An invisible, filter-passing virus, pathogenic for the guinea pig, and capable of cultivation on special media for at least seven generations, has been isolated from a tick of the species Dermacentor andersoni. One of two monkeys (Macacus rhesus) inoculated became infected, and in one rabbit the result was negative.

The virus has been transmitted from infected guinea pigs to ticks as also in one instance by tick feeding from an infected tick to a guinea pig The presence of the virus in the tick is more easily demonstrated by the inoculation of guinea pigs with a suspension of the tick viscera. Continuous high fever (104.5–106.5°F.) and enlargement of the spleen are the chief symptoms of the infection in guinea pigs. After the febrile attack the guinea pigs are not susceptible to reinoculation with the cultured virus but are subject to infection with the virus of spotted fever, and vice versa.

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