Complications of External Otitis in Horses

The physiological removal of foreign bodies in the horse’s external ear canal is best achieved by head-shaking. However, external otitis in the horse induces moderate to severe pain: therefore, the horse does not shake his head. The causes of external otitis are dust, water in the external ear canal, keratin, and ceruminous debris. The clinical symptoms are ear discharge, skittishness, facial nerve paresis, and/or head tilt. After the horse has been sedated, the most important diagnostic procedure is the endoscopy of the cartilaginous and osseous part of the external ear canal, including the evaluation of the transparency of the tympanic membrane. The clinical complications of external otitis are hearing loss, facial nerve paresis, head tilt, hypertrophy of the tympanostylohyoideum, and corneal ulcers. The most important treatment is soaking up the exudate in the osseous part of the external ear canal using small cotton balls which are held by the foreign body forceps of the endoscope. Based on the results of the culture of exudate and the antibiogram, an antibacterial drug must be administered orally for 3 to 4 weeks. At this time, an endoscopy of the external ear canals and guttural pouches also has to be done. Based on the outcome of the endoscopy, endoscopic and clinical investigations have to be performed six months later as well. Only 7/19 horses had a normal osseous part of the external ear canal with a transparent tympanic membrane, including normal hearing measured by the brainstem auditory-evoked response after one month of treatment.

External otitis is caused by dust in the stable, sweating during exercise, blood from head trauma or external parasites, keratin, and ceruminous debris [1]. Following exercise, riders or owners often splash water into the horse’s external ear canal. Sebum, water, or blood, together with the normal keratin and ceruminous debris of the external ear canal, form a sticky mass that can induce an obstruction at the entrance to the osseous part of the external ear canal. The normal accumulation of desquamated keratin scales, caused by epithelial migration from the tympanic membrane, forms a ring of cell debris around the lateral opening of the osseous part of the external ear canal [2]. The normal removal of foreign bodies (bedding material) or dry keratin and ceruminous debris from the cartilaginous part of the external ear canal is achieved by shaking the head. This is the only self-cleaning procedure available to horses. If the keratin scales and ceruminous debris or blood cannot be removed from the external ear canal, bacteria of the microbiome in the auricula and the cartilaginous part can induce an otitis externa exsudativa. More often Staphylococcus spp. was cultured from the exudate of the inflamed osseous part [3]. The aim of this study is to describe the diagnosed complications induced by external otitis in horses.

The history of the referred 19 horses was head-tilt, ataxia, keratitis, ear paresis, no reaction when called, becoming frightened, and/or ear discharge. The breeds included 6 Warmbloods, 4 Haflingers, 3 Tinkers, 3 Island Horses, 1 Quarter Horse, 1 Appaloosa, 1 Pony; 9 mares, 8 geldings, 2 stallions: the age 13.5 years (median, 4 min, 24 max). All horses were examined based on a general and neurological investigation plan. The horses were sedated using detomidine (Detomidin ad us. vet., 20 μg / kg b.w. i.v., Cepesedan RP, CP-Pharma, 31303 Burgdorf, Germany) before endoscopy of the external ear canals and the guttural pouches. A hygienically perfect and flexible endoscope with a diameter tip of 7 or 5.9 mm was used. The debris in the cartilaginous and osseous parts of the external ear canal was graded based on the tripartite grading system described by Sommerauer, et al. 2013 [4]. The tympanic membrane was characterized using the normal and abnormal criteria of the tympanic membrane described by Blanke, et al. 2014 [3]. The tympanostylohyoideum, the junction between tympanohyoideum and Processus styloideus, and the tympanic bulla medial of the junction were evaluated in the guttural pouch. A sterile cotton ball held by the foreign-body forceps of the endoscope was used to collect the exudate of the osseous part for both the bacterial culture and the antibiogram.

The clinical and endoscopic results are listed in Table 1. The cultured bacteria of the exudate from the osseous part were: Staphylococcus aureus, Staphylococcus delphinii, Streptococcus intermedius, Streptococcus dysgalactiae ssp. equisimilis, Acinetobacter baumannii. The most effective antibacterial drug was enrofloxacin (5 mg/kg b.w., p.o./d) which was administered for three to four weeks. The anti-inflammatory drug flunixin meglumine (1.1 mg/kg b.w., p.o./d) was included. If a progressive improvement occurred after two weeks, a half dosage was administered. The most important part was the removal of the exudate from the osseous part by soaking up the exudate with small cotton balls which were held by the foreign-body forceps of the endoscope. This procedure was performed every second day during the first week, and after this every 3rd to 4th day until the 3rd to 4th week of treatment. The goal was to see the tympanic membrane. However, in severely inflamed osseous parts of horses with left-sided external otitis, the tympanic membrane was not seen because the epithelium of the osseous part was severely swollen and the osseous part was occluded (Table 1). Nine horses with external otitis suffered also from facial nerve paresis on the same side and one horse had it on both sides. Nine horses had a head tilt to the side of external otitis. Horses with facial nerve paresis had ulcerated cornea or keratitis sicca based on reduced production of tear fluid. These horses were treated with carbomer eye drops three times a day. A horse with bilateral facial paralysis (bilateral ear paralysis, ptosis of the upper eyelid, ulcerated cornea) was euthanized on the owner’s recommendation. Six horses with right-sided external otitis and one horse with right-sided external otitis including facial nerve paresis and head tilt (with right-sided facial nerve paresis and moderate hypertrophy of the tympanostylohyoideum) were healed and discharged after four weeks. Three horses with left-sided external otitis and eight horses with external otitis including facial nerve paresis and head-tilt (six with left-sided facial nerve paresis and two with right-sided facial nerve paresis, severe hypertrophy of the tympanostylohyoideum) were discharged only following improvement of the external otitis but no improvement of the facial nerve paresis, head-tilt and hypertrophic tympanostylohyoideum after one to three weeks based on the owner’s request.

Horses with otitis externa exudativa purulenta have moderate to severe pain based on facial expression. A horse with this disease does not shake his head in order to expel the exudate from the external ear canal. A direct application of any drug into the external ear canal results in a mixture with the dirty material of the external ear canal which induces severe inflammation in this region. Therefore, the most important part of the treatment is the removal of the exudate by soaking it up the exudate with small cotton balls using foreign-body forceps through the working canal of the endoscope [5]. The goal is to see the tympanic membrane endoscopically in the first two weeks of treatment. The antibacterial drug administered has to be effective in the inner ear as well [6]. This drug has to penetrate the blood-brain barrier, especially this is important for horses with head tilt (vestibular syndrome). Bacteria in the exudate in the osseous part can move through the middle ear to the inner ear and labyrinthitis can be caused. This labyrinthitis can induce a vestibular syndrome with head tilt [7]. The canalis facialis is separated by a thin bone lamella from the meatus acusticus externus. This thin bone lamella could be damaged by the purulent exudate in the osseous part of the external ear canal. Neuritis of the facial nerve can cause a deficit of facial muscle activity and reduced tear production with induction of corneal ulcer or keratitis. The continuous treatment of these corneal diseases is very important. The successful treatment of purulent external otitis and corneal ulcer had a duration of four weeks based on the controlled eye investigation and endoscopy. The head tilt disappeared within six months, based on the owner’s feedback. The inflamed redness of hypertrophic tympanostylohyoideum was not visible after four weeks; however, the hypertrophy was still there. The hypertrophy of the tympanostylohyoideum can be induced by cytokines of T-cells which are in the inflamed tissue of the external ear canal. These cytokines act as growth factors in the tissue of the tympanostylohyoideum [8]. The most important complication in horses with external otitis is hearing loss or deafness. Horses had moderate to severe conductive hearing loss in the diseased ear, as measured by brainstem auditory-evoked response [9]. Only six horses with right-sided external otitis and one horse with right-sided external otitis including facial nerve paresis and head tilt returned to normal hearing after treatment for a period of one to three months.

A deficit of this study is the lack of endoscopy of external ear canals, brainstem auditory-evoked response investigations, and the endoscopy of guttural pouches in order to determine normal tympanostylohyoideum in the remaining 12 horses after their initial examinations, including treatment and the necessary follow-up controls in one to six months.

Horses with head tilt, ear paresis, keratitis, ear discharge, and/or skittishness have to have the external ear canals and guttural pouches endoscoped. The brainstem auditory-evoked response is an objective test to examine the hearing deficit in horses with external otitis. The complications of external otitis in horses include hearing loss, head tilt, facial nerve paresis, keratitis, and hypertrophy of the tympanostylohyoideum. The treatment of otitis externa exudativa purulenta involves the removal of the exudate from the osseous part of the external ear canal using small cotton balls applied by foreign-body forceps for soaking up the exudate over a period of one to two weeks!

Based on ethical considerations the authors declare that all investigations and treatments of equine patients were carried out in accordance with standard guidelines in animal care and treatment.

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