PATHOGENICITY AND EXPERIMENTAL INFECTION OF ANIMALS 425 



intraperitoneally may likewise prove fatal on occasion. Extensive lesions in the lungs 

 and kidneys are often seen both after intravenous and intraperitoneal injection (see Cobbett 

 1932). The rabbit, in fact, possesses a considerable resistance to the human type of 

 bacillus, but not so great as the ox or the goat. Rabbits are also fairly resistant to infection 

 with murine bacilli. Intravenous inoculation of 01 to 10 mgm. may cause death from 

 acut^ miliary tuberculosis. Smaller doses, 0-01 mgm. intravenously, or larger doses 

 subcutaneously, produce trivial lesions and the bacilli gradually die out ; an abscess 

 in the lung, however, may provide a nidus in which the bacilli may live for a long time. 

 The avian bacillus is less virulent for rabbits than the bovine, but more virulent than the 

 human bacillus. Subcutaneous injection gives rise to a chronic disease ; intraperitoneal 

 injection of large doses to a rapidly fatal peritonitis ; and intravenous injection to an 

 acutely fatal infection. The macroscopic lesions caused by the avian bacillus are much 

 less obvious tlian those caused by the bovine bacillus ; sometimes no tubercles are visible. 

 After intravenous inoculation Yersin (1888) found the spleen greatly enlarged and the 

 liver enlarged to a less extent ; no macrosco])ic tubercles were seen, but microscopically 

 there were numerous small tuberculous nodules in the liver and spleen containing large 

 numbers of tubercle bacilli ; the kidneys and lungs appeared practically normal. This 

 proliferation of the bacilli in the body without macroscopic tubercle formation is known 

 as the Yersin type of disease ; it is seen both in rabbits and guinea-pigs injected with avian 

 bacilli, and in rats injected with both mammalian and avian types. Chronic avian infec- 

 tions are generally characterized by infection of the joints (Griffith 1941c). 



After injection of the bovine or human type of bacillus, no matter what route is chosen, 

 the lesions in rabbits are most evident in the lungs and kidneys ; the spleen and liver 

 suffer less ; the lymphatic glands, with the exception of the regional glands, hardly at all. 

 Not infrequently the joints, mammary glands, and testes show lesions. For differentiating 

 between bovine and human baciUi in the rabbit, the best doses to employ are 10 mgm. 

 subcutaneously, 1 mgm. intraperitoneally, and 0-01 mgm. intravenously (Cobbett 1917). 



Feeding the rabbit with 1-10 mgm. of bovine baciUi sets up a disease that proves fatal 

 in 2 to 3 months. 



Guinea-pigs. — The guinea-pig is highly susceptible to experimental infection with 

 bacilli of the human and bovine type. Indeed it has been stated that the subcutaneous 

 injection of a single tubercle bacillus may succeed in producing disease — though only 

 of a slow type showing little tendency to generalization (Wamoscher and Stoeckhn 1927, 

 Doerr and Gold 1932). Less than 10 Uving bacilh, even of virulent strains, cannot be 

 relied upon to cause disease in every animal (Schwabacher and Wilson 1937). With 

 between 10 and 100 bacilU intramuscular inoculation into the thigh usually sets up a 

 slowly progressive disease, often characterized in the later stages by heahng of the initial 

 lesions. The bovine bacillus is rather more virulent than the human bacillus ; this can 

 be demonstrated, however, only by a series of comparative experiments in which carefully 

 measured doses are introduced. Table 26, from Griffith, quoted by Cobbett (1917), who 

 gives an excellent review of the pathogenicity of tubercle bacilli to animals, illustrates this. 



TABLE 26 



Showing greater Virulence of Bovine than Human Bacilli to Guinea-pigs. 



