Fine Structure of a Reptilian Malarial Parasite

Abstract

The erythrocytic stages of a reptilian malarial parasite, Plasmodium floridense, were studied for the first time with the electron microscope. The fine structure of the erythrocytic stages of this reptilian parasite was compared with the erythrocytic and exoerythrocytic stages of avian parasites and the erythrocytic stages of mammalian parasites. The overall structure and developmental sequence of P. floridense are essentially similar to those of avian and mammalian malarial parasites. The merozoite possesses all of the organelles which have been described in the merozoites of avian and mammalian parasites. However, we also noticed large less-electron-opaque rounded bodies of unknown nature located near the paired organelles in the merozoite of P. floridense. The dedifferentiation phase is also observed in P. floridense in that certain organelles of the merozoite are lost, such as an anterior end structure (conoid), paired organelles, dense bodies, spherical body and the thick inner membrane and a layer of microtubules of the pellicular complex. During the growth phase, the cytostome of P. floridense ingests host cell cytoplasm and forms food vacuoles. Nuclear division is also similar to that of avian and mammalian malarial parasites. The redifferentiation phase is initiated by the reappearance of the thick inner membrane, paired organelles and anterior end structure (conoid), as we have observed in other species of malarial parasites. The budding of the merozoites from a schizont occurs at the areas which are covered by the thick inner membrane. By the migration of various organelles into the budding areas, several merozoites are formed from a schizont as we have observed among avian and mammalian malarial parasites. A reptilian malarial parasite, Plasmodium floridense, was first described in Sceloporus undulatus and Anolis carolinensis by Thompson and Huff (1944) and the infection of P. floridense has been extensively studied by Goodwin (1951) and Jordan (1964). Though there are considerable studies on the fine structure of the erythrocytic and exoerythrocytic stages of avian (Rudzinska and Trager, 1961; Ristic and Kreier, 1964; Aikawa, 1966; Aikawa et al., 1967; Meyer and Oliveira Musacchio, 1965; Hepler et al., 1966) and mammalian (Rudzinska and Trager, 1959, 1968; Rudzinska et al., 1965; Ladda et al., 1966, Aikawa et al., 1966b) malarial parasites, no report has been made on reptilian malarial parasites. We have, therefore, undertaken a study of the reptilian malarial parasite, P. floridense, and investigated the possibility of morphological and developmental differences in ultrastructure between the reptilian parasite and avian and mammalian parasites. MATERIALS AND METHODS Sceloporus undulatus infected with P. floridense was obtained in Clinch County, Georgia, in the vicinity of the Okefenokee Swamp. Blood from these lizards with 1 to 5% parasitemia was fixed Received for publication 7 June 1968. * Contribution No. 399 from the Army Research Program on Malaria. in 1.25% glutaraldehyde solution with 0.05 M phosphate buffer at pH 7.3 and containing 4% sucrose for 1 hr at room temperature. The blood was centrifuged for 10 min at 200 g. The resulting pellets were washed several times in 0.05 M phosphate buffer solution for at least 1 hr. Following the wash, the pellets were postfixed in 1% Os04 for 1 hr. The pellets were dehydrated in an ascending ethanol series and in propylene oxide and embedded in Epon 812. Thin sections were cut on a Porter-Blum MT-2 ultramicrotome with a Dupont diamond knife. The sections thus obtained were mounted on 300-mesh copper grids and stained with 1% uranyl acetate and lead citrate. The material was examined with a Siemens Elmiskop 1A. OBSERVATIONS AND DISCUSSION The merozoites of the erythrocytic stages of Plasmodium floridense are similar to those of certain avian (Aikawa, 1966; Aikawa et al., 1967; Hepler et al., 1966) and mammalian (Aikawa et al., 1966b; Aikawa, 1968) parasites. These include P. fallax, P. lophurae, P. cathemerium, P. gallinaceum, P. elongatum, P. berghei, P. knowlesi, and P. cynomolgi. The organelles of the merozoites include an ante ior end structure (conoid*) with polar rings, pa red organelles, dense bodies, a mitochondrion, a spherical body, a nucleus, ribosomes, endoplasmic reticulum, a cytostome, a pellic* Our use of the term conoid differs from the original use of Gustafson et al. We use the term to refer to the truncated anterior end.