INSECTS AND HUMAN WELFARE 



disease is endemic over very extensive areas and is a common 

 malady in the Old World. In Africa a different form of re- 

 lapsing fever is carried by a tick (Ornithodoros moubatd) and 

 ticks are also thought to convey it in South America. For 

 a long time ticks were believed to harbor both types of re- 

 lapsing fever, but recent researches have disproved this idea. 

 Rocky Mountain spotted fever is the only definitely 

 characterized tick-borne human disease that occurs within 



FIG. 14. Rocky Mountain Spotted-fever Tick, male at left. 

 Unengorged female at right. 



the confines of the United States. In 1902 Wilson and 

 Chowning suggested that ticks might carry this disease, 

 and four years later Ricketts definitely proved such to be the 

 case. Rocky Mountain spotted fever is restricted to the far 

 western and northwestern states, whence 290 cases were re- 

 ported during the year 1916. Over half of these occurred in 

 Idaho, although the disease extends into the neighboring 

 states of Montana, Wyoming, Colorado, Utah, Nevada, 

 California, Oregon, and Washington, as well as northward 

 into Canada. As shown by the fatality rate, the disease is 

 most virulent in western Montana and northern Idaho, where 

 the mortality is said to reach 70 or 80 per cent. A single 

 species of tick, Dermacentor venustus (Fig. 14), common in 

 these regions is known to act as the vector. The Dermacen- 

 tor ticks occur abundantly on various small wild mammals 



