Morphology of colonies of Anaplasma marginale in nymphal Dermacentor andersoni

Colonies of Anaplasma marginale Theiler were studied in midgut epithelial cells of nymphal Dermacentor andersoni Stiles that had become infected by feeding on splenectomized calves with anaplasmosis. Colonies of A marginale were not observed in nymphal ticks killed during the 6-day feeding period, but were present in sections of midgut epithelial cells of ticks killed as early as 5 days after repletion. Colonies of A marginale also were present in ticks examined throughout development to the adult stage. Two distinct morphologic types of colonies were observed and categorized by light microscopy as nymphal types 1 and 2. Colonies that were morphologically indistinct with characteristics common in both types were termed transitional nymphal (TsN) colonies. Nymphal type 1 (Ny1) colonies were observed at 5 days after repletion and nymphal type 2 (Ny2) colonies were first observed at 20 days after feeding. Representatives of each colony type were selected by light microscopy and were sectioned for electron microscopy. The Ny1 contained small particles and large, round reticulated forms, some of which appeared to be dividing by binary fission. The Ny2 also contained reticulated organisms, but they were rod-like in shape, and there was no morphologic evidence of binary fission. Organisms within Ny2 appeared to be surrounded by 2 double-layered membranes. Electron-dense forms, commonly observed in adult ticks, were not seen in colonies (Ny1, Ny2, or TsN) from nymphal ticks.