SAPROPHYTIC AND PATHOGENIC MICROORGANISMS 17 



same form. The causative agent of leprosy, like certain pathogenic 

 actinomycetes, is believed to be a facultatively parasitic soil organism, 

 probably of wide but irregular distribution. Leprosy was thus looked 

 upon primarily as a soil infection, brought about presumably through 

 wounds i a secondary means of infection by contagion was not excluded. 

 A comparison of cultures obtained from rat leprosy, human leprosy, 

 and bacteria of soil origin led to the conclusion that the strains from all 

 three sources were identical j human and rat leprosy were said to have 

 the same etiology and endemiology, finding a normal habitat in the soil. 



An interesting relationship has been shown to exist between Texas 

 fever and the capacity of cattle tick {Boofhilus bovis)y the parasite car- 

 rier, to persist in the soil (865). The causative agent is an organism 

 with protozoan characteristics. It persists in southern pastures where 

 the carriers survive from one season to the next and keep the cattle con- 

 tinuously infected. The disease is of little importance in northern re- 

 gions, the ticks being destroyed during the winter. When northern cat- 

 tle are moved to southern pastures, they become subject to the disease. 



Pathogenic microorganisms capable of surviving in the soil have pre- 

 sented important economic problems to farmers raising hogs, cattle, 

 poultry, and other domestic animals, but disease incidence through this 

 source has been greatly diminished by the proper practice of sanitation. 

 The rotation of crops has been utilized for the purpose of overcoming 

 these conditions, several years usually being required to render infected 

 pastures safe for use. The fact that most pathogenic organisms rapidly 

 disappear when added to the soil makes this problem rather simple j the 

 prevention of infectious diseases would have presented far more diffi- 

 cult problems were the infecting agents to remain indefinitely virulent 

 in the soil. The few disease-producing agents that are capable of per- 

 sisting, such as anthrax, blackleg, and coccidiosis, have been the cause, 

 however, of considerable damage to animals. 



Of greater economic importance than the survival in the soil of hu- 

 man and animal pathogenic agents is the fact that the soil harbors a 

 number of plant pathogens, including not only fungi, bacteria, and 

 actinomycetes, but also nematodes and insects. Fortunately, the con- 

 tinued development of these organisms in the soil also leads to the ac- 

 cumulation of saprophytic organisms destructive to them. 



