Page 1497 - Small Animal Internal Medicine, 6th Edition

CHAPTER 95
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Polysystemic Rickettsial

Diseases

The most common rickettsial agents causing illness in dogs co-infections can occur. The vector must be attached for
and cats are in the families Anaplasmataceae (Anaplasma approximately 24 to 48 hours to transmit the agent. Clini-
spp., Ehrlichia spp., and Neorickettsia spp.) and Rickett- cal signs usually develop approximately 1 to 2 weeks after
siaceae (Rickettsia spp.) The organisms in the Anaplasma, infection. Neutrophils (and rarely other leukocytes) phago-
Ehrlichia, and Neorickettsia genera are classified genetically cytize the organism, and once intracellular, A. phagocyto-
and by cell tropism (monocytotropic, granulocytotropic, or philum prevents phagolysosome fusion. This mechanism
thrombocytotropic). The organisms of most importance to allows multiplication within the phagosome, which gives the
dogs and cats in the United States discussed in this chapter appearance of morula in neutrophils under light microscopy
include A. phagocytophilum, A. platys, E. canis, E. chaffeensis, (Fig. 95.1). The exact pathogenesis of disease is still undeter-
E. ewingii, N. risticii, R. felis, and R. rickettsii (Table 95.1). mined, and why some dogs but not others develop clinical
Prevalence rates in many countries have been determined for signs of disease is unclear. However, the disease-inducing
most agents; world maps are maintained by the Companion potential could be related in part to strain differences (De
Vector Borne Disease group sponsored by Bayer Animal Arcangeli et al., 2018; Rejmanek et al., 2012).
Health (http://www.cvbd.org/en/occurrence-maps/world-
map/). Maps showing prevalence rates for some of the agents Clinical Features
in the United States are published by the Companion Animal Although experimentally inoculated dogs can be PCR-
Parasite Council (www.capcvet.org). It is common for dogs positive for A. phagocytophilum DNA for weeks after expo-
and cats to be exposed to more than one vector-borne agent sure to infected Ixodes spp., clinical disease syndromes appear
(Yancey et al., 2014). to occur primarily during the acute phase of infection. Infec-
tion has been associated most commonly with nonspecific
signs of fever, lethargy, and inappetence. Stiffness and lame-
CANINE GRANULOCYTOTROPIC ness consistent with musculoskeletal pain are also common,
ANAPLASMOSIS and A. phagocytophilum has been associated with polyar-
thritis (Fig. 95.2). Vomiting, diarrhea, difficult breathing,
Etiology and Epidemiology cough, lymphadenopathy, hepatosplenomegaly, and central
Anaplasma phagocytophilum (previously known as E. equi, nervous system (CNS) signs (seizures and ataxia) have also
E. phagocytophila, canine granulocytic Ehrlichia, and human been reported. Dogs can be chronic subclinical carriers, so
granulocytic ehrlichiosis agent) is known to infect a variety of exacerbation of disease could occur in some dogs. However,
animals, including small mammals, mountain lions, coyotes, chronic disease syndromes such as those associated with E.
sheep, cattle, deer, dogs, horses, and human beings. Small canis infection are not well documented. In a recent study
mammals and deer are natural reservoirs. The distribution of dogs with neurologic diseases in Sweden, serologic evi-
of A. phagocytophilum is defined by the range of Ixodes ticks dence of exposure to A. phagocytophilum and B. burgdorferi
and in the United States is most common in California, Wis- was common, but neither organism was linked to the pres-
consin, Minnesota, and the Northeastern states. This agent ence of neurologic disease (Jaderlund et al., 2007). In one
is common in some other areas of the world including parts study of valvular endocarditis, all dogs with Bartonella spp.–
of Europe, Asia, and Africa. In one study of more than 4 associated disease were also seropositive for A. phagocyto-
million canine sera tested for antibodies against A. phagocy- philum (MacDonald et al., 2004). Whether the co-infection
tophilum spp., the Northeast and upper Midwest states were potentiated the Bartonella-associated disease is unknown.
most likely to have positive dogs (McMahan et al., 2016). Epistaxis, which occurs with E. canis, R. rickettsii, and Bar-
Borrelia burgdorferi is also transmitted by Ixodes ticks, so tonella spp. infections in some dogs, has also been reported.

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