Malaria 605 



arterioles into fragments small enough for phagocytosis. This leads to 

 substantial destruction of red corpuscles, parasitized and normal alike. 



After several erythrocytic cycles of merogony, two types of gametocytes 

 normally appear in the peripheral blood, macrogametocytes usually being 

 more abundant than microgametocytes. Gametocytes may be expected, 

 in a primary attack, some time after a definite fever develops. In experi- 

 mental vivax malaria gametocytes are often present on the fifth day of 

 the patent period, usually persist during the clinical attack, and may still 

 be present for some time after the fever disappears. Infection of mos- 

 quitoes may be possible even after the blood contains less than 10 game- 

 tocytes/mm^. In laboratory-induced falciparum infections, gametocytes 

 are observed about the tenth day of the patent period and sometimes not 

 until after the primary attack subsides. A gametocyte density of 60/mm'^ 

 is believed to be the minimum for infection of mosquitoes, and results 

 have usually been negative with less than lOO/mm^ (15, 26). In natural 

 infections with P. falciparum, gametocyte densities ranging from l/mm"^ 

 to 90/mm-^ have proven infective for mosquitoes (125). 



The factors responsible for differentiation of gametocytes are unknown. 

 Strains of P. vixiax vary in the numbers of gametocytes usually produced, 

 and the ability to produce gametocytes may decline during transmission 

 by blood inoculation exclusively (5). In addition, the ability to produce 

 gametocytes may be lost in an unnatural host. For instance, a strain of 

 P. elongatum, isolated from a sparrow and maintained in canaries and 

 ducks, stopped producing gametocytes at the fomteenth canary and the 

 fifteenth duck transfer. A return to sparrows failed to reverse the change 

 (72). Any strain inidergoing such a change under natural conditions 

 would necessarily perish at the end of its current infection. However, the 

 mere production of gametocytes in the vertebrate host does not insure 

 perpetuation of a strain. Mature gametocytes have a rather short life 

 in the vertebrate, perhaps only a day or so in the case of P. vivax (7), 

 and both types must be ingested by a suitable mosquito if the life-cycle is 

 to be completed. 



The various stages in the erythrocytic cycle — rings, growth stages, 

 mature schizonts, stages of merogony, and gametocytes — differ morpho- 

 logically from species to species and furnish the major criteria for differ- 

 entiation of malarial parasites (36, 118a). 



Erythrocytic phase in P. vivax (Fig. 13.2). The earliest stage in the 

 red corpuscle is a discoid form with a small nucleus. After development 

 of the usual vacuole, the ring measures about 2;^ in diameter and generally 

 contains a single chromatin inass, although sometimes two. A corpuscle 

 usually contains only one ring, occasionally two or three. Growing para- 

 sites appear as larger rings, and later on, as irregular amoeboid forms. 

 Refractile light brown pigment granules are deposited in the parasite 

 during growth. These inclusions show brownian movement in fresh 



