Notes on the characters of P. malariae oöcysts of possible value in mixed infections.

Abstract

Summary The development and diagnostic features of Plasmodium malariae oocysts in Anopheles gambiae have been studied at the Liberian Institute. Under these conditions the development of P. malariae is much more rapid than usually recorded, the cysts being half-grown by the 7th–8th day as compared with a more usual figure of 11 days or more. Development is still, however, distinctly slower than that of P. falciparum under the same conditions. Wide variations in the size of the oocysts of both species make the size difference of doubtful diagnostic value in individual cases. The characteristic clumping of the pigment in the oocysts of P. malariae from the 7th day onwards (corresponding to the 11th day of Shute and Maryon, 1955) is confirmed, but forms with two clusters or a bipolar arrangement are frequently seen in addition to the more usual single cluster form. The diagnostic value of this pigment pattern is rather limited by the occurrence of atypical falciparum oocysts with malariae-like pigment. When mixed infections are liable to occur—a distinct possibility even in experimental infections in West Africa—it is usually necessary to confirm the identity of P. malariae oocysts on a developmental basis which makes use of the fact that the characteristic pigment pattern of P. falciparum seldom persists after the 7th day, while that of P. malariae is still conspicuous up to the 10th and 11th day at least.