Plaque formation in tissue cultures by Rickettsia rickettsi isolated directly from whole blood and tick hemolymph.

A simple technique is described for isolation of Rickettsia rickettsi directly from tick hemolymph and whole blood of rickettsemic guinea pigs by means of the plaque assay technique in primary chicken embryo tissue cultures. Plaque-forming units per drop of hemolymph were almost 100-fold greater for partially engorged ticks than for unengorged ticks. Rickettsemia in guinea pigs fed upon by infected ticks was detected as early as 24 hr before fever. No morphological differences were noted between plaques formed by rickettsiae isolated from tick hemolymph or from whole guinea pig blood.