<: ir \ I' I i; i{ 



I 7 



Causation of Animal Diseases 



SapropIiN lisin and Parasitism 



The suh.strate on which miciohcs normally 

 live ha?^ fro(iuently been used as a basis for 

 classification and differentiation of these or- 

 ganisms. The normal existence of an oi'gan- 

 ism on dead organic and inorganic r(\sidues 

 has come to indicate its saprophytic nature. 

 Parasitism has come to indicate the normal 

 existence of an organism on living bodies of 

 higher plants and animals and of microor- 

 ganisms. On the basis of their ability to live 

 exclusively or electively on living substrates, 

 some of the parasites are classed either as 

 obligate or as facultatixe. 



A parasite may also be \irulent, if it has 

 the capacity to infect a living organism. Vir- 

 ulence varies greatly in nature and intensity, 

 depending not only upon the species of the 

 infecting organism, but also upon the strain 

 and its pre\'ious history, as well as upon the 

 nature of the host. The mode of infection, 

 the ability of the parasite to spread through 

 the various tissues of the host, its toxic mani- 

 festations, the degree of communicability, all 

 contribute to the intensity of virulence. 



An organism may be made to increase or 

 decrease its \'irulence by serial animal pas- 

 sage or by growth of the culture under sap- 

 rophytic conditions. Phenomena of dissocia- 

 tion in the culture and the development of 

 resistance to a particular treatment also con- 

 tribute to the degree of its virulence. Often 

 such changes are accompanied by a change 

 in the morphology of the organism or in its 

 immunological properties. 



The ab()\(' considerations ha\'e a particu- 

 lar application to the analysis of the patho- 

 genic properties of actinomycetes. The most 

 important and most highly significant com- 

 ment to b(> made in this connection is that 

 although actinomycetes are abundant and 

 widely distributed in nature, they are able 

 to cause only very few human and animal 

 diseases. On the contrary, many actinomy- 

 cetes are tible to produce antibiotic sub- 

 stances which have found extensive applica- 

 tion in the treatment of such diseases, 

 especially those caused by bacteria. 



Actinomycetes as Causative Agents of 

 Disease 



Although actinomycetes have been iso- 

 lated from various organs and excretions of 

 diseased human and animal bodies, they 

 have not always been the causes of the dis- 

 eases. At present, there are two major dis- 

 eases with which actinomycetes are usually 

 associated. One is caused by anaerobic or- 

 ganisms and is known as actinomycosis. The 

 other is caused by aerobic organisms and is 

 known as nocardiosis. Various other syno- 

 nyms, such as streptothricosis and madura- 

 mycosis were once used to designate these or 

 similar diseases, but these terms were grad- 

 ually discarded (Foulerton, 1910). 



Cope insisted upon adopting the generic 

 name Actinomijces for the "whole group of 

 organisms" and actinomycosis for the dis- 

 ease caused l)y them, since the name "is 



251 



