Transmitted by Dermacentor spp. (D. andersoni and D. 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. (I. pacificus and I. scapularis in the United States). Romanowsky stained canine blood smear containing morulae (arrow) of Anaplasma phagocytophilum within a neutrophil. Infections with A. phagocytophilum are most common in the northeastern United States, in the upper Midwest, and along the West Coast.
Transmistted by Rhipicephalus sanguineus in other parts of the world, which is the suspected vector in the United States. Romanowsky stained canine blood film containing Anaplasma platys (PCR confirmed), which are intraplatelet organisms within vacuoles.
Transmitted by Rhipicephalus sanguineus. Romanowsky stained canine blood smear containing morulae (arrow) of Ehrlichia canis within a circulating monocyte. Infections with E. 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 E. canis in blood films. However, E. chaffeensis is transmitted by A. americanum ticks. In dogs, E. chaffeensis infections are rarely clinical; In humans, however, ehrlichiosis caused by E. chaffeensis is considered the most common tick-borne disease in the southern United States.
Transmitted by Amblyomma americanum. Romanowsky stained canine blood smear containing morulae (arrow) of E. ewingii within a neutrophil. Infections with Ehrlichia ewingii tend to be clinically milder than with E. canis, and are most commonly seen in the middle 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)
Electron micrograph of Coxiella burnetii within an IDE8 cell. Photo kindly provided by Dr. Ed Shaw, Oklahoma State University.
Electron micrograph of Coxiella burnetii within a VERO cell. Photo kindly provided by Dr. Ed Shaw, Oklahoma State University.