Etiology

Hyperplasia

• Caused by infectious etiologies (bacteria, viruses, protozoa), when localized or systemic infection does not directly involve the lymph node(s).
• Antigenic stimulation by non-infectious causes.

Lymphadenitis

• Bacterial infection causing purulent lymphadenitis and may progress to abscessation:
  • Pseudotuberculosis Pseudotuberculosis, caused by the bacterium Yersinia pseudotuberculosis Yersinia pseudotuberculosis occasionally seen in rabbits from ingesting contaminated food by rodents. Caseous necrosis can be seen in the mesenteric lymph nodes as well gut-associated lymphoid tissue, liver and spleen.
  • Tularemia Tularemia, a zoonotic disease from the bacterium Francisella tularensis Francisella tularensis is rarely seen in domesticated rabbits in North America but is more commonly seen in jackrabbits (Lepus spp) and cottontails (Sylvilagus spp). Necropsy findings are consistent with septicemia, acute focal necrosis of the liver, spleen and/or bone marrow, hemorrhagic enteritis, typhlitis and intestinal lymphadenitis.
  • Mesenteric lymphadenomegaly observed in rabbits with colibacillosis Colibacillosis.
• Fungi: systemic infections:
  • Rare.
  • Single case report of disseminated histoplasmosis (Histoplasma capsulatum) in a pet rabbit with presence of intracellular and extracellular yeast organisms throughout the body, including the axillary lymph node.
• Eosinophilic:
  • May be associated with allergic inflammation of the organ being drained by the affected lymph node.
  • May be seen due to gastrointestinal eosinophilic disease.
  • Possible – reported in other species, but not the rabbit.

Neoplasia

• Lymphoma/lymphosarcoma Lymphosarcoma: overview is the most common neoplasia seen in young rabbits and the second most common neoplasia overall.

Predisposing factors

General

• Immunosuppression/impaired immune function predisposes to infection.
• Wounds.

Pathophysiology

• This is often due to reactive lymphoid hyperplasia but can also be caused by lymphadenitis (extension of the inflammatory process into the lymph nodes).

Hyperplasia

• Reactive hyperplasia of lymph node(s) because of proliferation of plasma cells and lymphocytes in response to antigenic stimulation.

Lymphadenitis

• Active migration of neutrophils, activated macrophages, or eosinophils into the lymph node.

Neoplasia

• May be primary (malignant lymphoma) or metastatic.
• Lymphosarcomas may be either B-lymphocyte or T-lymphocyte in origin. It may be classified as either low or high-grade depending on the degree of signs of malignancy and spread.
• Lymphoma may be described as lymphoblastic (large cell) or lymphocytic (small cell).