Electron Microscopy of Tick-borne Fever Agent in Bovine and Ovine Phagocytizing Leukocytes

Abstract

Two strains of tick-borne fever agent, one more virulent to cattle and the other more virulent to sheep, were studied in thin sections of leukocyte concentrates of an affected call and sheep, respectively. The calf was partially immune from previous infections. To obtain leukocyte concentrate, erythrocytes were lysed by ammonium chloride solution. Glutaraldehyde fixation and osmium tetroxide postfixation were used. Agent particles were detected in neutrophil and eoslnophil granulocytes (occasionally also in mono-cytes) inside cytoplasmic vacuoles. The preparation from the sheep revealed that the tick-borne fever agent has a life cycle; it appeared as large particles, intermediate forms, and small particles. In the preparation from the calf, only large particles were observed, probably as a result of partial immunity. Large particles had a cell wall, plasma membrane, cytoplasm with ribosome-like granules, and nucleoid. They were often elongated, bearing closer resemblance to rickettslae than to pslttacosis-lymphogranuloma-trachoma (PLT) agents. The arrangement of large particles in vacuoles was variable and provided an explanation for the different forms of light microscopic tick-borne fever bodies. The intermediate forms and small particles resembled those of PLT organisms. Release of particles occurred by rupture of inclusions. In the calf also, extrusion of whole inclusions was observed. This phenomenon was thought to reflect increased resistance of the cells. The large particle was considered to be the usual infective form of the agent. A middle-ground position of the agent between the genus Rickettsia and the PLT group is suggested.