Radiology of rabbits – Master class (Proceedings)

Normal and abnormal radiographs of a variety of conditions in the pet rabbit will be presented. Medical and surgical treatment of the conditions will be briefly presented as well. This is not intended to be an exhaustive presentation of physiology and treatment of all of these conditions, but rather an introduction to common radiographic findings.

Abdomen

Gastrointestinal tract - normal

It is normal to see some gas present in the gastrointestinal tract (GIT). This is due to the production of gas by bacteria as they process the high fibre foods that rabbits consume. Another normal feature of the GIT is that there is always material present in the stomach. The GIT transit time in rabbits is 3-5 days. The caecum can be small or large depending on the time of day and whether or not the rabbit is in the process of producing hard faeces or caecotropes.

Stomach – abnormalities

If anesthesia is used, a gas shadow is often seen around the ingesta due to aerophagia during induction. This can be indicative of dehydration. True hairballs are rare in rabbits. Hypomotility disorders are relatively common.

A common abnormality of the stomach that is diagnosed by physical exam and radiographs is bloat. The exact aetiology of this condition is unknown. Factors that contribute to this condition are the tight esophageal sphincter and the acutely curved pyloric outflow tract. It is possible to manage this condition medically, but great care needs to be taken to monitor these patients closely. Surgical intervention is often necessary. These patients need to be treated as an emergency. The bloated stomach creates pressure on the diaphragm, which makes it difficult for the rabbit to breath. These patients frequently presently severely compromised and hypothermic. Supplemental heat needs to be supplied and an intravenous catheter should be placed for fluid support immediately. The patient should be anaesthetized and placed in left lateral recumbency. A 12 Fr red rubber catheter with extra fenestrations in it is passed into the stomach. This will help to immediately relieve pressure by the removal of gas and fluid. After as much fluid and gas as possible is removed, 10cc of water with approx ¼ - ½ teaspoon of epsom salts is instilled into the stomach and the rabbit is recovered from general anaesthesia. The rabbit is then given 1-2mg/kg of diazepam to keep it calm and in left lateral recumbency. This helps to keep pressure off of the pyloric outflow tract. The rabbit needs to be supported with supplemental heat during this recovery time. The stomach is gently massaged and monitored every 15-30 minutes. If the stomach stays tight feeling and starts to enlarge again, surgery should not be delayed. It is usually a dried hard piece of ingesta that is obstructing the distal duodenum. In approximately 50% of the cases rabbits can be carefully managed medically to get this material softened and passed.

Intestinal obstruction vs. ileus

Distinguishing obstructions versus ileus in rabbits is not an easy call! Technically, if gas does not extend all the way to the rectum and if the loops of intestine make very tight turns, it is most likely obstruction. This is easier to distinguish if the obstruction is in the upper GIT. However, due to the amount of gas that accumulates in the rabbit intestine, it can be challenging to identify obstruction versus ileus. Obstruction of the GIT needs to be managed surgically. Rabbits, in general, do not tolerate surgery of the intestinal tract as well as other species. Great care should be taken for pain relief and supportive care. Ileus is managed medically with gastrointestinal motility drugs, fluids and syringe feeding.

Abdomen – urinary tract disorders:

Bladder sludge

It is normal for rabbits to have a significant amount of sediment in their urine. It is usually calcium carbonate. There is a condition referred to in layman's terms as "bladder sludge". This describes a condition whereby a large amount of tan colored sediment virtually fills the bladder. The radiographs of patients with this condition appear as though there is contrast agent in the bladder. The aetiology of this condition is unknown, but factors that are thought to contribute to it are obesity, a sedentary lifestyle, spinal pain and pododermatitis (2). Rabbits with this condition commonly present with discomfort and have perineal urine soiling.

Treatment of "bladder sludge" consists of flushing all the sediment out of the bladder. This removes the nidus for more precipitate. Limitation of dietary calcium has controversial benefit. Actions that do seem to help clinically are increasing the rabbit's water consumption. This can be done by offering leafy greens that are wetted down, flavouring the water with fruit juice, and offering a salt lick. Rabbits with this condition frequently have urinary tract infections as well. Identification and treatment of the infection is imperative. The author has used bethanechol at 6.25mg by mouth twice daily with questionable results. The objective in using the bethanechol is to hopefully increase the ability of the bladder to void the thick, sludgy urine.

After flushing all the sludge out of the bladder, the rabbit should be radiographed again to ensure that all the material has been removed and that there were no calculi being obscured by the sludge.

Bladder stones

Bladder stones are relatively common in rabbits, easy to diagnose via radiographs and can be resolved with a routine cystotomy.

Kidney stones

It is not uncommon for a rabbit with kidney stones to be asymptomatic. The stones are usually obvious on radiographs as they tend to be calcium carbonate. It is important to monitor kidney function, the presence of urinary tract infections and alert the owner to be vigilant for blood in the urine. The author has seen a case of a rabbit with a significant sized kidney stone spontaneously resolve over several months.

Other miscellaneous bladder conditions observed radiographically

Other conditions observed radiographically on rabbit bladders include urethral strictures, urethral stones, a large diverticulum of the bladder and an extremely enlarged lower motor neuron deficit bladder.

Hyperossification

Hyperossification of all the bones of the body as well as calcification of the aorta was observed in a rabbit with chronic renal failure. It is thought that it was due to the kidney's decreased ability to regulate blood calcium levels.

Thorax

Normal thoracic radiographs of rabbits can be challenging to diagnose. The cranial lung lobes are small and are obscured by a wide mediastinum. The caudal lung lobes have a pronounced vasculature. It can be easy to over interpret thoracic radiographs in the rabbit.

Several cases of rabbits with abnormalities of in their thorax will be presented. These include dilated cardiomyopathy, pneumonia, pleural effusion, a heart based fluid filled mass and a large thoracic abscess.

Bones

Dislocations, osteomyelitis, neoplasia, spondylosis and fractures will be demonstrated.

Dislocations

The most common joints to be dislocated in the rabbit are the coxofemoral joint, the carpus, tarsus and the elbow. They are most commonly secondary to trauma. Luxation of the coxofemoral joint is most effectively treated by a femoral head osteotomy. Luxation of the elbow can usually be reduced manually if the rabbit is presented soon after injury. Luxation of the carpus and tarsus respond well to splinting for two –three weeks until some scar tissue forms to stabilize the joint.

Osteomyelitis

The most common sites for osteomyelitis in the rabbit that his author has seen have been the jaw and the tarsus. Osteomyelitis of the jaw is most often secondary to tooth root infections. Aggressive management is necessary for resolution of osteomyelitis in any site. This includes surgical debridement, marsupialization, and systemic antibiotic therapy. Severe abscesses of the tarsus may require amputation.

Neoplasia

The author has only observed one case of confirmed bone neoplasia in a pet rabbit. The radiographic signs were both lysis and bony proliferation on both sides of the tarsal joint. Amputation and biopsy of the popliteal lymph node were performed. Diagnosis was osteosarcoma. The rabbit lived for a few more months.

Spondylosis deformans

This is the most common abnormality observed in the spine. Patients with this condition often present for urine soiling or urine scald around the perineum. These patients respond well to NSAIDS. It may be necessary to add narcotics to the treatment regimen over time.

Fractures

Rabbit bones make up only 7-8% of their body mass as compared to 50% of their body mass being comprised of muscle. (1) The relative delicacy of their bones make fractures common. Several cases of fractures will be presented. These include a metacarpal fracture, a Salter fracture, a pelvic fracture, a foreleg fracture, rear leg fracture and a spinal injury. Treatment modalities will be presented.

Skull

Skull radiographs are most frequently used to evaluate dentition. The normal appearance of the occlusal surface is a parallel "zig zag" pattern between the upper and lower arcades of molars. Characteristics of dental disease include increased periapical radiolucency, loss of internal structure of the tooth, elongation and distortion of tooth roots and penetration of the ventral mandible with or without abscessation.

Inner ear disease

Radiographs are useful when trying to distinguish whether a head tilt is most likely due to middle ear disease as opposed to Encephalitozoan cuniculi. Keep in mind that radiographic bony changes of the osseous bullae will be at least 14 days behind clinical changes. Sclerosis of one or both of the osseous bullae are indicative of a chronic middle ear infection.

Nasal passageways

Chronic unresponsive upper respiratory tract infections are an indication for skull radiographs. Masses and elongation of the maxillary tooth roots can be underlying causes of chronic infection. The nasolacrimal duct can also be evaluated with the aid of contrast agent. If a nasal mass is suspected, endoscopy and advanced imaging techniques such as an MRI may be necessary.

References

1 Donnelly, Thomas M., Rabbits – Basic Anatomy, Physiology, and Husbandry in Ferrets, Rabbits and Rodents 2nd ed.; Quesenberry K.E. and Carpenter, J.W.; Saunders, St. Louis; 2004; p137-138.

2 Harcourt-Brown, Frances; Textbook of Rabbit Medicine; Butterworth-Heinemann;Oxford, 2002, p 338.