28 HUMAN AND ANIMAL WASTES 



population or because of their actual destruction by the latter. Although 

 the pathogens seem to possess considerable resistance toward unfavor- 

 able soil conditions, they are unable to multiply at rates permitting 

 their indefinite survival in the soil. The anthrax bacillus and certain 

 other parasites infesting domesticated and wild animals belong to this 

 group. Certain insect and animal carriers make possible the survival 

 and spread of many pathogens in the soil. 



The great majority of disease-producing bacteria, however, are able 

 to survive only for very brief periods outside their respective hosts, 

 especially in soil and water. It is sufficient to cite the fact that typhoid 

 and dysentery bacteria, which are known to contaminate watersheds 

 and water supplies, disappear sooner or later. It has been estimated, for 

 example, that in sewage sludge free to undergo normal digestion, 

 typhoid bacteria probably survive for less than 7 days. It was sug- 

 gested, therefore, that sludge held in a digestion tank for about 10 days 

 might be applied to the soil for fertilizer purposes without detriment to 

 public health. 



The gram-negative bacteria of the typhoid-dysentery group die out 

 rapidly in septic material; the typhoid bacteria survive for about 5 

 days, the Flexner type of dysentery for about 3 days, and the Shiga 

 bacillus dies out even in a shorter period. If decomposition in the tank 

 has not advanced far enough, as shown by low alkalinity, the organisms 

 may survive for a much longer period. The efficiency of ripe tank ef- 

 fluent to destroy bacteria is believed to be due to both the alkaline re- 

 action and the presence of antagonistic metabolic products. The destruc- 

 tion of typhoid and dysentery bacteria in the soil depends on a number 

 of factors, chief among which are the moisture content and reaction, 

 and the nature and abundance of the microbiological population. In 

 moist or dry soils, most of the pathogenic bacteria were found to die 

 within 10 days (510). 



Numerous other pathogenic agents, including those causing some of 

 the most deadly human and animal scourges — tuberculosis, leprosy, 

 diphtheria, pneumonia, bubonic plague, cholera, influenza, mastitis and 

 abortion in cattle, the many poxes — constantly find their way into the 

 soil in large numbers. They disappear sooner or later, and no one now 



