Development and infectivity of Anaplasma marginale in Dermacentor andersoni nymphs.

The development of Anaplasma marginale was studied in Dermacentor andersoni nymphs after they had fed on a calf with ascending Anaplasma infection. Gut tissues were collected on day 4 of tick feeding, from newly replete (fed) nymphs and on postfeeding days (PFD) 5, 10, 15, 20, and were processed for light and electron microscopy to determine density of A marginale colonies. Homogenates of gut tissues were prepared from nymphs collected on the same days and inoculated into susceptible, splenectomized calves to test for infectivity. Anaplasma colonies were detected in gut cells on PFD 5, 10, 15, and 20. Although colony density appeared to be higher on PFD 10 and 15, differences were not significant. Nymphal type-1 colonies were detected in highest numbers on PFD 5 and 10, transitional colonies were seen in highest numbers at PFD 10 and 15, and nymphal type-2 colonies were observed only on PFD 20. Gut homogenates that were collected from ticks at 4 days of feeding, when newly replete, and on PFD 20 caused anaplasmosis when injected into susceptible calves, but homogenates made from ticks collected on PFD 5, 10, and 15 were not infective. The data indicate that of the colony types of A marginale that develop in replete nymphs, nymphal type-1 and transitional colonies may contain organisms that are not infective for cattle.

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.