Otitis in Rabbits

Key Points

  • The most common causes of head tilt in rabbits are Encephalitozoon cuniculi and bacterial otitis media/interna.
  • Lop-eared rabbits lack normal ear drainage and are at particular risk for developing otitis.
  • Visualization of the horizontal ear canal and tympanic membrane can be difficult even in the normal rabbit. Sedation or general anesthesia may be needed for a thorough otoscopic examination if the ear is painful, or if the canal is filled with exudate.
  • Bacterial infection caused by aerobic and anaerobic microbes is an important cause of otitis. Commonly isolated organisms include Pasteurella multocida, Staphylococcus aureus, Pseudomonas aeruginosa, and Streptococcus spp.
  • Ear mite infestation caused by Psoroptes cuniculi is intensely pruritic. A crusty exudate forms within the ear canal and may extend up the pinna. Removal of this crust is painful and unnecessary. Ivermectin or selamectin are effective treatments.
  • Provide analgesia to rabbits with otitis when indicated.
  • Non-steroidal anti-inflammatory agents are often helpful, but avoid corticosteroids since rabbits may be particularly sensitive to the immunosuppressive qualities of these drugs.
  • Use ear lavage to remove debris found deep within the ear canal.
  • Topical therapy should be based on the character of the disease. Always select a product that does not contain steroid. If an ear medication is not available without steroids, ophthalmic drops that contain an appropriate antimicrobial agent can be used in many patients.

Head tilt in rabbits

Head tilt that is often accompanied by nystagmus and circling is a common clinical problem in rabbits. The most important causes of head tilt in rabbits are bacterial otitis media/interna and
Encephalitozoon cuniculi infection of the central nervous system, although encephalitis or abscess, heat stroke, head trauma, and neoplasia, must also be considered (Fig 1). Toxoplasma gondii and Baylisascaris procyonis are rare causes of encephalitis and are most often diagnosed post-mortem.

head tilt rabbit MGrieb WC

Figure 1. Otitis media/interna is an important differential diagnosis for signs of pain or head tilt in the rabbit. Photo credit: MGrieb reworked by Sarbo via Wikimedia Commons. Click image to enlarge.

Differentiating peripheral and central causes of head tilt

It can be difficult to differentiate between central and peripheral causes of head tilt. Rabbits with central nervous system disease have additional clinical signs that may include changes in mentation, proprioceptive deficits, gait abnormalities, abnormalities in reflexes, cranial nerve deficits, and/or ataxia.

With otitis the head tilt may be severe, however no proprioceptive or mentation abnormalities will be seen. An affected rabbit may initially seem quieter or conversely more agitated than normal. Because the lesion is peripheral, the head tilt is towards the side of the lesion. Pain may cause the rabbit to periodically tilt its head with otitis externa, however the tilt can easily be corrected and there is no loss of balance.

When nystagmus is present, this may also provide a clue to the location of the lesion. With peripheral disease, the fast phase of nystagmus is away from the side of the lesion. Positional nystagmus is seen in central disease. Disease may be peripheral or central with horizontal or rotary nystagmus, however vertical nystagmus is always associated with central nervous system disease.

 

The rabbit ear

The anatomy of the rabbit ear canal is similar to other species. There is a pinna, and just cranial to the opening of the ear canal is a blind pocket called the pretragic incisure. The ear canal consists of a long vertical canal, a short horizontal canal, and then the tympanum, which separates the external ear from the middle ear. The middle ear contains the ossicles, and the inner ear contains the semicircular canals and cochlea.

The tympanic bulla sits at the base of the ear. The facial nerve runs near the bulla. In some rabbits the nerve lies directly ventral to the bulla and in others it may be directly superficial to the bulla.

Rabbits normally produce cerumen or earwax that is brownish or slightly off-white. This wax may serve a protective function, however, if the distal aspect of the canal cannot be visualized it may be necessary to remove this wax to facilitate a more thorough otoscopic examination.

 

Etiology

The pathogenesis of otitis is often multifactorial. For instance, predisposing factors such as ear conformation increase the risk of otitis in certain breeds. All rabbits have a relatively narrow ear canal, however in lop-eared rabbits (Fig 2) the fold in the ear cartilage is such that the lumen is entirely closed off preventing normal drainage of cerumen from the ear.

Lop-eared rabbits are at particular risk for developing otitis.

Figure 2. Lop-eared rabbits lack normal ear drainage and are at particular risk for developing otitis. Image by Zemlinkill. Click on image to enlarge.

Factors that either directly cause otitis or that perpetuate infection by preventing the resolution of disease include trauma, toxins, and the presence of microorganisms such as bacteria, fungi, and parasites.

  • The ear mite, Psoroptes cuniculi , can cause infection of the external ear canal that can extend to the middle ear. Ear mite infestations are particularly common in young, obese, or immunocompromised rabbits, or in large rabbitries.
  • A wide variety of bacteria, both aerobic and anaerobic, and fungi, can be associated with otitis in the rabbit. Bacteria most commonly invade the inner ear from the upper respiratory tract via the Eustachian tube. Concurrent rhinitis or sinusitis is highly suggestive for this route of transmission. Bacteria may also extend from the external ear, and in rare instances, microorganisms could also be introduced through the blood stream. Bacteria commonly involved in otitis include:  Pasteurella multocida, Pseudomonas aeruginosa, Staphylococcus aureus, and Streptococcus spp. Other microbes that have been reported include: Bordetella bronchiseptica, Bacteroides sp., Proteus mirabilis, Escherichia coli, and Mycoplasma spp.
  • Traumatic ear injury can occur secondary to migration of a foreign body, such as a grass awn or a concretion of wax, through the tympanum. Iatrogenic trauma can occur secondary to an overly aggressive ear flushes when the eardrum is ruptured, or in rare instances, head trauma may be associated with fractures to the tympanic bulla or petrosal bone.
  • Certain ear medications or antiseptics applied in the presence of a ruptured eardrum can damage to inner ear. Ototoxic items include iodine-based products, chloramphenicol, gentamicin, neomycin, and chlorhexidine.
  • Mass lesions, such as cerumen gland adenoma or inflammatory polyps, could potentially play a role but these conditions are rarely reported in dogs and cats and have not been described in rabbits.

Diagnosis of otitis

Obtain a detailed history:

  • Determine the onset and duration of disease.
  • Is there a history of any prior illness, especially respiratory disease?
  • Ascertain the possibility of exposure to toxins or parasites.
  • Is there a history of trauma?
  • How is the animal’s food and water intake? Is fecal production normal?

Perform a thorough physical examination:

  • Check the rabbit closely for any evidence of respiratory disease such as nasal discharge or dried discharge on the paws.
  • Check the eyes for corneal edema or ulcers. Rabbits with a severe head tilt can develop eye injuries because the “down” eye is prone to trauma.
  • Normal grooming behavior can spread mite infestation from the ears to other parts of the body. Mange can cause crusting and exudative skin lesions to develop on the perineal skin folds or the ventral abdomen.

Perform a careful neurologic exam:

  • Evaluate mentation. Is the rabbit alert and active, or dull and depressed?
  • If a head tilt is present, is it persistent? Which direction?
  • If nystagmus is present, is it spontaneous or positional? Which direction is the fast phase?
  • Does the rabbit exhibit ataxia or gait abnormalities? Unilateral (which side) or bilateral?
  • Damage to the cranial nerves, including the facial nerve, that run through the middle ear can lead to a host of deficits including:
    • Drooling
    • Reduced or absent blink reflex
    • Drooping ear
    • Deafness, which can be difficult to recognize in rabbits
    • Horner’s Syndrome: ptosis, relaxed third eyelid, and miosis
    • Contracture of the facial nerve and corresponding muscles of the affected side; sensation is generally normal but motor responses may be absent around the lips, nares, and sometimes eyes

 Perform a thorough otoscopic exam:

  • Evaluate the pinna for redness and odor. Thick crust and scale will develop with ear mite infestation.
  • Check the base of the ear. Soft tissue swelling at the base of the ear canal can sometimes be seen in cases of severe otitis media where there is a build up of pus within the ear. Gentle pressure at the ear base may express the exudate. Signs of excessive scratching or redness at the ear base or on the neck may also be noted.
  • Examine both the external ear canal and the blind pouch. Disease may be unilateral or bilateral. For animals with unilateral signs, examine the unaffected ear first to prevent iatrogenic contamination of the unaffected ear. Even in the normal rabbit, a waxy deposit often obscures the horizontal canal and eardrum. Gentle manipulation of the pinna during otoscope exam or use of an endoscope can help in evaluation of the ear canal.Look for evidence of inflammation such as redness, swelling, desquamation of the epithelium, an increase in cerumen, or the presence of exudate, which tends to be white and creamy. The ear canal may be painful or pruritic depending on the cause or duration of the ear infection, and sedation or general anesthesia may be needed for a thorough otoscopic examination.Evaluate the integrity of the tympanum. Look for evidence of disease or rupture. Is there any sign of fluid, blood or pus beyond the tympanic membrane? In many cases, the tympanic membrane cannot be visualized until the exudate is gently removed from the canal (see flushing below).
  • Endoscopy of the ear canal with a rigid scope may be useful if middle ear infection is present, or possibly to obtain cultures via myringotomy (see above).

 Cytology

Cytologic evaluation may provide immediate diagnostic information. Obtain samples before beginning treatment and cleaning. Ideally exudate or cerumen should be sampled from the horizontal ear canal. Obtain exudate with a cotton-tipped applicator. Roll the swab on a glass slide, heat fix, and then stain the slide using a modified Wright’s stain. Evaluate smears first under low-power magnification and then under high-power, preferably using immersion oil. Look for bacteria and/or yeast, noting their numbers and morphology, as well as white blood cells.

Ear mite lesions can be easily diagnosed by obtaining a small sample of crust and scale via scraping or tape cytology. Then evaluate the sample microscopically with mineral oil.

Culture

Many Gram-negative bacteria show resistance to various antimicrobial agents. Perform aerobic and anaerobic bacterial culture with antibiotic sensitivity and an antibiotic mean inhibitory concentration (MIC).

Complete blood count

A complete blood cell count or CBC may be helpful in cases with severe or overwhelming infection.

Imaging

Good quality skull radiographs are indicated when otitis media is suspected as a cause of relapsing bacterial otitis, or when neurologic signs accompany signs of otitis. Sedation or general anesthesia is necessary. Obtain at least four views: lateral, right and left obliques, and ventrodorsal or dorsoventral. In middle ear disease, the tympanic bulla may appear sclerotic or lytic with uneven edges. Radiographically, the medial and lateral walls of the normal bulla are slightly thicker than the dorsal and ventral walls in the normal rabbit. There may also be an increased soft tissue density within the bulla due to the presence of exudate. As in dogs and cats, radiographic detection of fluid in the tympanic bulla is only moderately sensitive, and radiographs are normal in many otitis media cases. Where available, computed tomography or MRI provides a greater degree of visualization and may detect more subtle soft tissue changes or fluid within the bulla.

One study by King et al looked at the use of ultrasound to evaluate the ears of New Zealand white rabbits. The prominent jawbones interfere with imaging from all approaches, however, using a lateral approach the external ear canal could be visualized to the level of the external acoustic meatus. The tympanic bullae could only be visualized from a ventral approach. A 12 MHz linear transducer was found to be most appropriate. More work will be needed to determine whether ultrasound examination of the tympanic bullae can be successfully performed in rabbits.

 

Therapy

Pain management

Treatment of otitis will vary with the underlying cause, however ear infections are painful so analgesia is invariably needed. Administer non-steroidal anti-inflammatory medications, such as meloxicam, carprofen, or ketoprofen, to reduce inflammation and control discomfort. These drugs may be needed only at the very beginning of therapy. Avoid corticosteroids because rabbits may be especially sensitive to the immunosuppressive qualities of these drugs, and their use may cause further complications.

Ear mites

Treat ear mites with ivermectin or selamectin and repeat in 2 weeks. Use ivermectin parenterally (400 µg/kg SC) or topically, and apply selamectin (Revolution, Pfizer) topically (6 mg/kg). Eprinomectin (Eprinex, Merial) (200 or 300 µg/kg SC) has also been used to eliminate P. cuniculi infection in rabbits. Do not clean the ears. The underlying skin is ulcerated and painful, and removal of this crust and scale is not necessary. After the inflammation and ulcers have resolved, any remaining debris can be gently removed. Sanitation is also important. Mites are transmitted by direct contact or contact with fomites, and they can survive off of the host for 21 days.

Ear cleaning

If the tympanum is intact and the accumulation of exudate is mild, then any ear solution that does not contain steroids may be used to clean the ear canal. Rabbits may be more sensitive than other animals to developing immunosuppression in the presence of corticosteroids. If the tympanum is ruptured, or its integrity cannot be determined, use only warm sterile saline to gently and carefully clean the ear canal.

Because of the rabbit’s deep vertical ear canal, it is often impossible to reach the bottom of the canal with cotton swabs for cleaning. Instead perform ear lavage or flushing to remove this debris using a 3.5 to 8 French red rubber catheter. Select the largest size that will fit in the ear canal. Gently lavage and suction warm sterile saline with a syringe (20 cc in most rabbits) until the canal appears clear. If there is debris that does not seem to break up, then repeat the procedure several days later. Sedation or general anesthesia may be required in painful or fractious rabbits.

Topical medication

Gently clean and dry the ears before starting topical treatment. Topical medications are inactivated by exudates, and excessive cerumen may prevent medications from reaching the epithelium.

Topical therapy should be based on the character of the disease. Always select a product that does not contain steroid. If an ear medication is not available without steroids, ophthalmic drops that contain an appropriate antimicrobial agent can be used in many patients.

Properly applied, the ideal medication coats the epithelium of the ear canal as a thin film. Monitor the rabbit for any changes in the skin of the ear canals during treatment as this may indicate a need for a different vehicle or base. Substances that normally are not irritating in may cause irritation in an ear that is already inflamed. This is particularly true of propylene glycol-based products.

Systemic antibiotics

Systemic antimicrobial therapy is indicated in many rabbits with otitis including most cases of chronic or recurrent ear infections, and all cases of otitis media. Antimicrobials are also indicated when neutrophils or rod-type bacteria are found on cytologic evaluation.

Base antibiotic selection on culture and sensitivity results whenever possible. If the exact organism involved cannot be identified, select a broad-spectrum antibiotic or consider combination therapy. Commonly used antibiotics include fluoroquinolones, trimethoprim-sulfa, chloramphenicol, metronidazole, and newer generation macrolides such as azithromycin, as well as injectable penicillin. Continue treatment for 3 weeks, then reevaluate the rabbit for the need of further treatment at that interval.

Myringotomy

If the infection is behind the tympanum, then a myringotomy can be performed with the aid of a rigid scope. Open the tympanum and lavage the middle ear with warmed sterile saline. Sedation or general anesthesia is required.

Surgical management

Consider procedures such as bulla osteotomy, lateral ear canal resection, and total ear ablation in cases that do not respond to medical treatment. Although surgical procedures are relatively aggressive, they may alleviate long-term pain as well as the need for repeated ear lavages. Potential postoperative complications include cellulitis, abscessation, and facial nerve paralysis. Placing doxycycline gel or antibiotic-impregnated polymethylmethacrylate beads within the tympanic bulla, along with long-term systemic antibiotic, may reduce the incidence of complications.

 

Ancillary therapy

Supportive care

in the form of fluids and nutritional support may be necessary in the acute stage of vestibular disease. Most rabbits rapidly regain a normal appetite unless facial nerve paralysis interferes with normal feeding. In these cases, long term or even permanent assisted feeding may be necessary.

Eye lubrication

Eye lubrication is useful in rabbits that have a severe head tilt. The “down” eye is very prone to injury due to the protruding nature of rabbit eyes.

Anti-nausea medication

Use of anti-nausea medication is controversial, however some veterinarians find diphenhydramine (2 mg/kg PO, SC q 12h) or meclizine (12.5-25 mg/kg PO q8-12h) useful in the rolling or anorectic rabbit.

 

Prognosis

The prognosis varies for rabbits with otitis interna/media. Clinical recovery may be good in some individuals. There may be a residual head tilt, but the rabbit can learn to reestablish balance and live a relatively normal life. The prognosis for return to normal function is guarded if facial nerve paralysis is present. Also in rare cases otitis media/interna can spread to the brain resulting in severe neurological signs including seizures.

 

Summary

Otitis media is characterized by debris in the ear canal, a head tilt towards the affected side, and if present, nystagmus with the fast phase away from the affected ear. Other neurologic deficits are absent. Always perform a thorough otoscopic evaluation with cytology and cultures, and if there is a suspicion of bulla involvement, radiographs or CT scan should also be performed. Clean ears with a commercial steroid-free cleaning solution if the tympanum is intact, but only use sterile saline if the tympanum is ruptured or cannot be visualized. For severe cases, myringotomy or bulla osteotomy can be performed. Topical therapy should always be utilized, and systemic therapy should be instituted if there is inflammation of the ear canal or rupture of the tympanum.

References