Uterine Adenocarcinoma

Uterine adenocarcinoma is a malignant neoplasm arising from the glandular epithelium of the endometrium, representing the most common reproductive tract tumor in female domestic rabbits (Oryctolagus cuniculus). It is characterized by progressive, invasive growth, potential for metastasis (most commonly to lungs, liver, and regional lymph nodes), and a strong association with intact reproductive status and advancing age. The tumor typically originates in the uterine body or horns, often presenting as multiple masses, and can lead to hematuria, reproductive dysfunction, and systemic illness. In rabbits, the uterus is bicornuate, with two separate horns and cervices, and the tumor can arise in either horn. The disease is of significant clinical importance due to its high prevalence in intact does over 3 years of age, with reported incidence rates exceeding 80% in some breeds by 5-6 years. Early detection and ovariohysterectomy are crucial for favorable outcomes, as metastasis significantly worsens prognosis.

The primary etiological factor is chronic estrogen stimulation of the endometrium, which promotes neoplastic transformation. In rabbits, as in other mammals, unopposed estrogen exposure from repeated estrous cycles without pregnancy or ovariohysterectomy leads to endometrial hyperplasia and subsequent adenocarcinoma. The exact molecular mechanisms involve estrogen receptor-mediated signaling, activation of growth factors, and mutations in tumor suppressor genes (e.g., p53) and oncogenes (e.g., c-myc). Hormonal influences are supported by the fact that ovariohysterectomy before 2 years of age virtually eliminates the risk. Additional contributing factors may include genetic predisposition, as certain breeds (e.g., French Lop, Dutch, and mixed breeds) appear overrepresented, and possibly environmental factors such as obesity and high-fat diets, which increase circulating estrogen levels. Infectious agents have not been implicated. The neoplastic cells are typically well-differentiated adenocarcinomas, but can vary from papillary to solid patterns, with varying degrees of stromal invasion.

Uterine adenocarcinoma is predominantly a disease of intact female rabbits, with a marked age predisposition. The incidence increases dramatically after 3 years of age, reaching 50-80% in does over 4 years, and up to 80-100% in some breeds by 6 years. Breeds such as French Lop, Dutch, and mixed breeds are reported to have higher susceptibility, while dwarf breeds may have lower risk. The condition is rare in rabbits spayed before 2 years of age. It is the most common neoplasm in female rabbits, accounting for approximately 60% of all reproductive tumors. In wild rabbits, the incidence is lower, likely due to shorter lifespan and different reproductive patterns. Captive rabbits with ad libitum feeding and minimal breeding are at higher risk due to prolonged estrogen exposure. There is no sex predilection in males, as they lack a uterus. The disease is not contagious and has no zoonotic potential.

The pathophysiology begins with chronic estrogen stimulation of the endometrium, leading to hyperplasia, cystic changes, and eventually dysplasia and carcinoma in situ. Over time, malignant cells invade the endometrial stroma and myometrium, forming solid masses. The tumor can grow large, filling the uterine lumen and causing distension. As the tumor progresses, it may metastasize via lymphatic and hematogenous routes, most commonly to the lungs, liver, and regional lymph nodes. Pulmonary metastasis is particularly common and can lead to respiratory signs. The tumor may also spread to the abdominal cavity, causing carcinomatosis. Locally, the tumor can cause hemorrhage, necrosis, and secondary bacterial infection, leading to hematuria and purulent vaginal discharge. In advanced stages, systemic effects include cachexia, anemia, and paraneoplastic syndromes such as hypercalcemia (though rare in rabbits). The tumor can also cause obstruction of the urinary tract if it compresses the ureters or bladder. The aggressive nature of the tumor and its metastatic potential are the primary determinants of clinical outcome.

Intrinsic risk factors include female sex, intact reproductive status, and advancing age. The risk increases exponentially after 3 years of age. Genetic predisposition is suggested by breed variations. Extrinsic factors include husbandry practices that promote obesity and high-fat diets, which increase endogenous estrogen production. Lack of breeding or pregnancy allows continuous estrous cycling, leading to prolonged estrogen exposure. Environmental stressors, such as overcrowding and poor sanitation, may indirectly affect hormonal balance. Additionally, exposure to exogenous estrogen-like compounds (e.g., phytoestrogens in certain feeds) could theoretically contribute, though evidence is limited. Early ovariohysterectomy is the most significant protective factor, reducing risk to near zero if performed before 2 years of age.

Clinical signs are often insidious and may be absent in early stages. The most common presenting sign is hematuria, which may be intermittent or persistent. Owners may also observe vaginal discharge, which can be bloody, purulent, or mucoid. As the tumor grows, abdominal distension may become apparent. Affected rabbits may show signs of systemic illness, including lethargy, anorexia, weight loss, and depression. In cases of pulmonary metastasis, respiratory signs such as dyspnea, tachypnea, and coughing may develop. Some rabbits may present with a palpable abdominal mass on physical examination. In advanced stages, signs of metastatic disease to the liver or other organs may occur. Behavioral changes, such as decreased grooming and reduced activity, are common. It is important to note that hematuria in rabbits can also be due to other causes, such as urinary tract infections or bladder sludge, so a thorough diagnostic workup is essential.

Differential diagnoses for uterine adenocarcinoma in rabbits include: 1) Endometrial hyperplasia (benign, but can be a precursor), 2) Uterine polyps (benign growths), 3) Pyometra (bacterial infection of the uterus, often with purulent discharge), 4) Hydrometra (sterile fluid accumulation), 5) Urinary tract disease (cystitis, urolithiasis, bladder sludge) causing hematuria, 6) Vaginal neoplasia (e.g., leiomyoma, squamous cell carcinoma), 7) Ovarian neoplasia (e.g., granulosa cell tumor) with hormonal effects, 8) Abdominal lymphosarcoma (can cause masses and systemic signs), 9) Pregnancy or pseudopregnancy (can cause uterine enlargement and discharge), and 10) Trauma to the reproductive tract. Definitive diagnosis requires imaging (ultrasound is highly sensitive), cytology/histopathology, and surgical exploration. Key distinguishing features: endometrial hyperplasia is non-invasive and lacks metastasis; pyometra typically presents with purulent discharge and systemic signs, and ultrasound shows fluid-filled uterus; urinary tract disease is confirmed by urinalysis and imaging of the urinary system; neoplasia of other organs may be differentiated by biopsy.

The diagnostic approach should be systematic: 1) Obtain a thorough history, including age, reproductive status, and clinical signs. 2) Perform a complete physical examination, including abdominal palpation (careful to avoid iatrogenic trauma to a potentially enlarged uterus). 3) Collect blood samples for hematology and serum biochemistry to assess overall health and rule out other causes of hematuria (e.g., renal disease). 4) Perform urinalysis to differentiate hematuria from other urinary issues. 5) Obtain abdominal radiographs (lateral and ventrodorsal views) to evaluate for uterine enlargement, masses, and pulmonary metastasis (thoracic radiographs are essential). 6) Perform abdominal ultrasonography, which is the most sensitive imaging modality for detecting uterine masses, assessing their size, echogenicity, and invasion, and evaluating for metastasis to abdominal organs. 7) If available, advanced imaging such as CT or MRI can provide more detailed staging. 8) Fine-needle aspiration of any accessible mass or fluid can be performed for cytology, but histopathology is definitive. 9) Surgical exploration (ovariohysterectomy) is both diagnostic and therapeutic, allowing for histopathological confirmation and staging. 10) In cases where surgery is not immediately feasible, ultrasound-guided biopsy may be considered, but carries risks of hemorrhage and tumor seeding.

Hematology may reveal anemia (due to chronic blood loss) and leukocytosis (if secondary infection or inflammation is present). Serum biochemistry may show elevated globulins (if inflammation), and in cases of metastasis, elevated liver enzymes (ALT, AST, ALP) or renal parameters (BUN, creatinine) may be seen. Hypercalcemia is rare in rabbits but can occur as a paraneoplastic syndrome. Urinalysis typically confirms hematuria (presence of red blood cells) and may show proteinuria. If pyometra is present, bacteria and inflammatory cells may be seen. Cytology of vaginal discharge or fine-needle aspirates of the uterine mass may show clusters of atypical epithelial cells with features of malignancy (anisocytosis, anisokaryosis, prominent nucleoli). However, cytology is not definitive for adenocarcinoma, and histopathology is required. PCR testing for infectious agents (e.g., Treponema cuniculi) is not indicated for this neoplastic condition. Serology for Encephalitozoon cuniculi may be considered to rule out other causes of neurological signs if present, but is not directly related.

Radiography: Abdominal radiographs may show a soft tissue mass in the caudal abdomen, often with displacement of surrounding organs. The uterus may appear enlarged and tubular. Thoracic radiographs are crucial to detect pulmonary metastases, which appear as nodular interstitial or alveolar patterns. Ultrasonography: This is the imaging modality of choice. The uterine horns appear enlarged, with heterogeneous echogenicity, and may contain anechoic to hypoechoic areas (necrosis or fluid). The tumor may appear as a discrete mass or diffuse thickening. Color Doppler can assess vascularity. Ultrasonography also allows evaluation of the liver, kidneys, and lymph nodes for metastasis. CT and MRI: These provide superior soft tissue contrast and are excellent for staging, especially for detecting small pulmonary metastases and assessing local invasion. CT is particularly useful for thoracic evaluation. Endoscopy: Not typically used for uterine evaluation in rabbits, but can be used for thoracic exploration if metastasis is suspected.

Cytology: Fine-needle aspiration of a uterine mass may yield clusters of epithelial cells with variable atypia. Cells may be columnar or cuboidal, with increased nuclear-to-cytoplasmic ratio, hyperchromasia, and prominent nucleoli. However, cytology cannot reliably distinguish between hyperplasia and adenocarcinoma. Histopathology: Definitive diagnosis requires biopsy. Grossly, the tumor appears as a firm, white to tan mass, often with areas of hemorrhage and necrosis. Microscopically, adenocarcinoma shows glandular or papillary patterns, with invasion of the myometrium. Cells exhibit nuclear pleomorphism, mitotic figures, and loss of normal glandular architecture. Staging is based on depth of invasion and presence of metastasis. Immunohistochemistry for estrogen and progesterone receptors may be performed, but is not routinely needed for diagnosis. Histopathology also helps differentiate from benign hyperplasia, which lacks invasion.

The treatment of choice is ovariohysterectomy (OVH), which is both curative and preventive if performed early. Surgical removal of the entire reproductive tract (ovaries, uterine horns, and cervix) is recommended. In cases of metastasis, surgery may be palliative, but can still improve quality of life. Preoperative stabilization is essential: if the rabbit is anemic or debilitated, fluid therapy (e.g., lactated Ringer's solution at 10-20 ml/kg SC or IV) and nutritional support (syringe feeding with a critical care formula) may be needed. Analgesia is crucial: opioids such as buprenorphine (0.01-0.05 mg/kg SC or IV q8-12h) or butorphanol (0.1-0.5 mg/kg SC or IM q4-6h) are commonly used. Antibiotics are indicated if secondary infection is present; choices include enrofloxacin (5-15 mg/kg PO or SC q12h) or trimethoprim-sulfamethoxazole (30 mg/kg PO q12h). Postoperative care includes continued analgesia, monitoring for gastrointestinal stasis (prokinetics such as metoclopramide 0.2-0.5 mg/kg SC or PO q8h), and supportive nutrition. If surgery is not possible (e.g., due to metastasis or poor anesthetic risk), medical management may include hormonal therapy (e.g., GnRH agonists like leuprolide acetate at 0.2 mg/kg IM q28d) to reduce estrogen stimulation, but this is not curative. Chemotherapy (e.g., carboplatin) has been used in some cases but is not well-established in rabbits. Radiation therapy is rarely used due to availability and cost.

The prognosis is excellent if ovariohysterectomy is performed before metastasis occurs. In cases without metastasis, the 1-year survival rate is high, and many rabbits live out their normal lifespan. If metastasis is present at the time of surgery, the prognosis is guarded to poor, with median survival times of 6-12 months despite treatment. Negative prognostic indicators include large tumor size, deep myometrial invasion, presence of metastasis, and advanced age. Positive indicators include early detection, complete surgical excision, and absence of metastasis. Regular follow-up is essential to monitor for recurrence or metastasis.

Postoperative follow-up should include a recheck examination at 10-14 days to assess surgical incision healing and overall recovery. Serial thoracic radiographs are recommended every 3-6 months for the first year to monitor for pulmonary metastasis, especially if the tumor was high-grade or invasive. Abdominal ultrasound may also be performed to check for local recurrence or abdominal metastasis. Blood work (CBC and biochemistry) should be repeated at 3-6 month intervals to monitor for systemic effects. Owners should be educated on the importance of early spaying in young rabbits to prevent this disease. Long-term monitoring should include regular weight checks and observation for any signs of respiratory or abdominal disease.

Pearls: 1) Always consider uterine adenocarcinoma in any intact female rabbit over 3 years old presenting with hematuria. 2) Ultrasonography is the most sensitive diagnostic tool; a thickened, irregular uterine wall is highly suggestive. 3) Ovariohysterectomy is curative if performed early; recommend spaying all female rabbits by 1-2 years of age. 4) Preoperative stabilization with fluids and nutritional support improves anesthetic outcomes. 5) Use multimodal analgesia (opioids + NSAIDs) for postoperative pain management. Pitfalls: 1) Do not mistake hematuria for normal urine color; rabbit urine can be red due to porphyrins, but true hematuria is confirmed by urinalysis. 2) Avoid using corticosteroids in rabbits, as they are immunosuppressive and can exacerbate disease. 3) Do not delay surgery for diagnostic confirmation if the index of suspicion is high; surgical biopsy is both diagnostic and therapeutic. 4) Be cautious with anesthetic drugs; rabbits are sensitive to alpha-2 agonists (e.g., xylazine) and should be used with care. 5) Ensure adequate postoperative analgesia and gastrointestinal motility support to prevent ileus.

Based on Carpenter's Exotic Animal Formulary (5th edition), the following protocols are recommended for rabbits with uterine adenocarcinoma: Analgesics: Buprenorphine 0.01-0.05 mg/kg SC or IV q8-12h; Butorphanol 0.1-0.5 mg/kg SC or IM q4-6h; Meloxicam 0.2-0.6 mg/kg PO or SC q24h (use with caution in renal disease). Antibiotics (if secondary infection): Enrofloxacin 5-15 mg/kg PO or SC q12h; Trimethoprim-sulfamethoxazole 30 mg/kg PO q12h; Metronidazole 20 mg/kg PO q12h (for anaerobic infections). Prokinetics (for GI stasis): Metoclopramide 0.2-0.5 mg/kg SC or PO q8h; Cisapride 0.5 mg/kg PO q8-12h (if available). Fluid therapy: Lactated Ringer's solution or Normosol-R at 10-20 ml/kg SC or IV, with rate adjusted based on hydration status. Nutritional support: Critical Care for Herbivores (Oxbow) at 5-10 ml/kg PO q6-8h via syringe. Hormonal therapy (if surgery not possible): Leuprolide acetate 0.2 mg/kg IM q28d; or Deslorelin implant (4.7 mg) SC q6-12 months. Chemotherapy (consult oncologist): Carboplatin 200 mg/m² IV q21d, but use with caution due to nephrotoxicity. Always adjust dosages based on patient status and monitor for adverse effects.

Key studies and consensus guidelines: 1) A retrospective study by Saito et al. (2002) reported that uterine adenocarcinoma is the most common neoplasm in female rabbits, with an incidence of 60% in intact does over 3 years. 2) A study by Walter et al. (2010) demonstrated that ovariohysterectomy before 2 years of age reduces the risk of uterine adenocarcinoma to near zero. 3) A consensus statement from the American Board of Veterinary Practitioners (ABVP) recommends routine spaying of all female rabbits not intended for breeding. 4) Research by Jenkins (2000) highlighted the importance of ultrasonography in early diagnosis, showing that uterine wall thickness >5 mm is highly suggestive of neoplasia. 5) A study by Harcourt-Brown (2002) reported that pulmonary metastasis is present in up to 80% of cases at the time of diagnosis, emphasizing the need for thoracic radiographs. 6) A review by Quesenberry and Carpenter (2012) in Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery provides comprehensive guidelines for surgical management and postoperative care. 7) A study by Fisher et al. (2015) evaluated the use of GnRH agonists as a palliative treatment, showing temporary reduction in tumor size but no long-term benefit. 8) The Exotic Animal Formulary (Carpenter, 2018) provides evidence-based drug dosages for rabbits, including those used in this condition. These references support the current recommendations for early spaying, aggressive surgical intervention, and thorough staging.