Transmitted by Dermacentor spp. (Dermacentor andersoni and Dermacentor variabilis in the United States). Romanowsky stained bovine erythrocytes containing Anaplasma marginale, which are dense, homogeneously staining blue-purple inclusions 0.3-1.0 µm in diameter that are typically located toward the margins of infected cells.
Transmitted by Ixodes spp. (Ixodes pacificus and Ixodes scapularis in the United States). Romanowsky stained canine blood smear containing morulae (arrow) of Anaplasma phagocytophilum within a neutrophil. Infections with Anaplasma phagocytophilum are most common in the northeastern United States, in the upper Midwest, and along the West Coast.
Transmitted by Rhipicephalus sanguineus. Romanowsky stained canine blood smear containing morulae (arrow) of Ehrlichia canis within a circulating monocyte. Infections with Ehrlichia canis can result in severe clinical disease in dogs (canine monocytic ehrlichiosis) and seem to occur most commonly in the southern United States, including areas of Arizona, southern California, Louisiana, Oklahoma and Texas. Ehrlichia chaffeensis is another tick-borne rickettia infecting monocytes that appears morphologically very similar to Ehrlichia canis in blood films. However, Ehrlichia chaffeensis is transmitted by Amblyomma americanum ticks. In dogs, Ehrlichia chaffeensis infections are rarely clinical. In humans, however, ehrlichiosis caused by Ehrlichia chaffeensis is considered the most common tick-borne disease in the southern United States.
Spotted fever group Rickettsia spp. are transmitted by a number of different tick vectors, including Amblyomma americanum, Amblyomma maculatum, Dermacentor spp., and Rhipicephalus sanguineus, and are obligately intracellular organisms. Photo kindly provided by Dr. Ed Shaw, Oklahoma State University.
Coxiella burnetii (no longer considered a rickettsia)