254 MANUAL OF MICROBIOLOGICAL METHODS 



viruses. Since this method is not covered in Chap. X, it will be described 

 briefly. In order that the progress of infection may be adequately 

 observed, it is desirable to remove any hair covering the scrotum. This 

 may be accomplished by clipping or by the use of a depilatory. The 

 scrotum of the animal should be cleansed and treated with a topical dis- 

 infectant. The inoculation is made by securing a testis between the 

 thumb and forefinger and introducing the needle to approximately the 

 center of the testis before expelling the inoculum. For small animals 

 0.05 ml and for larger ones up to 0.2 ml is customarily used. A 27-gauge 

 needle is used for small animals, and a 25-gauge for larger ones. 



Postinoculation observation. Precautions must be taken to prevent 

 cross infection among experimental animals. Such an occurrence is more 

 likely with some viral diseases than others. Suffice it to say that the 

 hands should be washed when going from one experiment to another and 

 proper precautions in regard to feed, water, and ventilation must be 

 observed to obviate the problem. Controlling aerosol hazards requires 

 considerable vigilance (Smadel, 1951; Reitman et aL, 1954). 



After having inoculated the animals by whatever method, you should 

 observe them daily under good light after they have settled down from 

 the initial disturbance of your first presence in the room. Nocturnal 

 animals tend to appear drowsy during the daytime and by the novice may 

 be pronounced ill. Respiration may appear labored during this time. 

 Only by observing normal experimental animals over a period of time 

 can one readily select the initially ill from the healthy. Daily observa- 

 tion is required for some viral and rickettsial infections in laboratory 

 animals because the signs are transient and would be overlooked other- 

 wise. In this regard it is particularly important to take daily rectal 

 temperatures on some animals, e.g., guinea pigs used for rickettsial study. 

 Guinea-pig temperatures vary considerably, but a fever may be con- 

 sidered present if the temperature definitely exceeds 39.6°C. 



A certain amount of differentiation among certain rickettsial diseases 

 is possible by careful examination of the testes after intraabdominal 

 inoculation. The testes and scrotum are examined daily for evidences of 

 edema, redness, necrosis, and the reducibility of the testes into the 

 abdomen. The time these changes make their appearance should be 

 recorded. 



Some viruses can produce encephalitis or paralysis following intra- 

 cranial inoculation. Not uncommonly a hyperkinetic state will be 

 observed preceding lethargy or paralysis. Convulsions can be provoked 

 by rotating the mouse to and fro by twisting his tail between thumb and 

 forefinger while he is suspended in air. Tics may also develop in the 

 animal. Since mice are commonly infested with mites, they scratch 

 often. A tic is a definite repeated action pattern of involuntary nature 



