120 PRACTICAL PARASITOLOGY 



different treatment to those which demand the agency of one or more 

 intermediate hosts before attaining sexual maturity. In the manage- 

 ment of all such experiments, however, there are certain precautionary 

 measures which must be observed. It is of paramount importance to 

 make sure that the animal to be used as a host is free from Helminthes ; 

 or, at any rate, from those which form the subject of the experiment. 

 Repeated microscopic tests of excreted material will show whether 

 eggs are present or not ; but the absence of eggs from the excreta is 

 by no means a certain proof of the absence of Helminthes from the 

 excretory organs. It may happen that the animal harbours parasites 

 which have not attained sexual maturity and which do not, as yet, 

 deposit eggs. Where infection is proved, it is rarely possible to free 

 the animal from worms, and, in any case, anthelminthic measures 

 could only take effect upon worms present in the intestine. Again, 

 the employment of young animals and sucklings does not guarantee 

 more than freedom from parasites, which might have been present in 

 media to which the animal has been denied access. It does not safe- 

 guard against infection by swallowing insects or other small animals, 

 or against infection with developmental stages (eggs containing 

 embryos, larvae) present in the excreta of the mother. Suckling 

 kittens and puppies, for instance, frequently harbour Dipylidium 

 caninum, transmitted by ectoparasitic insects, and Ascaris canis, the 

 eggs of which they have swallowed. 



The only possible method is to keep the animal isolated under 

 conditions of the most scrupulous cleanliness, and to feed it upon 

 perfectly untainted food. This, however, is more easily said than 

 done. Better results will be obtained if several animals are used for 

 experiment at the same time and examined at regular intervals ; or 

 a single individual may be treated with infective material over either 

 a prolonged period or several short ones, the examination being made 

 soon after the last infection. It is possible by these means to obtain 

 the parasite in its successive developmental stages, from the early 

 stage used as the agent of infection to the final adult worm. If, 

 during the entire course of the experiment, the possibility of infection 

 by other means is carefully guarded against, these results should be 

 conclusive. It is not always easy to prevent spontaneous infection, 

 but every precaution should be taken. The animals should be housed 

 singly in cages which are easily cleaned out, and which must be 

 scrupulously kept. Their food and drink must be above suspicion. 

 The domestic animals are most easily managed, especially those which 

 have been reared in captivity through several generations. Unfor- 

 tunately, however, their uses are limited, as they can be used for 

 experiment only when they happen to be the right host for the 

 particular species of parasite under investigation. 



