260 



PLASMODHJM, HAEMOPROTEUS AND LEUCOCYTOZOON 



FAMILY PLASMODIIDAE 



This family has the characters of the 

 suborder. Its taxonomy has been reviewed 

 by Garnhan (1953) and Bray (1957). These 

 authors preferred to split the classical 

 genus Plasniodtion into several genera, 

 based on the life cycles of their species, 

 but it is simpler not to do so. 



Genus PLASMODIUM Marchiafava 

 and Celli, 1885 



The gametocytes occur in the erythro- 

 cytes. Schizogony takes place in the ery- 

 throcytes and also in various other tis- 

 sues, depending on the species. The exo- 

 erythrocytic ("e.e.") schizonts are solid 

 or, at the most, vacuolated bodies. Mem- 

 bers of this genus are parasites of mam- 

 mals, birds and lizards. They are trans- 

 mitted by mosquitoes. Anopheles trans- 

 mitting the mammalian species, and 

 culicines or sometimes Anopheles the 

 avian and reptilian ones. 



Members of this genus cause malaria, 

 which is still the most important disease 

 of man. They also cause a similar dis- 

 ease in birds. Coatney and Roudabush 

 (1949) have cataloged the species of Plas- 

 modium, and other species are discussed 

 by Bray (1957). Man has 4 species, higher 

 apes 4, lower apes and lemurs 7, rodents 

 2, and bats 1. Birds have 14 or 15 valid 

 species (Hewitt, 1940; Bray, 1957; Laird 

 and Lari, 1958). 



Life Cycle : The life cycle of Plas- 

 modluin vivaxoi man is representative. 

 The sporozoites enter the blood thru a 

 mosquito bite. They stay in the blood less 

 than an hour, quickly entering liver par- 

 enchymal cells. Here they become schi- 

 zonts (known as cryptozoites from their 

 location), which enlarge and divide by 

 multiple fission to form metacryptozoites 

 (a type of merozoite). These enter new 

 liver parenchymal cells, undergo multiple 

 fission, and form new metacryptozoites. 

 This process may go on indefinitely in P. 

 vivax, but in another human species, P. 

 falciparnni , there is only a single genera- 

 tion of metacryptozoites. 



The metacryptozoites break out of the 

 liver cells, pass into the blood stream and 

 enter the erythrocytes about a week to 10 

 days after infection. Here they round up 

 and develop a large vacuole in their cen- 

 ter. They are called ring stages because 

 in Romanowsky stained smears they re- 

 semble a signet ring, with a red nucleus 

 at one edge and a thin ring of blue cyto- 

 plasm around the vacuole. These grow 

 and are now called schizonts or tropho- 

 zoites. 



The trophozoites were formerly 

 thought to obtain their nutriment sapro- 

 zoically, but Rudzinska and Trager (1957) 

 showed in an electron microscope study 

 of P. lophurae of the duck that they are 

 holozoic as well. They form food vacuoles 

 containing host cell cytoplasm by invagina- 

 tion. The hematin pigment granules are 

 formed within these food vacuoles by di- 

 gestion of the hemoglobin. This study, 

 incidentally, settled once and for all the 

 question which is raised perennially as to 

 whether Plasmodium occurs within or on 

 the surface of the host cell; it is within it. 



The trophozoites undergo schizogony 

 to produce merozoites, the number depend- 

 ing on the species. These break out of the 

 erythrocytes, enter new ones, and repeat 

 the cycle indefinitely. 



The length of each cycle depends on 

 the parasite species. It is 2 days in P. 

 vivax and P. falciparum, and 3 days in 

 another human species, P. malariae. 

 Practically all the parasites are generally 

 in the same stage of the cycle at the same 

 time, so all the merozoites break out of 

 the red cells and pass into the blood at the 

 same time. Along with them go the hema- 

 tin granules and other waste products pro- 

 duced by the parasites' metabolism. These 

 are toxic, and cause a violent reaction or 

 paroxysm in the host- -the chills and fever 

 characteristic of malaria. 



After the infection has been present 

 for some time and after an indefinite num- 

 ber of asexual generations, some mero- 

 zoites entering the erythrocytes develop 

 into macrogametes and others develop into 

 microgametocytes. The former are 



