28 HUMAN AND ANIMAL WASTES 



may survive there and thus become a constant and important source of 

 infection. The introduction by Koch, in 1881, of the gelatin plate 

 method placed in the hands of the investigator a convenient procedure 

 for measuring the abundance of the soil population and determining the 

 survival in the soil of agents causing serious human diseases. In spite of 

 the fact that this method revealed only a very small part of the soil 

 population, it enabled the medical bacteriologist to establish beyond 

 doubt that such organisms tend to disappear in the soil. This resulted in 

 definite conviction on the part of the public health and medical world 

 that the soil is seldom a source of infection. It was soon demonstrated 

 that disease-producing agents die out in the soil at a rather rapid rate, 

 depending on the nature of the organisms, the soil, climate, and other 

 conditions. 



Organisms that Survive for Long Periods 



Only a few disease-producing microorganisms are able to survive in 

 the soil for any considerable periods of time. These few include the or- 

 ganisms causing tetanus, gas gangrene, anthrax, certain skin infections, 

 actinomycosis in cattle, coccidiosis in poultry, hookworm infections, 

 trichinosis, enteric disorders in man, blackleg in cattle, and Texas fever. 

 To these may be added the botulinus organism and others producing 

 toxic substances, as well as bacteria, actinomycetes, and fungi that cause 

 plant diseases such as potato scab, root rots, take-all disease of cereals, 

 and damping-off diseases. 



Anthrax, a scourge of cattle and sheep, is a persistent survivor in 

 soil j spores of this organism are known to retain their vitality and viru- 

 lence for fifteen years. Anthrax survives particularly well in damp re- 

 gions, especially in soils rich in decomposing organic matter 5 the hay 

 and feed from these lands may transmit the disease to animals. The fact 

 that certain fields carry anthrax infection was recognized in Europe long 

 before the nature of the disease was known. Human infection results 

 from contact with diseased animals or animal products. 



The anaerobic, spore-forming bacteria that cause gas gangrene are 

 widely distributed in nature. They are found extensively in soils and in 

 decomposing plant and animal residues. The causation of disease by 



