Page 967 - Clinical Small Animal Internal Medicine

93 Ehrlichiosis and Anaplasmosis 905

or monoclonal hypergammaglobulinemia. Immune‐
VetBooks.ir mediated consumption of platelets is thought to con-
tribute to thrombocytopenia in dogs infected with these
organisms. Other mechanisms of thrombocytopenia
such as splenic and vascular sequestration may also be
involved. Thrombocytopathia may also occur due to
antiplatelet antibodies. Deposition of immune com-
plexes and infected cells in the vascular endothelium of
kidneys, joints, and eyes may predispose to glomerulo-
nephritis, polyarthritis, and uveitis. Systemic inflamma-
tion associated with E. canis infection may also
contribute to myocarditis, meningitis, thrombosis,
hyperviscosity, and vasculitis. The development of high
circulating levels of antibodies against Ehrlichia or
Anaplasma spp. does not promote protective immunity
against reinfection.
The progression of the disease appears to vary accord-
ing to the cell type targeted by the parasite. Granulocytic
anaplasmosis caused by A. phagocytophilum appears to
be an acute illness in naturally infected dogs that can be
self‐limiting. However, chronic persistent infection has
been shown in experimentally infected dogs, and has
been suspected in some naturally infected dogs. Similarly,
granulocytic ehrlichiosis caused by E. ewingii appears to
be self‐limiting, but persistent infection was detected in
experimentally infected dogs.
In contrast, monocytic ehrlichiosis associated with E.
canis infection causes an acute illness, which may vary
from mild to severe depending upon the pathogenicity of
the parasite, followed by subclinical phase of months to
years in duration. It is unclear if dogs are able to elimi-
nate E. canis or if the organism remains sequestered in
the spleen, bone marrow or other primary niche indefi-
nitely. Some dogs develop a chronic phase characterized
by hyperglobulinemia and bone marrow suppression,
which is more difficult to treat. E. chaffeensis also appears
to cause chronic infection in dogs, but dogs are generally
asymptomatic.

Epidemiology

The presence of Ehrlichia and Anaplasma spp. primarily
follows the geographic distribution of their vectors and
mammalian reservoir hosts (see Figure 93.3). E. canis has
worldwide distribution, being one of the most common
tick‐borne pathogens in dogs in tropical and subtropical
regions. The ubiquitously distributed brown dog tick
(R. sanguineus) is well adapted to urban environments.
Consequently, dogs with no access to wooded areas are
still at risk of E. canis infection. In contrast, the Lone Star
tick (A. americanum), vector for E. ewingii and E. chaf-
Figure 93.3 Distribution of Amblyomma americanum, Dermacentor feensis, is most frequently found in woodland habitats,
variabilis, Ixodes pacificus, Ixodes scapularis, and Rhipicephalus
sanguineus ticks in the USA. Source: Centers for Disease Control and especially where a high number of white‐tailed deer are
Prevention, www.cdc.gov/ticks/geographic_distribution.html. present. The geographically limited distribution of dogs