Survey for the Arthropod Vectors and Mammalian Hosts of Rocky Mountain Spotted Fever and Plague at Fort Ord, California1

Abstract

The diagnosis of Rocky Mountain spotted fever in a soldier at Fort Ord, California, in March 1969, an area not previously known to be endemic for the disease, prompted field studies of the actual or potential tick and mammalian reservoirs. Studies for sylvatic plague were undertaken concurrently. Six hundred and fifty-three mammals (feral and domestic) were examined. Sera from a majority of the animals were tested for rickettsial group complement-fixing antibodies ( Rickettsia akari antigm) and for antibodies against Yersinia pestis (= Pasteurella pestis ) using the passive hemagglutination test. Tick and flea ectoparasites were identified, and attempts were made to isolate rickettsiae from tick pools. Five mammalian species, the brush rabbit ( Sylvilagus bachmani ), gray fox ( Urocyon cinereoargenteus ), den mouse ( Peromyscus maniculatus ), ground squirrel ( Spermophilus beecheyi ), and a domestic sheep ( Ovis aries ), had Rocky Mountain spotted fever (RMSF) group CF antibodies. A known arthropod vector of RMSF, the dog tick ( Dermacentor variabilis ), was found in abundance; and numerous rabbit ticks ( Haemaphysalis leporispalutris ), known to be important in the maintenance of RMSF in other areas, were found on brush rabbits ( S. bachmani ) and jackrabbits ( Lepus californicus ). Evidence for the presence of rickettsiae in D. variabilis, H. leporispalustris , and perhaps other tick species was found by inoculation of tick poob into guinca pigs. Although the presence of sylvatic plague could not be demonstrated, several flea species potentially important in the transmission of sylvatic plague were found.