PATHOLOGY OF HAEMIC AND LYMPHOID SYSTEM

Poikilocytes

Broad term for RBCs with abnormal shapes, classified by specific shape changes, caused by various diseases, significance depends on number, shape, and clinical context, few poikilocytes may be normal in healthy or sick animals and should be interpreted with clinical findings.

Acanthocytes

Spherical RBCs with 3–12 irregular blunt or club-shaped asymmetric spicules, caused by liver disease and lipid metabolism disorders, irregular spicules distinguish them from echinocytes, commonly associated with hepatic disease.

Echinocytes/Burr Cells/Crenated Cells

RBCs with sharp evenly spaced uniform spicules, commonly an artifact from stored or old blood but may occur in disease, always rule out artifact before interpretation.

Dacryocytes

Tear-drop-shaped RBCs associated with bone marrow fibrosis, hemolytic anemia, and drug reactions including phenothiazine and chloramphenicol, suggest marrow pathology or RBC distortion during marrow release.

Codocytes/Target Cells/Leptocytes

RBCs with a central hemoglobinized area surrounded by a pale ring producing a bullseye appearance, caused by excess membrane or decreased hemoglobin, associated with iron deficiency anemia, obstructive liver disease, and cirrhosis.

Schizocytes

Fragmented RBCs caused by mechanical injury from fibrin strands such as thrombosis, common in DIC, also seen in iron deficiency anemia, normal finding in young ruminants.

Drepanocytes/Sickle Cells

Sickle-shaped RBCs associated with blood parasites including malaria and trypanosomiasis, also caused by a recessive gene defect in humans, normal finding in deer and should be distinguished from pathologic cases.

Elliptocytes

Elongated or elliptical RBCs associated with various diseases, Type I is slightly oval, Type II is rounded to oval, Type III is elongated elliptical, classification has little clinical significance.

Spherocytes

RBCs with increased central thickness and loss of normal biconcave shape, caused by membrane depletion or accelerated RBC aging especially IMHA, strong indicator of immunohemolytic anemia and immune-mediated RBC destruction.

Erythrocytosis/Polycythemia

Increased RBC mass, PCV/HCT, hemoglobin, and peripheral RBC count, caused by increased RBC production or decreased plasma volume, results in hyperviscosity and hypervolemia, RBC count is the best indicator of RBC mass.

Relative Erythrocytosis

Apparent increase in RBC concentration with normal RBC mass due to decreased plasma volume, most commonly dehydration from vomiting, diarrhea, polyuria, hypodipsia, or inappetence, most common form in veterinary patients.

Absolute Erythrocytosis

True increase in RBC mass caused by increased erythropoiesis, divided into Primary and Secondary erythrocytosis.

Primary Erythrocytosis/Polycythemia Vera

Autonomous bone marrow RBC production caused by a myeloproliferative disease, EPO-independent.

Secondary Erythrocytosis – Appropriate

Increased RBC production secondary to chronic hypoxia through increased EPO release, seen with chronic respiratory disease, brachycephalic airway syndrome, and right-to-left cardiac shunts.

Secondary Erythrocytosis – Inappropriate

Increased RBC production despite normal oxygenation due to excess EPO, hormones, or cytokines, consider renal disease, endocrine disease, EPO-producing lesions, or drugs.

Anemia

Decreased RBC mass, PCV/HCT, and hemoglobin causing reduced oxygen-carrying capacity, results from blood loss, hemolysis, or decreased RBC production, clinical signs mainly due to tissue hypoxia, commonly detected on CBC.

Regenerative Anemia

Bone marrow responds by releasing reticulocytes, usually due to blood loss or hemolysis, indicates functional bone marrow.

Regenerative Anemia – Blood Loss (Hemorrhage)

Caused by trauma, organ rupture, perforation, or coagulopathies including rodenticide toxicity, chronic GI bleeding may eventually become nonregenerative because of iron deficiency.

Regenerative Anemia – Hemolysis

Premature RBC destruction caused by IMHA, DIC, caval syndrome, and hemophagocytic histiocytic sarcoma, clues include:

• Spherocytes = IMHA

• Schizocytes = DIC

• Icterus + Pigmenturia = hemolysis

Nonregenerative Anemia

Inadequate RBC production with absent or minimal reticulocyte response, caused by bone marrow disease or systemic suppression of erythropoiesis, indicates impaired erythropoiesis.

Pre-regenerative Anemia

Early stage after recent blood loss or hemolysis, initially appears nonregenerative, reticulocytosis develops after 2–5 days.

Medullary Nonregenerative Anemia

Failure of RBC production within the bone marrow due to primary marrow disease or secondary marrow suppression, bone marrow is the primary site of pathology.

Extramedullary Nonregenerative Anemia

Bone marrow is intact but erythropoiesis is inadequately stimulated by systemic disease such as CKD, chronic inflammatory disorders, or decreased EPO.

Bone Marrow Pool

Leukocyte reserve awaiting maturation and release, part of normal leukopoiesis, includes bone marrow reserves with lymphoid contribution from lymph nodes and spleen.

Marginal Pool

Leukocytes attached to vascular endothelium, normal physiologic distribution, rapidly mobilized during inflammation or infection.

Circulating Pool

Leukocytes freely circulating in blood, measured on CBC.

Leukocytosis

Increased circulating leukocyte count caused by infection, tissue necrosis, non-fatal intoxication, neoplasia, hemolysis, severe hemorrhage, trauma, or hypersensitivity, identify which leukocyte population is increased.

Leukopenia

Decreased circulating leukocyte count caused by bone marrow degeneration, depression, depletion, or destruction, suggests bone marrow failure or suppression, includes neutropenia, lymphopenia, eosinopenia, and panleukopenia.

Panleukopenia

Decrease in all leukocyte types caused by severe bone marrow suppression or destruction, hallmark of severe marrow disease and viral infections such as feline panleukopenia.

Leukemia

Presence of neoplastic leukocytes in blood or bone marrow due to hematopoietic malignancy, classified as acute or chronic.

Acute Leukemia

Aggressive bone marrow cancer of hematopoietic stem cells, rapidly fatal without treatment.

Chronic Lymphocytic Leukemia (CLL)

Neoplastic proliferation of small mature lymphocytes, often presents with persistent lymphocytosis and may be an incidental CBC finding.

Bazophila

Increased absolute basophil count, usually associated with inflammatory or hypersensitivity disorders, rare and commonly occurs with eosinophilia.

Neutrophils

Phagocytic WBCs produced in bone marrow, mature cells have lobulated nuclei, immature cells have band nuclei, first responders in bacterial infection and acute inflammation, band neutrophils indicate a left shift.

Neutrophilia

Increased neutrophil count caused by bacterial infection, acute inflammation, stress, or glucocorticoids, most common leukocyte response in acute bacterial disease.

Neutropenia

Decreased neutrophil count caused by viral infections, toxins, carbimazole, methimazole, or immune-mediated destruction, increases risk of secondary bacterial infections.

Eosinophils

WBCs with large pink or red granules, produced in bone marrow, inactivate histamine, reduce edema, important in allergic reactions and parasitism.

Eosinophilia

Increased eosinophil count caused by hypersensitivity, parasites, tissue injury, mast cell tumors, estrus, and pregnancy or parturition in bitches, clue is itchy animal + eosinophilia = allergy or parasites.

Eosinopenia

Decreased eosinophil count caused by glucocorticoids, classic stress or steroid leukogram finding.

Basophils

Rare granulocytes with blue or violet granules, produced in bone marrow, release histamine, closely related to mast cells, more common in cattle.

Basophilia

Increased basophil count caused by hypersensitivity, parasitism, tissue injury, mast cell tumors, estrus, and pregnancy or parturition in bitches, frequently accompanies eosinophilia.

Lymphocytes

Small WBCs with round nucleus and scant cytoplasm, mediate adaptive immunity, develop mainly in lymph nodes, spleen, and GALT, only leukocytes that recirculate, B cells produce antibodies and T cells mediate cell-mediated immunity.

Lymphocytosis

Increased lymphocyte count caused by physiologic excitement especially in cats or lymphoid leukemia, persistent marked lymphocytosis suggests CLL or leukemia.

Lymphopenia

Decreased lymphocyte count caused by corticosteroids, stress, Cushing's disease, and viral infections including parvovirus, classic component of the stress leukogram, may accompany neutropenia in viral disease.

Monocytes

Large WBCs with blue-gray vacuolated cytoplasm and kidney-shaped or lobulated nucleus, produced in bone marrow, become tissue macrophages, important in phagocytosis and chronic inflammation.

Monocytosis

Increased monocyte count caused by chronic inflammation, chronic disease, neoplasia, and glucocorticoids in dogs, characteristic of chronic inflammatory conditions and part of the canine steroid leukogram.

Thrombocytopenia

Platelet count below reference interval, most common hemostatic disorder in dogs and cats, inherited forms are usually mild to moderate, acquired causes include decreased production, increased consumption, increased destruction, sequestration, and excessive loss, key phrase is spontaneous bleeding with petechiae, ecchymoses, epistaxis, hematochezia, melena, hematuria, hyphema, and prolonged bleeding after trauma or venipuncture, severe bleeding usually indicates an acquired cause.

Inherited Thrombocytopenia

Congenital decrease in platelet count caused by genetic disorders in certain dog breeds, usually mild to moderate, severe thrombocytopenia suggests an acquired disease.

Acquired Thrombocytopenia

Decreased platelet count secondary to disease caused by decreased production, increased consumption, immune-mediated destruction, splenic sequestration, or hemorrhage, often involves multiple mechanisms simultaneously.

Thrombocytosis

Increased platelet count caused by primary bone marrow disease such as megakaryocytic leukemia, chronic blood loss, iron deficiency, or chronic corticosteroid exposure, may be primary or secondary reactive.

Myeloid Hyperplasia

Increased production of myeloid precursors in bone marrow due to increased demand during infection or inflammation, may occur at the expense of other blood cell lines.

Bone Marrow Aplasia (Pancytopenia)

Bone marrow failure causing decreased production of RBCs, WBCs, and platelets, caused by irradiation, chemical toxins, bacterial toxins, prolonged chloramphenicol use, and bracken fern, key phrase is pancytopenia.

Bone Marrow Necrosis

Death of bone marrow tissue caused by direct injury from toxins or impaired blood supply due to microcirculatory failure.

Myelofibrosis

Replacement of normal bone marrow with fibrous tissue, part of the repair process, commonly secondary to hemolytic anemia, results in decreased hematopoiesis.

Myelophthisis

Replacement of normal bone marrow by metastatic neoplastic cells, causes decreased hematopoiesis.

Myelodysplasia (Dysmyelopoiesis)

Abnormal maturation and development of bone marrow cells due to defective cell division, commonly occurs during increased demand for RBCs and WBCs, immature blast cells may enter circulation.

Myeloproliferative Disorder

Abnormal proliferation of bone marrow cells, includes myelodysplasia and neoplasia, characterized by numerous neoplastic myeloid cells, marrow infiltration may cause hepatomegaly and splenomegaly.

Lymphocytic Leukemia

Leukemia with predominance of lymphocytes due to neoplastic proliferation of lymphocytic cells, classified by the predominant abnormal circulating cell.

Granulocytic Leukemia

Leukemia with predominance of granulocytes due to neoplastic proliferation of granulocytic cells.

Myelocytic Leukemia

Leukemia with predominance of myelocytes caused by neoplastic proliferation of myelocytic cells.

Myeloblastic Leukemia

Leukemia with predominance of myeloblasts due to neoplastic proliferation of immature myeloid precursors, characterized by increased blast cells in peripheral blood.

Hairy Cell Leukemia

Leukemia characterized by hairy cell morphology due to neoplastic proliferation of abnormal hematopoietic cells, recognized in domestic animals and humans.

Splenomegaly

Enlargement of the spleen that may be diffuse or localized, caused by neoplasia, inflammation, hyperplasia, congestion, or infiltration.

Splenic Atrophy

Decrease in spleen size due to severe blood loss, producing a dry, shrunken, and fibrous spleen.

Localized Splenic Enlargement – Neoplastic

Focal splenic enlargement caused by lymphosarcoma, hemangiosarcoma, or other splenic tumors.

Localized Splenic Enlargement – Non-neoplastic

Focal enlargement caused by localized hematoma, segmental congestion, or nodular hyperplasia.

Diffuse Granulomatous Splenitis

Granulomatous inflammation of the spleen caused by infectious microorganisms, produces uniform splenomegaly and may form focal or multifocal nodules.

Diffuse Splenomegaly

Uniform splenic enlargement caused by splenitis, red pulp hyperplasia, white pulp hyperplasia, right-sided congestive heart failure, or myeloproliferative disorders.

White Pulp Hyperplasia

Hyperplasia of splenic white pulp due to antigenic or immune stimulation, causes diffuse splenomegaly.

Red Pulp Hyperplasia

Hyperplasia of splenic red pulp caused by increased hematologic demand or congestion.

Splenic Infarction

Ischemic necrosis of splenic tissue caused by thromboembolic disease, acute lesions are red-black, wedge-shaped, and blood-filled, chronic lesions are pale gray-white, wedge-shaped, firm, and contracted from fibrosis.

Accessory Spleens

Ectopic splenic tissue sealed by the omentum after previous non-fatal abdominal trauma with splenic rupture, usually an incidental necropsy finding.

Splenic Pigmentation (Siderofibrotic Plaques)

Yellow calcareous plaques on the splenic capsule caused by chronic pigment and mineral deposition.

Lymphadenopathy

Non-specific enlargement of lymph nodes caused by lymphadenitis, lymphoid hyperplasia, or neoplasia, determine whether inflammatory, hyperplastic, or neoplastic.

Lymphoid Hyperplasia

Increased proliferation of lymphoid tissue caused by antigenic stimulation, major cause of lymphadenopathy.

Sinus Histiocytosis

Hyperplasia of the monocyte-macrophage system within lymph node sinuses caused by increased phagocytic activity, form of lymph node hyperplasia.

Follicular Hyperplasia

Hyperplasia of B-lymphocyte follicles caused by antigenic stimulation.

Paracortical Hyperplasia

Hyperplasia of the T-cell region of lymph nodes caused by cell-mediated immune stimulation.

Lymphoid Atrophy

Decrease in lymphoid tissue and lymph node size caused by congenital disorders, lack of antigenic stimulation, viral infections, cachexia, malnutrition, aging, radiation, immunodeficiency, or lymphoid exhaustion, produces small lymph nodes.

Acute Lymphadenitis

Acute inflammation of a lymph node caused by bacterial or other infectious agents, node is firm, enlarged, tense, bulging, with a wet cut surface containing blood, edema, and inflammatory cells.

Chronic Lymphadenitis

Chronic inflammation of a lymph node caused by persistent infection or inflammation, common cause of chronic lymphadenopathy.

Acute Suppurative Lymphadenitis

Acute pus-forming lymphadenitis caused by pyogenic bacteria, in horses the retropharyngeal lymph nodes are markedly distended with pus.

Focal Granulomatous Lymphadenitis

Localized granulomatous inflammation caused by granuloma-forming infectious agents.

Diffuse Granulomatous Lymphadenitis

Widespread granulomatous inflammation replacing lymph node tissue due to chronic granulomatous infections, causes diffuse lymph node enlargement.

Caseous Lymphadenitis

Abscess replaces the entire lymph node due to suppurative bacterial infection, represents the early stage before pus becomes laminated.

Tuberculous Lymphadenitis

Caseating granulomatous lymphadenitis caused by tuberculosis, characterized by numerous yellow-brown caseating granulomas and loss of normal lymph node architecture.

Cryptococcal Lymphadenitis

Granulomatous lymphadenitis caused by cryptococcosis, in cats commonly affects the right mandibular lymph node with complete loss of architecture.

Secondary (Metastatic) Neoplasms

Metastatic tumor infiltration of lymph nodes, common cause of lymphadenopathy.

Primary Lymphoma

Primary malignant neoplastic proliferation of lymphoid cells within lymph nodes.

Jowl Abscess (Pig)

Marked enlargement of mandibular lymph nodes in pigs caused by Streptococcus porcinus, resulting in suppurative lymphadenitis.

Thymic Involution

Normal reduction of thymic parenchyma after birth due to physiologic aging and maturation, also seen in congenital immunodeficiencies.

Thymic Agenesis

Complete absence of the thymus due to congenital developmental defect, nude mice lack a thymus and T cells, widely used in immunology research.

Thymic Hypoplasia

Incomplete development of the thymus due to congenital developmental defect, associated with immunodeficiency from reduced T-lymphocyte maturation.

Thymic Hemorrhage

Hemorrhage within the thymus associated with the agonal process and septicemic diseases.

Thymitis (Thymic Inflammation)

Inflammation of the thymus caused by canine distemper, feline panleukopenia, and equine viral rhinopneumonitis.

Thymic Atrophy

Reduction in thymic size and lymphoid tissue caused by cortisone administration or hyperadrenocorticism, corticosteroid-induced.

Epithelial Thymoma

Primary epithelial tumor of the thymus, often cystic at the thoracic inlet or base of the heart, may obstruct lymphatic drainage causing hydrothorax.

Thymic Lymphoma (Thymic Lymphosarcoma)

Primary malignant lymphoma of the thymus, forms a large cranial mediastinal mass displacing the lungs, often with widespread organ involvement, synonymous with malignant lymphoma or lymphosarcoma when the thymus is the primary site.