JPC SYSTEMIC PATHOLOGY
HEMOLYMPHATICS SYSTEMS
April 2021
H-V10

SIGNALMENT (JPC #2286727): 9-year-old male Appaloosa

HISTORY: This horse presented with anorexia, ataxia, behavioral changes, and photosensitivity.

HISTOPATHOLOGIC DESCRIPTION: Liver: Diffusely, portal areas are infiltrated by moderate numbers of lymphocytes and plasma cells that extend into expanded sinusoids and multifocally infiltrate the capsule. Often, hepatic cords are attenuated (atrophic) with dilated sinusoids; in other areas hepatocytes are multifocally separated, individualized, or lost and replaced by the infiltrate.  Multifocally, hepatocyte cytoplasm is expanded by lacy vacuoles (glycogen type degeneration). Increased numbers of Kupffer cells and macrophages scattered throughout the sinusoids or in portal areas contain intracytoplasmic, fragmented erythrocytes (erythrophagocytosis) or yellow-brown, granular, cytoplasmic pigment (hemosiderin).  There are multifocal areas of hemorrhage. Multifocally, bile canaliculi are distended with green-brown, globular material (bile stasis).

MORPHOLOGIC DIAGNOSIS: Liver: Hepatitis, lymphoplasmacytic, portal, diffuse, moderate, with erythrophagocytosis, hemosiderosis, and mild hepatocellular degeneration and loss, Appaloosa, equine.

ETIOLOGIC DIAGNOSIS: Lentiviral hepatitis

CAUSE: Equine infectious anemia virus (EIAV), equine lentivirus

CONDITION: Equine infectious anemia

SYNONYMS: Swamp fever

GENERAL DISCUSSION:

• Virus in the Lentivirinae subfamily of Retroviridae (RNA virus); infects cells of the monocyte-macrophage system
• All species of Equidae are susceptible to infection (horses, mules, and donkeys); donkeys become persistently infected but do not develop clinical disease
• Mechanical transmission through the transfer of contaminated blood or blood products by biting insects or contaminated needles; transmission also transplacental, or by insemination with contaminated semen
• Viral replication does not occur in insect vectors (mechanical); vectors include the biting fly (Stomoxys calcitrans), horse flies (Tabanus), and mosquitoes (Anopheles spp., Culex spp.)
• Infection is lifelong: starts as acute severe illness that may be fatal; if the animal survives, there will be cyclic viremia with thrombocytopenia and anemia; eventual asymptomatic infection (and reservoir of infectious virus)
• Worldwide distribution; outbreaks usually in the summer in wet, marshy areas ("swamp fever"); clinical signs develop within 2 to 7 days of infection
• Virus causes anemia by immune-mediated hemolysis and decreased erythropoiesis

PATHOGENESIS:

• Mechanism of injury is inflammation and proliferation of the monocyte-macrophage and lymphocytic systems
• Infection of blood monocytes (without viral replication) > monocytes disseminate virus to tissues throughout body without host immune detection > differentiation of monocytes to macrophages as they enter tissue > activation of viral replication > high levels of virus in macrophage-rich tissues (liver, spleen) > proinflammatory chemokine/cytokine production by macrophages recruits additional leukocytes, resulting in splenomegaly/lymphadenomegaly > infected tissue macrophages in various tissues continue to release viral antigen (reservoir)
• Hemolytic anemia
  • Immune-mediated erythrocyte destruction (phagocytosis or complement lysis): Circulating viral antigen adsorbed to erythrocyte membranes (act as haptens) > antibody and C3 bind to the antigen > extravascular hemolysis via macrophage recognition of complement or haptens/ intravascular hemolysis (hemolysis is predominantly intravascular) due to type II hypersensitivity response in acute phase (Ab-viral hapten complex on red cell membrane activates complement cascade); “innocent bystander” destruction of erythrocytes
  • Anemia also due to direct suppression of early-stage erythroid cells by virus
  • Immune-mediated thrombocytopenia – platelets destroyed by similar mechanism or are activated and then bound by fibrinogen and phagocytosed; indirect viral suppression of megakaryocytes is also thought to contribute; platelets produced have decreased function in vitro
• Virus does not kill cells
• Recurrence in chronic EIA is due to antigenic variance of surface glycoproteins (gp90, gp45 and p26) that bind to equine lentivirus receptor-1 in cell membranes on monocytes/macrophages

TYPICAL CLINICAL FINDINGS:

• Acute: High fever (up to 108^oF), marked depression, anorexia, weakness, ataxia, weight loss, dependent edema, petechiae (ventral tongue, ocular and vulvar mucosa) due to thrombocytopenia, anemia (severe and sufficient to cause death), icterus (pre-hepatic)
• Chronic: Recurring 1-2 week cycles of fever, emaciation, weakness, ventral edema, serous atrophy of fat with bilirubin staining, anemia, and thrombocytopenia; lasts 8 to 12 months with decreasing frequency and severity of disease until asymptomatic infection; asymptomatic carriers may suffer relapse

CLINICAL PATHOLOGY:

• Anemia – PCV 14-20%
  • Acute – macrocytosis, anisocytosis, poikilocytosis (abnormally shaped RBCs)
    • Regenerative anemia without reticulocytes (reticulocytes not found in horses)
    • Anemia severe enough to cause death
  • Chronic – normochromic, normocytic, nonregenerative
• Sideroleukocytes – circulating monocytes with phagocytized erythrocytes and hemosiderin; indicate intravascular hemolytic anemia; highly suggestive of EIAV
• Bone marrow:
  • Acute – hypercellular with erythroid shift
  • Chronic – reduced cellularity with dyserythropoiesis; macrophage and plasma cell proliferation
• Thrombocytopenia
• Decreased serum albumin/globulin ratio with normal total protein
• Hyperbilirubinemia – unconjugated (indirect)
• CSF – elevated protein and leukocytes; xanthochromic

TYPICAL GROSS FINDINGS:

• Bone marrow: The increase of reddening (hyperplasia of the hematopoietic marrow) is directly proportional to disease duration
  • Dark red and yellow, focal hemorrhagic infarcts within the proximal and distal segments of the medullary cavity and along the subendosteal surface of the diaphysis
• Icterus, widespread hemorrhagic foci, subcutaneous edema, pallor, and renal petechiae (capsule, cortex, and medulla)
• Spleen, lymph nodes: Splenomegaly and lymphadenomegaly with capsular hemorrhages, and abundant lymphoid follicles that bulge from the cut surface
• Liver: Enlarged, dark, and turgid with capsular hemorrhage

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Lesions most prominent in the heart, lungs, liver, spleen, kidney, bone marrow, and lymph nodes; severity is dependent on the chronicity of infection
• Acute:
  • Hemosiderosis in major organs; prominent lymphoid hyperplasia
  • Ischemic necrosis, hemorrhage, or edema of the heart, liver, and kidneys with perivascular and interstitial lymphocytic infiltration
  • Bone marrow: Hypercellular with productive synchronous erythroid hyperplasia and reduction in mature granulocytes (eosinophils are prominent); megakaryocytes are not increased
• Chronic:
  • Monocyte/macrophage hyperplasia with hemosiderosis; lymphoid involution and atrophy
  • Liver: Periportal lymphocytic infiltrates, atrophic cords with sinusoidal dilation, Kupffer cell hyperplasia, hemosiderosis, hemorrhage, and hepatocellular necrosis in acute phase
  • Spleen: Large follicles but hypocellular with sharp distinction between follicular center and mantle and marginal zones and sinusoidal congestion ("bull's eye" appearance)
  • Kidney: Immune complex glomerulonephritis; IgG and C3 in mesangium and basement membranes
  • Lymph nodes: medullary hemosiderosis; persistence of follicles and thin moth-eaten paracortical areas; sclerosis and lymphoid atrophy later in disease
  • Bone marrow: Erythroid hyperplasia replaces adipose tissue; large macrophages containing hemosiderin, erythrocytes, or rubricytes; dyserythropoiesis with binucleate interphase rubricytes
  • Due to decreases in cellular packing, marrow will appear hypocellular; stromal cells are increased, endothelial nuclei are prominent; diffuse plasmacytosis
• CNS: Granulomatous ependymitis and choroid plexitis; inflammation surface related (central canal, ventricular system and leptomeninges) and most prominent in spinal cord
• Heart: Myocyte fiber atrophy

ADDITIONAL DIAGNOSTIC TESTS: 

• Agar-gel immunodiffusion test – AGID (Coggins test)
• Western blot, ELISA, PCR

DIFFERENTIAL DIAGNOSIS:

For anemia in the horse:

• Purpura hemorrhagica
• Equine granulocytic ehrlichiosis (Anaplasma phagocytophilum)
• Babesiosis (Theileria equi)
• Equine viral arteritis (Arterivirus)
• Red maple leaf toxicosis
• Neonatal isoerythrolysis
• Immune-mediated hemolytic anemia; idiopathic thrombocytopenia
• Gastric ulcers
• Parasitism (external or internal)
• Leptospirosis

COMPARATIVE PATHOLOGY:

Lentiviruses:

• Immunosuppressive (virus replicates in macrophages & lymphocytes)
  • Human immunodeficiency virus (HIV); feline immunodeficiency virus (FIV); simian immunodeficiency virus (SIV)
  • Bovine immunodeficiency virus (BIV); Jembrana Disease (JD) – cattle (mild, transient immunosuppression)
  • Immunostimulatory (virus replicates in macrophages)
  • Caprine arthritis-encephalitis virus (CAEV) – goats
  • Ovine progressive pneumonia virus (Maedi-Visna virus) – sheep
  • Equine infectious anemia virus (EIAV) – horses

References:

1. 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: Mosby Elsevier; 2017:758.
2. Campbell RSF, Robinson WF. The comparative pathology of lentiviruses. J Comp Path. 1998;119:333-395.
3. Constable PD, Hinchcliff KW, Done SH, Gruenberg W. Veterinary Medicine; a Textbook of the Diseases of Cattle, Horses, Sheep, Pigs and Goats. 11th ed. Philadelphia, PA: Saunders Elsevier; 2017:795-799.
4. Cullinane A, et al. Diagnosis of equine infectious anaemia during the 2006 outbreak in Ireland. Vet Rec. 2007;161(2):647-652.
5. Valli VEO, Kiupel M, Bienzle D. Hematopoietic system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. 6th ed. Vol 3, Philadelphia, PA: Saunders Elsevier; 2016:114-116.
6. Zachary JF. Mechanisms of microbial infections: In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Mosby Elsevier; 2017:218.

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