Ear base swellings in rabbits

Chronic ear disease is a very common pathological process encountered in all small animals. A variety of different ear conditions are seen in the rabbit, and some are exacerbated by anatomical traits found in certain breeds. Chronic ear disease can cause pain and reduced quality of life in canine and feline patients. However, ear disease and its impact can go unnoticed by owners and veterinary staff because of the rabbit's ability to hide pain. Therefore, veterinary professionals should familiarise themselves with the different anatomy in rabbits, along with up-to-date medical and surgical management techniques to provide the best care for rabbits.

Comparative anatomy

Unlike canine and feline patients, rabbits do not have a distinct vertical or horizontal ear canal. The external ear possesses cartilaginous plates and a blind-ending diverticulum which is separated by the tragus. The canal itself attaches to the acoustic meatus of the tympanic bulla. In dogs and cats, the meatus is positioned horizontally, whereas this is vertically orientated in the rabbit (Meredith, 2014). In all species, the acoustic meatus leads to the tympanic membrane, which separates the external canal and middle ear. In rabbits, unlike dogs and cats, the rounded tympanic bulla does not project beyond the occipital bone, and the wall of the bulla is much thicker, particularly on the lateral aspect. The bulla is closely associated with the facial nerve that emerges from the stylomastoid foramen, as this nerve travels across the ventral aspect of the lateral wall. The bulla cavity is divided into two compartments called the epitympanic recess and the tympanic sinus (Mancinelli and Lennox, 2017). The tympanic recess is in close association with the jugular foramen and cranial nerves IX, X and XI, which are the glossopharyngeal, vagus and accessory nerves respectively (Mancinelli and Lennox, 2017).

The anatomy is different in lop-eared rabbits. Deformity of the auricular cartilages results in a small gap between these cartilages which allows the pinna to ‘lop over’, creating this characteristic in lop-eared rabbits. This deformity occludes the external canal and allows a build-up of cerumen which the rabbit has difficulty removing because of the overlying pinna. There is also increased stenosis of the external canal, further precipitating disease. Johnson and Burn (2019) found that lop-eared rabbits have a significantly higher level of ear disease than rabbits with erect ears, and show frequent pain responses during examination, increased levels of cerumen, erythema and stenotic ear canals.

Clinical signs and examination

An increasingly common finding during clinical examinations in lop-eared rabbits is the palpation of an ear base swelling that occurs because of the accumulation of ceruminous material. If the pinna is lifted, large swellings are noticeable and can be palpated, which is pathognomonic for the condition (Figure 1). However, otoscopic examination of the tympanic membrane in these animals is often impossible because of the accumulation of ceruminous material within the canal (Chitty, 2019). Therefore, professionals should familiarise themselves with the normal ear base texture to identify issues. Evidence of pain such as reactive head shaking and/or a facial grimace may be elicited when examining the swellings in these rabbits.

Historically, ear base swellings have been referred to as abscesses. An abscess is ‘localised pus, surrounded by inflamed tissue’ (Boden, 2005). This definition infers that all of these swellings are purulent, but this may not always be the case. Cytology is required in most cases to diagnose infection, as normal cerumen is light beige in colour and can be mistaken for dried pus which has a similar appearance (Chow, 2011). True purulent debris is creamy and often white in colour (Chow, 2011). While rabbits are predisposed to infection and abscess formation, in the early course of the disease, some rabbits may have a lateral ear base pouch full of normal cerumen, the management of which will be discussed later in this article. However, it is important that these are not treated the same as a feline abscess by sedation and lancing into the swelling. This would lead to an imprecise incision in the rabbit's ear canal without adequate analgesia/anaesthesia and healing and associated pain will not mimic that of a abscess seen in a cat. These pouches can also often be missed by owners and may lead to progression of the disease, resulting in bulging/perforation of the tympanic membrane with movement of material into the middle ear (otitis media).

Otitis media is inflammation of the middle ear which includes the opening of the auditory tube, ear ossicles and tympanic bulla (Woodward, 2022). It can be unilateral or bilateral and typically results from migration of infection from otitis externa through the tympanic membrane. In rabbits, migration from pharyngeal bacteria through the eustachian tube is also seen, typically in those with concurrent respiratory disease. Clinical signs of otitis media in rabbits range from being completely asymptomatic, to decreased hearing, facial nerve contracture or palsy, with secondary exposure keratitis and corneal ulceration, head shaking and scratching of the affected ear (Woodward, 2022). In some cases, progression of disease can lead to otitis interna which often has more dramatic symptoms including nystagmus, head tilt and loss of balance. In humans, facial nerve involvement occurs late in disease progression and is thought to be caused by changes in the middle ear microenvironment, osteitis, elevated pressures and demyelination of the nerve via bacterial toxins (Prasad et al, 2017). While rabbits with otitis media can be asymptomatic, and the disease can be found incidentally, this is true in a range of conditions in all species and should not imply the animal is not in pain. The ear is heavily innervated by sensory nerves, and irritation to any portion of these nerves can result in severe pain, as reported by humans (Earwood et al, 2018).

Diagnostic methods

To diagnose otitis media and the integrity of the tympanic membrane behind the swelling, computed tomography (CT) scanning is the most accurate diagnostic method (Richardson et al, 2019). Therefore, referral to an institution capable of CT evaluation is warranted in cases where otitis media is suspected, which includes any rabbit with an ear base swelling. Richardson et al (2019) developed a grading system (I–IV) for both otitis media and otitis externa based on retrospective evaluation of CT images. This system is complete with a descriptive reference, which helps to objectify the severity of disease and subsequent treatment pathways. CT scanning is superior to both ultrasonography and radiography and should be considered first line by clinicians to help plan the treatment once severity of disease has been assessed (Figure 2). However, in general practice this may not be possible. King et al (2012) found that while CT scanning was the most accurate, ultrasonography was more reliable than radiography when assessing fluid-filled tympanic bullae in rabbit cadavers. While the practicality of this method is yet to be evaluated in live rabbits, the tympanic bullae should be a gas-filled cavity, and the presence of anything other than gas in this cavity should raise suspicions for otitis media.

Alternatively, radiography can be used, where a straight dorsal-ventral view of the skull allows imaging of both tympanic bullae (Figure 3). While lateral projections are less useful as there is superimposition of the tympanic bullae directly onto each other, some oblique angles may be of use (King et al, 2010). As explained previously, ear-base swellings may or may not be infected and cytology remains an invaluable tool for investigation. As with cytology of any other lesions, hallmarks of active infection include large numbers of bacteria and/or yeast and inflammatory cells (Eatwell, 2013). Antibiotic culture and sensitivity may be useful if the sample area reflects the true site of pathology, but systemic antibiosis is not required unless there are indications of bony inflammation or otitis media (Chitty, 2019). Antibiotic sensitivity data reflects the serum level needed systemically, but may not reflect the sensitivity of bacteria/yeasts to topical administration (Koch, 2017).

Treatment

Medical management may involve flushing of the canal under general anaesthesia and grasping of large pieces of cerumen with crocodile forceps, especially in cases of diverticulum (Chitty, 2019). Video endoscopy offers the option of performing a myringotomy to flush the middle ear in case of a concurrent otitis media. Other options include ear wicks soaked in antibiotics and ear drops combined with cleaning (Eatwell, 2013). Tris-EDTA has been used by Chitty (2019) with no side effects. In mild cases, ear cleaners such as CleanAural cat ear cleaner can be used to aid with cerumen dilution. Where there is an ear-based swelling, flushing may be attempted and if successful, allows examination of the tympanic membrane. However, these cases are difficult to manage medically in the long term and can often progress to middle or inner ear disease, as relapse is extremely common (Csomos et al, 2016).

Csomos et al (2016) suggested that resection of the lateral wall may be useful in select cases of ear base swellings to provide a draining board. However, the authors reported that case selection is critical and should ideally follow advanced imaging to avoid missing underlying pathology in the middle ear. As in other species, more invasive surgery is indicated in cases that are refractory to medical management or where there is evidence of otitis media with changes to the tympanic bullae. A partial ear canal ablation with lateral bulla osteotomy may be selected in some cases and Csomos et al (2016) reported that it is faster and carries less risk than a total ear canal ablation. However, Dobberstein et al (2023) found that the incidence of complication of this procedure was 65% in 20 rabbits.

At home analgesic options

While analgesia can only be prescribed by a veterinary surgeon, veterinary nurses play a vital role in pain management and recognition. Analgesia in rabbits with ear disease can often be overlooked by owners because rabbits are often asymptomatic, and disease is picked up incidentally. It is vital that both the veterinary surgeon and veterinary nurse help the owner to recognise subtle signs of discomfort, such as daily head shaking or head shyness when the rabbit's ears are touched. In humans, otalgia can be primary or secondary, with secondary otalgia often associated with dental or temporomandibular joint disease (Earwood et al, 2018). The ear is innervated by several sensory nerves and has neural connections to regions of the head, neck and chest (Earwood et al, 2018). Equally, inflammation of the glossopharyngeal, nervus intermedius and vagus neuralgias may result in otalgia, demonstrating the potential for complex referred pain.

Meloxicam is palatable and well-tolerated by rabbits. It is a selective COX-2 inhibitor that is weakly acidic and highly protein bound. It is indicated for acute and chronic inflammation including postoperative pain and chronic musculoskeletal disorders in dogs. The literature supports the safety of meloxicam at both 0.2 mg/kg and 1 mg/kg daily for healthy rabbits, based on liver and kidney biochemical analytes (Nield and Govendir, 2019). The authors typically use 0.6 mg/kg twice daily with otherwise healthy individuals with otalgia. In rabbits with symptoms of facial nerve damage and possible neuropathic pain, the use of gabapentin is likely to be appropriate. Neuropathy is usually associated with injury or disease that damages the axon or soma of sensory neurons or causes demyelination (Mathews, 2008). While gabapentin is a structural analogue of gamma-aminobutyric acid (GABA), it does not bind to GABA receptors. It is a schedule 3 controlled drug and found to selectively inhibit voltage-gated calcium channels and decrease nociceptive input via suppression of dorsal horn neurons and stimulation of descending inhibition, by increasing glutamatergic transmission in the locus coeruleus (Guedes et al, 2018). A study by Conway et al (2023) evaluated the pharmacokinetics of gabapentin after oral administration in rabbits at 25 mg/kg. The study concluded that gabapentin at this dose peaked after 2 hours and reduced stress to facilitate handling. This may indicate that sedation is more profound in the home environment, and therefore lower doses are usually prescribed. In a guinea pig model of neuropathic pain, gabapentin produced anti-allodynic effects (Fox et al, 2003). However, pharmacological data in rabbits are limited and doses used for analgesia are empirical.

Paracetamol has been increasingly used in rabbits, although there are limited safety studies on clinical doses, pharmacokinetic studies or robust evidence of efficacy beyond anecdotes. A study by Maciejewska-Paszek et al (2007) concluded that giving 50 mg/kg four times a day induced impaired liver function in rabbits, measured by concentration of ketone bodies and arterial ketone body ratio value. Extrapolation from canine medicine has resulted in the use of paracetamol in rabbits at 10–30 mg/kg with anecdotal clinical success and safety (Hedley, 2020). Until there are further studies concluding efficacy, empirical use of paracetamol with knowledge extrapolation from other species is accepted.

Tramadol is a synthetic opioid and a schedule 3 controlled drug that is typically indicated for mild to moderate pain. Clinical doses are recommended at 3–10 mg/kg per dose every 8–12 hours in rabbits, but the analgesic dose has not been confirmed. Souza et al (2008) found that at 11 mg/kg per dose, plasma levels did not reach a level that would be considered therapeutic in humans. However, in a surgical model involving rabbits, Udegbunam et al (2015) concluded that tramadol at 10 and 20 mg/kg of body weight subcutaneously injected in two groups of rabbits controlled objective measurements of pain (heart rate, respiratory rate, temperature) compared to the control group.

Prevention

No single preventative measure can be advised for otitis as recommendations must be based on full clinical examination of individual animals, with assessment of severity of the disease progression. It is important for the veterinary nurse or surgeon to examine the ears of lop-eared rabbits to identify this potentially debilitating condition at an early stage. Mild cases of ear cerumen build up without ear canal dilation may respond to medical management such as flushing or the use of cleaners such as CleanAural cat ear cleaner. However, where dilation of the canal is identified, cases are unlikely to respond as well to medical treatment and may require surgical intervention.

Conclusions

Ear base swellings are commonly seen in lop-eared rabbits and may contribute to rabbit morbidity and mortality. Selective breeding has resulted in deformity of the auricular cartilage and canal, predisposing the rabbit to chronic otitis. CT scanning is the preferred imaging modality to rule out concurrent otitis media, and many rabbits will require surgical intervention to prevent progression of disease. Analgesic options prescribed by a veterinary surgeon which can be administered at home include the use of non-steroi-dal anti-inflammatory drugs, paracetamol, gabapentin and tramadol. Veterinary nurses should familiarise themselves with the disease process and analgesic options for rabbits to provide optimum care.

KEY POINTS

• Otitis externa with or without otitis media are common conditions seen in lop-eared rabbits and ear base swellings are pathognomonic for ear disease.
• Ear disease is painful and analgesics should be prescribed by a veterinary surgeon to affected rabbits.
• Mild diverticula that are not infected may respond to medical management however progressive disease often requires surgical intervention.
• All rabbits ears should be examined at every opportunity in practice to avoid missing chronic disease and potential pain.