Pathogenesis and Transmission
Anaplasmae are transmitted by tick vectors of different species. The Castor Bean tick, Ixodes ricinus, is the main vector for A. phagocytophilum in Central and Northern Europe, while the Deer tick (Ixodes scapularis) and the Western black-legged tick (I. pacificus) are the main transmitting vectors in North America. A. platys is transmitted by the Brown Dog tick (Rhipicephalus sanguineus) and Dermacentor spp.
A. phagocytophilum is an obligate intracellular bacterium which primarily attacks neutrophil granulocytes but also eosinophils of different species, including dog, cat, cattle, sheep, goat, horse, deer, rodents and birds. Monocytes and lymphocytes act as secondary host cells.
The elementary body is the infective stage and enters the host cell by phagocytosis. Elementary bodies are individual bacteriae about 1 µm in diameter and usually coccoid or ellipsoid in shape. Once they are captured inside the phagosomes, the pathogens replicate by binary fission, forming clusters of tightly packed elementary bodies termed initial bodies. Additional growth and replication leads to the formation of microcolonies, so called morulae, the configuration that typifies the genus. Apoptosis of the host cell causes a dissolving of the morulae so that the bacteria are released to the blood and are able to infect new cells. Furthermore bacteria can also be released by exocytosis too.
A. phagocytophilum infection is associated most commonly with the development of mild to moderate thrombocytopenia, also other cytopenias may occur, including neutropenia, lymphopenia, and mild anemia.
The pathogenic impact of A. platys in dogs is not clearly defined. The species infects platelets leading to abnormalities of primary hemostasis. Differences in severity of illness have been described depending on the geographic origin of the A. platys strain. As only a few experimental studies have been performed, it could be suggested that the clinical variability could be due to other factors like immune status, stress conditions, breed, etc. Furthermore, the common detection of A. platys in dogs co-infected especially with E. canis may lead to misunderstanding of the real pathogenic impact of the infectious agent.
- Shaw SE, Day MJ, Birtles RJ, et al.: Tick-borne infectious diseases of dogs. Trends Parasitol. 2001, 17, 74-80