The causative agents in humans are four species of Plasmodium protozoa -- P.falciparum, P.vivax, P.ovale and P.malariae. Of these, P.falciparum accounts for the majority of infections and is the most lethal.

Anopheline mosquito

Malaria is by far the world's most important tropical parasitic disease, and kills more people than any other communicable disease except tuberculosis. In many developing countries, and in Africa especially, malaria exacts an enormous toll in lives, in medical costs, and in days of labour lost.

Parasites are transmitted from one person to another by the female anopheline mosquito. The males do not transmit the disease as they feed only on plant juices. The parasites develop in the gut of the mosquito and are passed on in the saliva of an infective mosquito each time it takes a new blood meal. The parasites are then carried by the blood in the victim's liver where they invade the cells and multiply.

The spread of the disease is linked with activities like road building, mining, logging and new agricultural and irrigation projects, particularly in "frontier" areas like the Amazon. Elsewhere, disintegration of health services, armed conflicts and mass movements of refugees worsen the malaria situation.

It induces bouts of fever and anaemia in the infected individual along with shivering, pain in the joints and headache. In cerebral malaria, the infected red cells obstruct the blood vessels in the brain. Other vital organs can also be damaged often leading to the death of the patient.

Previously extremely widespread, the disease is now mainly confined to the poorer tropical areas of Africa, Asia and Latin America. Malaria is endemic in 91 countries with small pockets of transmission occuring in a further eight countries.

More than 120 million clinical cases and over 1.5 million deaths occur in the world each year. More than 90% of all malaria cases are in sub-Saharan Africa. Two-thirds of the remainder are concentrated in six countries -- India, Brazil, Sri Lanka, Viet Nam, Colombia and Solomon Islands, in decreasing order of prevalence.

It can normally be cured by antimalarial drugs. The symptoms, fever, shivering, pain in the joints and headache, quickly disappear once the parasite is killed. In certain regions, however, the parasites have developed resistance to certain antimalarial drugs, particularly chloroquine. Patients in these areas require treatment with other more expensive drugs. Cases of severe disease including cerebral malaria require hospital care.

In endemic regions, where transmission is high, people are continuously infected so that they gradually develop immunity to the disease. Until they have acquired such immunity, children remain highly vulnerable.

A detailed knowledge of the ecology of the local insect vector and the behaviour of people in endemic areas is essential for determining the most effective methods to control the disease. The problems of controlling malaria in tropical countries are aggravated by inadequate health structures and poor socioeconomic conditions. The situation has become even more complex over the last few years with the increase in resistance to the drugs normally used to combat the parasite that causes the disease.

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