JPC SYSTEMIC PATHOLOGY
HEMOLYMPHATIC SYSTEM
April 2018
H-B11 (NP)
Signalment (AFIP # POLA 27): German shepherd dog, age and gender not specified
HISTORY: A blood smear was obtained from an emaciated German shepherd dog.
HISTOPATHOLOGIC DESCRIPTION: Peripheral blood smear: The sample is of low cellularity. Rarely, monocytes contain round to oval, 4-6 um diameter, basophilic intracytoplasmic rickettsial morulae.
MORPHOLOGIC DIAGNOSIS: Cytologic specimen, peripheral blood smear: Intramonocytic ehrlichial morulae, few, German shepherd dog, canine.
CAUSE: Ehrlichia canis
CONDITION: Canine monocytotropic ehrlichiosis (CME)
SYNONYMS: Canine tropical pancytopenia, tracker dog disease, canine hemorrhagic fever
GENERAL DISCUSSION:
- Ehrlichia sp. are tick-transmitted, aerobic, gram-negative, obligate intracellular pleomorphic coccobacilli in the family Anaplasmataceae that infect leukocytes
- E. canis specifically infects monocytes, macrophages and lymphocytes
- Arthropod vector of E. canis is the brown dog tick (Rhipicephalus sanguineus)
- Transmission is transstadial (between life stages) only
- Transovarial transmission (between generations) does not occur
- Ticks carrying E. canis can transmit infection to susceptible dogs for up to 155 days post-acquisition, allowing the organism to overwinter within the tick
- Because transmission is transstadial and not transovarial, the tick is not considered to be a true reservoir
- Reservoir hosts for E. canis: Domestic dog, coyote, fox, jackal
- German shepherd dogs with ehrlichiosis develop a severe hemorrhagic syndrome because of decreased cell-mediated immunity to E. canis
PATHOGENESIS:
- Infection occurs when an infected tick ingests a blood meal and salivary secretions contaminate the feeding site
- During the initial incubation period (lasting 8-20 days), organisms multiply in macrophages by binary fission and spread throughout the body
- Ehrlichia spp. replicate in membrane-bound vacuoles that protect them from the host’s immune system, then survive in macrophages by inhibiting phagosome-lysosome fusion
- The bacteria can evade the immune system through repeated recombination in the outer membrane major antigenic protein-genes
- E. canis infection often results in a polyclonal hypergammaglobulinemia, and there is increasing evidence that this hyperimmune condition is involved in the pathogenesis
- Bone marrow production of blood elements is inhibited in chronic disease
- Thrombocytopenia occurs because of increased platelet consumption and decreased platelet half-life resulting from splenic sequestration and immune-mediated destruction (bound and circulating anti-platelet antibodies have been detected in the acute phase), as well as bone marrow hypoplasia; platelet dysfunction also occurs in addition to decreased platelet numbers
- Platelet migration-inhibition factor (PMIF) is a serum cytokine produced by lymphocytes exposed to infected monocytes that inhibits platelet migration; high levels of PMIF are associated with more virulent strains of E. canis
- An IgG2 reaction is the principal response in all phases of CME, which is associated with a T-helper type 1 response and a corresponding production of IFN-γ; INF-γ and TNF-α exert an anti-rickettsial effect via the induction of nitric acid synthesis
- Closely related to Ehrlichia chaffeensis (human monocytotropic ehrlichiosis), which can cause similar disease as E. canis in dogs and goats
TYPICAL CLINICAL FINDINGS:
- Three phases of disease: Acute, subclinical, and chronic
- Acute phase (2 - 4 weeks): Fever, oculonasal discharge, anorexia, depression, petechiae, ecchymoses, lymphadenomegally and splenomegaly
- Subclinical phase (40-120 days): Dog may appear clinically normal; if immunocompetent the animal may eliminate the organism
- Chronic phase: Impaired bone marrow production resulting in pancytopenia; can be asymptomatic, mild or severe; mild – weight loss, mild hematologic changes; severe – epistaxis, generalized hemorrhage, pallor, severe weight loss, anterior uveitis, retinal hemorrhage, neurologic signs, protein-losing nephropathy
CLINICAL PATHOLOGY:
- Thrombocytopenia is the most consistent hematologic finding in the acute and chronic stages of ehrlichiosis
- Acute phase: Thrombocytopenia, large platelets, nonregenerative anemia, mild leukopenia; may have concurrent Babesia canis, Anaplasma platys, Hepatozoon canis, or Bartonella vinsonii infection
- Chronic phase: Pancytopenia
- Lymphocytosis develops as disease progresses with large granular lymphocytes
- Examination of the buffy coat reveals Ehrlichia sp. morulae in lymphocytes and monocytes
- Examination of bone marrow in chronic phase reveals depletion of erythroid, myeloid, and megakaryocytic cells with plasmacytosis
- Hypergammaglobulinemia (usually polyclonal, but may be oligoclonal; does not parallel the magnitude of corresponding specific E. canis-antibody titers) and hypoalbuminemia, elevated ALT and ALP
- Proteinuria, hematuria, prolonged bleeding time (even in dogs with normal platelet counts)
TYPICAL GROSS FINDINGS:
- Widespread petechial and ecchymotic hemorrhages on serosal and mucosal surfaces of most organs, and on gingiva and conjunctiva; large subcutaneous hemorrhages
- Lungs: Mottled and edematous; focal hemorrhages
- Acute phase: Generalized lymphadenomegaly, splenomegaly, hepatomegaly, red bone marrow
- Chronic phase: Loss of body condition, emaciation, subcutaneous and interstitial edema, pale bone marrow
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Morulae of E. canis: Round, basophilic structures 1/3 to 1/2 the diameter of erythrocytes, with a granular internal basophilic elementary bodies (0.5-1.0 um in diameter); within mononuclear leukocytes
- Prominent perivascular plasmacytic infiltrate and vasculitis in many organs
- CNS: Multifocal, nonsuppurative meningoencephalitis
- Eye: Lymphoplasmacytic and monocytic anterior uveitis, chorioretinitis, retinal hemorrhage, conjunctivitis, conjuctival or iridal petechiae and ecchymosoes, inflammation most common in ciliary body
- Lungs: Interstitial pneumonia, interstitial and alveolar hemorrhages
- Kidneys: Glomerulonephritis and interstitial plasmacytosis
- Lymph nodes: Depleted germinal centers, marked medullary cord plasmacytosis and sinus histiocytosis, prominent erythrophagocytosis
- Bone marrow: In acute cases, it is hyperplastic and densely cellular with few fat cells; in chronic cases it is hypoplastic with dilated sinusoids and hemorrhagic infarcts; aplastic in terminal stages; may have marked plasmacytic infiltration
ULTRASTRUCTURAL FINDINGS:
- Morulae: Intracytoplasmic inclusions composed of numerous pleomorphic (round, ovoid or elongated) organisms surrounded by a double membrane
ADDITIONAL DIAGNOSTIC TESTS:
- Ehrlichia morulae in leukocytes from thin blood smears (made from peripheral capillary bed of ear margin); tissue aspirates (spleen, lung, lymph node); buffy coat smears
- Clinical signs, hematologic changes, demonstration of antibodies (serology, Western immunoblotting, IFA, ELISA test), and PCR-BP (biotinylated E. canis-specific primers) confirm the diagnosis
- Wright-Giemsa-stained impression smears of fresh tissues (lung, liver, lymph nodes, spleen): more effective for detection of morulae in macrophages
DIFFERENTIAL DIAGNOSIS:
- Rocky Mountain spotted fever (Rickettsia rickettsii): Acute phase signs of ehrlichiosis are identical to RMSF
Ehrlichia canis |
Rickettsia rickettsii |
|
Incidence all year |
Tick season (April-September) |
|
All ages of dogs |
Young dogs (< 2yrs) |
|
Purebreds susceptible; esp. German shepherds |
Purebreds susceptible |
|
Chronic and progressive |
Rapid and severe |
|
Anemia, leukopenia, thrombocytopenia |
Leukocytosis and thrombocytopenia |
|
Hyperglobulinemia and hypoalbuminemia |
Hypoalbuminemia, high serum alk phos & cholesterol |
|
Lymphoplasmacytic cuffing |
Necrotizing vasculitis |
|
Serology; PCR |
Tissue IFA; PCR |
COMPARATIVE PATHOLOGY:
Other rickettsial bacteria that infect leukocytes:
- Ehrlichia ewingii: Canine granulocytic ehrlichiosis; thrombocytopenia, lameness, muscular stiffness, arthritis, neutrophils with intracytoplasmic morulae in synovial fluid
- Anaplasma platys (formerly E. platys): Infectious canine cyclic thrombocytopenia
- Anaplasma phagocytophilum: Three strains with a close, but not identical, genetic relationship; each strain differs in species affected and geographic range
- Equine granulocytic ehrlichiosis (formerly E. equi); edema (ventral midline, limbs, prepuce), icterus, ataxia, petechiae, orchitis; cytoplasmic inclusions in neutrophils and eosinophils; vasculitis; low mortality unless secondary infection
- Tick-borne fever in young lambs and calves (formerly E. phagocytophila)
- Neorickettsia risticii (formerly E. risticii): Equine monocytic ehrlichiosis; fever, colitis
- Ehrlichia ruminantium (formerly Cowdria ruminantium): Causes heartwater in cattle, sheep and goats in Africa and on a few islands in the Caribbean; scientists have identified an organism that is identical or closely related to E. ruminantium in dogs (dogs could possibly transport organism from Africa)
Rickettsial bacteria that infect erythrocytes:
- Anaplasma marginale, Anaplasma centrale: Bovine anaplasmosis; anemia via immune-mediated extravascular hemolysis; calves younger than one year rarely develop clinical disease; cattle three years and older are more likely to develop severe illness; Indian cattle (Bos indicus) are more resistant to disease than European cattle (Bos taurus); surviving cattle become chronic carriers and develop cyclic bacteremia
- Anaplasma ovis: Ovine and caprine anaplasmosis
`REFERENCES
- Boes KM, Durham AC. Bone marrow, blood cells, and the lymphoid/lymphatic system. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017:749-50.
- Sykes JE. Ehrlichiosis, anaplasmosis, Rocky Mountain spotted fever, and neorickettsiosis. In: Ettinger SJ, Feldman EC, Cote E, eds Textbook of Veterinary Internal Medicine. 8th ed. St Louis, MO:Elsevier Saunders; 2017:944-951.
- Harrus S, Waner T, Neer TM. Ehrlichia canis In: Greene CE, ed. Infectious Diseases of the Dog and Cat. 4th ed. St. Louis, MO: Saunders; 2012:227-238.
- Valli VEO, Kiupel M, Bienzle D. Hematopoietic system: In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016:111.
