ROBERT HEGNER 66$ 



by this infected fecal material. Cysts have been described in the life-cycle of most 

 of these species, and transmission no doubt usually occurs in the cyst stage. Tricho- 

 monas, however, apparently exists only in the active trophozoite stage and, therefore, 

 must pass from one host to another in this condition; it is possible that the tropho- 

 zoites of other intestinal protozoa may also be infective. Successful transmission de- 

 pends largely on the viability of the cysts and trophozoites during the period when 

 they are outside of the body. It also depends on the behavior of the host rather than 

 on the activity of the parasite, since the latter plays no part in its own distribution but 

 reaches the drinking water or food of man because of man's own carelessness in dis- 

 posing of infected fecal material. Another point that is of some interest is that 

 protozoa do not increase in numbers while outside of the body of the host although 

 several of them are not infective when passed but develop to the infective stage within 

 a few days. 



PARAS ITOLOGICAL AND CLINICAL PERIODS 



In any discussion of host-parasite relations it is necessary to distinguish between 

 parasitological and clinical periods. The clinical periods are well known. The incu- 

 bation period extends from the time of the entrance of the parasite until symptoms 

 appear. This is followed by the period of symptoms, and these two periods together 

 constitute what may be called the primary attack. The primary attack is often fol- 

 lowed by a latent period which may change to a second period of symptoms which is 

 known as a "relapse." The parasitological periods may be called "prepatent," 

 "patent," and "subpatent." The prepatent period extends from the introduction of 

 the parasites until these organisms can be recovered from the host by a given labora- 

 tory procedure. This period is usually shorter than the incubation period, but in at 

 least one case, that of Isospora kominis, it is longer. The patent period, during which 

 parasites can be recovered, follows. Although the parasites cannot be found after the 

 patent period, they may still be present in the body in small numbers; this may be 

 called the "subpatent period." An increase in the number of parasites may again 

 inaugurate a patent period which brings about a relapse. 



DISTRIBUTION AND LOCALIZATION OF PARASITES WITHIN THE HOST 



The protozoa of the mouth, vagina, and urinary tract that are transmitted by 

 direct contact find themselves in their definitive locations immediately. The other 

 intestinal protozoa are ingested by the host in water or food and passively carried by 

 the host into the stomach. From the stomach they escape into the small intestine 

 where Giardia and Isospora remain, the others being carried by peristalsis through the 

 small intestine and into the large intestine. It is worth noting that this distribution is 

 largely brought about by the host since the parasite, which is in the cyst stage, is in- 

 active during this time. Why Giardia and Isospora remain in the small intestine is 

 not definitely known although various suggestions to account for the primary site 

 of infection of these forms have been made. 



NATURAL RESISTANCE OF HOST AND PARASITE 



Conditions within the digestive tract of man that are unfavorable for the existence 

 of protozoa may be considered to constitute the natural resistance of the host. No 



