194 PARASITISM 



The environmental conditions which parasites have to meet and 

 overcome are well stated in principle by Manson in the following 

 excerpt: "The pathogenic protozoa are responsible probably for a 

 very large number of diseases. Many appear to be able to pass directly 

 from host to host, unaffected apparently by the atmospheric conditions 

 they encounter on the passage; that of smallpox and of most of the 

 exanthematous fevers probably belong to this category. Others, on 

 the contrary, demand special climatic conditions. Such are the germ 

 of scarlet fever, which does not spread in the tropics, and the germ of 

 dengue which, conversely, does not spread in cold climates. That of 

 the first is killed or paralyzed by heat; that of the latter by cold. Or, 

 it may be, they do not find appropriate transmitters except in special 

 climatic conditions. Many of the protozoa acquire the power of suc- 

 cessfully invading the human body only after certain developmental 

 changes, which take place after they leave their first host. Thus, 

 according to Schaudinn, the germ of amebic dysentery has to pass 

 through a sporulating stage before it becomes infective, and this stage 

 is accomplished outside the body and in conditions of tropical heat. 

 Hence, amebic dysentery is a tropical disease. Other protozoan dis- 

 ease germs, notably those of malaria, yellow fever, trypanosomiasis, 

 and relapsing fever, require an animal intermediary to remove them 

 from the body of their original host, foster them during a necessary 

 stage of development, and reimplant them in the human host. These 

 animal intermediaries being tropical, the diseases they disseminate 

 are also necessarily tropical." (Introduction to Vol. II, Part II, of 

 Allbutt and Rolleston's System of Medicine, 1907.) 



The majority of facultative parasites (some species of entameba, 

 cercomonas, copromonas, etc.), and many obligatory parasites, find 

 their best environment for further development in the digestive tract 

 of difl^erent animals, and the spores, when formed, are discharged with 

 the feces. Protected by their tough sporocysts, they may resist drying 

 for long periods or until taken again into some digestive tract, infection 

 being due to the more or less gregarious mode of life of the hosts and 

 to their indiscriminate feeding. An essentially similar result is 

 obtained in the case of cannibalistic animals, where, as in centipedes, 

 the weaker forms are eaten by the stronger and with them whatever 

 parasites they happen to harbor; it is in large part for this reason 

 probably that centipedes are rarely found without sporozoan para- 

 sites of some kind. In water-dwelling animals the spores of mvxo- 

 sporidia are usually disseminated through the water, so that infection 

 is brought about in the same way through the digestive system. In 

 land-dwelling or air-breathing animals of clean habits such sources 

 of infection are rare, and comparatively few protozoan parasites 

 occasionally found in them acquire a new host in this way. Other 

 means, however, especially in the higher animals and man, are eft'ective 



