430 MYCOBACTERIUM 



Type Differentiation by Pathogenicity. — At the risk of repetition we give for the 

 sake of clearness the methods by which the three main types of tubercle bacilli may 

 be differentiated. 



The human and the bovine types of tubercle bacilli are able, when injected 

 subcutaneously in minute doses, to give rise to a progressive and fatal disease in 

 guinea-pigs. But whereas the bovine type is able likewise to produce a progressive, 

 generalized, and fatal disease in rabbits, cats, voles, goats, and calves, the human 

 bacillus cannot do so. This fundamental distinction can be elicited in practice only 

 by strict regard to certain factors, such as dosage and route of inoculation. The 

 experience of the Royal Commission showed that the best differentiating dose for 

 calves was 50 mgm. subcutaneously, and for rabbits 10 mgm. subcutaneously. The 

 bacilli are taken from a 1 to 3- weeks old culture on inspissated serum and weighed 

 moist. Under these conditions the bovine bacillus sets up a generalized fatal 

 disease, while the human bacillus causes at most a localized and retrogressive 

 disease, confined to the site of inoculation, and sometimes to one or more of the 

 internal organs ; often no lesions are visible at all. Animals vary so much in their 

 susceptibility to experimental inoculation that it is advisable to inject two or three 

 simultaneously, and to repeat the test on a fresh series of animals if the results are 

 not precise. The time taken to produce death is probably not of such importance 

 as the extent of the lesions at necropsy (Park and Krumwiede 1910) ; but generally 

 speaking calves inoculated with a bovine strain may be expected to die in about 

 6 to 8 weeks, and rabbits in 6 to 12 weeks. A more rapid method of differentiation 

 consists in the intravenous inoculation of rabbits with 0-001 mgm. or even less. 

 In such a dose the human bacillus has little effect : the bovine kills the animal 

 in 4 to 6 weeks, and at post mortem miliary tuberculosis is found affecting par- 

 ticularly the lungs and kidneys. According to Wells (1944), an equally satis- 

 factory and perhaps even more rapid method consists in the subcutaneous inocula- 

 tion of 1 mgm. into the Orkney vole {Microtus orcadensis). The human bacillus 

 is practically non-pathogenic to this animal, giving rise to no more than a local 

 lesion with perhaps minimal lesions in the focal lymphatic nodes and occasionally 

 a few scattered tubercles in the lungs, whereas the bovine bacillus causes generalized 

 glandular and visceral tuberculosis, proving fatal as a rule in 3 to 5 weeks. 



The separation of the avian from the mammalian bacilli is generally quite 

 straightforward. The avian bacillus is pathogenic for fowls and pigeons, but not 

 for the guinea-pig ; the mammalian bacilli, on the contrary, are pathogenic for the 

 guinea-pig, but not for fowls and pigeons. Cobbett (1917) recommends for differen- 

 tiating purposes the injection of 10 mgm. of culture subcutaneously into guinea- 

 pigs, and 10 mgm. intraperitoneally into fowls or pigeons. The mammalian bacilli 

 prove fatal to guinea-pigs in about 4 to 8 weeks, and at necropsy there is generalized 

 tuberculosis with extensive caseation of the glands and large necrotic areas in the 

 spleen and liver. The avian bacilli often give rise to no lesions at all, or only to a 

 slight local collection of caseous material ; but sometimes, if the animals are killed 

 after a few weeks, the focal glands will be found to be hypersemic, and the spleen 

 enlarged and congested ; smears of the spleen, liver, and lungs from these cases 

 may reveal a few acid-fast bacilli (Straus and Gamaleia 1891). After injection 

 with avian bacilli, fowls and pigeons die in a variable time, often considerably 

 emaciated ; if they are still alive at the end of 3 months they should be killed. 

 Disease is most evident in the spleen, liver, and kidneys, which show numerous 

 tubercles or hard caseous areas ; bacilli are abundant in smears of these organs. 



