24 METHODS OF EXAMINING FECES FOR PARASITISM. 



contain no ova. Bass suggested the possibility of old age as an 

 explanation of this. Dock and Bass (1910) state that they found 

 9.25 per cent of 397 female A 7 , americanus barren in cases where the 

 infection was of long standing, while only one-third of 1 per cent 

 of 120 female hookworms from the dog were without eggs. They 

 state, however, that Dr. Stiles does not think that hookworms 1'ive 

 long after old age induces cessation of ovulation. 



(4) A thorough purging of the intestinal tract may remove an 

 accumulation of parasite eggs, with the result that numbers of eggs 

 will be found at this time and none a day or two later. Dock and 

 Bass (1910) comment favorably on Ashford and King's statement 

 that hookworms cause an increase of mucus at the site of their feeding 

 ground, and as this mucus often comes off en masse and contains 

 most of the eggs the actual feces would contain relatively few. They 

 also note that Ashford and King say that eggs are more difficult to 

 find in diarrheal stools, and that they saw cases of heavy infection 

 that at times had no ova in the feces. Dock and Bass (1910) call 

 attention to the fact that the amount of feces varies with different 

 persons and from time to time in the same person. They also state 

 that it is a common experience to find no eggs in the feces within a 

 few r days after a course of thymol and subsequently to find many 

 eggs. They say that Dieminger found that the number of eggs was 

 very much diminished when patients were drinking hard. 



It follows from the above that little weight can be laid on a col- 

 lection index. In the writer's experience, consecutive and careful 

 examinations of the feces of a given animal show days when eggs 

 of various kinds are abundant and days when they are scarce or 

 missing, and the collection index for the various days would present 

 a striking disagreement. This would be particularly true of cestodes. 



Negative examinations must be considered doubtful and must be 



o 



checked at intervals if infection is suspected. 



TESTS OF UNPUBLISHED METHODS. 



Numerous tests of unpublished modifications of technique were 

 made in connection with the development of the writer's method. 

 Among these was an attempt to increase the specific gravity of para- 

 sites and their eggs by adding mercuric chlorid before centrifuging. 

 Comparative tests did not indicate that this method was of value, and 

 any adaptation of technique which would make it so would probably 

 not repay in results what it required in time. Tentative attempts to 

 use a differential stain were unsatisfactory. In one of these, sul- 

 phuric acid and iodin were added to slide preparations to distinguish 

 cellulose substances which might otherwise have to be examined as 



