History of Malaria

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Malaria is a protozoal disease transmitted by the Anopheles mosquito, caused by minute parasitic protozoa of the genus Plasmodium, which infect human and insect hosts alternatively. It is a very old disease and prehistoric man is thought to have suffered from malaria. If probably originated in Africa and accompanied human migration to the Mediterranean shores, India and South East Asia. In the past it used to be common in the marshy areas around Rome and the name is derived from the Italian, (mal-aria) or "bad air"; it was also known as Roman fever. Today some 500 hundred million people in Africa, India, South East Asia and South America are exposed to endemic malaria and it is estimated to cause two and a half million deaths annually, one million of which are children.

Fishermen and traders, long before British colonisation, probably introduced the disease into northern Australia and in the past malaria was not uncommon in the northern parts of the country.

In Western Australia an explosive outbreak of falciparum malaria occurred at Fitzroy Crossing in 1934 which at first was mistaken for influenza and resulted in 165 deaths. WHO declared Australia free of malaria in 1981, however that time 9 patients have contracted locally acquired malaria.

The so called "airport malaria" has become a problem in recent years. A publican working in an establishment close to London's Heathrow Airport became acutely ill and was found to be suffering from falciparum malaria, he had never been out of the country.

A lady driving her car past the same airport became ill with malaria although she too had never been out of the country. Four workers unloading a cargo plane at Amsterdam airport became infected with malaria. It is assumed that infected mosquitoes were carried on planes from Africa and released at the destination airport.

While it was recognised that the Anopheles mosquito played a key role in the transmission of the disease it was not until 1948 that all the stages in its life cycle were identified. The parasite undergoes a development stage in the mosquito and the female of the species requires a blood meal to mature her eggs.

She bites a human and injects material from her salivary glands, which contains primitive malarial parasites called sporozoites, before feeding. These sporozoites circulate in the blood for a short time and then settle in the liver where they enter the parenchymal cells and multiply; this stage is known as pre-erythrocytic schizogony. After about 12 days there may be many thousands of young parasites known as merozoites in one liver cell, the cell ruptures and the free merozoites enter red blood cells.

The blood stages of the four species of malaria can be seen in the section on diagnosis. In the case of P. vivax, and P.ovale the liver cycle continues and requires a course of primaquine to eliminate it. P.falciparum on the other hand does not have a continuing liver cycle.

In the red blood cells the parasites develop into two forms, a sexual and an asexual cycle. The sexual cycle produces male and female gametocytes, which circulate in the blood and are taken up by a female mosquito when taking a blood meal.

The male and female gametocytes fuse in the mosquito's stomach and form oocysts in the wall of the stomach. These oocysts develop over a period of days and contain large numbers of sporozoites, which move to the salivary glands and are ready to be injected into man when the mosquito next takes a meal.

In the asexual cycle the developing parasites form schizonts in the red blood cells which contain many merozoites, the infected red cells rupture and release a batch of young parasites, merozoites, which invade new |red cells.

In P.vivax, P.ovale and probably P.malariae, all stages of development subsequent to the liver cycle can be observed in the peripheral blood. However, in the case of P.falciparum only ring forms and gametocytes are usually present in the peripheral c|o8d. Developing forms appear to stick in the blood vessels of the large organs such as the brain and restrict the blood flow with serious consequences.

While all four species have a haemolytic component ie. when a new brood of parasites break out of the red blood cell this is usually of little consequence.

The exception is falciparum malaria where the parasites multiply very rapidly and may occupy 30 or more of the red blood cells causing a very significant level of haemolysis. One reason for this is that P.falciparum invades red cells of all ages whereas P.vivax and P.ovale prefer younger red cells, while P.malariae seeks mature red cells. n

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