50 OCCURRENCE OF PATHOGENIC BACTERIA IN NATURE 



Cryptogenetic Origin. Pathogenic bacteria may first invade the 

 body through wounds, by inhalation, or by ingestion, and be taken 

 up by the blood or lymph current. They may then be deposited in the 

 lymph glands and later carried to other places, where they multiply 

 considerably to produce their most important pathologic lesions and 

 changes. This happens in glanders where the important lesions are 

 found far from the original place of entrance, and it occurs also in 

 such diseases as pyelonephritis bacillosa bovis and necrotic liver 

 abscesses in cattle due to the Bacillus necrophorus. Pus-producing 

 microorganisms which sometimes produce very insignificant lesions 

 at the portal of entrance are carried on and in another place produce 

 profound purulent lesions. This occurs in bone abscesses (osteo- 

 myelitis) and in abscesses of the ovary. In fact, in many cases of 

 this kind all trace of the small portal of entrance is lost. This is 

 known as an infection of cryptogenetic origin. 



Auto-infection. Pathogenic microorganisms may enter through 

 the respiratory or gastro-intestinal tract, and may not produce an 

 infection at all. They may simply remain on the surface of the mucous 

 membranes without penetrating the tissues. In this manner they may 

 remain latent for a time, and then, when circumstances favor them, 

 due to the animal becoming debilitated by overwork, poor food, 

 chilling, etc., break into the tissue, there multiply and produce their 

 specific infectious disease. Sometimes bacteria like the colon bacillus 

 which live in the gastro-intestinal tract as harmless commensales 

 may acquire pathogenic properties, invade the wall of the intestine, 

 the peritoneum, the liver, etc., and cause an infectious disease with 

 all of its stages and features. Such an infection which comes from 

 within, that is, one that has no well-established outside source, is 

 called an auto-infection. 



Placental Circulation. Bacteria may enter the embryo in utero 

 through the placental circulation. This is a comparatively rare 

 occurrence (as far as bacterial infections are concerned), since the 

 maternal and fetal circulations are separate, and intact villi will 

 generally not permit the passage of bacteria from mother to offspring. 

 However, calf embryos are frequently infected with tuberculosis from 

 the mother through the placental circulation, and such infection in 

 human syphilis is quite common. 



Insects. Biting insects and other arthropods are responsible for 

 a particular kind of wound infection. Sometimes these insects which 

 have previously fed on infected material may spread an ordinary wound 

 infection. Flies which have fed on contaminated, material or cadavers 

 may also spread disease by subsequent feeding on material destined 

 as food for man or domestic animals. While these occurrences are 

 by no means rare, they are what may be called accidental in their 

 nature. There are, however, some insects which act as intermediary 

 hosts for certain pathogenic microorganisms, and, therefore, are par 



