Surgical Diseases of the Rabbit

Rabbits can be challenging surgical patients. Fear, pain, and stress have a profound effect on survival in the perioperative period. Anorexia, depression, and death can occur following minor surgical procedures. This is believed to be a result of increased catecholamine release [1].

Rabbits ferment cellulose in their cecum. Stress, antibiotics, anesthetics, and diet can alter gastrointestinal function and physiology, which can lead to serious gastrointestinal (GI) complications, especially gastrointestinal ileus, postoperatively. Rabbits are unable to vomit so fasting to prevent aspiration pneumonia is not necessary. Rabbits have a relatively large GI volume. A fast prior to laparotomy in rabbits is not routinely performed; however, some suggest a fast of 6 to 12 hours prior to laparotomy in order to decrease GI volume, improve surgical visualization and exposure, and improve the ability to manipulate GI structures. Yet, a fast of this duration will have little effect on cecal volume where the majority of ingesta (about 40%) is located [2,3].

It is important to have airway and venous access during anesthesia in the event of cardiac and/or respiratory arrest. Intubation is recommended for major surgeries in rabbits, and is essential during thoracotomies and procedures lasting greater than 20 minutes. Techniques for intubation are not covered in this chapter but are found elsewhere [4]. Repeated attempts to intubate rabbits can result in swelling and edema of the glottis that can result in fatal upper airway obstruction. If the intubation is not successful after 3 or 4 attempts, the rabbit should be maintained on mask anesthesia or, if appropriate, the procedure should be postponed for 2 to 3 days, allowing the glottis to recover.

Rabbits have a blood volume of 57 ml/kg body weight [5]. Loss of 15 to 20% of total blood volume in a rabbit causes massive cholinergic release. An acute blood loss of 20 to 30% of the total blood volume causes hemodynamic collapse and shock [5]. Fresh whole blood can be collected and prepared from donor rabbits for transfusion. Rabbits do have blood groups and transfusion reactions do occur, because of this a cross match is recommended before a blood transfusion is given. Purified bovine hemoglobin with oxygen-carrying capacity can be used temporarily to restore tissue perfusion and oxygenation while hemorrhage is controlled intraoperatively.

The success of surgery is often dependent on minimizing pain, stress, and fear. The use of analgesics, sedatives, anxiolytics, and shorter hospitalization times will minimize the negative effects of increased catecholamines on the surgical outcome. A positive outcome is also dependent on getting the rabbit to eat within 1 to 2 hours after surgery. If the rabbit is not eating after surgery, it should be force fed every few hours. Intravenous or subcutaneous fluids should be administered and continued until the rabbit is eating and drinking on its own.

Uterine Adenocarcinoma

Endometrial adenocarcinoma is the most common tumor of female rabbits [6,7]. In intact female New Zealand White rabbits that are greater than 3 to 4 years of age, incidence approaches 80% [6,8].

Disease Mechanism

As the rabbit ages, the endometrium of the uterus undergoes cellular changes, characterized by increases in endometrial collagen content [9]. The increase in collagen and decrease in cells are considered senile changes and are associated with the development of uterine adenocarcinomas [9,10]. An identical process occurs in humans [11].

Uterine adenocarcinomas in rabbits tend to be slow growing tumors and are slow to metastasize (Fig. 119-1A and Fig. 119-1B). Local invasion of the tumor into the myometrium can occur early in the development of the tumor, and the neoplasm can extend through the uterine wall to invade other visceral structures [6]. Relevant medical history may include reproductive abnormalities such as decreased litter size, abortion, retention of feti, and stillbirth [8,10].

Figure 119-1. Uterine adenocarcinoma in the rabbit. At the time of diagnosis, the tumor is commonly found in both uterine horns. Note the bicornuate uterus in figure B.

Clinical Signs

Clinical signs include depression, anorexia, hematuria, and bloody vaginal discharge [8,10]. Hematuria is the most common presenting complaint but the hemorrhage is from the reproductive tract. Owners are usually not able to differentiate between hematuria and a hemorrhagic vaginal discharge. Cystic mammary glands can also develop concurrently with uterine endometrial hyperplasia and adenocarcinoma [11,12].

Diagnostics

Larger tumors are easily palpated on physical examination. Adenocarcinoma is often present in both horns at the time of diagnosis [6,9,13]. A complete blood count, serum biochemistry panel, and urinalysis should always be performed. Abdominal and thoracic radiographs are useful in detecting primary tumors as well as metastatic disease. Metastatic uterine carcinoma has been reported in the lungs, liver, bone, brain, and throughout the peritoneal cavity (Fig. 119-2) [8,13-15]. Hypertrophic osteopathy associated with metastasis to the lungs has also been reported in rabbits [8]. Abdominal ultrasonography is useful in differentiating between uterine masses and other causes of reproductive tract enlargement or disease (endometrial hyperplasia, endometrial polyps, pyometra, and endometritis), metastatic disease in the abdomen, or concurrent abdominal disease.

Figure 119-2. Metastatic uterine carcinoma in the rabbit. Note the circular white lesions in the lungs. Although metastatic spread of uterine adenocarcinoma in the rabbit is uncommon, disseminated disease can be found in the lungs, liver, bone, brain, and throughout the peritoneal cavity.

Treatment

Prevention is the best treatment for uterine adenocarcinoma. It is recommended that rabbits undergo ovariohysterectomy (OHE) between the ages of 6 and 12 months.10 The treatment of choice for rabbits with uterine adenocarcinoma is OHE [8,13-15]. The uterus of young rabbits is found just dorsal to the bladder, coiled in the caudal abdomen. In older rabbits, the cervix is dorsal to the bladder and the horns extend laterally but not very cranially. The doe reproductive anatomy lacks a uterine body. The uterus is bicornuate and each horn has its own cervix that empties into the relatively large vagina. The mesometrium is a major adipose storage site in rabbits, which makes identification of the ovarian and uterine vessels difficult during OHE. The oviduct in rabbits encircles the ovary and must be removed during OHE. If a remnant of the oviduct is present, it can become cystic [16]. The cranial vagina is ligated caudal to the cervices to avoid the development of cervical aneurysms, which may rupture resulting in fatal hemorrhage [16].

Prognosis

If no macro/micrometastatic disease is present at the time of surgery, OHE may be curative. If pulmonary metastatic disease is present, the rabbit is given a grave prognosis [10]. It has been suggested that rabbits with uterine adenocarcinoma at the time of OHE be examined for pulmonary and abdominal metastasis every 2 to 3 months after OHE for 1 to 2 years [10].

Urinary System and Cystic Calculi

The urine of normal rabbits varies in color from yellow, to creamy, to red or brown, which can be linked to pigments in the diet. The reddish hue of some rabbits’ urine is normal but will lead rabbit owners to present their rabbit for hematuria [2]. The most common surgical disease of the urinary tract in rabbits is urinary bladder sludge and associated cystic calculi. Both conditions are likely related to the diet of the rabbit. Urinary sludge and calculi are composed mostly of calcium carbonate.

Disease Mechanism

It is reported that the total serum calcium levels in rabbits reflects the level of calcium in the diet and is not regulated within a narrow range [2], however, recent research has identified that both ionized and total serum calcium concentrations in rabbits are maintained in a narrow reference range regardless of diet, although normal levels are higher than those in dogs and cats (personal communication, KR Rosenthal). Urine is a major route of calcium excretion in rabbits. The fractional excretion of calcium in rabbits is 20 to 30 times that of most mammals [2]. The calcium carbonate excretions may accumulate in the urinary bladder and form a thick sludge or cystic calculi. The color and consistency of rabbit urine can be greatly affected by dietary calcium levels, often appearing thick and white while ingesting high levels of calcium. Prolonged ingestion of a diet high in calcium levels can lead to mineralization of the aorta and kidneys [2].

Clinical Signs

Clinical signs of cystic calculi may include straining to urinate, decreased appetite with subsequent decreased fecal production, or, if a lower urinary tract obstruction is present, the rabbit may become depressed, anorectic, and recumbent.

Diagnostics

Routine diagnostics should be performed in any rabbit demonstrating signs of lower urinary tract disease. These include a complete blood count, serum biochemistry profile, urinalysis, abdominal radiographs, and abdominal ultrasound. Mineralized calcium excretions (sludge) are commonly detected in the bladder on abdominal radiographs as an incidental finding while another disease process is being investigated. The material in the bladder is usually radiopaque because the composition is mainly calcium carbonate (Fig. 119-3). Abdominal ultrasound is also helpful in evaluating the entire urinary tract, especially if a partial or complete urinary tract obstruction is suspected.

Figure 119-3. A right lateral radiograph of a rabbit with cystic calculi. The material in the bladder is usually radiopaque because the composition is mainly calcium carbonate.

Treatment

In some patients, this sludge-like material can be removed by lavaging the urinary bladder using a urinary catheter, sterile saline, and manual expression. This procedure is performed under general anesthesia. Occasionally, the material becomes thick enough that it will not pass or it forms a stone and causes lower urinary tract obstruction. Cystotomy in rabbits is analogous to that in other species.

Prevention and Prognosis

Some controversy exists as to whether dietary calcium restriction is beneficial in preventing urinary sludge or stone formation in rabbits. Commercial alfalfa pellets and alfalfa hay are relatively high in calcium. They were originally formulated for meat-production rabbits to cause rapid weight gain. As such, they are also high in protein and can predispose to obesity. Reducing the daily intake of pelleted alfalfa diets and feeding rabbits a diet composed mostly of timothy hay and timothy hay pellets, which are lower in calcium, is currently recommended. Additionally, treats high in calcium should be avoided. The prognosis is good after cystotomy but recurrence of sludge and of calculi formation is common.

Dental Abscesses

The dental formula in rabbits is 2 x (I 2/1 C 0/0 PM 3/2 M 3/3), making the total number of teeth 28 [17,18]. The rostral incisors of the upper and lower jaw are modified to form a chisel-like cutting tool [2,3]. Rabbits keep these teeth sharp by gnawing on objects in their environment. The thicker enamel on the labial surface of the incisors allows the front surface of the teeth to wear down more slowly, creating the sharp edge [3]. The peg teeth (maxillary I2) are smaller than the other incisors and are just caudal to the rostral incisors. The points of the lower incisors rest on the peg teeth. Therefore, in a normal rabbit’s mouth, the lower incisors rest caudal to the upper incisors. The premolars and molars are often grouped together and called cheek teeth. The diastema is the toothless space between the last incisor and the first cheek tooth [2].

The rabbit’s nasolacrimal duct narrows at the maxilla bone and as it passes over the roots of the upper incisor teeth. The duct opens at a nasal punctum, depositing tears in the nostril as in dogs and cats. Abscessation of the upper incisors can lead to occlusion of the ipsilateral nasolacrimal duct.

All rabbit teeth grow continuously, and the permanent teeth are completely erupted by 3 to 5 weeks of age [2]. The teeth continue to grow from the highly metabolically active apical germinal cells. The cheek teeth of adult rabbits consist mostly of crown. The clinical crown is exposed in the oral cavity [18]. The reserve crown makes up most of the tooth but is within the bone of the mandible or maxilla. In rabbits, the growth of the permanent teeth is never completed, so no tooth roots are formed. The process of continual growth leaves rabbits susceptible to dental disease throughout their lives.

Disease Mechanism

In rabbits eating natural vegetation, the power stroke of chewing, the lingual edges of the mandibular cheek teeth occlude against the food on the buccal edges of the maxillary cheek teeth [18]. This repetitive movement of the cheek teeth during chewing enables even wear. Rabbits being fed a pellet diet have a decreased lateral excursion of the cheek teeth against the pellet and an increase in the vertical movement of the teeth. This altered motion of the jaw leads to abnormal tooth wear. Abnormal tooth wear can lead to erosions or ulcerations of the oral mucosa (maxillary cheek teeth) and tongue (mandibular cheek teeth) from the sharp points that develop. Anorexia, oral pain, arrested tooth development, and infection of the oral cavity occur secondary to abnormal tooth wear and growth. Systemic disease and metabolic abnormalities may also lead to inappropriate tooth growth and development.

The most common cause of periodontal disease in rabbits is the impaction of food debris into the peridontium [18]. This occurs secondary to abnormal or arrested tooth growth from inappropriate diet and systemic disease. The inflamed and infected peridontium, periodontal ligament, and surrounding tissues remodel and reabsorb. Tissue loss leads to deeper periodontal pockets, advanced infection into surrounding tissues, and local spread of bacteria toward the apex and bone of the mandible and maxilla. If not addressed, osteomyelitis and abscessation ensue.

Historically, abscesses in rabbits were thought to be caused by Pasteurella multocida; however, a variety of other organisms are routinely cultured. It is actually rare to isolate Pasteurella sp. from abscesses in rabbits. Anaerobic bacteria, Staphylococcus sp. and Streptococcus sp. are most commonly cultured [19]. Abscesses of the head in rabbits may develop secondary to dental disease, food or foreign body impaction between the gingiva and tooth, periodontal disease, tooth fracture, or tooth root abscess. Abscesses occur in rabbits of all ages and can be soft or firm. When the bone is involved, there is often bony swelling owing to the periosteal reaction. If the maxillary cheek teeth are involved, ocular discharge is observed owing to the involvement of the ipsilateral nasolacrimal duct. The wall of the abscess is commonly thick and contains bacteria. The pus is usually caseous, thick, and creamy-white in appearance. Of all the abscesses occurring in rabbits, retrobulbar and dental abscesses are likely the most difficult to cure.

Diagnostics

Many rabbits with dental abscesses present with a swelling or a mass palpable during a physical examination. During a routine physical examination, the ventral mandible is palpated for any bumps. Bumps or scalloping along the ventral mandible is indicative of mandibular cheek teeth root disease resulting in cortical penetration.

Clinical work up includes a minimum data base including a complete blood count, serum biochemistry panel, and urinalysis. An oral examination is helpful in evaluating the crowns of the teeth for elongation, points, and other structural abnormalities; however, the extent of involvement of the reserve crowns, germinal tissue, and bone cannot be assessed by oral examination alone. Additionally, many attempt to evaluate the crowns in an awake rabbit. This is often difficult because the rabbit’s mouth does not open very far, it chews constantly while the oral examination is being attempted, and it is difficult to evaluate all surfaces of the crowns. Because some type of imaging modality (radiography or computed tomography) is essential when evaluating dental abscesses, an oral examination is best conducted under general anesthesia.

Skull radiographs under anesthesia are useful to evaluate for the presence and extent of infection. The four or five views recommended are a ventrodorsal or dorsoventral, a lateral, a left lateral oblique, and a right lateral oblique at a minimum. A rostrocaudal projection allows evaluation of the occlusal surface of the cheek teeth and for any bony proliferation of maxillary bone. Computed tomography (CT scan) has become more available in veterinary medicine and is far more useful for evaluating the extent of dental disease.

Aspirates of the abscess are obtained to confirm the presence of pus and for cytologic and microbiologic evaluation. Cultures of aspirated pus have yielded higher quantities and more species of bacteria than intraoperative cultures of the abscess walls [19]. A positive preoperative culture is useful for starting antibiotic therapy prior to surgery as well as for deciding which antibiotics would be best for local therapy. Cytology, including a Gram’s stain, is useful to confirm that the mass is an abscess and define the types of bacteria present. This is useful in deciding on antibiotic therapy prior to receiving culture results. Additionally, some aspirates will fail to grow bacteria and the Gram’s stain may be the only objective data on which to base antibiotic therapy.

Treatment

A variety of surgical therapies has been used in treating dental abscesses in rabbits with variable results. The traditional treatment of abscesses in mammals (lancing, draining, and flushing) is not effective in rabbits because the pus is too thick to drain effectively. Also typically fibrous tracts are present that contain bacteria, making recurrence common. As a result, the skin surface usually heals over before the infection has completely been eliminated. This traps infected tissue below the skin surface and a new abscess develops at the same site. Failure to completely remove any teeth that are associated with the abscess will result in recurrence of the abscess regardless of the technique used because bad teeth and surrounding infected bone will serve as a nidus of infection.

The basic principles employed when treating a dental abscess in a rabbit are to remove all infected soft tissue including the abscess capsule, debride all infected bone, remove any teeth involved, minimize contamination of remaining tissues with pus, irrigate extensively, implant antibiotic-impregnated polymethylmethacrylate (AI-PMMA) beads (Fig. 119-4A, Fig. 119-4B, and Fig. 119-4C), and administer systemic antibiotic to treat any bacteremia that may result from the surgery.

Figure 119-4. A. AI-PMMA is rolled out into fine tubular strips. B. The tubular strips are then cut into appropriate size pieces for the wound. The pieces are cut before the PMMA hardens. A scalpel blade works well for this. Bead makers are also available but are limited to one bead size. C. The beads are then placed in the wound. In this photograph, the beads are being used in a thoroughly debrided mandibular abscess in a rabbit.

The use of antibiotic impregnated polymethylmethacrylate beads, which release relatively high concentrations of antibiotic locally with little systemic absorption, is currently the recommended surgical option for rabbits with oral cavity abscesses [20-25]. It appears that antibiotics that are not safe when administered systemically in rabbits, such as cefazolin and clindamycin, are safe in AI-PMMA beads. Different antibiotics should not be mixed together in the same bead as this has been shown to unacceptably alter the elution rate [22]. It is also important to refrain from using antibiotics in beads for which the elution rate and appropriate concentration are unknown, for example, enrofloxacin will not mix into PMMA. The rabbit should still be placed on systemic antibiotics for 2 to 3 weeks but long-term therapy is not needed as the beads release antibiotic for many months. The antibiotics chosen are based on culture and sensitivity or Gram’s stain results.

Some teeth, even after they are removed, will grow back if the germinal bud remains viable. Because rabbit teeth grow continuously, the question arises, if teeth are removed, is it necessary to remove the opposing teeth? This is not usually necessary. The maxillary teeth tend to grow and erupt at a slower rate than the mandibular teeth. Often, after the mandibular teeth are removed, the maxillary teeth stop erupting [18]. Some removed cheek teeth grow back because the germinal tissue is still healthy. Because of the extensive movement during chewing, even 2 to 3 teeth are often enough to wear the opposing teeth. Sometimes, the opposing teeth overgrow, in which case they either need to be trimmed on a regular basis (every 6 to 8 weeks) or they need to be removed.

Even after aggressive surgical debridement, rabbits recover quickly. Most begin eating within a couple of hours of surgery. It appears that removing the abscess helps resolve the pain associated with the abscess.

Prognosis

In general, the prognosis for complete recovery without recurrence is fair to good using this technique. The best chance for surgical cure is with the use of CT imaging, aggressive removal of all grossly abnormal tissue including teeth and bone based on the CT images, and the implantation of PMMA beads impregnated with an appropriate antibiotic based on preoperative culture and sensitivity results. Even with all of this, dental abscesses in rabbits can be difficult to completely eliminate. Owners should be advised before surgery about the nursing care, frequency of follow-up visits, and financial commitment involved.

Gastric Stasis Syndrome (Ileus), Trichobezoar, and Gastrointestinal Foreign Body

The most common clinical problems in rabbits involve the gastrointestinal tract. Most of these are directly or indirectly related to diet [26]. Rabbits are unable to vomit because of the anatomic arrangement of the cardia and stomach [26,27]. The pylorus is easily compressed by the duodenum because it exits the stomach at a sharp angle [2]. The ileocecal junction is a common site for foreign body obstruction in rabbits [2]. The cecum of rabbits is thin-walled and holds 40% of the total gastrointestinal contents [2]. Rabbits are hindgut cecal fermentors that efficiently ingest large quantities of poorly digestible fiber that is essential for normal peristalsis more than it is important for nutrition [26,27].

Disease Mechanism of Trichobezoar and Gastric Stasis

It has previously been suggested that, when rabbits are fed a diet deficient in fiber or if gastric stasis occurs for another reason, trichobezoars can become a clinical problem by obstructing the pyloric outflow tract and proximal duodenum [28,29]. Although trichobezoars are described as a clinical problem, it is now accepted that the presence of a trichobezoar is normal in rabbits and the illness is not directly related to the presence of a mass of hair and fiber. Rabbits that present with clinical signs of anorexia, lethargy, and mild abdominal distention that have radiographic evidence of a hairball are commonly ill for other reasons, most likely gastrointestinal ileus. Historically, rabbits with a trichobezoar were treated by gastrotomy to remove the hairball. Most of these rabbits did not survive because the mass of hair was not causing the illness and little attention was paid to managing the underlying ileus and dehydration. It is now accepted that the hairball is an incidental finding and this syndrome is more accurately termed gastric stasis. It is best managed medically, not surgically.

In rabbits with dental disease, the discomfort associated with chewing food will cause rabbits to quickly decrease daily food intake and appears to be a common cause of ileus. Ileus is also commonly associated with a high-carbohydrate/low-fiber diet, stress, pain, and a lack of exercise.

Clinical Signs

Gastric stasis syndrome (ileus) is common in rabbits and is defined clinically by anorexia, decreased or no stool production, and a large stomach typically containing a mass of fiber and hair along with gastric fluid [26]. If an underlying disease is present, clinical signs associated with that condition should be evaluated and treated concurrently with the gastric stasis.

Diagnostics

A detailed dietary history is an important part of the diagnostic investigation in rabbits with GI ileus. A complete blood count, serum biochemistry panel, and urinalysis should be performed to look for the underlying cause for the ileus. A dental examination should also be performed. If indicated, abdominal radiographs and ultrasound should be performed.

Treatment

The most important aspects of medical management for rabbits with gastrointestinal stasis are fluid support, getting fiber into the GI tract to stimulate motility, administering GI motility-enhancing drugs, and providing nutritional support to minimize the risk of secondary hepatic lipidosis. Medical management usually includes intravenous fluid support because rabbits with GI stasis are typically dehydrated. Although most patients have a decreased appetite, many will still eat fresh leafy green vegetables. Eating such materials will provide additional fluids, compared with dry hay or pellets, as well as fiber. If the patient will not eat, nasogastric intubation for alimentation may be necessary. Analgesics are often beneficial to palliate concurrent abdominal discomfort and increase the activity level of the rabbit, which may help stimulate GI motility.

The condition may be complicated by the presence of hepatic lipidosis secondary to the anorexia. Pelleted diets are relatively high in protein and carbohydrates, resulting in obesity and predisposing to hepatic lipidosis. Correcting the nitrogen balance will decrease the acidosis and ketosis, and counter the negative effects of the lipid hepatopathy.

Disease Mechanism of Gastrointestinal Foreign Body

Carpet fiber is a common cause of foreign body obstruction seen in rabbits. Another cause of intestinal foreign body is a piece of gastric hairball that breaks off and migrates into the duodenum. Rabbits with GI foreign body obstruction can present with clinical signs similar to those of ileus, and both conditions can occur simultaneously. The disease process associated with GI foreign body is generally an acute condition that does not allow time for the rabbit to adjust systemically. The acute distention of the stomach with gas and fluid, the inability of rabbits to vomit, and the pain associated with a foreign body obstruction make this disease in rabbits a rapidly life-threatening emergency.

Clinical Signs

Rabbits with gastrointestinal foreign body obstruction commonly present with acute depression and severe and rapid abdominal distention. They are typically much more critically ill than are rabbits with GI ileus alone.

Diagnostics

A complete blood count, serum biochemistry panel, and urinalysis should be performed. Abdominal radiographs and ultrasound are useful in determining the presence of a foreign body obstruction. The presence of a mass of hair in the stomach is considered normal in rabbits and does not confirm a diagnosis of obstruction. Obstruction can cause an ileus pattern on radiographs; however, if the obstruction is at the pylorus or orad duodenum, only the stomach will be distended.

Treatment

Systemically ill rabbits must be stabilized with intravenous fluids and analgesics before definitive surgery is performed. Gastric decompression by nasogastric or orogastric intubation or by trocharization is vital in critically ill patients. Routine exploratory laparotomy and gastrotomy with two-layer closure, as performed in dogs and cats, should be performed in rabbits with a gastric foreign body causing pyloric outflow obstruction. For an intestinal obstruction, enterotomies and intestinal resection and anastamosis are performed as needed. The small intestine of rabbits has a much thinner wall than that of dogs and cats. Surgical magnification and appropriate suture and atraumatic needle sizes are used. Postoperatively, rabbits should be treated with analgesics and for gastrointestinal stasis as previously described.

Prognosis

Rabbits with GI foreign body obstruction do poorly postoperatively and have a high perioperative mortality rate compared with dogs and cats. These patients need to be rapidly stabilized and decompressed on presentation. Short anesthesia and operative times, aggressive analgesia, fluid therapy, and postoperative nutritional and GI motility support are vital.

Thymoma

The thymus in rabbits persists into adult life, lies ventral to the heart, and extends to the thoracic inlet [2]. Thymomas in rabbits are infrequently reported, but make up as much as 8% of all neoplasms [8,30]. Two age groups appear to be affected. Genetically based thymomas occur in rabbits 1 to 4 years of age and have been associated with a systemic immunopathy and hemolytic anemia [6,8]. Older rabbits, 7 to 9 years of age, are affected with primary thymoma. Most of these tumors are diagnosed post mortem.

Disease Mechanism

The thymus is responsible for the normal development and regulation of the immune system [31]. Thymomas are uncommon tumors in rabbits, dogs, and cats [8,30,32,33]. In dogs, cats, and humans, paraneoplastic syndromes associated with thymoma include myasthenia gravis, cytopenia, thyroiditis, hypogammaglobulinemia, and polymyositis [32,33]. Potential paraneoplastic conditions associated with thymoma in rabbits include hypercalcemia, systemic immunopathy, and hemolytic anemia [6,8].

Clinical Signs

Clinical signs are variable and are related to a space-occupying mass in the cranial mediastinum or to the paraneoplastic processes. Rabbits can present with clinical signs including respiratory distress and precaval syndrome (swelling of the head, exophthalmos, and prolapse of the third eyelids) [34-36].

Diagnostics

Complete blood count, serum biochemistry panel, and urinalysis should be performed. Thoracic radiographs of rabbits with thymoma reveal a cranial mediastinal mass (Fig. 119-5). Because the radiographic density of the mass is the same as the heart, it is difficult to distinguish the two structures [34-36]. Ultrasound-guided transthoracic fine-needle aspiration and needle-core biopsy are helpful to evaluate the extent of the mass, and to evaluate the heart and other thoracic structures as well as to rule out other causes of a cranial mediastinal mass. Although cystic and necrotic areas are commonly associated with thymomas, a diagnosis via fine-needle aspiration or needle-core biopsy is usually possible with ultrasound guidance.

Figure 119-5. A right lateral thoracic radiograph in a rabbit with a thymoma. The cranial borders of the heart are obscured by a soft tissue opacity in the cranial mediastinum.

CT or MRI is highly useful in evaluating the invasiveness of the mass and its association with vital thoracic structures. This can be valuable in planning the surgery and offering a prognosis.

Treatment

Thymomas are surgically resected using a ventral midline approach (Fig. 119-6). The sternebrae of rabbits are usually too narrow to be able to split longitudinally. Therefore, it is best to disarticulate the ribs from the sternebrae on one side, allowing the chest to be opened. Invasion of the thymic capsule, vena cava, pericardium, pleura, and lung parenchyma can make complete resection difficult in some cases.31 Care is taken to avoid important neurologic and vascular structures in the cranial thorax. Complications of thymoma removal include fatal hemorrhage, pneumothorax, and inadvertent damage to vital thoracic structures.8,34-36

Figure 119-6. A ventral approach to the thoracic cavity has been performed in a rabbit. The sternebrae of rabbits are usually too narrow to be able to split longitudinally. Therefore, it is best to disarticulate the ribs from the sternebrae on one side allowing the chest to be opened. A cranial mediastinal mass is noted with adhesions to the pericardium, aortic root, and cranial aspect of the heart.

Prognosis

If rabbits survive the postoperative period and the mass is completely removed, prognosis is good. Radiographic rechecks are recommended every 3 to 4 months for the first year to assess for evidence of recurrence. Recurrence following complete excision of a thymoma in rabbits has not been reported.

Vertebral Fracture, Subluxation, or Luxation

The most common cause of acute paraparesis or paraplegia in rabbits is vertebral body fracture, subluxation, or luxation [37]. Fractures are more common than luxations, with the most common site of injury being the caudal lumbar vertebral bodies.

Disease Mechanism

Injury results from improper handling, from accidentally stepping on the rabbit, or when a caged rabbit is startled while in its enclosure. The heavily muscled hindquarters are allowed to twist at the lumbosacral junction, which acts as a fulcrum in applying leverage to the caudal lumbar vertebral column [38]. Also, the bones of rabbits are hard and brittle compared with those of dogs and cats. The bones of rabbits comprise only 8% of the total bodyweight, compared with 13% in cats [39]. The spinal cord in rabbits extends all the way to the sacrum so even lower lumbar vertebral fractures in rabbits can results in upper motor neuron signs.

Clinical signs

Clinical signs are dependent on the location and severity of the injury to the spinal cord. In addition to paraparesis or paraplegia, loss of urinary and fecal continence, urine retention, and myelomalacia can occur. Again, it is important to note that, because the spinal cord ends at the sacrum, neurolocalization is dissimilar to that in cats and dogs.

Diagnostics

Diagnosis is based on a thorough neurologic examination and survey spinal radiographs. Magnetic resonance imaging (MRI), myelography, and/or computed tomography (CT scan), are useful for defining the nature of the injury and prognosis for neurologic recovery in preparation for surgery. MRI is preferred if available.

Treatment

Both medical management and surgical intervention have been applied to rabbits with spinal cord injury. Medical management includes strict cage rest, management of urine retention (UMN lesion) or dribbling (LMN lesion), analgesics, nonsteroidal anti-inflammatories, and physical therapy. Surgical options depend on the location of the injury and whether the injury involves a fracture, instability, or both. Fixation of vertebral body fractures or instabilities can be performed with plates, pins and polymethylmethacrylate, screws and polymethylmethacrylate, or spinal stapling.

Prognosis

Prognosis depends on the severity of the neurologic deficits present during the perioperative period and the degree of displacement of the fracture and/or luxation. Owners should be informed that nursing care will be intensive regardless of whether medical or surgical options are pursued. Loss of deep pain perception in the limbs is usually considered a poor prognostic indicator for the return of neurologic function in rabbits suffering from a vertebral body luxation or fracture.