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Definition.— A specific fever resembling symptomatically typhus exanthematicus, supervening on the bite of a tick— Dermacentor venustus, Banks.

History.— For upwards of thirty years a peculiar disease variously named "spotted fever," "blue disease," " black fever," has been recognized as endemic in limited districts of Montana and Idaho, United States of America. It was first described by Maxey, in 1899, as a distinct disease, and in the spring of 1902 it was investigated by Wilson and Chowning, who attributed it to the presence in the blood of a parasite akin to the babesia of Texas cattle fever. Their results were endorsed by Wesbrook, Cobb and Anderson; but in May, 1904, Wardell Stiles failed to recognize the parasite, either in fresh preparations of the blood or in the preparations of previous investigators. Other observers have been equally unsuccessful; the existence of the so-called Pyrosoma hominis is now discredited, and the germ of Rocky Mountain fever is still unknown.

In 1906 King definitely ascertained that the disease supervened on the bite of" a certain tick, Dermacentor venustus (D. andersoni) (Figs. 64, 65), a discovery subsequently confirmed by Ricketts, who succeeded in conveying the infection to a monkey by means of an infected tick. McClintick in 1911, in researches conducted at the instance of the United States Government, failed to communicate the disease to guineapigs, but unfortunately himself contracted the infection and died.

Geographical distribution.— The disease has been reported for several of the Western States of the American Union Idaho, Montana, Wyoming, Utah, Nevada, Oregon, and Washington State. It is