From hearing and health issues to how they tell a horse’s feelings, Kieran O’Brien MRCVS delves into the outer ear of the horse
A HORSE has 10 muscles to control ear position and movement, enabling more manoeuvrability than in any other domesticated animal. Each ear is independently mobile and so they can be moved in different directions at the same time.
These adaptations, combined with highly sensitive hearing, allow what was originally a prey animal to be extremely responsive to approaches by other animals that signal threat – and to locate these threats very precisely.
The external ear, termed the pinna, comprises a funnel-shaped vertical tube (the vertical canal), made of cartilage with overlying skin. This is joined at the base via a right-angled bend to a short, horizontal tube, which in turn connects to the eardrum.
A thin coating of wax is produced by glands that line the canal. This wax not only serves to keep the ear surface moist and to trap small particles that may enter the ear, but also has antimicrobial activity against bacteria and fungi. A natural ear wax “escalator”, assisted by jaw movement, transports the wax along the canal to the outside.
Because a horse’s ears are vertically placed, there is the potential for rain or objects such as seeds, dust, pollen and insects to become lodged inside them. For this reason, the ears are lined with a dense coating of interlocking hairs that filter out everything except sound waves.
Accumulations of wax on the hairs below the ear can be snipped off with a pair of scissors, but clipping of the hair inside the ear for cosmetic purposes is strongly discouraged – the hair is there for a very good reason.
If the inside of the ears must be wiped, only a damp cloth should be used because liquid should not be allowed to run down into the ear.
Be vigilant, however, for any sign of ear irritation or discharge, or any abnormal growths on the ear surface, both externally and internally.
What can a horse’s ears tell you?
EAR position may indicate pain, fear, curiosity and anger. Horses use body language rather than vocalisation during their social interaction with other horses, and ear position is a key indicator of mood during these interactions – signalling aggression or submission.
A study of free-ranging ponies on the Isle of Rhum in Scotland found that 80% of aggressive encounters between animals consisted of threats with the head alone, by pinning the ears back and extending the neck. It is also assumed that when horses fight, the ears are held pinned back to protect them from injury.
Recent studies looked at the facial expressions, including ear position, of lame and non-lame horses when being ridden. It was found that the ears were behind the vertical in 43% of non-lame, but in 77% of lame horses. We are only now beginning to understand how horses signal their “wellness” via their facial expressions. Ear position may be a key element in this.
The drooping of one ear to the side and loss of motor control may indicate paralysis of the nerves controlling the ear muscles. This can occur due to damage to the facial nerve, infection in the guttural pouch or inner ear, and arthritis of the joint between the hyoid bones (that sit between the two halves of the lower jaw) and the skull.
Common ear problems in horses
ANY irritation in the ears can cause a horse to shake his head. Although headshaking behaviour is often attributed to ear mites, the prevalence of these in the UK is now low.
Where present, the mites cause irritation, ear rubbing and shaking of the head in random directions. This contrasts with the true equine headshaking syndrome, where there is no ear rubbing, the “shaking” action is almost solely a vertical “flip” of the head (mainly when ridden) and the irritation occurs in the nose, not the ears.
But the face and ears are common biting sites for midges. An allergic response to their saliva is triggered in horses suffering from sweet itch. In advanced cases, self-trauma will result as the horse tries to relieve the irritation caused by histamine release in the skin. The skin can be rubbed almost raw, which can cause bleeding sores, so protect this region with a fine-meshed face mask that covers the ears.
Blackflies can attack the skin on the inside of the horse’s ears and in the midline, in front of the udder or sheath. They suck blood and in doing so, can spread a papilloma virus that causes flat, scaly and initially circular white-grey zones in the ears.
Sometimes, these warty “aural plaques” proliferate and protrude from the skin. Some regress spontaneously, but others persist for life. They are rarely treated as they do not cause a clinical problem, and any treatment will usually result in a head-shy horse.
Thought to be caused by a cattle wart virus, probably spread by flies, sarcoids can develop anywhere on a horse’s body, including occasionally on the ears. They may look like an area of hairless, scaly skin or appear as growths that enlarge over time. Melanomas and squamous cell carcinomas can also occur in the ear area.
These tumours must be treated early when small, to ensure the treatment is effective and the horse does not become head-shy as a result. Topical chemotherapy may be successful, but some growths require removal; a surgical laser is often used on sarcoids and tumours, but this can be hazardous in the ear region due to the risk of damage to the cartilage underlying the skin.
A rare occurrence at a young age is an opening on the front margin of the ear flap that oozes a sticky fluid. Called a dentigerous cyst, this communicates with an abnormal area of dental tissue – sometimes an entire tooth – usually close by in the head. This developmental abnormality rarely causes any other symptoms and usually can be safely left untreated.
Why ears must be handled with care
HEAD-SHY horses are commonly encountered in veterinary work. While some animals will accept their ears being handled, others instinctively resent the touch. These horses must be desensitised by careful and skilful repetition, using behavioural techniques from a young age.
Any veterinary procedure involving the application of creams and other treatments to the ears must not be embarked upon lightly, because even with the greatest care, head-shyness can result.
Applying a twitch (a restraining device) to the ear is strongly discouraged. A study found that while a twitch applied to the upper lip lowers the heart rate and is calming, an ear twitch causes a rise in heart rate and blood cortisol level – indicating the horse is in pain.
This Vet Clinic feature was first published in 15 July issue of Horse & Hound magazine
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