92 RESEARCH IN PROTOZOOLOGY 



experiment in progress at one time and thus minimize the danger 

 of contamination from other sources. 



In summarizing it may be stated that negative animals can be 

 selected only by employing the greatest care and that a combina- 

 tion of the above-mentioned methods is often desirable. The impor- 

 tance of building up a stock of negative animals for experimental 

 purposes is recommended. 



B. Methods of Infection 



After negative animals have been determined the question at 

 once arises as to the best methods of infecting the animals. This 

 W\\\ depend on the animal in question but should be accomplished 

 by the most natural procedure. When one is endeavoring merely 

 to establish infection, the maximum effort may be made. Since the 

 viability and infectivity of the protozoa in question are important 

 factors and since we are not sure of the maximum conditions for 

 the same, i.e., w^hether encysted protozoa require a certain incuba- 

 tion period outside the body before they v^ill reach their maximum 

 infectivity to other animals, it is advisable to collect ample amounts 

 of feces containing cysts, feed some as soon as possible after the 

 collection is made and allow the remainder to stand at room tem- 

 perature and feed more of the material at intervals of one, two, 

 or three days for a week. If no particular desire is present to estab- 

 lish an infection from a single case, protozoa from several cases 

 may be mixed and fed. 



The simplest and most natural method of infection is to mix 

 the infective material with the food of the animal to be infected. 

 When several animals are present in a single cage, however, it is 

 not always possible to be sure that all animals will acquire a uni- 

 form amount of the material to be infected. Also, if spread upon 

 or mixed with food some of the cysts may be killed by drying 

 before being ingested. Consequently the writer in most cases has 

 adopted the procedure of giving to individual animals by a Luer 

 syringe to which a rubber catheter is attached, uniform amounts 

 of dilute material previously passed through a wire screen to re- 

 move coarse debris. It takes two people to perform a satisfactory 

 feeding by this method. The animal must be held firmly and the 

 person holding it must grip the jaw firmly with thumb and index 

 finger of one hand to hold upper and lower jaws of animal apart 

 and keep the teeth from cutting the catheter. The catheter should 



