CROSS-INFECTION EXPERIMENTS 91 



cecum to warrant the assumption that an animal is actually nega- 

 tive when found to be negative by this method. 



(e) Treatment method followed by repeated examination over 

 extended periods. This method may be resorted to in extreme cases 

 when negative animals are not obtainable. It, however, has certain 

 drawbacks, first the difficulty in assuring a complete cure of the 

 animal, there being the possibility that an infection thought to be 

 cured will flare up again and second the possibility that animals 

 once infected have established at least a partial immunity to a 

 second infection. Negative experiments when animals of this type 

 are employed would not afford final evidence that parasites were 

 not transferable. Further, the importance of using young animals 

 in cross-infection experiments should be pointed out. Older animals 

 are, as a rule, more difficult to infect and often appear to exhibit 

 an immunity to infection. 



Material collected in the above ways should be subjected to ex- 

 amination by the following methods : 



(a) Preliminary examination of smears in Donaldson's iodine- 



eosin stain. 



(b) Examination of smears permanently stained in Heiden- 



hain's iron hematoxylin. 



(c) Study of cultures made in L.E.S. or R.E.S. medium of 



Boeck and Drbohlav, or suitable modifications of the 

 same. 



(d) Cultures for bacteria should also be made to rule out the 



presence of pathogenic bacteria when the question of 

 pathogenicity is involved. 



II. Breeding of laboratory stock from parasite-free parents. A 

 negative pregnant female may be selected and the young kept under 

 conditions which render contamination impossible. In order to in- 

 sure definitely that the mother was negative she may be sacrificed 

 when the young are old enough to wean, and her intestinal contents 

 examined carefully by smear and cultural methods. 



At this point the necessity of isolation rooms for each type of 

 experiment should be mentioned. These should be so constructed 

 that they are insect proof. It seems superfluous to mention the 

 necessity of care in restricting utensils to individual cages, in feed- 

 ing uncontaminated food and in washing hands when passing from 

 one isolation room to another. It is safest to have only one type of 



