Heidi L. Hoefer, DVM, Dipl ABVP
Upper Respiratory Tract Disease
Because rabbits are obligate nasal breathers, any obstruction of the nasal cavity will result in respiratory compromise. Open-mouth breathing in a rabbit is a very poor prognostic sign and is most often seen in the agonal patient.
The most common cause of upper respiratory disease (URD) in the rabbit is pasteurellosis. P. multocida is a gram-negative, coccobacillus that varies in serotypes and virulence. Other bacteria like Moraxella catarrhalis, Bordetella bronchiseptica, and Staphylococcus aureus may also be present in the nasal cavity.
Clinical signs of upper respiratory infection or "snuffles" in rabbits include rhinitis with sneezing and conjunctivitis, and is the most common form of pasteurellosis. Nasal discharge can be serous or mucopurulent. Rabbits are fastidious groomers and use the forepaws to remove debris from the nares, resulting in matted fur and exudate on the medial aspect of these paws.
Conjunctivitis is common with upper respiratory tract infection in rabbits and results in epiphora, discharge, and periorbital alopecia. Inflammation of the nasolacrimal duct is common and the duct may become obstructed. Because rabbits only have one lacrimal puncta and duct in each eye, occlusion results in chronic epiphora.
There are other causes of upper respiratory disease in rabbits. Maxillary tooth root overgrowth can inpinge on the nasal cavity and result in partial obstruction and inspiratory rales. Ocular discharge and a retrobulbar mass-effect can also be produced with tooth root overgrowth, abscesses, and odontomas of the caudal maxillary teeth. This resembles "snuffles" (pasteurellosis) in some cases.
Diagnosis of upper respiratory tract disease in rabbits is based on clinical signs and physical examination initially, followed by laboratory testing and diagnostic imaging. Auscultation of the trachea and nares may help to distinguish upper airway from lower airway disease. Deep nasal cuture is performed by placing a small-tipped sterile culture swab (Mini-tip culturette®) 2-4 cm. into the medial aspect of each nare. Some rabbits may violently resist this procedure and may need to be sedated prior to culture using this technique. Innoculation of the swab onto a blood agar plate as soon as possible will enhance recovery of Pasteurella multocida. False negatives are possible and several attempts can maximize isolation of the organism. Of course, other bacteria may be involved as well.
Routine blood testing in affected patients may not show any abnormalities, even with abscessation. It is important to check the hemogram for subtle changes. It is unusual for total white blood cell count elevations to occur with infection in bunnies but changes in the lymphocyte:heterophil ratio may be seen. An elevated heterophil count and lymphopenia may be the only CBC abnormality and may be difficult to differentiate from a "stress leukogram".
Serology testing for Pasteurella multocida is available as an ELISA (enzyme-linked immunosorbent assay). Antibody detection has been used to screen colonies of rabbits for pasteurellosis. There are inherent problems with interpreting serologic tests; false negatives may be present in an immune-suppressed individual and positive tests may indicate exposure and not necessarily active infection. As with other antibody tests, paired sera samples taken 3 weeks apart can help in evaluating a rabbit's immunoglobulin response.
Skull radiographs are an important diagnostic tool in cases of chronic, non-responsive upper respiratory disease. Erosion of the nasal turbinates occurs with chronic Pasteurella rhinitis in some rabbits. Concurrent otitis externa and media may be seen radiographically as an increased opacity in the external ear canal and in the tympanic bulla. Tooth root evaluation can also be made from a series of skull films. For a more detailed study, computed tomography (CT) scanning can be performed.
Treatment of respiratory tract disease is dependent on etiology, if identifiable. Bacterial infection can be assumed until otherwise proven, and therapy directed against Pasteurella multocida should be initiated. Without culture and sensitivity results, antibiotic therapy may have to be on a "hit or miss" basis. Antibiotics with a known sensitivity towards Pasteurella include the quinolones (enrofloxacin and ciprofloxacin), chloramphenicol, the aminoglycosides, penicillin G (parenteral only), and trimethoprim-sulfa drugs. The use of antibiotics that induce bacterial dysbiosis (the beta-lactams, and the macrolides e.g.) should be avoided regardless of sensitivity results. Therapy should be for a minimum of 14 days and in some cases, 2-3 months is indicated.
Removal of potential environmental irritants like dusty bedding and moldy hay and increasing ventilation is helpful. Nebulizing with sterile saline and antibiotics can be done by most owners at home using a store-bought nebulizer (PulmoAide®, Devilbiss Health Care, Somerset PA). Antihistamines rarely work because allergies are uncommonly identified in the rabbit but can be used if an allergic component is strongly suspected. Avoid the use of steroids.
Conjunctivitis must also be addressed. Good choices for topical therapy include gentamicin, ciprofloxacin opthalmic drops (Ciloxan®, Alcon Laboratories, Fort Worth), chloramphenicol, and tetracycline. Nasolacrimal ducts should be flushed in all cases of epiphora or ocular discharge.
Prognosis is variable and depends on the bacterial virulence, host response, and therapeutic modality chosen. With Pasteurella, infections can be life-long and treatment may be for control and not elimination of the organism. Deep sinus infections and abscesses are the most difficult to control. There are generally several courses Pasteurella UTI can take in the rabbit: infection and spontaneous elimination, infection with response to antibiotics, infection with initial antibiotic response and subsequent recurrence, and chronic infection non-responsive to treatment. Which course an individual takes is unpredictable but an informed owner is best equipped to deal with chronic upper respiratory disease in their rabbit.
Lower Respiratory Tract Disease
Because rabbits are obligate nasal breathers, it may be difficult on initial presentation to determine if a dyspneic rabbit is suffering from upper or lower airway disease. Careful auscultation of the upper (trachea) and lower airways (lungs) can help differentiate between an upper airway obstruction and lung disease in most cases. Nervous or overheated rabbits will present with an increased respiratory rate and may be panting. This can also obscure normal pulmonary sounds. Ultimately, radiographs may be the only way to confirm thoracic disease and this differentiation is vital to diagnosis, treatment, and prognosis.
Bacterial infections account for the majority of bronchopneumonias in rabbits and once again, Pasteurella multocida tops the list of pathogens. Because pasteurella can spread hematogenously, rabbits with respiratory disease can subsequently develop pneumonia, lung abscesses, and infection essentially anywhere in the body. Other bacterial pathogens include Moraxella catarrhalis, Bordetella bronchiseptica, and Staphylococcus aureus. Other pathogens like Mycoplasma, and Chlamydia, and viruses in general, are not considered significant.
Noninfectious causes of lower respiratory diseases are common in the rabbit and must be differentiated from the bacterial etiologies. Environmental irritants and allergans may play a role as a primary etiology or as a predisposing factor to bacterial opportunists. Rabbit urine is particularly irritating and in poorly ventilated areas may be a problem. Proving an allergic response is difficult; a transtracheal wash may provide a cytological diagnosis. Avoid the use of steroids in the respiratory rabbit and consider antihistamines and bronchodilators if allergies are strongly suspected.
Neoplasia is an important differential in the dyspneic rabbit. Primary pulmonary tumors are uncommon but thymomas are reported and lymphosarcoma is a possibility. More common is metastatic uterine adenocarcinoma. Some of these affected intact females only present when metastasis developes.
Cardiovascular disease can be seen in the rabbit. Most rabbits with clinically identifiable heart disease present in heart failure (the other subset suffers from acute death). Dilated cardiomyopathies have been seen and can produce pulmonary edema and pleural effusion. Arteriosclerosis and aortic mineralization are not uncommon in geriatric rabbits and may contribute to cardiovascular compromise.
Diagnosis of lower airway disease is based on clinical examination and radiographic imaging. Thoracic ultrasound or echocardiography can determine if lung consolidation or abscesses are present, and give important cardiac parameters. In general, blood tests are not as helpful in elucidating etiologies but should always be performed for metabolic profiling and the realistic possibility of concurrent disease. Serodiagnosis of P. multocida can be performed but it has it's limitations and like other serological tests, is subject to interpretation.
Treatment of lower respiratory disease is based on etiology. There is no therapy for neoplasia, so once that has been eliminated as a cause, antimicrobials are the first line of defense (see above). It may be difficult initially to distinguish heart failure from bacterial pneumonia; these bunnies should be stabilized in oxygen and treated with antibiotics until thoracic films can be taken. It is difficult to auscult murmurs in rabbits with heart disease but arrhythmias are detectable in some cases. Heart failure with effusions are approached much like any other animal: pleural effusions can be tapped, diuretics (furosemide) administered, and oxygen provided for the acute crisis. Long term therapy follows that for a dog, cat, or ferret and would ideally be based upon results of echocardiographic studies.
Prognosis for lower airway disease is often poor. Most affected rabbits do not show early signs of disease and are often in the advanced stages of respiratory compromise when presented clinically.
Managing The Dyspneic Rabbit
Rabbits, as prey species, are particularly sensitive to stressful environments and situations. It is vital that these sick cases be handled quickly, efficiently, and with a minimum of stress. In hospital situations, rabbits must not be placed near dogs, or loud noises, and all rabbit cases should be discharged for home care as soon as it is medically safe to do so.
Respiratory cases can be pre-treated with oxygen and stabilized with fluids and antibiotics for 24 hours prior to diagnostic testing. Sedation may be needed for radiographs; this author prefers to administer midazolam (Versed® @ .5 mg/kg IM) to calm dyspneic or panicky rabbits and add oxygen and isoflurane by facemask as needed.
Deeb, BJ: Respiratory disease and the Pasteurella complex. In E. V. Hillyer and K. E. Quesenberry (eds): Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery. Philadelphia, WB Saunders, 1997, 189-201.