CHAPTER XL 



THE USE OF ANIMALS FOR DIAGNOSTIC AND TEST PURPOSES. 



SUITABLE animals are necessarily employed for many bacteriological 

 purposes. 1. To obtain a development: Thus they may be used as a 

 soil for bacterial growth, when, as in the case of tubercle bacilli, we 

 cannot get a growth in the dead culture media. For this reason material 

 suspected to contain tubercle bacilli is injected into rabbits or guinea- 

 pigs, with the knowledge that, if present, although in too small numbers 

 to be detected by microscopic or culture methods, they will develop 

 in the animals' bodies, and thus reveal themselves. The same may be 

 true of glanders, tetanus, and anthrax bacilli, of pneumococci, of other 

 bacteria, and of protozoa. 2. To cause an increase of one variety of 

 organisms in a mixture: An injection of sputum subcutaneously in 

 rabbits may give rise to a pure pneumococctis septicaemia or a pure 

 tuberculosis. 3. To test virulence: Animals are used to test the viru- 

 lence or toxin production of organisms, where, as in the case of diph- 

 theria, we have very virulent, attenuated, and non-virulent bacilli of, 

 so far as we know, identical cultural characteristics. Here the injec- 

 tion of a susceptible animal, such as the guinea-pig, is the only way 

 that we can differentiate between those capable of producing diseases 

 from those that are harmless. Still another use of animals is to dif- 

 ferentiate between two virulent organisms, which, though entirely 

 different in their specific disease poisons, are yet so closely allied mor- 

 phologically and in culture characteristics that they cannot always be 

 separated except by studying their action in the animal body both 

 without and under the influence of specific serums upon them. In 

 this way the typhoid and colon bacilli may be separated, or the pneu- 

 mococcus and streptococcus. 4. To test the antitoxic or bactericidal 

 strength of sera: Diphtheria antitoxin in different amounts is added 

 to one hundred fatal doses of diphtheria toxin and injected into guinea- 

 pigs, and streptococcus immunizing serum is mixed with living strepto- 

 cocci and injected into the vein of a rabbit. 5. To produce antitoxic, 

 bactericidal, or agglutinating sera. 



The Inoculation of Animals. The inoculation of animals may be 

 made either through natural channels or through artificial ones: 



1. Cutaneous. Cultures are rubbed into the abraded skin. 



2. Subcutaneous. The bacteria are injected by means of a hypo- 

 dermic needle under the skin, or are introduced by a platinum loop 

 into a pocket made by an incision. 



3. Intravenous. The bacteria are injected by means of a hypo- 

 dermic needle into the vein. This is usually carried out in the ear vein 



