MICROSYMBIOSIS 83 



known, it has been demonstrated that they are capable of 

 producing disease in man and some of the higher animals, 

 when introduced through bites or other forms of injury. 

 These organisms are known collectively as ''Rickettsia 

 bodies." The Rickettsia bodies of the wood tick {Derma- 

 centor venustus) are well known through the researches of 

 Wolbach ('19), and many others. The microorganism 

 appears to be an occasional parasite of the wood tick, 

 since all individuals do not seem to harbor them. In so 

 far as can be learned, they produce no injurious effects in 

 the larva, nymph or adult wood tick. They have been 

 demonstrated in the egg of the host. The particular 

 disease in man associated with the microorganism is known 

 as "Rocky Mountain spotted fever," and is more or less 

 limited to certain sections of INIontana. The host, how- 

 ever, is widely distributed over the Rocky Mountain region. 

 Dr. F. E. Becker, working in this laboratory, has been 

 able to demonstrate structures indistinguishable from these 

 Rickettsia bodies in wood ticks that were collected near 

 Boulder, Colorado. He also has examined the tissues of 

 a number of guinea pigs upon which the native wood ticks 

 have been placed. In the tissues of these guinea pigs he 

 was able to demonstrate the Rickettsia-like bodies, but in 

 a different situation from that affected by the Montana 

 strain. The Montana strain, which produces Rocky 

 Mountain spotted fever, localizes in the peripheral blood 

 vessels. The Colorado strain, according to Becker's find- 

 ings, appears to select the intestinal tract. It appears 

 probable that the Rickettsia bodies are symbiotic micro- 

 organisms in the wood tick, that vary in their specific 

 reactions in different locahties. These organisms are 

 extremely deUcate, and difficult to demonstrate on a mic- 

 roscopical slide, due to their great fragihty. 



In the examples of microsymbiosis that have been re- 

 viewed in the preceding pages, a large number of the defin- 



