Diseases of the Rabbit Respiratory Tract

Key Points

  • Pathogens commonly cultured from the rabbit respiratory tract include Pasteurella multocida, Staphylococcus aureus, Enterobacter spp., and Pseudomonas aeruginosa.
  • A subtle sign of upper respiratory disease in rabbits may be discharge matted on the medial aspect of the forepaws due to the rabbit’s tendency to fastidiously clean its face with its forepaws.
  • Consider nasal foreign body in rabbits with persistent, intractable upper respiratory tract infection.
  • Dental disease can be an important underlying cause of upper respiratory tract disease. Overgrowth of maxillary cheek tooth roots can impinge upon the nasal cavity or nasolacrimal duct causing obstruction and bone remodeling.
  • The only clinical signs seen with pneumonia or pulmonary abscess may be tachypnea and a mild to moderate decrease in activity level.
  • Unfortunately pulmonary neoplasia such as metastastic uterine adenocarcinoma is an important cause of lower respiratory tract disease in intact does.
  • A useful adjunct to systemic antibiotic therapy is nebulization, which serves to humidify the respiratory passages and/or deliver antibiotics.

Introduction

Respiratory disease is a common clinical problem in the rabbit (Oryctolagus cuniculus). Although respiratory disease may often be caused by a bacterial infection, there are a host of possible factors that should be considered. Obtain a detailed history and perform a careful physical examination, in order to formulate an appropriate differential list and diagnostic plan.

 

Differential diagnoses for respiratory signs in the rabbit

Although respiratory disease is often caused by bacterial infection, rabbits that present with respiratory signs may have other underlying problems such as neoplasia, nasal foreign body, or even heart disease.

Bacterial infection

Bacterial infection in the rabbit respiratory tract may cause rhinitis, sinusitis, or even dacryocystitis. Lower respiratory tract disease may take the form of pneumonia and/or pulmonary abscesses that can become very large in size, significantly compromising lung capacity.

Pathogens commonly cultured from the rabbit respiratory tract include Pasteurella multocida, Staphylococcus aureus, Enterobacter spp., and Pseudomonas aeruginosa. Although Bordetella bronchiseptica has been reported as part of the normal flora of the rabbit respiratory tract, I have seen significant upper and lower respiratory tract disease in rabbits associated with pure growth of B. bronchiseptica from deep nasal and tracheal wash cultures. Although we may learn in the future that Bordetella spp. is a primary pathogen in rabbits, there are other possible explanations for these culture results. These rabbits with may have been immunocompromised by some other condition that then allowed Bordetella to overgrow, or Bordetella may have prevented the growth of other organisms.

Viral infection

Myxoma virus, a member of the poxvirus group, causes myxomatosis. Myxoma virus is transmitted via arthropod vectors like fleas and mosquitoes. The distribution of myxomatosis is worldwide, however in the United States disease is seen in the West. Clinical disease typically involves cutaneous lesions, non-specific signs of illness, and a purulent oculonasal discharge. For more information on myxomatosis see the Merck Veterinary Manual.

Respiratory signs such as dyspnea are sometimes seen in rabbits infected with rabbit viral hemorrhagic disease (VHD) or rabbit calicivirus disease, however other presentations are more important. Many rabbits develop a fever and die within 12-36 hours, often with bloody fluid emerging from the nose and mouth. Survivors may exhibit neurologic signs. Rabbit calicivirus disease was first reported in China in 1984. It is now widespread throughout Europe, and has also been reported in Mexico, parts of Northern Africa, and the United Kingdom (UK).

Nasal foreign body

It is possible for a rabbit to get grass seeds, or pieces of hay or fur stuck in its nose. Irritation causes nasal discharge that is initially serous, but is often mucopurulent and copious by the time of presentation. Rabbits may paw at the affected side of their nose and sneeze forcefully or have stertorous breathing.

Allergic disease

Allergic rhinitis and bronchitis has been described in rabbits but appears to be uncommon and information is largely anecdotal.

Neoplasia

The most common tumors of the chest cavity in the rabbit are metastatic mammary adenocarcinoma and primary thymoma of either lymphoid or epithelial origin. Since the thymus persists throughout life in the rabbit, thymomas may be seen in rabbits of all age. Lymphoma may also be present as a mediastinal mass.

Cardiac disease

Although not a respiratory condition, heart disease is a possible cause of respiratory signs in the rabbit. For instance, I have seen rare cases of dilated cardiomyopathy in the rabbit.

 

Diagnostic testing

Recognizing respiratory disease in the rabbit

Handle sick rabbits carefully. If you suspect respiratory disease, consider allowing the individual to rest in oxygen or at least a quiet, dark area prior to physical examination.

Rhinitis or sinusitis, sometimes called “snuffles” in laymen’s terms, is usually characterized by unilateral or bilateral, mucopurulent nasal discharge, sneezing, and congestion. A subtle sign of upper respiratory tract disease can be discharge matted on the paws or the medial aspect of the forelimbs. Discharge may collect here as the rabbit fastidiously cleans its face with its forepaws. In the early stages of disease, discharge may not be evident on the nose or even on the paws, however close examination of the nares with an otoscope or rigid endoscope may confirm the presence of nasal discharge.

With an underlying foreign body is present, nasal discharge is initially serous. By the time of presentation, nasal discharge is often mucopurulent and copious. Rabbits tend to paw at the affected side of their nose and sneeze forcefully.

Heavy nasal discharge that interferes with the rabbits’s sense of smell may lead to anorexia, as rabbits rely heavily on scent to stimulate appetite. As rabbits are obligate nasal breathers, severe obstruction of breathing by heavy nasal discharge is distressing and will also lead to anorexia.

Signs of dacryocystitis may include epiphora, nasal discharge, and a white discharge at medial canthus of the eye. Gentle pressure on the lacrimal sac at the medial canthus may cause white discharge to ooze from the nasolacrimal punctum.

The only clinical signs of pneumonia, pulmonary abscess, or thoracic neoplasia may be tachypnea and non-specific signs of illness such as anorexia and a mild to moderate decrease in activity level. As disease advances, dyspnea, cyanosis, and exophthalmos, due to impedance of venous return from the large venous plexus at the back of the orbit, may be observed. Coughing is extremely rare.

Dental disease may be an important underlying cause of upper respiratory tract disease. Overgrowth of maxillary cheek tooth roots can impinge upon the nasal cavity or nasolacrimal duct causing obstruction and bone remodeling. Carefully evaluate the teeth, saving the dental exam for the end of the physical examination since some rabbits may need to rest afterwards.

Minimum database

Perform hematology, and possibly serum biochemistry, as indicated. Complete blood count results may show an inflammatory leukogram, but are often within normal limits. Serum chemistries are usually unremarkable unless infection involves other organ systems.

Obtain skull radiographs in rabbits with chronic, persistent upper respiratory tract disease. Carefully evaluate the nasal turbinates, the tympanic bullae, and the teeth, particularly the roots of the maxillary cheek teeth. Although survey radiographs may be adequate to identify bony changes, computed tomography is a more sensitive diagnostic test.

Although their interpretation can be quite challenging because of the rabbit’s small thorax, obtain survey chest radiographs when lower respiratory tract disease is suspected. Ultrasonography is very useful in identifying pulmonary or pleural masses and pleural effusion. Ultrasound can then be used to obtain biopsy specimens or needle aspirates. Magnetic resonance imaging is useful for imaging soft tissues such as thoracic masses.

If heart disease is suspected, echocardiogram and electrocardiogram are useful diagnostic tools in the rabbit. Normal measurements have been reported by Redrobe and Lord et al.

Otoscopic exam

Use a scope to look for nasal discharge in early upper respiratory tract disease, or to explore the rostral nares when a nasal foreign body is suspected. To perform an otoscopic exam, roll the rabbit onto its back in your arms. Stabilize its head with one hand and look up the nares using a small otoscope cone. Although visibility is limited, when the rabbit expires sometimes material may be observed.

If the rabbit has persistent, intractable rhinitis, exploration of the nasal passageways with a rigid endoscope is indicated under general anesthesia. Rhinoscopy in many rabbits is best performed with the use of a 1.9 mm scope. For rabbits over 2 kg, a 2.7 mm scope may be used.

Deep nasal swab

Unless a bacterial culture sample is obtained from a lesion identified via rhinoscopy, a nasal swab may likely grow contaminants. Nevertheless, samples obtained via nasolacrimal flush or deep nasal swab have a better chance of representing the bacterial population of the upper respiratory tract than swabs or nasal discharge.

To perform a deep nasal swab, restrain the rabbit on its back in a slight trance or with light sedation. Insert a mini-tip culturette (Marion Laboratories) ventromedially at least 2 cm into the nasal cavity to the level of the medial canthus of the eye (Fig 1).

Deep nasal swab

Figure 1. Deep nasal swab in a rabbit (Oryctolagus cuniculus). Image provided by Dr. Mark Grobner. Click image to enlarge.

Tracheal wash

The technique used for tracheal wash is similar to that described in cats except that intubation of rabbits is much more difficult. Administer an induction agent to the rabbit; I like to use dexmedetomidine (Dexdomitor, Pfizer) (0.1 ml/kg). Then intubate the rabbit with a sterile endotracheal tube. Feed a sterile 3.5 or 5 French red rubber catheter down the tube, then gently squirt in about 3 ml of sterile saline. Quickly aspirate fluid back. Aspiration sometimes requires coupage and turning the rabbit so that its nose points down. Even then only about 25% of the fluid may be recovered. Use the tip of the catheter to make cytologic impression smears and culture the remaining fluid.

 

Therapy

Antimicrobials

Topical therapy

In cases of dacryocystitis or early or simple rhinitis, instill ophthalmic antibiotic drops directly into the nares and/or eye after nasolacrimal flushing (see below). Popular choices include gentamicin, tobramycin, ciprofloxacin, and fusidic acid. NEVER include corticosteroids, as rabbits are exquisitely sensitive to their adverse effects.

Systemic therapy

Carefully consider systemic antibiotic choices for the treatment of respiratory infection in rabbits. Life threatening dysbiosis and enteritis can occur if the wrong antibiotic is chosen. Antibiotics commonly used include:

  • Sulfa antibiotics such as trimethoprim-sulfadiazine (30mg/kg PO BID),
  • Fluoroquinolones, enrofloxacin and ciprofloxacin (5-10mg/kg PO, SQ BID),
  • Chloramphenicol (50mg/kg PO TID), and
  • New-generation macrolides like azithromycin (50mg/kg PO QD X 10-14 days).

Although beta-lactams should never be administered orally, penicillin may be given parenterally (42,000-84,000IU/kg SQ, IM q 24h) for some conditions. One to two doses of tilmicosin have been used successfully in rabbits with pasteurellosis, however human health risks have prevented widespread use of this antibiotic in private practice.

Continue treatment for a minimum of 14 days in mild cases and up to several months in chronic infections. Repeated cultures and hematology may aid in determining treatment duration.

Nasolacrimal flushing

Nasolacrimal flushing is an invaluable technique for rabbits with dacryocystitis. To perform a flush, you will need:

  • A cannula of appropriate size or a 24 gauge catheter with the needle removed
  • Local anesthetic such as proparacaine
  • A 3-6 ml luer lock syringe aseptically filled with eyewash or sterile saline.
  • Fluorescein stain may optionally be added to the flush solution.

Depending on the individual rabbit and its clinical history, nasolacrimal flushing may be performed under general anesthesia, sedation, or in the conscious patient. Restrain the rabbit in sternal recumbency. It can help to securely wrap up . the rabbit in a large towel. Gently retract the lower eyelid to identify the slit-shaped opening or nasolacrimal punctum. Gently introduce the tip of the cannula in a ventromedial direction. Relax your hold on the eyelid, and gently infuse the fluid.

Fluid will pass out of the nostril on that side if the duct is patent. If the duct is blocked, it is not uncommon for purulent material to initially well back out of the punctum. It can sometimes help to gently cover the slit-like punctum with a moistened cotton-tipped applicator while flushing. Fluid may be collected on a culture swab or into a Petri dish for culture and cytology. The nasolacrimal duct can also be flushed retrograde by locating and inserting a cannula into the nasal punctum of the duct on the dorsomedial aspect of the nares. Use of both approaches can sometimes aid in dislodging purulent material.

Management of non-infectious conditions

Nasal foreign bodies may be removed during rhinoscopy. Alternatively, intubate the rabbit. Maintain the rabbit on 100% oxygen in sternal recumbency. Place moistened gauze sponges around the glottis, then flush the nasal passages with sterile saline.

The most common tumors of the chest cavity are metastatic mammary adenocarcinoma and thymoma of either lymphoid or epithelial origin. There have been reports of surgical removal of thymomas via thoracotomy as well as medical management in the form of radiation chemotherapy.

Non-specific therapy

Nebulization is a useful adjunct for both upper and lower respiratory tract disease. Saline alone is great for humidifying the respiratory tract, I also find the following recipe very useful for rabbits with pneumonia:

  • Saline (5ml)
  • Acetylcysteine 200 mg/ml (0.25ml)
  • Aminophylline 25 mg/ml (0.5ml)
  • Amikacin (1 ml)

Nebulize the rabbit with a mask over its nose or in a chamber for 15-30 minutes; repeating this treatment every 4-12 hours. Even the most distressed rabbits seem to tolerate this treatment quite well. Other antimicrobial agents, mucolytics, and bronchodilators can also be administered by nebulization.

Supportive care may include supplemental oxygen, fluids, and nutritional support. Affected rabbits may also benefit from use of non-steroidal anti-inflammatory agents. House the rabbit in a quiet, darkened room away from predator species like cats, dogs, or ferrets.

Regardless of the underlying cause of respiratory disease, changes in husbandry can also be very helpful Recommend low-dust bedding such as recycled paper product (CareFRESH, Absorption Corp) or aspen shavings. Encourage scrupulous sanitation, and ensure the rabbit is not exposed to humidity or temperature extremes. It may also help to provide an air purifier and/or air filtration.

 

Prevention/control

Stress and poor husbandry play important roles in the development of infectious respiratory disease in the rabbit. Encourage caretakers to practice scrupulous sanitation and ensure adequate ventilation as high ammonia levels and dust can irritate the respiratory mucosa.

 

Prognosis

Unfortunately by the time lower respiratory tract disease is diagnosed in most rabbits, disease is often quite advanced and the prognosis is grave. Rabbits with bacterial rhinitis or dacryocystitis may respond well to treatment, but always warn owners that clinical signs may recur later in life. In fact some rabbits may suffer from waxing and waning signs throughout their lives, particularly when diagnostic testing is not pursued.

 

Conclusion

Rabbits with respiratory disease may present with a variety of clinical signs. Respiratory disease may also be caused by a variety of etiologies, however bacterial infection and neoplasia are the two most important differential diagnoses. A thorough physical examination and diagnostic testing such as radiographs, tracheal wash, deep nasal culture, endoscopy, and ultrasound are helpful in identifying the cause of illness.

References