Morphologic characteristics of colonies of Anaplasma marginale Theiler in midgut epithelial cells of Dermacentor andersoni Stiles.

Colonies of Anaplasma marginale Theiler were studied in midgut epithelial cells of adult Dermacentor andersoni Stiles that were infected as nymphs. Colonies were categorized by light microscopy into 5 morphologic types that appear to represent stages in a developmental sequence. The mean diameter of the colony types ranged from 5.64 micrometer to type 1 colonies to 10.49 micrometers of type 5 colonies. Representatives of each colony type were selected by light microscopy and were sectioned for examination by electron microscopy. The morphologic features of individual A marginale organisms within colony types varied and included: (i) small electron-dense forms, (ii) larger reticulated forms, (iii) pleomorphic reticulated forms, and (iv) small particles. Types 1 and 2 colonies contained small, electron-dense forms. Type 3 colonies contained electron-dense forms, early reticulated forms, and small particles that were often outside of the parasite's limiting membrane. Type 4 colonies contained many reticulated forms that often had small particles within cell membranes. Type 5 colonies contained fewer well-formed reticulated forms than did type 4 and, in some cases, large masses of pleomorphic reticulated forms.