Studies of the Exoerythrocytic Stages of Simian Malaria. III. Plasmodium inui

Abstract

Liver biopsies were taken at laparotomy from squirrel monkeys (Saimiri sciureus) 7, 14, 18, and 21 days after intrahepatic and intravenous inoculation with sporozoites of Plasmodium brasilianum from Anopheles freeborni mosquitoes. Exoerythrocytic schizonts found in the tissue taken on days 14, 18, and 21 are described in detail. The morphologic features of the exoerythrocytic stages of P. brasilianum distinguish this species from other simian malarias, including the quartan parasite, P. inui, and indicate a close resemblance to the human quartan species, P. malariae. These observations suggest the importance of exoerythrocytic stage morphology as an additional criterion for understanding the relationships among the various strains and species of the genus Plasmodium. This paper, the fourth in a series on the exoerythrocytic (EE) stages of simian malaria, deals with the EE stages of Plasmodium brasilianum in the liver of the squirrel monkey (Saimiri sciureus). The present studies were initiated to gain basic information about the morphology and growth rates of the EE stages of a variety of simian malarias. This ultimately should be useful both in the study of prophylactic and curative effects of antimalarial drugs on EE stages of malaria parasites and as an aid in the taxonomy of the simian parasite species. Plasmodium brasilianum was originally described in a Cacajao (=Brachyurus) calvus from Brazil by Gonder and von BerenbergGossler in 1908 and was noted to exhibit a quartan periodicity. It was subsequently described from a number of New World monkeys of Central and South America, among them the squirrel monkey. Garnham et al. (1963) isolated a strain of P. brasilianum from a squirrel monkey and succeeded in transmitting the strain by sporozoite to a Cebus capucinus; a single 11-day exoerythrocytic body was found. To date, no other fixed tissue stages have been demonstrated. MATERIALS AND METHODS The strain of P. brasilianum used, designated as S strain, was isolated from a naturally infected squirrel monkey which had originated in the upper Amazon basin. It has been maintained by successive transfer of the erythrocytic stages to squirrel Received for publication 1 July 1969. * Centro Panamericano de Zoonosis, Casilla 23, Ramos Mejia, PCIA, Buenos Aires, Argentina, South America. monkeys. The methods used for the maintenance of strains, infection of mosquitoes, and examination of liver biopsies follow closely those described for P. cynomolgi by Eyles (1960) to which reference should be made for detailed information. Sporozoites were harvested in 10% monkey serum in saline from the salivary glands or from mature oocysts on the midgut of Anopheles freeborni mosquitoes (F-1 strain) after 17, 19, and 20 days of extrinsic incubation. These sporozoite suspensions were inoculated directly into the livers of malaria-free squirrel monkeys at laparotomy and the sites tagged with stainless steel wire after the technique of Held et al. (1967). At the same time, each monkey was inoculated intravenously with serum-saline suspensions of the triturated bodies of the same mosquitoes. Monkey S-4 was inoculated at 2 sites in the liver with sporozoites from 11 pairs of salivary glands and liver biopsies were taken on the 14th and 18th days after inoculation. Monkey S-9 was inoculated at one site in the liver with sporozoites from 6 pairs of salivary glands and liver biopsies were taken at laparotomy on the 7th and 21st days after inoculation, both from the same site. Tissues acquired at biopsy were fixed in Carnoy's fluid, embedded in paraffin, and sections made at 3 and 6 ,t. These sections were stained with Giemsa by the technique of Shortt and Cooper (1948) as modified by Eyles (loc. cit.). The photomicrographs in this article were made with a Zeiss photomicroscope.