Spotted Fever Group Rickettsiae in the Lone Star Tick, Amblyomma Americanum (Acari: Ixodidae)1

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Abstract
To determine the prevalence of spotted fever group rickettsiae in the lone star tick, Amblyomma americanum , 2,333 adult A. americanum collected from Mississippi and Kentucky were individually tested in 1983 and 1984. In 1984 a total of 734 specimens was also collected from Oklahoma and Texas. Results indicated that 41 (9.5%) of 421 ticks tested from Mississippi were hemolymph-test (HT) positive, and 23 of the HT positive ticks were fluorescent-antibody (FA) positive. From Kentucky, 195 (10.2%) of 1,912 were HT positive, and 96 of these were FA positive. In Oklahoma, 45 (7.0%) of 645 ticks were HT positive, and 15 of these were also FA positive. Of the 89 ticks examined from Texas, 12 (13.5%) were HT positive, and 3 of these were FA positive. Micro-immunofluorescence assay of 138 ticks positive by FA revealed that 38 produced titers 2:1:32 (21 seropositive to Rickettsia parkeri and 17 to the WB-8-2 strain of Rickettsia sp.); the remaining 100 sera revealed low titers (1:8 or 1:16) to R. parkeri , WB-8-2, R. rickettsii, R. sibirica , or R. conorii . One rickettsial isolate obtained in Vero cell culture from an FA-positive tick was identified as WB-8-2. This study did not reveal virulent spotted fever rickettsiae in the ticks examined. Our results suggest that A. americanum may not be a major vector of R. rickettsii , the causative agent of Rocky Mountain spotted fever.