Natural Infection. 517 



results in 37 guinea pigs that were made to inhale dust produced from 

 sputum which had dried in the dark. A similar experiment with 11 

 rabbits gave negative results, while 36 guinea pigs and 8 rabbits which 

 were forced to inhale moist material in the form of a fine Spray, all 

 acquired tuberculosis. Analogous experiments by Peterson gave similar 

 results, while investigations by Koehlisch showed that in inhalation of 

 a dry spray (dust) at least 50,000 bacilli must be inhaled in order to 

 produce tuberculosis. 



The floor, food stuffs and air are liable to be infected wdth 

 bacilli particularly in dark, poorly kept and crowded stables 

 wdien tlie air is saturated with moisture which retards the 

 precipitation of the droplets expelled by coughing'. (Ostertag 

 found that the air of a cattle stable contained a relative humidity 

 of 80 to 95%.) The danger of infection in such cases is in 

 direct proportion to the number of animals that are housed 

 together, because the larger the herd the greater the probability 

 that diseased animals are in it. The generally observed fact 

 that large herds of cattle are rarely free from tuberculosis 

 is in accordance with this statement as is also the fact that a 

 much larger percentage of small herds is found free from 

 tuberculosis. And this is true in the face of the further fact 

 that the sanitary arrangements, care, etc., of large stables is 

 usually much better than that of smaller stables (see p. 503). 



In the open air the danger of infection is in general very 

 slight. The bacilli which are expelled with the expired air 

 of diseased animals are rapidly disseminated and are destroyed 

 in a short time after reaching the ground by the action of 

 sunlight. Accordingly tuberculosis is almost unknown among 

 animals that are continually on pasture. The disease spreads 

 much more slowly in stal)ied animals if they are pastured a 

 greater portion of the time. 



Infection through the act of copulation may occur wdien 

 the generative organs of either the male or the female are 

 diseased, in males especially the epididymis causing infection 

 of the seminal fluid, in females a tuberculous uterus causing 

 infection of the vaginal secretion. 



Tuberculosis of the uterus may be produced in guinea pigs 

 by injection of cultures into the intact vagina. Observations, 

 though rare, confirm the possilnlity of this mode of infection 

 in domestic animals. Thus Franck observed tuberculosis of 

 the vagina in several cows that had been served by a bull with 

 a tuberculous penis. Similar observations are recorded by 

 Zippelius, Haarstick, Eber. Goerig and Schmidt record the 

 development of a primary tuberculosis of the epidid^miis follow- 

 ing copulation with infected cows. 



The primary infection of the udder through the milk ducts 

 can occur by the virus whicli is present on soiled litter gaining 

 entrance into the milk cisterns through the milk ducts Avhere 

 it will propagate and disseminate (Bang, Modius, Schmidt- 

 Miihlheim, Conte, Alessandro). 



