222 ANIMAL PARASITES 



Morphology. These are probably all transmitted by ticks or 

 related insects. They have lately been cultivated and retain some- 

 what of their virulence for monkeys and rodents. Close studies 

 have placed them among the Spirochetes, since they possess an 

 undulating membrane, some divide in longitudinal manner and 

 since an insect is necessary for their transmission. They are 

 elongated, flexible, corkscrew-like, serpentine and vibratory in 

 motility, and do not form spores. They are stained with reasonable 

 ease by polychrome methods but not by Gram's method. They 



FIG. 81. Spirilla of relapsing fever from blood of a man. (Kolle and 



Wassermann.) 



measure from 10-40/4 in length and about i/i in breadth. Coils 

 vary from 6-20. The American type is smaller than the rest. 



Transmission. The tick which transmits these organisms 

 becomes infective in one week after biting a patient and remains 

 so all its life; its young are also infective. The types of disease 

 vary but little. In all these is a relapsing fever with periods of 

 apyrexia in between. During the fever the spirochaetes are swim- 

 ming free in the blood and disappear in the afebrile interval. 



