STKEPTOTHKICOSIS. 481 



a slower growth on artificial media and sometimes in slight changes in 

 the color and quantity of pigment production. 



Some of the so-called saprophytic species may in reality be found to be 

 pathogenic, when improved technique is used. 



Modes of transmission. — Although experimental evidence is still far 

 from satisfactory regarding modes of transmission, it seems to be entirely 

 reasonable to assume from the known biology and the available evidence, 

 that transmission may take place both directly and indirectly. So far 

 as we are aware, no positive examples of direct transmission from person 

 to person have been reported, but that such infection does not take place' 

 in localities with high incidence of the disease, seems probable. Eather 

 convincing evidence of indirect transmission through the agency of food- 

 stuffs, water, etc., has been furnished by several observers. Several 

 authors whose work has already been reviewed, have shown that patho- 

 genic species may be isolated from water, air, soil, and food-stuffs and the 

 history of many of the reported cases indicates infection from some such 

 source. The frequent presence of the infection in the lower animals 

 must not be forgotten in considering the manner of transmission of the 

 parasites. 



Infection probably takes place in two general ways, first, by direct 

 or wound infection, as is shown in many cases by the history of injury 

 before the development of the symptoms of the external forms; and 

 secondly, infection by way of the respiratory or gastro-intestinal tracts, 

 which is shown in many of the internal forms of the disease. 



Species. — The number of species important in this disease is probably 

 a considerable one. Those which we have been able to recognize from the 

 literature and our own work are given in Part I. 



It seems probable that all the species have not, as yet, been discovered 

 and that some, which have been described, are specifically different from 

 the ones mentioned here. 



Pathology. — In general the morbid changes found in this disease 

 are those of a peculiar, low-grade, chronic infective process. Anaemia, 

 atrophy of tissues, and mild, chronic degenerations of parenchymatous 

 organs are present. The special pathology depends somewhat on the part 

 of the body involved and on the extent of this involvement. The strep- 

 tothritic unit, as it were, is a granule which is usually surrounded by a 

 zone of peculiarly appearing suppuration, and this in turn by an area of 

 inflammation characterized by connective tissue proliferation and cell 

 infiltration. Cells of the usual character are found, with a relative in- 

 crease of fixed tissue cells and occasionally a giant cell. In other instances 

 the suppurative zone is absent and the streptothritic unit takes on more 

 the general and histologic appearance of a tubercle. In fact, the whole 

 morbid process of Streptothrix infections resembles more or less closely 

 that produced by Bacillus tuberculosis. Tissue destruction often spreads 



