Page 1502 - Small Animal Internal Medicine, 6th Edition

1474 PART XIV Infectious Diseases

lasts months to years in naturally infected dogs. Although TABLE 95.2
some dogs clear the organism during the subclinical phase,
VetBooks.ir the organism persists intracellularly in some, leading to the Clinical Abnormalities Associated With Ehrlichia canis
chronic phase of infection. Many of the clinical and clini-
Infection in Dogs
copathologic abnormalities that develop during the chronic
phase are from immune reactions against the intracellular STAGE OF
organism. The variable duration of the subclinical phase of INFECTION ABNORMALITIES
disease explains why E. canis infection does not have a dis- Acute Fever
tinct seasonal incidence as does Rocky Mountain spotted Serous or purulent oculonasal discharge
fever (RMSF). However, acute-phase disease is recognized Anorexia
most frequently in the spring and summer when the vectors Weight loss
are most active. The pathogenesis of acute and chronic Dyspnea
ehrlichiosis is complex and composed of both agent and Lymphadenopathy
host effects. Tick infestation often evident
Subclinical No clinical abnormalities
Clinical Features Ticks often not present
Clinical disease from ehrlichial infection can occur in any Chronic Ticks often not present
dog, but its severity varies depending on the organism, host Depression
factors, and presence of co-infections like A. platys and Weight loss
Bartonella spp. Virulence is thought to vary with different Pale mucous membranes
field strains of E. canis. Dogs with depressed cell-mediated Abdominal pain
immunity develop severe disease. However, E. canis itself Evidence of hemorrhage: epistaxis, retinal
did not cause immunosuppression in young, experimentally hemorrhage, etc.
infected dogs within the first several months of infection Lymphadenopathy
(Hess et al., 2006). Splenomegaly
Dyspnea, increased lung sounds,
Clinical findings in dogs with E. canis infections vary interstitial or alveolar lung infiltrates
with the timing of infection (Table 95.2). The clinical mani- Ocular: perivascular retinitis, hyphema,
festations of acute-phase disease are quite similar to those of retinal detachments, anterior uveitis,
RMSF as a result of the development of vasculitis. Ticks are corneal edema
most commonly found on dogs during the acute phase of Central nervous system: meningeal pain,
infection. Fever can occur in both clinical phases of infection paresis, cranial nerve deficits, seizures
but is more common in dogs with acute ehrlichiosis. Pete- Hepatomegaly
chiae or other evidence of bleeding noted during the acute Arrhythmias and pulse deficits
phase is generally caused by a combination of mild thrombo- Polyuria and polydipsia
cytopenia (consumption or immune-mediated destruction) Stiffness and swollen, painful joints
and vasculitis; thrombocytopenia (consumption, immune-
mediated destruction, sequestration, decreased production),
vasculitis, and platelet function abnormalities (Brandao and proteinuria are reported in some dogs that develop renal
et al., 2006) occur in the chronic phase. The thrombocyto- insufficiency.
penia in the acute phase is generally not severe enough to Stiffness, exercise intolerance, and swollen, painful joints
result in spontaneous bleeding, so bleeding may be primar- occur in some dogs with suppurative polyarthritis (see Fig.
ily from vasculitis and decreased platelet function. Some 95.2). Most dogs with polyarthritis from which the organism
dogs with E. canis infection induced experimentally show has been demonstrated have been infected with E. ewingii or
activated platelets, which may lessen the tendency to bleed A. phagocytophilum. Ophthalmic manifestations of disease
in some dogs (Shropshire et al., 2018). are common; tortuous retinal vessels, perivascular retinal
Pale mucous membranes usually only occur in the infiltrates, retinal hemorrhage, anterior uveitis (see Fig.
chronic phase during the development of pancytopenia. 95.4), and exudative retinal detachment occur (Komnenou
Hepatomegaly, splenomegaly, and lymphadenopathy are et al., 2007). CNS signs can include depression, pain, ataxia,
from chronic immune stimulation (i.e., lymphoreticular paresis, nystagmus, and seizures.
hyperplasia) and are detected most frequently in dogs in
the chronic phase. Interstitial or alveolar edema second- Diagnosis
ary to vasculitis or inflammation, pulmonary parenchymal Clinicopathologic and radiographic abnormalities consis-
hemorrhage secondary to vasculitis or thrombocytopenia, tent with E. canis infection are summarized in Table 95.3.
or secondary infections from neutropenia are mechanisms Neutropenia may occur during acute-phase vasculitis
resulting in dyspnea or cough in some dogs with ehrlichio- and after bone marrow suppression in the chronic phase.
sis. Pulmonary hypertension may occur in some dogs with Chronic immune stimulation causes monocytosis and lym-
chronic disease (Locatelli et al., 2012). Polyuria, polydipsia, phocytosis; lymphocytes often have cytoplasmic azurophilic