Malaria

I

Introduction

Malaria, disease of humans caused by infection with protozoan parasites belonging to the genus Plasmodium, and characterized by chills, fever, and, in the most severe cases, coma leading to death. The parasites are transmitted by the bite of female mosquitoes (the vector, or carrying organism, of the parasite) of about 60 species belonging to the genus Anopheles.

II

The Plasmodium Life Cycle

To understand the nature of the disease, it is necessary to understand the life cycle of the parasite responsible for it. The life cycle of Plasmodium involves both sexual reproduction within the host mosquito, and asexual reproduction in the liver and red blood cells within the human being. The multiplication of the parasite within human blood cells produces the characteristic recurrent attacks of the disease.

The infection begins when a mosquito vector injects infectious stages, called sporozoites, present in the mosquito’s saliva, into the bloodstream when it feeds on a human being. These enter liver cells where they multiply by asexual reproduction (pre-erythrocytic schizogony) for about 7 to 14 days (the incubation period of the disease) before producing between 10,000 and 30,000 daughter cells, called merozoites, which invade red blood cells. The parasites multiply in the red cells, again by asexual reproduction (erythrocytic schizogony), to produce between 8 and 16 merozoites every 48 or 72 hours, depending on the species of Plasmodium. These merozoites are released by the bursting of the infected red blood cells and the cycle is repeated. The bursting of the red blood cells causes anaemia and the release of toxic substances from within the cells causes the febrile attacks of the disease. After a few such cycles, sexual stages, male and female gametocytes, are produced, and these are taken up by a feeding mosquito, in which the Plasmodium life cycle is completed by sexual reproduction, resulting in the production of new sporozoites in the salivary glands.

Human malaria is caused by infections from four species, Plasmodium falciparum, P. vivax, P. ovale, and P. malariae, each responsible for a different form of the disease. Other Plasmodium species infect primates, rodents, birds, and lizards, and several of these, particularly those infecting rodents, have been used in experimental studies and for the testing of drugs and vaccines.

III

Occurrence of Malaria

The distribution of malaria is limited by conditions that are inimical to the development of the mosquito vector, such as temperature and altitude, mainly in tropical and subtropical regions of the world. About 85-90 per cent of cases occur in Africa, but it is also present in parts of Asia, Central and South America, Oceania, and some Caribbean islands. There are estimated to be between 350 and 500 million cases of malaria each year and about 1.5 to 3 million deaths, mainly of children in sub-Saharan Africa. In some parts of sub-Saharan Africa the situation is actually deteriorating, and there have been resurgences of the disease in several countries in Asia that were once thought to be free from malaria. The optimistic concept of the mid-20th century of the eradication of malaria has had to be modified, and the emphasis is now on control.

Malaria is one of the most important diseases in tropical countries and its spread has been enhanced by factors such as population growth, urbanization, the opening up of previously sparsely inhabited areas, deforestation, irrigation schemes, migrations, and the movement of refugees and others as a result of conflicts and natural disasters. Widening opportunities for travel have also increased the numbers of cases of malaria imported into temperate countries. There is also some concern that with global warming the disease might spread into such more temperate areas.

IV

Malaria in Human Beings

In all four Plasmodium species that cause human malaria, the infection is characterized by regular periodic fever, occurring every 72 hours in the case of P. malariae, and every 48 hours in the others. The attacks produce sweating and sometimes rigor. The intervals between the outbreaks of fever are accompanied by chills. The mildest forms of malaria are the tertian forms (attacks recurring every 48 hours) caused by P. vivax and P. ovale; the severest form is malignant tertian malaria caused by P. falciparum. P. malariae causes quartan malaria (attacks recurring every 72 hours).

Malignant tertian malaria is responsible for most of the deaths attributable to malaria, mainly because in this form of disease the parasitized red blood cells stick to the walls of blood vessels and lodge in capillaries in the brain, setting in train a cascade of pathological changes leading to coma and, unless treated, death. Hence, this form of the disease is also called cerebral malaria.