Respiratory Disease and Pasteurellosis

Chapter 16 Respiratory Disease and Pasteurellosis

Respiratory disease is common in pet rabbits and, as in other species, is caused by a number of primary and secondary etiologies. Although the anatomy of the respiratory tract is discussed in more detail in Chapter 12, it should be kept in mind that rabbits are obligate nasal breathers because of the position of the elongated epiglottis engaged over the caudal margin of the soft palate. For this reason, mouth breathing is an indication of a severe abnormality. In general, disease of the upper respiratory tract is more severe and stressful in rabbits than in other species. Respiratory disease can be classified as affecting the upper or lower respiratory tract or both.

Diseases of the Upper Respiratory Tract


Numerous infectious diseases are associated with disease of the upper respiratory tract; they are due mainly to bacterial agents, but in rare cases, viral, fungal, or parasitic pathogens may be involved as well. The rabbit is a common research model for rhinitis and sinusitis. Sinusitis is established after mechanical blockage of the ostium connecting the nasal and sinus cavities, resulting in impaired mucociliary function and clearance of organisms from the sinus cavity.2 Ostial dysfunction appears to be an important requirement for the establishment of infection in laboratory models, especially for organisms not considered primary pathogens, and may have significance for naturally infected animals as well.

Otitis media is associated with respiratory disease in rabbits, as infection can spread via the eustachian tube to the tympanic bulla and middle and possibly inner ear.11,17

Bacterial Pathogens

While Pasteurella multocida is often implicated as a cause of respiratory disease, other pathogens must be considered. A recent epidemiologic study of 121 rabbits with symptoms of upper respiratory tract disease (nasal discharge and sneezing) indicated that the most common bacterial isolates (deep nasal samples) were P. multocida (54.8%), Bordetella bronchiseptica (52.2%), Pseudomonas species (27.9%), and Staphylococcus species (17.4%). Mixed infections were also seen.30

Pasteurellosis is associated with many disease processes in rabbits including rhinitis, sinusitis, conjunctivitis, nasolacrimal duct infection, otitis, tracheitis, pneumonia, and abscesses (dental, skin, other). While not identified as a pathogen in wild rabbit populations, Pasteurella is a significant pathogen in laboratory animals, leading to the development of “Pasteurella-free” animals for research use. Because of its ubiquitous nature, many pet rabbits are already infected with Pasteurella multocida. Many exhibit symptoms in response to stress, such as stress of weaning, transport, and purchase from the pet store as well as poor sanitation and ventilation and concurrent illnesses.11

Strains of Pasteurella vary in pathogenicity,27 with some being more likely to spread via the hematogenous route and causing acute septicemia, generalized disease, and death. P. multocida gains entry to the host primarily through the nares or wounds via aerosolization or direct contact with infected rabbits or fomites. Does with venereal infections may pass the organism to their young, who may remain apparently uninfected for several weeks until signs of infection develop. Infected rabbits can resist infection or become subclinical carriers, thus complicating determination of an incubation period. In experimental studies, rhinitis occurred 1 to 2 weeks after intranasal inoculation.24 After infection, the organism spreads to the sinuses, nasolacrimal duct, eustachian tube and middle ear, trachea, and lungs. Factors facilitating disease expression include exposure to ammonia through improperly cleaned enclosures, administration of corticosteroids, and the psychologic stress or stress of concurrent disease.10 Recent studies have indicated promise for the development of vaccines as an aid to management of this disease.33

B. bronchiseptica is a common inhabitant of the rabbit respiratory tract, and disease prevalence may increase with age.11 Experimental inoculation of organisms may or may not produce disease. This organism is pathogenic in guinea pigs, dogs, cats, and pigs. B. bronchiseptica is suspected to be a copathogen or predisposer to infection with P. multocida. However, Deeb has reported severe infection in the absence of P. multocida in a colony of inbred laboratory rabbits, suggesting that more pathogenic strains may exist (BJ Deeb and RF DiGiacomo, unpublished data).

Pseudomonas species infections in rabbits are common. Rabbits are used as a laboratory model for human Pseudomonas infections in multiple body systems, including the cardiovascular system.21

Staphylococcus species is commonly isolated from the nares of rabbits and is thought to be a secondary invader. Pathogenicity depends on host susceptibility and bacterial virulence. Disseminated staphylococcosis has been linked with fibrinous pneumonia or abscesses in the lung or heart.10

While not a primary respiratory pathogen, Treponema cuniculi (rabbit syphilis) can produce crusting of the mucocutaneous borders of the nose, lips, and eyelids, in some cases resembling an upper respiratory infection. Marked crusting of the nares can produce mild to moderate respiratory distress.

A number of other pathogens can produce upper respiratory disease in rabbits. Pasteurella species other than P. multocida can be cultured from rabbits but may be nonpathogenic. However, pure cultures associated with clinical disease should be treated as true pathogens. Other potential pathogens include Moraxella species, Yersinia pestis, and Escherichia coli. Mycoplasma pulmonis was isolated by Deeb in the rhinopharynx of rabbits with evidence of upper respiratory disease. Infected rabbits were housed in close proximity to rats, which may have been the source of the infection.10 S. Kelleher (personal communication) reported a case of nasal granuloma produced by Mycobacterium species.



Traumatic injury to the upper respiratory tract includes blunt force trauma, predator injury, and damage to the glottis and trachea after endotracheal intubation. There are numerous reports of inflammation of the glottis and stenosis of the trachea in the laboratory literature.15,29 In one report of three cases of tracheal stenosis secondary to intubation, evidence of respiratory disease appeared within 17 to 21 days postintubation. Risk factors could not be identified.29 The author has regularly performed blind endotracheal intubation in rabbits and taught the technique to students and support staff for over 10 years. The most severe complications have been rare cases of mild respiratory stridor that did not persist beyond 24 hours postprocedure. In normal position, the epiglottis is above the soft palate in the rabbit and other obligate nasal-breathing species, preventing both mouth breathing and access to the trachea via the oral cavity. Hyperextension of the head causes the epiglottis to disengage and reposition below the soft palate, allowing access to the trachea (Fig. 16-1). Other keys to successful intubation appear to be correct tube size (2-3 mm) and gentle technique without the use of pressure during placement.

All mammalian species are susceptible to airway irritation due to chemical exposure, and mucosal damage may predispose to infection. Numerous studies have demonstrated upper airway and lung damage secondary to irritants such as tobacco and dried dung smoke.14 Other potential sources of respiratory irritants include household chemicals and ammonia from poorly cleaned enclosures.

Foreign materials in the nares, pharynx, or trachea have been reported. In one rabbit with chronic sneezing, its clinical signs ceased following removal of hay from the nares.32

Diseases of the Lower Respiratory Tract


A number of neoplasms produce metastasis to the lungs or, in the case of thymoma or thymic lymphoma, indirectly affect the lower respiratory tract. Neoplasms producing metastasis to the lungs include uterine adenocarcinoma, osteosarcoma, lymphoma, and mammary carcinoma.36 Primary carcinoma of the lungs of a 7-year-old rabbit has also been reported.19

Thymoma is a primary tumor of lymphoid or epithelial origin and is increasingly reported in the literature.26 Common clinical features include decreased activity, increased respiratory rate and effort, and bilateral exophthalmos, which can be intermittent and often increases in severity over time. Exophthalmos is thought to occur secondary to vascular compression and reduction of vascular return to the heart caused by the growing mass.26

Diagnosis and Differentiation

Physical Examination

Symptoms of respiratory disease can be similar to those noted in other traditional pet species; they include nasal and/or ocular discharge and increased respiratory effort and rate, often worsening with exertion. Ocular and nasal discharge frequently produces wet or matted fur beneath the eyes and nares. Rabbits with ocular or nasal discharge often attempt to remove the material with the forefeet, resulting in an accumulation of debris in the fur of the medial aspect of the feet.17 In addition, rabbits with tracheitis will often cough when the trachea is gently palpated. Because rabbits are obligate nasal breathers, diseases producing nasal discharge or other occlusion of the nares can lead to severe respiratory distress.17 Other nonspecific symptoms that should arouse suspicion of respiratory disease include weight loss, decreased appetite, lethargy, and exercise intolerance.

Auscultation in rabbits is extremely useful. Airway and lung sounds in the normal rabbit are similar to those in other small traditional pet species. However, small thoracic cavity and lungs can make identification of lung sounds challenging. Diseases affecting upper airways often result in increased airway noise or wheeze, which is often referred to the thorax. Pulmonary disease produces predictable changes in lung sounds, including decreased or increased sounds, which may be bilateral or unilateral. It should be noted that some rabbits under stress are particularly vocal and will produce a characteristic honking noise that should not be confused with a sign of respiratory disease.

Breathing patterns are also important. As in other species, rabbits with pleural disease (pneumothorax, hydrothorax) may demonstrate an asynchronous breathing pattern, with the thorax and abdomen appearing to move opposite to each other. The author has noted a case of hydrothorax in a rabbit where removal of fluid resulted in immediate restoration of synchronous breathing movements.

Sep 6, 2016 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on Respiratory Disease and Pasteurellosis

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