Page 1501 - Small Animal Internal Medicine, 6th Edition

CHAPTER 95 Polysystemic Rickettsial Diseases 1473

Later in infection, thrombocytopenia can be severe, but the Zoonotic Aspects and Prevention
organism may not be recognized cytologically or by PCR The strategies discussed for control of A. phagocytophilum
VetBooks.ir with blood (Eddlestone et al., 2007). In these experimentally plasma platys DNA has been amplified from the blood of
infection of dogs should also be effective for A. platys. Ana-
infected dogs, microbial DNA could be amplified from bone
human beings, so tick control should always be maintained
marrow and splenic aspirates. Anemia and thrombocytope-
nia in dogs experimentally infected with either A. platys and/ (Breitschwerdt et al., 2014)
or E. canis were more persistent in the co-infected dogs
(Gaunt et al., 2010).
CANINE MONOCYTOTROPIC
Clinical Features EHRLICHIOSIS
Dogs with A. platys infections in the United States are usually
subclinically infected or have mild fever. More severely Etiology and Epidemiology
affected dogs have exhibited fever, uveitis, and clinical evi- Organisms that are associated with monocytotropic ehrlichi-
dence of bleeding, including ecchymosis, petechia, epistaxis, osis in naturally infected dogs include E. canis, E. chaffeensis,
melena, gingival bleeding, retinal hemorrhage, and hema- and Neorickettsia risticii var atypicalis. Cases with E. canis
toma formation. Co-infection with other tick-borne agents and E. chaffeensis are detected most frequently and will be
such as E. canis and Babesia spp. is common and may reflect discussed herein. An individual dog can be infected by more
a shared vector. Fatalities have been reported in the Mediter- than one ehrlichial agent, and co-infection with other tick-
ranean basin, suggested that some strains are more patho- borne pathogens is common.
genic than others (Bouzouraa et al., 2016). Ehrlichia canis is the most common of these agents and
causes the most severe clinical disease; it is maintained in the
Diagnosis environment from passage from ticks to dogs. Rhipicephalus
Anemia, thrombocytopenia, and neutrophilic leukocytosis sanguineus and Dermacentor variabilis are the known vectors.
can occur. Morulae may or may not be present within plate- The organism is not passed transovarially in the tick, so
lets. In endemic areas A. platys infection, alone or in combi- unexposed ticks must feed on a rickettsemic dog in the acute
nation with other tick-borne agents, should be suspected in phase to become infected and perpetuate the disease (Ipek
dogs with anemia or thrombocytopenia. Cross-reactivity et al., 2018). Dogs seropositive for E. canis have been identi-
with E. canis is thought to be minimal, but A. platys antibod- fied in most regions of the world and most of the United
ies are detected in some serologic assays for A. phagocytophi- States, but the majority of cases occur in areas with high
lum, including one commercially available kit (SNAP 4DX concentrations of R. sanguineus, such as the Southwest and
Plus; Chandrashekar et al., 2010). Agreement between dif- Gulf Coast. There are different genogroups within E. canis,
ferent tests for A. phagocytophilum and A. platys antibodies which may partially explain differences in pathogenicity
are not 100% (Liu et al., 2018). Antibody assay results can be (Nambooppha et al., 2018).
falsely negative in acute cases, so a convalescent test 2 to 3 Ehrlichia chaffeensis is a cause of human mononuclear
weeks later may be required to confirm exposure. PCR assays ehrlichiosis. White-tailed deer, voles, coyotes, and opossums
performed on blood collected in EDTA can be used to are reservoirs, and Amblyomma americanum, D. variabilis,
confirm infection and differentiate A. platys infections from and some Ixodes ticks are vectors. Infections by E. chaffeensis
other infections, and microbial DNA can also be amplified are detected primarily in the southeastern United States.
from healthy dogs and can be negative in clinically ill dogs Clinical manifestations in dogs are currently being detailed
(Eddlestone et al., 2007). Most dogs infected by A. platys (Breitschwerdt et al., 1998; Zhang et al., 2003) and appear to
have subclinical infections, most infected dogs only have an be rare. In experimentally infected dogs, fever is common
acute phase, exposure rates in endemic areas are high, and (Nair et al., 2016). In one study of dogs infected by natural
the disease syndromes associated with infection have mul- exposure to ticks in Oklahoma, clinical abnormalities were
tiple other causes. Thus antibody test results and PCR assay not detected in the dogs that became positive for E. chaffeen-
results alone cannot be used to prove clinical disease associ- sis (Starkey et al., 2014). In two large serological surveys of
ated with A. platys infection. 8662 and 6582 dog serum samples in North America and the
Caribbean, antibody prevalence rates for E. canis and E.
Treatment chaffeensis were approximately 1% and 3%, respectively
The doxycycline and tetracycline treatment protocols dis- (Beall et al., 2012; Qurollo et al., 2014). As little is known
cussed for A. phagocytophilum infections of dogs should also about clinical disease associated with E. chaffeensis, the fol-
be effective for A. platys infections. If co-infection with E. lowing discussion focuses on E. canis.
canis exists, treatment duration should be at least 4 weeks Ehrlichia canis infection causes acute, subclinical, and
(Neer et al., 2002). In one study of dogs that were experi- chronic phases of disease. Infected mononuclear cells mar-
mentally inoculated, PCR-positive test results for A. platys ginate in small vessels or migrate into endothelial tissues,
or E. canis remained negative after doxycycline administra- inducing vasculitis during the acute phase. The acute phase
tion in spite of attempted immune suppression (Gaunt et al., begins 1 to 3 weeks after infection and lasts 2 to 4 weeks;
2010). most immunocompetent dogs survive. The subclinical phase