558 PATHOGENIC MICROORGANISMS 



Pathogenicity. — In man, plague is acquired^ by entrance of the bacil- 

 lus either through the skin or by the respiratory tract. The period of 

 incubation is about three to seven days. Two distinct chnical types 

 of the disease occur, depending upon the mode of infection. When 

 cutaneous infection has occurred the disease is first locahzed in the 

 lymph nodes nearest the point of inoculation. If the respiratory tract 

 has been the portal of entrance the disease primarily takes the form of a 

 pneumonia. 



Infection may take place through the most minute lesions, hardly 

 visible to the naked eye. Even the unbroken skin may admit the 

 microorganisms if these are rubbed in with sufficient energy. From the 

 primary lymphatic swelUngs, the bacilli enter the blood and may pro- 

 duce secondary foci. 



The pneumonic form of plague usually begins with sjonptoms not 

 unlike a typical pneumonia and is usually fatal within four or five or 

 even fewer days. This form of the disease is especially menacing as a 

 means of dissemination, because of the enormous numbers of plague 

 bacilli in the sputimi. 



One of the chief characteristics of the general systemic plague infec- 

 tion is the very marked cardiac depression. 



The bacteriological diagnosis during life may be made by finding the 

 bacilli in the sputum or in aspiration fluid from a bubo. The micro- 

 organisms are identified morphologically, culturally, by animal experi- 

 ment, and by agglutination reaction. Blood cultures from plague pa- 

 tients often yield positive results, especially when the blood is well 

 diluted in neutral broth to prevent any inhibiting action of the anti- 

 bodies in the serum. 



At autopsy, in man, the bacilli are found in the primary lesions, in 

 the blood, and in the spleen, the liver, and the lymphatics. There may 

 be hemorrhages into the serous cavities. When pneumonia exists, it 

 usually takes the form of a bronchopneumonia with extensive swelling 

 of the bronchial lymph nodes. 



In cases in which the disease is prolonged, there are often tubercle- 

 hke foci in the spleen, liver, and lungs. Histologically these foci show 

 central necrosis surrounded by the usual inflammatory cell reactions. 

 In more chronic cases connective-tissue encapsulation may appear. 



Bacillus pestis is extremely pathogenic for rats, mice, guinea-pigs, 

 rabbits, and monkeys. The most susceptible of these animals are rats 



' GoUschlich, Zeit. f . Hyg., xxxv, 1900. 



