536 Host-Parasite Relationships 



The siibpatent period varies in significance. It may represent a marked 

 decrease in number of parasites as the infection is brought under control 

 prior to elimination. In tertian malaria, on the other hand, a subpatent 

 period may parallel continued development of exoerythrocytic parasites. 

 A new patent period may follow the subpatent period, and the sequence 

 may be repeated several times before the infection is terminated. 



Infections which induce the appearance of definite symptoms in the 

 host also may be characterized in terms of several clinical periods — 

 period of incubation, period of symptoms and period of convalescence. 



The incubation period, initiated by introduction of the parasites, 

 ends when symptoms are recognizable. In malignant tertian malaria, 

 symptoms may appear at or near the end^of the prepatent period. In 

 various other infections, the correlation between incubation period and 

 prepatent period is not necessarily close. Parasites are often detectable 

 some time before the appearance of symptoms. At the other extreme, 

 characteristic symptoms appear and reach a peak before the end of the 

 prepatent period in infections with Isospora hominis. 



The period of symptoms opens typically with the appearance of mild 

 (or prodromal) symptoms. As the infection progresses, the symptoms 

 become progressively more severe and more characteristic of the particu- 

 lar host-parasite association. During this phase, the parasites are pro- 

 ducing more or less specific effects, the nature of which varies with the 

 parasite. Variations in the severity of the effects produced may reflect 

 differences in resistance of the hosts and in virulence of the parasites 

 (Chapter XIV). Mechanical irritation may be caused by movements of 

 intestinal Protozoa, and tissues are destroyed by many parasites. Invasion 

 of individual cells may lead to extensive destruction of tissues — an epi- 

 thelium by Coccidia, or blood cells by malarial parasites. Tissues also 

 may be destroyed without invasion of cells, as in ulceration of the in- 

 testine by Entamoeba histolytica and Balantidium coli. Whether such 

 ulceration is brought about solely by histolytic enzymes of the parasites 

 or partly by mechanical means is uncertain. The production of toxic 

 substances has been suggested for some parasites, although specific toxins 

 have not been isolated. However, the production of a potent toxin has 

 been reported for a free-living dinoflagellate. Gonyaulax catanella appar- 

 ently is the source of the poison found occasionally in the edible Cali- 

 fornia mussel (87), and concentrates of this substance have shown a 

 toxicity of 1.65 mouse units per microgram (83). 



The period of convalescence, marked by the gradual disappearance of 

 symptoms, extends to clinical recovery of the host. In some protozoan 

 infections, apparent convalescence may be merely a period of latency 

 during which a low-grade infection persists. Latency may be interrupted 

 sooner or later by a relapse, in which symptoms reappear following re- 

 newed multiplication of the parasites. 



