318 PARASITES OF THE DOMESTIC ANIMALS 



A brief review of these conclusions, drawn from the results of ex- 

 perimental research, will be sufficient to direct attention, not only to 

 the powers which insects have as carriers of disease, but to their poten- 

 tial powers in the making of disease as well. 



Methods of Reproduction. — Sexual and asexual methods of repro- 

 duction alternate in free forms of Protozoa, but the asexual method is 

 usually limited to simple division or budding. Parasitic forms, on the 

 other hand, have acquired a more prolific means of multiplication, the 

 simple division and budding being replaced by asexual spore formation 

 as exempHfied among the Sporozoa. In the parasitic Protozoa, there- 

 fore, two kinds of spores may be present, the one occurring asexually 

 during the vegetative life in the host and giving rise to auto-infection 

 in the same host, the other sexual, occurring at the end of the vegetative 

 life of the parasite, preparing its germs to withstand the unfavorable 

 conditions of an external environment, and giving rise to infection of 

 new hosts. 



The asexual method of multiplication, taking place during the veg- 

 etative life in the host, is termed schizogony or schizogenesis, while the 

 term sporogony or sporogenesis has been given to reproduction by the 

 sexual method. The first is sometimes referred to as the multiplicative, 

 the second as the propagative cycle. 



Life History of the Malaria Organisms. — With a view to an ele- 

 mental conception of these reproductive and infective processes in the 

 Sporozoa, the Hfe history of the organisms producing malaria in man 

 affords a clear example for study. 



Malaria was the first of the human diseases in which it was proved 

 that a protozoan is the direct cause, and by 1901 the disease was as 

 thoroughly understood as perhaps any other due to a germ. The malaria 

 parasites belong with the genus Plasmodium, so named from their early 

 supposed resemblance to some of the plasmodia-forming fungi. They 

 are usually considered under three forms with which three well-marked 

 types of malaria are associated. These may be briefly summarized as 

 follows: 



1. Plasmodium vivax. — Cause of tertian fever; paroxysms occur every 

 forty-eight hours; incubation period about two weeks. Temperate cli- 

 mates, also in tropics and subtropics. 



2. Plasmodium falciparum (P. prcecox). — Cause of estivo-autumnal 

 fever; paroxysms every twenty-four hours; incubation period usually 

 from ten to twelve days. Tropics and subtropics. 



3. Plasmodium malarice. — Cause of quartan fever; paroxysms every 

 seventy-two hours; incubation period about three weeks. Tropics and 

 subtropics. 



Two distinct cycles are involved in the life history of the malaria 

 organisms. The first takes place in the blood of the human patient and 



