250 MANUAL OF MICROBIOLOGICAL METHODS 



sone, X ray, vitamin-deficiency states, artificial selection, and other 

 methods may be used to alter the natural susceptibility. It seems advis- 

 able to confine this treatise to normal animals and to suggest that those 

 interested in special methods of altering resistance consult the original 

 articles. 



The choice of the experimental animal depends on many factors. 

 Usually mice, guinea pigs, rabbits, monkeys, and chickens offer a suffi- 

 cient species spread for propagating many viruses and rickettsiae. Gen- 

 erally speaking, young animals are more susceptible than older animals. 

 SuckUng animals are necessary for propagating some viral agents, and 

 pregnancy may alter resistance to certain viral infections. The expense 

 in procuring the animals and in feeding and caring for them as well as the 

 available space for their housing may limit the proposed small list even 

 further. Since a discussion on the use of laboratory animals appears in 

 Chap. X, the material covered there will not be repeated. Working with 

 viruses and rickettsiae, however, does require some considerations that, 

 for obvious reasons, do not appear there. 



Natural inapparent infections. The necessity of obtaining animals 

 which are not carrying certain viruses in their tissues cannot be over- 

 emphasized. Animals plagued with any other disease are undesirable, 

 but bacterial, parasitic, and fungal diseases among animals are usually 

 quite apparent if one looks for them. A herd of mice carrying lympho- 

 cytic choriomeningitis or mouse pneumonitis viruses as well as other viral 

 agents may appear healthy. It becomes very important therefore to 

 attempt to provoke the inapparent state to become the obvious one and 

 to check periodically for neutralizing antibodies against viruses causing 

 any natural infections in a colony. Inapparent infections sometimes 

 may be made apparent by serial passage of tissue from a suspected site of 

 such an infection. For example, brains removed from several sacrificed 

 mice are pooled and ground to a 10 per cent suspension. This suspension 

 is inoculated intracranially into several mice. After an appropriate 

 incubation period has been allowed, which will vary with the disease 

 suspected, the inoculated mice are sacrificed and a suspension of their 

 brains passed on to additional mice. Some inapparent infections become 

 overt only after six or more passages. The same general procedure is 

 followed in passing lung tissue, except the inoculations are made intra- 

 nasally. The regular inoculation of control animals under identical con- 

 ditions of the experimental ones, minus the infectious agent in the 

 inoculum, is a procedure which must be followed religiously. 



Market vs. local animals. If the investigator is procuring his mice 

 from one breeder, the state of health may be fairly constant. If a jobber 

 is involved who secures mice from a number of breeders, the chance of 

 running into trouble is proportionately greater. If it is feasible, main- 



