Middle and inner ear (otitis media and interna)
Esther van Praag, Ph.D.
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Otitis media and interna, Latin names for inflammation of the ear chambers located behind the tympanic membrane (ear drum), involve about 50% of all cases of acute vestibular disease. The middle ear is the region located directly behind the tympanic membrane (eardrum). It is made of the different bones and nerves responsible for the diffusion of the sound coming from the outer ear to the brain. The middle ear is connected to the nasal cavity by the Eustachian tube, opening that enables the adjustment of the air pressure inside the middle ear. It is responsible for balance.
Otitis media, or middle ear infection, is typically located behind the eardrum. The presence of bacteria, fungi, yeast, or parasites triggers the production of fluid and pus, which results in inflammation and pain, and may lead to the loss of hearing.
When the infection is severe, rupture of the tympanic membrane can occur. The pus contained in the middle ear will flow into the ear canal and the infection can spread to the outer ear. The infection can also spread from the middle ear to the inner ear (otitis interna or labyrinthitis). This development of the disease is marked by head-tilt, and ataxia (lack of balance).
Pasteurella multocida, a natural host of the nasal cavity of rabbits, is often associated with middle and inner ear infection. Healthy rabbits can be carriers of this bacterium, without showing clinical signs. The development of the disease depends on the general resistance of the host and the virulence of the Pasteurella sp. strain. The bacterium is believed to migrate from the nasal cavity to the middle ear along the Eustachian tube or mandibular molar root abscess with exposure to the Eustachian tube.
Staphylococcus aureus is considered an opportunistic pathogen of the nasopharyngal cavity of rabbits. Its presence in the ear can lead to severe middle or inner ear infection. The Staphylococcus aureus bacterium can present resistance to one or more antibiotics.
Further bacteria known to cause inner ear infection include Streptococcus sp., Escherichia coli, Enterococcus sp, Proteus sp., Pseudomonas sp.. Sporadic cases of yeast infection, e.g. Candida sp. or Pityrosporum sp., are found in rabbits. Fungal infection, e.g. Cryptococcus sp., is rare.
Clinical signs for otitis media can be absent. More often, they cannot be distinguished from an external ear infection: shaking of ears, scratching with paws, rubbing, anorexia, depression and pain. Discharge is observed in the external ear canal if the eardrum has rupture under the internal pressure caused by the infection.
Middle ear infection is characterized by head shaking and drooping of the ear. Facial paralysis is possible when there is swelling and compression of the facial nerve. Rabbits may hear less well, due to pus in this part of the ear. The condition is painful.
Inner ear infection is accompanied by ataxia (circling, rolling stumbling), leaning to one side and head-tilt. Some rabbit sway their head from side to side. This is caused by the pressure of the infected tissue and inflammation of surrounding tissues leading to the compression of nerves passing through the vestibular region of the brain.
Appearance of nystagmus (involuntary rhythmic eye movement) is observed when treatment is late or inappropriate. When observed, it can be indicative of inner ear infection or E. cuniculi. Depending on the location of the damage, differences of eye movement are, indeed, observed:
- Bacterial infection of the inner ear generally leads to peripheral vestibular disease. This is characterized by horizontal and rotary nystagmus, but never vertical nystagmus.
- E. cuniculi is generally related to central vestibular disease, which shows typically vertical and positional nystagmus, more rarely horizontal nystagmus. The vertical nystagmus is the one mainly observed in E. cuniculi suffering rabbits that do not suffer from secondary inner ear infections.
- Rotary nystagmus (in vertical and horizontal directions). This relates to lesions of the cerebellum, the brainstem or the vestibular connections; causes can be the presence of a tumor or a bacterial infection (encephalitis), to name the main ones.
The direction of repetitious involuntary eye movement should not be a basis for a final diagnosis between the two disorders. Nystagmus is a clinical feature of various diseases, including metabolic disorders, eye disorder (glaucoma, cataract, retina problems, and albinism), nutritional deficiencies (e.g. magnesium, thiamin, medication (e.g. barbiturates), the presence of brain lesions, or trauma.
It is important to differentiate ear infection from other causes of vestibular diseases. See: Head tilt and their various causes.
Otitis media is visible on X-rays, changes on the level of the bullae, on the contrary of otitis interna and E. cuniculi lesions. Changes of the soft tissue density are observed in the middle ear, with appearance of an opaque grayish mass. It is sometimes accompanied by sclerosis and bone proliferation, which may reach as far as the temporal bone or the temporo-mandibular joint. X-ray can, furthermore, help rule out dental problems or E. cuniculi.
If discharge is present in the outer ear, the presence of bacteria, yeast or fungi should be determined by means of a culture, followed by a sensitivity culture in order to determine the most effective antibiotic or antifungal treatment.
Cytological methods will help determine the presence of bacteria, yeast, fungi and certain types of cancer.
A complete blood count (CBC) and biochemistry panel can help determine the presence of an infection or E. cuniculi, with neutrophilia or changes in values related to the kidney function (BUN, creatinine) respectively.
Serological tests help determine exposure to E. cuniculi or Pasteurella sp. during its life. A high titer is linked to an active infection. These tests are indicative only.
The antibiotic treatment should be based on the result of the sensitivity culture. This is not always possible; in that case, antibiotics known to pass the blood-brain barrier must be administrated.
Chloramphenicol and penicillin (bicillin) antibiotics pass the blood-brain barrier and have successfully treated middle or inner ear infection in rabbits. Trimethoprim sulfate is sometimes advised, but appears to bring poor improvement in rabbits. This could relate to the fact that half-life of this drug is about 40 min in rabbits. Ciprofloxacin has been successful to treat a case of inner ear infection in a dwarf rabbit. Combined antibiotic therapies can be administered, such as enrofloxacin/chloramphenicol or marbofloxacin/penicillin.
The treatment must be aggressive and long, a minimum of 4 to 6 weeks, or continued another 2 weeks after full disappearance of the symptoms. If no improvement is observed after 14 days, it is possible to switch to another antibiotic. In order to minimize the appearance of resistance in the pathogenic bacteria, it is best to administrate a cocktail including the old antibiotic and the new one.
An otoscopic examination is necessary to determine if the eardrum is ruptured. If this is the case, antibiotic-containing eardrops will lead to ototoxicity. The consequence is permanent deafness, loss of balance or death. A safe alternative to remove pus and debris is to wash out the outer and middle ear with a saline solution.
The antibiotic therapy should be accompanied by NSAIDS (non-steroidal anti-inflammatory drugs) pain medication. Meloxicam can be used over a longer period of time, without reported side effects in rabbits.
The use of glucocorticosteroids in the treatment of ear infection is controversial. They are advisable during the first days of treatment, in order to reduce the inflammation, but their use of should not extent over 5 days, due to their immunodepressive properties.
The administration of meclizine, a motion sickness drug, is advisable in case of otitis interna.
If the rabbit has trouble eating and drinking, force-feeding and administration of SC fluids are necessary.
If the middle ear or the nerves are damaged, deafness or head-tilt is irreversible.
Prognosis of surgical drainage, e.g. bulla osteotomy, is poor and is accompanied by post-operative complications in rabbits. It should be used in cases of severe infection of the middle or inner ear, when antibiotics fail to keep the situation under control.
My deepest gratitude to Dr. Zahi Aizenberg, (The Koret School for Veterinary Studies, The Hebrew University of Jerusalem, Bet-Dagan, Israel) and to Dr. Estella Böhmer (Chirurgische u. Gynäkologische Kleintierklinik, Ludwig-Maximilians-Universität München, Germany) for the permission to use their pictures.
Many thanks also to Renee Brennan for sharing the video of her head-tilted rabbit Rudy and to Kei Rivers for sharing her video of Holly.
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