720 INFECTION AND IMMUNITY. 



immunity ration. According to the French Commission, a 

 Properties, certain degree of immunity could be conferred by 

 the injection of infected serum which had been 

 heated to 55 C. for five minutes, or of defibri- 

 nated blood which had been kept under vaselin 

 oil at room temperature for eight days. They also 

 claimed that the serum of convalescents has pro- 

 phylactic and ciirative properties to a certain de- 

 gree. 



in. "SPOTTED FEVER" or THE ROCKY MOUNTAIN 



STATES. 



In the valley of the Bitter Eoot Eiver of Mon- 

 tana, and in certain sections of Idaho, Wyoming 

 and Washington an acute febrile disease, known 

 in these localities as spotted fever, is encountered 

 in the months of spring. The disease is denned by 

 Maxey as "an acute, endemic, non-contagious, but 

 probably infectious, febrile disease, characterized 

 .clinically by a continuous moderately high fever, 

 severe arthritic and muscular pains, and a pro- 

 fuse petechial or purpural eruption in the skin, 

 appearing first on the ankles, wrists and forehead, 

 but rapidly spreading to all parts of the body/' 



In 1902-03, Wilson and Chowning studied many 

 cases of the disease in Montana, and described as 

 the cause a protozoon organism which they con- 

 sider as a piroplasma (Piroplasma homim*) . The 

 organism is a hematozoon, occurring within the 

 erythrocytes. Young cells resemble the "hyaline 

 bodies" of malaria, are of ovoid shape, 1 micron 

 thick and 1 to 2 microns long, and usually occur in 

 pairs, but sometimes in numbers of 4 to 16, within 

 an erythrocyte. The smaller ends of pairs often 

 are directed toward each other, and they may be 



