Canine anaplasmosis is caused by one of two gram-negative, obligate, intracellular bacterial agents, Anaplasma phagocytophilum or Anaplasma platys. Infection with A. phagocytophilum, the species more commonly associated with anaplasmosis, causes lameness and is often confused with Lyme disease. Infection
with A. platys results in a cyclic thrombocytopenia. Both types of anaplasmosis are likely spread by ticks and can occur worldwide.
A NOTE ON CLASSIFICATION
Both A. phagocytophilum and A. platys are in the order Rickettsiales, which includes members of the genera Ehrlichia, Anaplasma, Cowdria, Wolbachia, and Neorickettsia. In 2001, a major restructuring of the classification of organisms occurred in the order Rickettsiales.1 As a result of these investigations, three separate species of Ehrlichia—Ehrlichia equi, Ehrlichia phagocytophila, and the previously unnamed agent causing human granulocytic ehrlichiosis—were found to be insufficiently different to warrant
separate species designations. These organisms were phylogenetically most closely related to species in the genus Anaplasma, so they were reclassified as A. phagocytophilum. In addition, Ehrlichia platys was found to be more closely related to Anaplasma species and was subsequently renamed A. platys.1
ANAPLASMA PHAGOCYTOPHILUM INFECTION
A hallmark of this form of anaplasmosis is that it may result in subclinical disease or a chronic carrier state.
Infection with A. phagocytophilum was first reported in dogs from Minnesota and Wisconsin in 1996.2
Anaplasma phagocytophilum infection is a vector-borne zoonotic disease, and its appearance in dogs in those areas closely coincided with recognition
of the disease in people.3
The organism has a worldwide geographic distribution and is endemic in the upper Midwest, East, and Northeast regions of the
United States as well as the western coastal regions. European countries such as the United Kingdom, Norway, Sweden, Switzerland,
and Germany also have reported infections in ruminants, dogs, and people. The disease has been less frequently reported in
Asia and South America. In the United States, most disease outbreaks are seasonal and coincide with the emergence of tick
vectors in spring and early summer (May and June) and then again in the fall (September).
Tick vector and mammalian hosts
Several ixodid ticks can serve as vectors for A. phagocytophilum. In the United States, Ixodes scapularis is the primary vector in the upper Midwest and the Northeast, and Ixodes pacificus is the primary vector in the West. Ixodes ricinus is the primary vector in Europe. The organism can infect a wide range of mammals, including dogs, cats, horses, ruminants,
people, and many wildlife species. White-tailed deer and several species of small rodents are considered the primary reservoir
hosts. It is suspected that transmission to susceptible mammalian hosts requires prolonged tick attachment and feeding of
24 hours or more.4