Chap. 21 THE PROTOZOANS 445 



in the blood and are transmitted solely by female anophcline (Anopheles) 

 mosquitoes. During its complete life history, the malarial parasite passes one 

 part of its existence in man and another part in the mosquito. Although other 

 vertebrates have malaria-like parasites and symptoms, the parasites causing 

 human malaria have been found only in man and anopheline mosquitoes. 



The paroxysms of malaria known as chills and fever may occur every day, 

 every other day, or every third day. These differences are due to peculiarities 

 in the life cycle of different species of malarial parasites which occur in human 

 blood. More than one of these may live in the blood at once and thus a person 

 may have more than one type of malaria at the same time and the distinctness 

 in the succession of temperature changes may be irregular. 



Immunity. Human beings have some natural resistance to all malarial para- 

 sites and certain races show a greater degree of it than others. In the United 

 States, the Negro race has a greater immunity to Plasmodium vivax than the 

 white. Some degree of acquired immunity is evidently developed by people 

 living in tropical regions where they have been subjected to malaria since 

 babyhood. Immunity to malaria artificially acquired by a vaccine as it is in 

 smallpox has not been accomplished and the prospects for it are not regarded 

 as promising. There are several reasons for this. One of them is the existence 

 of so many malarial parasites each of which may create its own type of im- 

 munity, and the immune person is prone to carry latent infections of them. 



Plasmodium Causing Human Malaria. There are four species of Plasmo- 

 dium that cause malaria: Plasmodium ovale is very rare; P. vivax is the cause 

 of "tertian" or "benign tertian" malaria, has a 48-hour cycle of development in 

 man, and is widely distributed in tropical and temperate zones; P. malariae 

 is the cause of "quartan" malaria and has a 72-hour cycle; and P. falciparum 

 is the cause of "malignant tertian" malaria and has a 40- to 48-hour cycle. 



Life History of Plasmodium vivax. Benign tertian malaria caused by the 

 parasite, Plasmodium vivax, is the commonest type of the disease in the 

 United States. When they bite human beings, female anopheline mosquitoes 

 carrying these parasites introduce them into the blood in an infective stage 

 of development known as sporozoites (Fig. 21.15). The sporozoites travel to 

 the liver cells and divide for 6 to 10 days (exoerythrocytic stages). When they 

 are released from the liver cells they enter the red blood cells and give rise to 

 nonsexual and sexual forms. In a red blood cell, each sporozoite grows and 

 divides into from 15 to 20 new individuals, the merozoites, nonsexual forms, 

 within about 48 hours. During this time, the parasite splits the hemoglobin in 

 the blood cell, and absorbs the hematin part of it known as malarial pigment 

 which accumulates in the parasite. After 48 hours, the red blood cell bursts 

 and the contained merozoites are freed in the blood plasma along with the 

 debris of the broken cell. This is the period of fever and general disturbance of 

 temperature in the person who has the disease. The merozoites soon attack 



