844 IMMUNOLOGY 



tissue cells. We have omitted from discussion the so-called exo-erythro- 

 cytic and fixed tissue cell stages described in the life cycles of some 

 Plasmodia because of the lack of agreement which now exists among 

 malariologists as to their nature (Boyd and Coggeshall, 1938, review). 



The course of untreated malarial infections has been most thoroughly 

 studied in avian malaria. This work commenced with the careful statis- 

 tical studies of the Sergents (1918) and was extended by Ben Harel 

 1923), L. G. Taliaferro (1925), G. H. Boyd (1929a), Hartman 

 (1927), Gingrich (1932), Lourie (1934), and others. Treated cases 

 will not be considered in this analysis because treatment itself has been 

 shown greatly to affect the length of the asexual cycle and the number 

 of merozoites produced (G. H. Boyd, 1933; G. H. Boyd and Allen, 

 1934; Lourie, 1934; and Boyd and Dunn, 1939). 



Infections with P. cathemerium in canaries are extremely stereotyped 

 and therefore afford an excellent base line for considering the so-called 

 benign infections, which tend to recover and which constitute the ma- 

 jority of malarial infections. When a few parasites are injected into a 

 bird, an incubation period follows during which no parasites can be 

 detected in the peripheral blood. As soon as parasites appear, they 

 increase from day to day at a constant rate (the intersporulation death 

 of parasites will be taken up later), according to a geometrical progres- 

 sion, until sometimes as many as half of the red blood cells are infected 

 (acute rise of infection). At this point, if the bird does not succumb to 

 the infection, recovery is initiated and is manifested by the rapid disap- 

 pearance of many of the parasites from the peripheral blood (crisis). 

 Following the crisis, parasites remain few in number, but may fluctuate 

 to some extent (developed infection). Sooner or later, the number of 

 parasites is reduced to a level at which they can no longer be detected 

 in peripheral blood films; but a few persist, since transfers of large 

 amounts of blood will infect other birds. This latent period may last 

 for several years, but it may be interrupted periodically by spontaneous 

 or induced relapses (much rarer in P. cathemerium than in many other 

 species ) , which are similar to, though generally quantitatively less than 

 the acute rise of the initial infection, and which are terminated by a 

 crisis. Occasionally, such a relapse may be fatal. By Hegner's (1926) 

 terminology such an infection is divided into prepatent (incubation), 

 patent (acute rise, crisis, and developed infection), and subpatent 



