PROTOZOA IN DISEASE 319 



To this group belong the widely distributed Paramecia, which are all 

 true scavengers, feeding upon decayed and decaying organic matter, 

 vegetable as well as animal, including the organisms which give rise to 

 the decomposition changes. They show marked preference for decay- 

 ing vegetable matter. For example, decaying fish food or fish meal 

 infusion, well diluted with water, is likely to contain a pure culture of 

 Paramecia. The accompanying rotting bacteria are actively devoured 

 by the paramecia. 



The complexity (physiological as well as morphological) of the para- 

 mecial cell has suggested the use of this group of organisms for the 

 purpose of making comparative toxicological tests, in place of the higher 

 animals now almost universally employed for such tests. 



The infusoria proper (Ciliata), while exceedingly abundant and widely 

 disseminated; are mostly non-pathogenic. The Balantidium colt is a com- 

 mon hog parasite which may also cause serious dysentery in man. 

 V. Sporozoa. — Have no cilia, move by plasmic contraction of the cell 

 and reproduce by spores. Of this group, the most important species is the 

 Plasmodium malaria which is the primary cause of ague or malaria. The 

 carriers of the infection are certain mosquitos (species of Anopheles). 

 If the Anopheles group of mosquito could be exterminated throughout the 

 world, malaria would disappear also.' The organism is introduced into the 

 blood by the sting of the insect. In the blood it undergoes certain cycles 

 of development. The fever paroxysms are due to the sporulation of the 

 organisms in the circulatory system. During the intervals (non-sporula- 

 tion) there is no marked febrile disturbance. There are several species of 

 Plasmodium causing the several forms of malaria. The tertian form 

 {P. vivax) has a cycle of forty-eight hours; the quartan (P. malaria) has a 

 cycle of seventy- two hours; and the malignant tertian {P. falciparum) 

 has a cycle of forty-eight hours. In the latter type the paroxysms are so 

 severe as to give rise to a continued fever. Quinine is fatal to the Plasmo- 

 dium and this remedy should be given as a prophylactic and as a cure. 



The draining of swamps and other breeding places for mosquitos has 

 reduced malaria. The use of mosquito netting, screens, etc., has also 

 checked this disease. Small water areas may be treated with crude petro- 

 leum oil which kills the mosquito larvae. 



The primary cause of yellow fever is as yet unknown but it has been def- 

 initely determined that the carrier is a mosquito, the Aedes (Stegomyia) 

 calopus. Yellow fever is essentially a tropical disease, though it may 

 flourish in temperate zones until checked by frost which is so readily fatal 

 to the carrier, the mosquito. It has been ascertained that the Aedes does 

 not occur far from human habitations, that it breeds generally in barrels 

 and cisterns containing rain water, rather than in ponds or larger bodies of 



