All Ears
The equine ear is a marvel of utility. No need for a horse to turn his head toward the origin of a sound; he can simply swivel his ear to pick it up. Equine ears also show emotions such as affability, apprehension, and anger, which help us understand them better.
The equine outer ear includes the pinna (ear flap) and the external auditory canal (the cuplike ear canal). The pinnae collect sound waves and channel them into the external auditory canal to the middle ear. Here the ossicles (auditory bones) transmit the tympanic membrane’s (eardrum’s) resulting movements to the inner ear. The inner ear, which includes the auditory nerve, cochlea, and labyrinth, transfers the waves into nerve impulses, which are sent to the brain’s hearing center.
Though their ears are similarly complex in structure, unlike some species, such as dogs, horses aren’t susceptible to many ear problems. We’ll describe a few that can crop up.
Insect-Induced Dermatitis
Insect-induced dermatitis is a hypersensitivity to gnat and midge bites that can occur in a variety of locations over the horse’s body, including his ears. “The immune system gets overly excited by the insects’ saliva,” says Michelle Abraham, BVSc, BVMS, a resident in large animal internal medicine at the University of Pennsylvania School of Veterinary Medicine’s New Bolton Center. “The bite causes an immune response, leading to an inflammatory reaction. You’ll see scabby ears and little dots of blood where the insects have bitten. Insect-induced dermatitis is fairly easy to manage by using insect protection, such as long-lasting fly sprays, topical medications, and fly masks with ear protection. Horses that are badly affected may require an oral anti-inflammatory treatment.”
The problems bugs cause can extend beyond irritation. “Biting insects can also spread a papillomavirus that appears as crusty plaques (also known as aural plaques) on the inside of the ear,” Abraham adds. “These plaques can be treated by topical ointments; however, it is important to manage the insects to prevent reoccurrence.”
Some plaques are simply unsightly, whereas others can cause severe ear sensitivity or headshaking.
Abraham suggests keeping skin-sensitive horses in the barn when insects feed, which is early morning and evening.
Ear Ticks
Some disease-harboring tick species attach and feed on the horse’s inner ear. This can cause the horse to become ear shy and sensitive, as well as resistant to wearing a bridle, halter, or anything that extends over the poll.
Several tick species can live in the ear canal, such as the tropical horse tick, Gulf Coast tick, and the Lone Star tick. But the spinous ear tick is the most common and problematic species. “Spinous ticks are common in areas of the United States that are warm much of the year,” says Leslie Easterwood, DVM, MA, assistant clinical professor at Texas A&M University’s College of Veterinary Medicine. “The ticks travel into the ear canal and attach there to engorge on blood. Ticks can hatch out right in the ear. It’s hard to see these ticks. If they have attached high enough on the ear pinna you can see them and remove them, but the ones that cause the most problems are in the ear canal. In this case we have to sedate the horse to take a look.”
Fortunately, ticks are relatively easy to remove, although an infestation (attachment of more than a few) does require a veterinarian’s care. Treatment includes applying medication, usually pyrethrin-based, to the ear canal of a sedated horse. With this approach, “there’s no need to remove the ticks,” says Easterwood. “They will release fairly quickly and then the horse will shake them out. Usually we’ll re-treat seven days later, but the horse should be tick-free within a few days. Sometimes there’s a secondary ear infection, but this isn’t usual. Once the ticks are gone, the ear heals quickly. If there is an outer ear infection, an owner can treat the ear by administering an antibiotic and anti-inflammatory (prescribed by a vet)—such as Panalog, which is a combined antibiotic and steroid.”
Prevention includes outfitting the horse in a fly mask that encloses the ears. Fly sprays can help repel ticks, too. Minimize tick populations by mowing grass where horses graze and prevent exposure by keeping them away from trees and wooded areas.
Guttural Pouch Problems
Located at the back of the pharynx in the horse’s throat are the auditory tubes (Eustachian tubes, or the passageways that take in soundwaves from the ears). These include a pocket nestled at the base of the ears, near the throatlatch area, called the guttural pouch, the purpose of which is unknown. “People still go back and forth on the function of the guttural pouches,” says Abraham. “One theory is that they help cool the brain and assist with breathing during exercise.”
Whatever the pouches’ purpose, sometimes infections develop within them. Although the practice is uncommon, veterinarians can look for evidence of infection by maneuvering an endoscope through the horse’s nose (nasopharyngeal endoscopy). The most common guttural pouch infection is strangles, an upper respiratory/throat infection caused by the bacterium Streptococcus equi subspecies equi. Strangles is a highly contagious disease and infection can mean serious consequences for the animal.
“Strangles gets into the lymph nodes around the throat area,” says Abraham. “When the lymph nodes get large they compress on the trachea, obstructing airflow. They can abscess and burst, filling the guttural pouches with the infected pus. Treatment includes lancing the abscesses and flushing the pouch out. The pus can also form solid concretions in the guttural pouches that become a chronic problem. And because the guttural pouch contains important arteries, veins, and nerves, there is sometimes a secondary nerve problem.”
Asymptomatic strangles carriers harbor the bacteria in their guttural pouches and can shed the disease without ever showing clinical signs. “If your foals are getting strangles every year, you should look around for the adult horse carrying strangles asymptomatically in its guttural pouches,” says Easterwood, by testing all the horses. “Your vet will run sterile saline up into the nasal pouches and catch what comes out. Or she will run a flush pipette up into the guttural pouches and culture the result. The carrier can be treated systemically with oral or injectable antibiotics or the pouch can be flushed and treated with antibiotics.”
Adult horses can experience side effects from strangles vaccination, but Easterwood says this shouldn’t deter owners from vaccinating susceptible horses; some of the worst outbreaks have been among mature horses in boarding facilities. “Rarely is it fatal,” she says, “but it’s a mess and a real scourge for the stable owner,” emphasizing the extended time it can take to clear up infections, detect and treat carriers, and overcome the stigma that can come with experiencing an outbreak.
Another rare ailment impacting these structures in horses of all ages is a life-threatening fungal infection called guttural pouch mycosis. In these cases a fungus infects the pouch lining, which then fills with pus.
“The fungus can potentially erode large blood vessels in the guttural pouch.” says Abraham. “This can lead to significant blood loss through the nose. Surgery is often required to ligate the artery. You can imagine if a horse is losing a lot of blood, by the time you get him to a hospital, there is a lot of risk. And the vessels aren’t far away from the bones that protect the brain. So the infection can erode the bones and set up another infection in the brain. We don’t really know how or why the fungus sets up in the pouch.”
And finally, guttural pouch tympany (GPT) is a congenital problem that becomes apparent in the early months of a foal’s life. In this condition the guttural pouch accumulates extra air and causes distinctive swelling in the throat latch region. “We see this distention on one or both pouches,” she says. “That causes some breathing difficulties, problems swallowing, and even aspiration pneumonia. (The problem) requires surgery to open up the pouch and let the air out. It can be life-threatening if left alone, but it’s less emergent than guttural pouch mycosis.”
“Sarcoids can get large enough to fold the ear over, so it’s important to treat a tumor when it is small.”
Dr. Leslie Easterwood
Tumors
Sarcoids (also called blood warts) are noncancerous tumors that can disfigure ears. Easterwood says researchers do not yet know what causes sarcoids, but they have associated the tumors with the bovine papillomavirus. She notes that although the same papillomavirus can be isolated from both aural plaques and sarcoids, veterinarians still don’t clearly understand why some horses with the virus get sarcoids or aural plaques but others don’t. They do know, however, that the histopathology (microscopic appearance) of both sarcoids and aural plaques appears to be distinct.
“There are a tremendous number of treatments and none work very well,” she says. “If there is enough of a base we can remove the sarcoid flush with the ear. We can also freeze the sarcoid and/or treat it with chemotherapy-type agents. Sometimes we have to remove part of the ear. Sarcoids can get large enough to fold the ear over—as big as a tennis ball. So it’s important to treat a tumor when it is small.”
Ear Teeth
Ear teeth (dentigerous cysts) are rare cysts that occur in young horses. “The cyst has dentine-type material in it,” says Easterwood. “The material inside can have some tooth characteristics, but it’s not as graphic as it sounds. It’s a defect of development when the fetus is forming. It occurs at the base of the ear, typically, but the appearance can be different from case to case.”
There is usually a draining tract at the base that oozes a clear material. Veterinarians treat these cysts by dissecting them and removing the whole tract near the base of the ear or the skull. “Ear teeth are generally cosmetic,” says Easterwood. “They can get infected, but that’s not usual. Surgery is usually done as a weanling. We don’t know what direction the tract is going so we put a contrast material in the hole and take a radiograph.” (See page 48 for an ear tooth illustration.)
Did You Know?
Horses possess an additional unique way of hearing. As a horse grazes, he can pick up vibrations through the ground via his teeth, which are then sent to the inner ear through the jawbone.
Hearing Loss
Hearing loss isn’t common in horses, but it is possible. Horses that have white blazes past the eyes carry a gene that can cause deafness. Also, Abraham says recent research results suggest older horses can experience hearing loss: “Researchers are investigating tests that can be used to evaluate hearing loss.”
“It’s a little tricky, but we can test for hearing loss with an auditory nerve test,” adds Easterwood. “Electrodes are placed at the base of the ears, and a machine produces wavelength sounds that measure the auditory nerve function.”
Abraham says hearing loss in horses makes sense because old age in other animals is associated with deteriorating auditory function. “However, I don’t think hearing loss creates much of a problem for them,” she says. “Horses have other senses to alert them, and they can rely on horses in the herd, too.”
Take-Home Message
Abraham says that while horses have few ear issues, signals such as ear drooping or itching can tip owners off to more serious problems that might or might not be ear-related. “The ears are just one tool of a physical exam that can indicate a problem elsewhere,” she says.
