THE PROTOZOA 



occurs by multiple fission or merogony. The resulting new generation of 

 parasites (merozoites) are liberated with the destruction of the invaded 

 erythrocytes, and in turn enter new cells in which the process is repeated. 

 In this manner a very large number of erythrocytes may be destroyed and the 

 population of the parasites greatly increased. In the form of the disease 

 produced by P. malanae, the patient suffers chills and fever which recur at 

 intervals of about 72 hours. This periodicity coincides with the maturation 

 and liberation of successive generations of merozoites in the red cells, and 

 it is probable that the symptoms are precipitated by the toxins released by 

 the disintegrating cells. After a considerable period of such asexual repro- 

 duction, the parasite forms macrogametocytes and microgametocytes, which 

 remain in the red cells of the vertebrate host until the blood is ingested by 

 a mosquito. In the stomach of this host the gametocytes differentiate into 

 macrogametes and microgametes, and syngamy occurs. The resulting motile 

 zygote (ookinete) passes through the epithelium of the gut and takes up a 

 position on the outer surface of the mosquito's digestive organs, where it 

 becomes invested by a cyst wall. Within this cyst multiple fission again 

 occurs, and eventually many spindle-shaped cells, the sporozoites, are formed. 

 The cyst wall finally bursts, and the sporozoites thus liberated into the 

 mosquito's body cavity migrate into its salivary glands. Here they remain 

 until ejected with saliva when the mosquito bites a human. The life cycle of 

 the malaria parasite is outlined in Figure 8.15. 



A long-standing problem in malariology has been posed by the fact that 

 within half an hour of the injection of sporozoites into the blood stream, 

 they disappear from the circulating blood. Infected erythrocytes cannot then 

 be found until some days later, when the symptoms of the disease begin to 

 appear. It has recently been discovered that injected sporozoites do not 

 penetrate erythrocytes but attack fixed tissue cells of the spleen, liver, and 

 bone marrow (Fig. 8.16). Within these cells merogony proceeds, producing 

 thousands of merozoites from each original sporozoite. Only after this "exo- 

 erythrocytic" period of incubation are the erythrocytes of the circulating 



Fig. 8.16. Merogony in the 

 exo-erythrocytic stages of 

 Plasmodium cynomolgi, in liver 

 cells of a monkey. The re- 

 sulting merozoites will invade 

 other liver cells, as well as 

 erythrocytes, and in these 

 fixed tissue cells will persist 

 as a chronic source of infec- 

 tion. (Adapted, after H. E. 

 Short t, from R. P. Hall, 

 Protozoology, copyright 1953 

 by Prentice-Hall, Inc., 



printed by permission.) 



