74 THE PROTOZOA AS PARASITES OF MAN 



of needle-like sporozoites together with some residual protoplasm. 

 The ripe cysts burst and scatter their contents into the body 

 cavity of the gnat, from which the sporozoites pass into the 

 salivary glands. In these the gnat secretes a Hquid which is 

 injected into its prey when it bites ; this saliva contains an anti- 

 coagulant, which prevents the clotting of the blood. When next 

 it feeds, the little parasites pass with the saliva along the proboscis 

 into the blood of the man on whom the gnat is feeding. 



It has been known for a long time that for about ten days 

 after infection there is a latent period, during which there is 

 no fever and no parasites can be found in the blood. It has now 

 been shown that in P. vivax and P. falciparum they spend part 

 of this time in the liver. No sporozoites can be found in the blood 

 half an hour after the gnat bites, and by the fourth day they are 

 recognisable in the cells of the liver. Here they grow as schizonts 

 to four times their original diameter, and on the seventh or eighth 

 day from the bite divide to form merozoites, about a thousand 

 coming from each schizont. Some of the merozoites invade the 

 red cells of the blood and begin the cycle of fever which we have 

 already described in infections of P. vivax, others remain in the 

 liver and repeat the cycle there, but in P. falciparum this organ 

 is emptied. Neither species causes any pathogenic reaction in 

 the liver. 



Long after apparent recovery from an attack of malaria caused 

 by P. vivax or P.. malarice, a patient may suffer a recurrence of 

 the disease. This is probably due to persistence of a few^ of the 

 trophozoites in the liver, where they continue to carry out the 

 exoerythrocytic cycle, i.e. that which goes on outside the red cells. 

 Plasmodium vivax may live in man for as long as ten years, 

 though it usually disappears after two. P. falciparum does not 

 usually persist for more than twelve months. 



Malaria is very widely distributed ; it is found in almost all 

 tropical and sub-tropical lands, and to a varying degree in the 

 temperate zones. In England it was formerly well known under 

 the name of ague, but fortunately this was nearly always of the 

 benign sort caused by P. vivax or P. malaricB. The reason for this 

 is that P. falciparmn needs a relatively high average temperature 

 — about 21° C. in summer and 8° C. in winter — to carry out 

 the insect part of its life-cycle, so that although the disease can 

 continue when a traveller brings it here from the tropics, and may 

 infect a few gnats and so a few more people, long-continued 



