Electron microscopy of Cowdria (Rickettsia) ruminantium (Cowdry, 1926) in the endothelial cells of the vertebrate host

Abstract

The ultrastructure of Cowdria ruminantium was studied in thin sections of choroid plexus from experimentally infected sheep. Glutaraldehyde fixation and osmium tetroxide postfixation were used. The organism developed within the confines of a membrane-bound vacuole in the cytoplasm of endothelial cells. Four different forms, or particles, of the organism could be identified: small, intermediate, large and very large or giant forms. The various forms differed not only in size but also in the appearance and distribution of the nucleoid and "cytoplasmic" material within their inner structure. Within any one vacuole, in the host cell cytoplasm, the organisms were of the same form or type. Multiplication of the organism took place mainly by binary fission of the small and intermediate forms. Infrequent evidence was found that reproduction may also take place by the process of multiple budding and endosporulation. Small forms of the organism were embedded in a well developed matrix. Small and large forms were seen extracellularly, lying free in the lumen of blood vessels. Rarely small forms were seen in vacuoles in the cytoplasm of monocytes. The taxonomic position of C. ruminantium is discussed.