Page 1504 - Small Animal Internal Medicine, 6th Edition

1476 PART XIV Infectious Diseases

lymph nodes of dogs with chronic monocytotropic ehrlichi- blood sample for testing and place it in an EDTA tube before
osis than other causes of lymphadenopathy (Mylonakis et al., treatment. In one recent study tissues (lymph nodes, spleen,
VetBooks.ir 2011b). Polyarthritis due to E. canis was not detected in one liver, bone marrow, and blood) from naturally infected dogs
were assayed by PCR. Blood and lymph nodes were the most
experimental study (Theodorou et al., 2015). If nondegen-
erate neutrophils are detected in synovial fluid from dogs
mately 30% of the samples (Gal et al., 2007). In one study,
with polyarthritis, E. ewingii and A. phagocytophilum may be likely to be positive but were falsely negative in approxi-
the more likely causes. Bone marrow aspirates in dogs with PCR performed on blood and splenic aspirates were equiva-
chronic ehrlichiosis typically reveal myeloid, erythroid, and lent for making the diagnosis of E. canis infection (Faria
megakaryocytic hypoplasia in association with lymphoid et al., 2010). Repeated testing with serology and PCR assays
and plasma cell hyperplasia. However, myelofibrosis was may be required to accurately determine the infection status
not detected in one study of 10 affected dogs (Mylonakis of some individual dogs (Kidd et al., 2017) and is recom-
et al., 2010). Morulae from E. canis are rarely detected in mended in general for most vector borne diseases (Maggi
the cytoplasm of mononuclear cells. Ehrlichiosis generally et al., 2014).
causes mononuclear pleocytosis and increased protein con-
centrations in cerebrospinal fluid. Antiplatelet antibodies, Treatment
antinuclear antibodies, antierythrocyte antibodies (by direct Supportive care should be provided as indicated. Several
Coombs test), and rheumatoid factors are detected in some different tetracycline, doxycycline, chloramphenicol, and
dogs with ehrlichiosis, leading to an inappropriate diagnosis imidocarb diproprionate protocols have been used. The
of primary immune-mediated disease (Smith et al., 2004). ACVIM Infectious Disease Study Group currently recom-
No pathognomonic radiographic signs appear in dogs mends doxycycline (10 mg/kg PO q24h for at least 28 days).
with ehrlichiosis. The polyarthritis is nonerosive, and dogs Doxycycline administered at 5 mg/kg, PO, q12h has also
with respiratory signs most commonly have increased pul- been studied and can be effective. In one study of experimen-
monary interstitial markings, but alveolar patterns can occur. tally infected dogs, ticks still could acquire E. canis from
Identification of morulae in cells documents Ehrlichia feeding on dogs previously treated with doxycycline for 14
infection, but it is uncommon with monocytotropic strains. days (Schaefer et al., 2007). In an experimental study, mino-
Examination of buffy coat smears or blood smears made cycline at 10 mg/kg, PO, twice daily for 28 days had similar
from blood collected from an ear margin vessel may increase results to doxycycline at 10 mg/kg, PO, once daily for 28 days
the chances of finding morulae. Some Ehrlichia spp. can be (Jenkins et al., 2018). Whether E. canis infection persists
grown on cultured cells, but the procedure is low yield and appears to vary in part on the basis of when treatment is
expensive and so is not clinically useful. initiated. For example, experimentally infected dogs treated
Most commercial laboratories (using IFAs) and point- during the acute or subclinical phases became PCR-negative
of-care diagnostic tests use reagents that detect antibodies as clinical parameters improved, but dogs treated during the
against E. canis in serum. These tests are generally used as chronic phase were intermittently PCR-positive after treat-
the first screening procedures in dogs suspected to have ment (McClure et al., 2010).
ehrlichiosis. If serum antibodies against E. canis are detected Clinical signs and thrombocytopenia should rapidly
in a dog with clinical findings consistent with ehrlichio- resolve. If clinical abnormalities are not resolving within
sis, a presumptive diagnosis of canine ehrlichiosis infection 7 days, other differential diagnoses should be consid-
should be made and appropriate treatment begun. However, ered. Results of studies that used imidocarb diproprionate
detection of antibodies alone is not diagnostic of ehrlichio- (5-7 mg/kg intramuscular (IM) or SC repeated in 14 days)
sis because many dogs are subclinically infected. In addi- to treat canine ehrlichiosis have been variable. Some patients
tion, negative test results do not totally exclude ehrlichiosis develop pain at the injection site, salivation, oculonasal dis-
from the list of differential diagnoses because clinical disease charge, diarrhea, tremors, and dyspnea after administration
can be detected before seroconversion and not all Ehrlichia of this drug. Quinolones are not effective for the treatment
spp. induce antibodies that are consistently detected in all of E. canis infections in dogs.
E. canis assays. If not stated by the company, veterinarians Positive antibody titers have been detected for up to 31
should contact specific test providers to determine whether months after therapy in some naturally infected dogs. Dogs
the assay to be used detected antibodies against E. canis, E. with low (<1:1024) antibody titers generally revert to nega-
chaffeensis, and E. ewingii. tive by 1 year after therapy. Dogs with antibody titers greater
PCR assays are now available commercially and can be than 1:1024 often maintain positive antibody titers after
used to detect organism-specific DNA in peripheral blood. therapy. Whether these dogs are persistent carriers of the
It can be performed on joint fluid, aqueous humor, cerebro- organism is undetermined. On the basis of these findings
spinal fluid, and tissues. Blood PCR assay results can be antibody titers are considered to be ineffective for monitor-
positive before seroconversion in some experimentally inoc- ing response to therapy. Monitoring resolution of thrombo-
ulated dogs and positive results document infection, whereas cytopenia and hyperglobulinemia as markers of therapeutic
positive serologic tests only document exposure (Moroff elimination of the organism may be more effective.
et al., 2014). Because antibiotic treatment rapidly induces Results of studies have been variable on whether ehrlichial
negative blood PCR results, the clinician should draw the infections are cleared by treatment and repeated infections