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Plasmodium malariae causes quaan malaria, in which febrile paroxysms occur every fouh day, with 72 hours' interval between the bouts. The disease is generally mild, but is notorious for its long persistence in circulation in undetectable levels, for 50 years or more.The sporozoites are relatively thick. Pre-erythrocytic schizogony takes about 15 days, much longer than in other species. Each schizont releases about 15,000 merozoites. Hypnozoites do not occur. The long latency of the infection is believed to be due to persistence of small numbers of erythrocytic forms in some internal organs. P.malariae preferentially infects older erythrocytes and the degree of parasitisation is low TEXTBOOK OF MEDICAL PARASITOLOGY,CKJ PANIKER,6TH EDITION,PAGE NO 81