444 CHRONIC INFECTIOUS DISEASES 



the tubercle bacilli morphologically and in staining properties, 

 that a differentiation with the microscope is impossible. 

 (See Bacteriology.) 



C. Diagnostic Inoculations. — The inoculation of experi- 

 mental animals is always advisable where the microscopic 

 examination has been indecisive. The guinea pig is usually 

 chosen, as it is very susceptible to tuberculosis. In practice, 

 negative results in this animal speak for the absence of 

 tubercle bacilli in the suspected material injected. Positive 

 results are recognized by the formation of a true tubercle 

 which appears not only at the point of injection but spreads 

 to the neighboring lymph glands and internal organs (liver, 

 spleen, lungs). Acid-fast, paratubercle bacilli, on the other 

 hand, induce in guinea pigs merely a local lesion at the 

 point of injection which does not tend to spread from the 

 primary focus. The inoculations may be made subcutane- 

 ously, intramuscularly, intramammarily (in nursing females), 

 and intraperitoneally. (See Bacteriology.) 



Course. — The course of tuberculosis in animals is chronic. 

 An infected calf may show no clinical symptoms until it has 

 reached maturity or even old age. Too frequently advanced 

 tuberculosis which has led to the near destruction of 

 important organs (lungs, liver) or caused great areas of the 

 pleura or peritoneum to be invaded, is first discovered in the 

 slaughter house. Only in the last stages of the disease, and 

 then usually due to the invasion of secondary organisms 

 (pus cocci), is it possible by the customary methods of 

 physical examination to recognize the disease during life. 

 A sudden generalization of the disease which may follow 

 parturition, an attack of some acute disease, exposure and 

 privation is sometimes noted. It may lead to death in a 

 few weeks. As a rule, however, for months or years following 

 infection the tubercular ox seems in good health. Even cases 

 of open tuberculosis may appear in normal health and 

 condition. When clinical symptoms finally appear their 

 development is slow and the decline of the patient gradual. 



Tuberculosis at first spreads slowly through a herd, but 

 with each new victim another source of infection is supplied 

 and the spread becomes more rapid. Finally, but usually 



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