Page 1507 - Small Animal Internal Medicine, 6th Edition

CHAPTER 95 Polysystemic Rickettsial Diseases 1479

thrombocytopenia and anemia, also occur. Morulae can be associations are less apparent (see the Other Rickettsial
detected in neutrophils and eosinophils in peripheral blood Infections section). In one survey of ticks in Missouri, R.
VetBooks.ir and in neutrophils from synovial fluid. However, presence amblyommatis was the predominant SFG species, and it was
detected in Amblyomma americanum, Dermacentor variabi-
of morulae is transient and so easily missed cytologically.
A peptide-based, E. ewingii serologic assay is now available
members of the SFG that infect dogs may induce antibodies
(O’Connor et al., 2010; SNAP 4DX Plus, IDEXX Labora- lis, and Ixodes scapularis (Santanello et al., 2018). Other
tories, Portland, ME). However, because antibodies can be that cross react with R. rickettsii (see Diagnosis later), which
detected in healthy as well as diseased dogs, presence of E. may lessen the risk of developing RMSF (Santanello et al.,
ewingii–specific antibodies cannot be used alone to diagnose 2018). Canine RMSF is recognized predominantly in the
clinical granulocytotropic ehrlichiosis. If not stated by the Southeastern states from April through September when the
company, veterinarians should contact specific test providers tick vectors are most active. Dermacentor andersoni (Ameri-
to determine whether the assay to be used detected antibod- can wood tick), Dermacentor variabilis (American dog tick),
ies against E. canis, E. chaffeensis, and E. ewingii. and A. americanum (Lone Star tick) are the principal vectors,
Some dogs with acute disease will have negative test host, and reservoir of R. rickettsii. A reemergence of RMSF
results on presentation, so a convalescent test to shown sero- in the southwestern states has recently occurred, and R. san-
conversion is required for diagnosis. PCR assays are now guineous ticks are the vector (Demma et al., 2005, 2006;
used to differentiate between members of the Ehrlichia, Ana- Nicholson et al., 2006). Rickettsia rickettsii has also been
plasma, and Neorickettsia genera, and should be performed detected in R. sanguineous in California, and infection of this
on blood collected in EDTA before administration of antibi- tick has been confirmed experimentally (Piranda et al., 2011;
otics in acute cases or those negative for specific antibodies. Wikswo et al., 2007). Strains of R. rickettsii that infect dogs
However, healthy dogs can be positive for E. ewingii DNA and human beings are closely related genetically (Kidd et al.,
in blood so positive test results do not prove the clinical 2006). Seroprevalence rates for antibodies against SPG
syndrome was from the infection (Starkey et al., 2014). organisms are high in endemic areas. In one study of dogs
in the southeastern United States 14.1% and 29.7% of healthy
Treatment and clinically ill dogs, respectively, had antibodies that
Supportive care should be provided as indicated. The tetra- reacted to R. rickettsii (Solano-Gallego et al., 2004).
cycline, doxycycline, and chloramphenicol protocols recom- The organism is maintained in nature in a cycle between
mended for E. canis infections are generally effective. ticks and small mammals such as voles, ground squirrels,
Doxycycline administered at 5 mg/kg PO q12h or at 10 mg/ and chipmunks, and it is transmitted transovarially in some
kg PO q24h for at least 14 days is recommended by most ticks, so nymphs and larvae can be infected without feeding.
clinicians for the treatment of canine granulocytotropic Rickettsia rickettsii replicates in endothelial tissues (causing
ehrlichiosis. However, few data from the field are available vasculitis) and can lead to diverse and sometimes severe
that document treatment efficacy of these protocols. clinical manifestations of disease as soon as 2 to 3 days after
exposure. Antiplatelet antibodies can be detected in many
Zoonotic Aspects and Prevention infected dogs, suggesting an immune-mediated component
Dogs and human beings are both infected by E. canis, E. to the thrombocytopenia that is frequently present (Grindem
ewingii, and E. chaffeensis (Buller et al., 1999). Although et al., 1999). The course of RMSF in dogs experimentally by
people cannot acquire ehrlichiosis from handling an infected tick infestation was recently described (Levin et al., 2014).
dog, dogs may be reservoirs for these agents and may play a Although seropositive cats have been detected, whether
role in the human disease by bringing vectors into the human clinical illness occurs is unclear (Bayliss et al., 2009; Case
environment. Ticks should be removed and handled with et al., 2006).
care. The ACVIM recommended to optimally use only sero-
negative and PCR-negative dogs as blood donors, particu- Clinical Features
larly in areas endemic for Amblyomma americanum (Wardrop Any dog not previously exposed to R. rickettsii can develop
et al., 2016). Whether to treat healthy, seropositive dogs has RMSF. The tick frequently feeds on and leaves the dog before
pros and cons similar to those for E. canis infections of the development of clinical signs. In one study, only 5 of 30
healthy dogs but is probably not needed (see the Canine owners knew their dogs had been infested by ticks (Gasser
Monocytotropic Ehrlichiosis section). et al., 2001). After infection, the majority of dogs are sub-
clinical; some develop acute disease with a clinical course of
approximately 14 days. No age or sex predilection exists.
ROCKY MOUNTAIN SPOTTED FEVER In dogs infected with R. rickettsii after tick infestation,
the most significant findings were fever (starting 3-7 days
Etiology and Epidemiology after infestation), lethargy, anorexia, ocular lesions, tremors,
In the United States, the spotted fever group (SFG) organism skin rash, thrombocytopenia and leukocytosis (Levin et al.,
most likely to be associated with clinical disease in dogs is 2014). In naturally occurring cases, interstitial pulmonary
R. rickettsii, the cause of RMSF. Other SFG organisms like R. disease, dyspnea, and cough have been reported as well as
parkeri and R. felis are known to infect dogs, but disease vomiting or diarrhea. Because the disease is generally acute,