CLINICAL AND PARASITOLOGICAL PERIODS 



(1) PARASITOLOGICAL PERIODS. (See Fig. I9.) 



(a) The Prepatent Period * extends from the time 

 the infective parasites enter the body of the host until 

 their offspring can be recovered by specified laboratory 

 methods. The length of this period obviously depends 

 to some extent on the character of the laboratory tech- 

 nique employed. 



(b) The Patent Period covers the interval during 

 which the parasites can be demonstrated by the tech- 

 nique employed. The parasite number undergoes a Rise 

 during this period, reaches a Peak and then suffers a 

 Fall. The patent period ends when the parasites can no 

 longer be found in the feces. 



(c) In many infections the patent period is followed 

 by a Subpatent Period of indefinite length. During this 

 period parasites can not be recovered by the usual labo- 

 ratory methods but their presence can be proved in vari- 

 ous ways depending on the species of parasite. For ex- 

 ample, protozoan cysts may disappear from the stools 

 but reappear after a few days, weeks or months have 

 elapsed. 



The subpatent period may be followed by a second 

 patent period during which the parasite number rises, 

 reaches a peak, and falls, but often does not rise as high 

 as in the primary attack. 



(2) Clinical periods. (See Fig. 19.) 



(a) The Incubation Period extends from the time of 

 the entrance of the parasites until symptoms appear. 

 This period is usually longer than the prepatent period, 

 but may be shorter. For example, in man infected with 



* The terms prepatent, patent and subpatent were first suggested by Dr. 

 Justin Andrews. The word patent is derived from the latin word patens 

 meaning evident, apparent, unconcealed. 



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