Their life cycle is outlined here in a very simplified form.

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 quickly follows.

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 the liver.

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 natu
re 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 by malana.

A "balance" has therefore become established between homozygous sickie cell sufferers (who may die as a resu
lt), 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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