Respiratory disease is one of the most common diseases in rabbits. Several challenges arise when evaluating a rabbit with respiratory problems. The rabbit's respiratory system has not such a high degree of ventilatory reserve as that of dogs, which can make respiratory diseases difficult to treat. Thorough knowledge of rabbit anatomy and physiology is necessary to differentiate between upper and lower respiratory disease and between cardiovascular and other systemic factors which could impair respiration. Breathing difficulties can affect rabbits of any age, and the disease can quickly become life threatening.

Rabbits have a well developed sense of smell. Under normal circumstances, nostril twitching may be seen only in up or down directions. Each part of the nasal cavity has dorsal, medial and ventral cartilaginous conchae and maxillary and ethmoid paranasal sinuses. The relatively small glottis is engaged over the caudal rim of the soft palate, sealing the oral pharynx from the lower airways. Rabbits are susceptible to laryngeal spasm making the intubation difficult. As obligate nasal breathers, rabbits with upper airway disease will attempt to breathe through their mouths, which prevents feeding and drinking and could be quickly fatal.

The rabbit thoracic cavity is small and the thymus, which is located in the cranial mediastinum, persists in the adult. Rabbit lungs have no septa; the right lung has four lobes and the left lung has three lobes; air flow volume is higher in the left lung. Rabbits at rest respire mostly through the activity of the diaphragm. The threshold of lung inflation (which initiates afferent impulses that depress the vasomotor centre and cause vasodilation) is an air pressure as low as 2.5 cmH₂O. In cases of thermal polypnoea, the minute tidal volume increases about 15-fold.

Diagnostics

Accurate diagnosis is based on thorough history, physical examination, diagnostic imaging and laboratory results (e.g., haematology, plasma chemistry, bacteriology). Dyspnoea is one of the earliest symptoms even in mild conditions, such as rhinitis. Severity may be judged by assessing breathing effort, respiratory rate, rhythm and character. Affected rabbits display a hunched posture with obvious abdominal respiratory movement. It is imperative that animals with severe respiratory distress are stabilised before time is taken to obtain a thorough history. Stabilisation may include oxygen therapy, appropriate medications (diuretics, fluid therapy) and then a brief history. Gentle physical restraint or sedation with benzodiazepines (monitor blood pressure) is essential.

The physical examination is extremely important. Respiratory disease could be associated with a restrictive or obstructive pattern. Determination of which pattern the animal is exhibiting can help to narrow the list of differential diagnoses. Restrictive diseases, which prevent the lungs expanding, lead to short, rapid and shallow breaths. These signs could be associated with pneumonia, lung oedema, pleural effusion, pneumothorax, mediastinal and lung tumours or abscesses, or be due to increased pressure of abdominal organs on the diaphragm (gastric distension). Obstructive disease, which is caused by narrowing of the airways, leads to slower and deeper breaths. Upper obstructive respiratory disease is associated with increased inspiratory effort, whereas lower respiratory disease is associated with increased expiratory effort. Wheezes are usually heard on expiration and indicate obstruction or narrowing of small airways. Crackles are caused by the opening of small airways during inspiration and are heard with bronchial inflammation and pneumonia.

The mucous membranes should be checked for any indication of cyanosis or pallor. Due to the rabbit's tendency for regular grooming, signs of nasal discharge may be seen only as wet hairs on the front paws. Facial symmetry, oral cavity and all maxillary teeth should be evaluated for the presence of any pathology associated with the nasal cavity. Palpation of the skull, cervical area, thorax and abdomen could reveal tumours, skeletal injury or enlargement / distension of abdominal organs.

During auscultation the entire thorax should be examined to identify the exact source of abnormal sound. Wheezes (musical sounds) are generated primarily by airway narrowing, stenosis or obstruction. Crackles (short, explosive, non-musical sounds) are typically produced by a delayed opening of small airways attributable to an abnormal fluid-air interface (pneumonia, pulmonary oedema).

Knowledge of the timing (systolic, diastolic) and location of abnormal heart sounds (murmurs, arrhythmias) allows the practitioner to establish a differential diagnosis rapidly. In rabbits, the arterial pulse is evaluated bilaterally to assess heart rate, rhythm and pulse quality. However, heart disease can be present without auscultable abnormalities, and not all murmurs are associated with heart disease.

It is imperative with small mammals that the potential hazard of any test be considered, because even minor stress can lead to collapse of these patients. Haematology, plasma chemistry and urinalysis in respiratory diseases are often unremarkable; however, the most important contribution is to reveal systemic diseases that might be affecting respiration, such as acidosis, anaemia, leukaemia or the presence of liver and kidney failure. An enzyme-linked immunosorbent assay (ELISA) test on paired samples is available for the increased antibody detection for Pasteurella multocida.

Radiography should be performed with the patient under sedation or anaesthesia to ensure proper positioning. In thoracic radiographs, cranial cardiac borders are often indistinct, especially if a large amount of fat is present. Remember that the thoracic thymus is present throughout life. Interpretation of radiographs may document the presence of cardiomegaly, pulmonary oedema, pleural effusion or a prominent pathological lung pattern. Both computed tomography (CT) and magnetic resonance imaging (MRI) provide tomographic images of the skull, which allow improved anatomical information concerning many regions that are difficult to assess with conventional radiography.

Endoscopy of the respiratory tract is very useful especially in cases of nasal or tracheal foreign bodies, chronic respiratory diseases or neoplasia. This imaging method allows guided biopsy. Echocardiography and electrocardiography are useful to document cardiac function and to assess mediastinal masses. Cytology (nasal swab, bronchoalveolar lavage, fine-needle aspiration, effusion deter­mination) and bacteriology / mycology are important for diagnosis, treatment and also prognosis. Histopathological examination should be carried out on any biopsy specimens or any mass removed.

Treatment

Frequently, after evaluation of the historical and physical examination findings, laboratory examination and interpretation of the thoracic radiographs, it is possible to determine whether heart or lung disease or other disease is the most likely cause for the respiratory distress. If pulmonary infiltrates compatible with oedema are present on thoracic radiographs, the rabbit should be treated with diuretics, oxygen, cage rest and possibly vasodilators. In our practice, furosemide (1–5 mg/kg i.m. or i..v q2–6h) is most frequently used. Placement of an intravenous catheter is essential and this may often be done with minimal restraint into the cephalic or marginal ear vein. Oxygen is most easily administered by placing the animal within an oxygen cage. If a moderate to large volume of pleural effusion is present, it should be removed in order to improve the stability of the patient as well as to help in reaching the final diagnosis. Thoracocentesis (21-gauge needle) is recommended, performed under ultrasound guidance.

Antibiotics or other drugs should be administered systemically or topically based on the disease severity and exact cause. For nebulisation it is preferable to use aminoglycosides alone or in combination with bronchodilatators. Moreover nebulisation itself will keep the mucous membranes hydrated and acts as an excellent expectorant.

Pasteurellosis

Pasteurella multocida is a gram-negative, bipolarly staining rod. Infection is nearly ubiquitous among pet and laboratory rabbits. Transmission is by direct contact and, to a lesser extent, fomite, aerosol and sexual exposure. Disease susceptibility depends upon host, environmental and bacterial factors. The most pathogenic strains are of serogroup A, D and F. The majority of rabbits infected with P. multocida are asymptomatic carriers. The most common presentations are rhinitis (snuffles), conjunctivitis, pneumonia, otitis media, otitis interna, abscesses, genital tract infections and septicaemia.

Colonisation often occurs initially in the pharynx. The infection quickly spreads to the nasal cavity, from which it disseminates via direct or hematogenous spread to the lungs, middle ear, conjunctival sac, subcutaneous tissues and visceral organs. Regardless of the organ system affected, the hallmark of P. multocida infection is suppurative inflammation. The accompanying exudate is most often purulent. Microscopically, affected tissues may be oedematous, hyperaemic, congested and necrotic. Large amounts of thick pus may also place direct pressure on adjacent tissues, such as in the lungs, and may further compromise organ function. Some other bacteria that could be present in the respiratory infection of rabbits are Staphylococcus aureus, Bordetella bronchiseptica, Moraxella catarhalis and Klebsiella pneumoniae.

The diagnosis is based on clinical signs, thoracic and tracheal auscultation, radiography, rhinoscopy and bacterial culture.

Therapy includes supportive care, nebulisation (aminoglycosides) and antibiotic treatment. Use of trimethoprim-sulpha or fluoroquinolones is recommended. In more severe cases parenteral administration of beta-lactams provides better outcomes.

References

1.  Harcourt-Brown F. Textbook of Rabbit Medicine. Oxford: Reed Educational and Professional Publishing, 2002.

2.  Johnson-Delaney C, Orosz SE. Rabbit respiratory system: clinical anatomy, physiology and disease. Veterinary Clinics of North America: Exotic Animal Practice 2011;14:257–266.