Have you ever strained to hear your trainer over a blustery wind, pouring rain on the grandstand roof, or a crowded saddling paddock?
Guess what – you might not have been the only one. Although not often reported, horses can have a hard time hearing, as well.
University of California associate professor Monica Aleman reviewed auditory loss in adult horses at the 2014 American Association of Equine Practitioners Convention in December.
“Hearing loss occurs in horses but is seldom reported,” Aleman said. “It could impact behaviour, training, and performance.”
Although hearing loss isn’t always complete, horses with even some degree of auditory loss can feel the impact. “One clap can help determine if a horse has full hearing,” Aleman explained. “Horses need full auditory function in both ears to localize sound.
“Horses with partial hearing loss can hear the sound, but they can’t localize it. They can get anxious due to hearing loss especially if it develops acutely. Horses with chronic hearing loss might be adapted to their environment; therefore, this deficit might go undetected until exposed to an unfamiliar situation.
“Hearing loss should be included as a possible cause of altered behaviour in horses. Though not the most common cause, it is one to consider.”
Aleman said there are several ways to diagnose hearing loss, including:
1. A neurologic exam including a sound test, in which the horse hears a loud sound once or repeatedly. If the horse reacts to the sound and is able to localize it, he likely has good auditory function.
If he reacts to the sound, but can’t seem to determine where it’s coming from, he likely has partial hearing loss. And if the horse shows no reaction whatsoever, his hearing must be further evaluated for possible complete hearing loss.
2. An endoscopic exam to evaluate the horse’s ear structures. The veterinarian can performed this either under general anesthesia or standing sedation. The former is safer for the delicate equipment vets use and allows easier visualization. However, inherent risks of general anesthesia makes standing sedation the preferred method.
3. An otoscopic (ear) examination. Like the endoscopic exam, this is better carried out under general anesthesia.
4. A brainstem auditory evoked response (BAER) test. This hearing test uses small electrodes placed under the skin of the scalp to detect electrical activity from the inner ear all the way to the brain (auditory pathway) when the horse is exposed to a noise via head- or insert phones.
Aleman and colleagues recently completed a retrospective study to evaluate BAER readings from 76 horses evaluated at the University of California from 1982 to 2013. The team determined that 57 had an auditory deficit, 74% of which were bilateral and 26% unilateral.
The most common causes of hearing loss they identified were:
A Temporohyoid osteoarthropathy (THO 35%). All horses diagnosed with THO (a bony growth on the stylohyoid bone in the temporohyoid joint associated with the skull and located near the middle ear and guttural pouch) in the study had some degree of hearing loss; 52% of affected horses had bilateral hearing loss. The team found no breed or sex predilection, and affected horses averaged 11 to 14 years old. Aleman said common signs of THO include vestibular and facial neuropathy, corneal ulcers, and hearing loss.
B Congenital sensorineural deafness (30%). This condition is associated with a mutation in the endothelin B receptor, and all horses have been found to have complete bilateral hearing loss. Sensorineural deafness is often seen in American Paint horses with lots of facial white, and blue or heterochromic (blue and brown) irides. Clinical signs include performance or behaviour deficits and an easy-to-startle horse. Sensorineural deafness does not carry a good prognosis for hearing.
C Multifocal brain disease (23%). While some horses with brainstem disease had some degree of auditory dysfunction, hearing loss is often the least of these horses’ problems.
D Otitis (7%)—Otitis media (inflammation of the middle ear) or interna (inflammation of the inner ear) can also cause hearing loss.
Other causes of auditory dysfunction include head or sound trauma, hypoxic ischemic injury, ototoxic drugs, skeletal malformation, and old age.
The prognosis for auditory recovery depends on the cause and, if present, the severity and location of the lesion. In people, for instance, discontinuing some ototoxic drugs, such as aspirin or furosemide, can undo temporary hearing loss; other drugs, such as aminoglycosides and cisplatin, can cause permanent hearing loss.
Although it’s an uncommon problem, hearing loss in horses should always be investigated. Consult your veterinarian if you suspect your horse could have an auditory issue.