ANTIMALARAL DRUGS

Malaria is one of the most serious protozoal infection in manand is caused by infection by any four species of Plasmodium. The disease is characterized by successive chills, fever with rigor, anaemia and sweats. The Plasmodium vivax (benign tertian). Plasmodium falciparum (estivo-autumnal; malignant tertian) and Plasmodium malariae (quartan) are widely distributed and occur most frequently in tropical and sub-tropical countries. Plasmodium ovale (ovale tertian) is not very common....

All species of Plasmodium have two hosts, a vertebrate and a mosquito that acts as both vector and definitive host. 

The malaria organism requires times in both hosts to complete its multistage life cycle. Infection in man is caused by injection of sporozoites by the bite of a female anopheles mosquito. The salivary glands of the infected mosquito contain a large number of sporozoites and they enter into blood stream of the host. Some of the sporozoites rapidly enter into liver cells where they undergo exoerythrocytic propagation forming tissue schizonts which mature and release thousands of merozoites into the blood on rupture of the cell. When the erythrocytes burst then chilling is felt and fever is produced. The merozoites can infest more red-blood cells and start a new life-cycle. After several erythrocytic cycles, some erythrocytic forms develop into sexual gametocytes which eventually give rise to the sexual cycle in the mosquito.

Again during the mosquito bite of an infected person these merozoites (known as male and female gametocytes), enter mosquito from the peripheral blood. In the gut of the mosquito the female gametocyte fertilizes to zygote to form oocyst which utimately gives rise to sporozoites and they eventually reach the salivary glands of the mosquito.

Antimalarial drugs may kill the sporozoites as soon as they are introduced into the bloodstream by the bite of a mosquito (sporozoitocide drugs); they kill the parasite as it exists in the schizont stage (exoerythrocytic schizontocide drugs); they inhibit the development of schizonts during the erythrocytic stage (erythrocytic schizontocide drugs); they kill the parasite as it exists in the gametocyte stage (gametocytocide drugs); they prevent sporogony in the mosquito by their effect on the gametocytes in the blood of the vertebrate host (sporozoitocide drugs).

Cinchona alkaloids (quinine), 4-aminoquinolines (chloroquine, amodiaquine), 8-aminoquinolines (primaquine), 8aminoacridines (mepacrine), biguanides (proguanil), diaminopyrimidines (primethamine), o quinoline methanol (mefloquine) and some miscellaneous drugs (sulfonamides. sulfones, tetracycline) are used as antimalarial agents.