Practical Equine Medicine Studies of 2015

Photo: Anne M. Eberhardt/The Horse

Scientific journals abound with cutting-edge equine research—far more information than the average practitioner can take in. The Kester News Hour presented at the annual American Association of Equine Practitioners (AAEP) Convention provides a venue to discuss interesting and practical topics from the past year’s professional journals. At the 2015 Convention, held Dec. 5-9 in Las Vegas, Nevada, Carol Clark, DVM, Dipl. ACVIM, of Peterson & Smith Equine Hospital, in Ocala, Florida, presented the medicine topics, including ophthalmology, gastrointestinal disease, and pain management.

Ophthalmology

In the first paper Clark discussed, researchers looked at 22 cases of keratomycosis (fungal infection of the cornea, the transparent outer coat of the eye) presented to the University of Florida Veterinary Medical Center from 2001 to 2013. In a healthy eye, fungus shouldn’t be present on a corneal scraping embedded within the cornea. But when they evaluated scrapings from horses with keratomycosis, researchers saw fungal hyphae in 100% of the samples when viewed under the microscope; when they cultured the samples, only 60% grew fungi, indicating that corneal cytology was more sensitive than culture in diagnosing the condition, Clark said.

Further, the veterinarians treated 91% of the horses with at least one antifungal and an antibiotic ophthalmic medication. They also treated all the horses for secondary uveitis (inflammation of pupillary tissues), which often occurs secondary to corneal inflammation. Additionally, 91% of the horses underwent a keratectomy (trimming away the plaque where the corneal tissue was undermined). This surgical approach promoted blood vessel migration into the defect and epithelial cell growth to repair damaged corneal tissue. A small corneal scar remained in many cases. Treatment took, on average, 6 ½ weeks, and it took eight weeks for 73% of the cases to regain vision. Clark said this study’s take-home message is that cytology (examination of a cellular swab) is important for making accurate diagnoses of eye infections in order to implement appropriate treatment.

The next paper Clark reviewed concerned a newly recognized ocular inflammatory disease called heterochromic iridocyclitis—which at its very simplest means the iris loses its pigment and becomes inflamed, along with the ciliary body (which produces transparent liquid in the eye)—with secondary keratitis (corneal inflammation). In the retrospective study researchers examined 21 affected eyes in 16 horses. All the horses had pigmented kerato-precipitants (pigment dispersion from the iris on the corneal epithelium, or lining). Initially, many affected horses present with corneal edema (fluid swelling) but no pain, at it appears as if they have glaucoma. This is a progressive disease and, in 24% of cases, a fibrous membrane develops behind the cornea that can impair a horse’s vision. Researchers believe that an immune-mediated uveitis attacks the iris’ melanin pigment, causing the pigment dispersion and secondary dysfunction of corneal endothelial cells (the single layer of cells on the inner surface of the cornea). About 50% of the horses had retained vision on short-term follow-up after treatment. The researchers reported that the best treatment was a combination of immunosuppressive cyclosporine implants placed beneath the conjunctiva and a topical non-steroidal anti-inflammatory drug (NSAID). Clark reported that treatment is long-term and likely will not have a good outcome because these cases don’t seem to go into remission.

On the topic of breed-associated ocular problems, Clark discussed various genetic possibilities:

  • Haflinger horses tend to develop limbal (on the border of the cornea and the white of the eye) squamous cell carcinoma earlier than horses of other breeds – 8-9 years of age as compared to 12-13 years of age. This eye lesion appears to have an autosomal recessive mode of inheritance, which means a horse must inherit two copies of the abnormal gene for the mutation to occur.
  • Bilateral corneal stromal (the thickest middle layer of the cornea) loss in Friesians might occur along with other collagen diseases in this breed. Initially, the problem is obvious in one eye although it does involve both eyes. Half the cases go on to perforate, but if treated surgically they have a good outcome. The average onset is 11 years of age. In light of the fact that most of the patients are males, researchers suspect a chromosome X-linked inheritance pattern.
  • Hereditary equine regional dermal asthenia (also known as HERDA) in Quarter Horses is accompanied by collagen dysfunction that leads to a thinner cornea and increased corneal curvature and diameter. These characteristics compromise distance vision by making affected horses nearsighted.
References:

Gastrointestinal Disease

Ileal impactions within the equine small intestine can lead to colic associated with smooth muscle contractions. Researchers evaluated the in vitro (in the lab, on tissue samples) use of Buscopan (butylscopolamine, an anti-spasmodic drug) for its effect on ileal smooth muscle and found that it inhibited spontaneous contractions, regardless of whether it was administered pre- or post-treatment.

“This study was done in tissue baths with normal intestine harvested from healthy horses,” Clark explained. “The tissues were treated with medications to make them contract. When Buscopan was added either before or after the contraction drug, the tissue relaxed.”

Treatment following ileal impaction more closely resembles those cases seen in a real-life colic situation. Clark noted that this drug might be useful to administer with impaction colic, and because its half-life is short (it takes less than 25 minutes for its blood levels to reduce by half, an indication of drug clearance), it is not likely to have an adverse effect on colic cases. She suggested that more studies are needed to determine dosing strategies and the effects of repeated administration.

Clark then described a study evaluating acorn toxicity in horses, which is practical for areas of the United States, such as Florida and California, that have an abundance of oak trees. To be included in the study, horses had to meet three of four criteria: access to acorns, acorns found in the GI tract, clinical signs of gastrointestinal and kidney illness, and if there was a necropsy, typical findings of acorn toxicity. The researchers examined nine cases 2004-2013 that occurred in autumn months, but only in two of the 10 years studied. They hypothesized that these cases probably correlated with bumper crops of acorns. The affected horses presented with colic or diarrhea. Horses initially presenting with only colic developed bloody diarrhea within 36 hours; most had hemorrhagic diarrhea. Six horses died or were euthanized within 36 hours. The three that survived had a slower onset and progression of clinical signs; these three improved after 72 hours. Acorn toxicity induces acute tubular necrosis (meaning the tubular epithelial cells that form the kidney’s renal tubules are damaged or die) of the kidneys and edema (fluid swelling) of the intestinal lining, especially of the cecum and colon. Horses with rapid onset have a poorer outcome.

Next up? Ulcers. The Clark said that the new European College of Equine Internal Medicine’s consensus statement about equine gastric ulcer syndrome (EGUS) summarizes the extensive information found in the equine veterinary literature. Some key points she listed:

  • It is important to differentiate between squamous and glandular ulcers (these comprise different areas of the stomach), which have different pathophysiology (physiological processes associated with disease or injury).
  • Nutritional and exercise risk factors are important (e.g., fasting between meals and high-speed exercise increase EGUS incidence.
  • Clinical signs of gastric ulcers are not necessarily obvious.
  • Ulcer lesion grade does not correlate well with clinical signs.
  • Treatment failure does not rule out the presence of ulcers.
  • Ulcer pain can impact performance due to decreased stride length and decreased ventilation (shallow or fewer breaths per minute) related to abdominal pain.
  • Omeprazole is an acid-labile drug (it breaks down readily in the presence of acid), so it must be buffered in a paste or as an enteric-coated form and, therefore, is not effective in a compounded form.
  • Glandular ulcer treatment hinges not only on stomach acid suppression but also on mucosal protectants. Only 25% of glandular ulcers healed when treated with omeprazole (which only suppresses stomach acid) alone. “These ulcers need mucosal protectants in addition to omeprazole,” Clark said.
  • It’s important to perform gastroscopy (examine the stomach with an endoscope), especially if clinical signs are still apparent despite treatment and also to follow-up on cases of severe ulcer disease.
  • The veterinarian might prescribe lower doses of omeprazole if he or she has mitigated other ulcer risk factors.
  • There is no reason to use antibiotics for routine first-line treatment of EGUS. Researchers have not found Helicobacter species to be part of EGUS pathophysiology in horses, in general.
References:

Foal Care

Changing gears, Clark described the role of macrolide antibiotics, such as erythromycin, in impairing sweat responses in foals to the point that they develop hyperthermia (heat stress). The study looked at foals treated with erythromycin and kept in shaded, but not climate-controlled, stalls for 10 days, followed by turnout (with access to shade and sun) without treatment for 10 days. All erythromycin-treated foals had higher average rectal temperatures and changes in sweat patterns, with six out of 10 developing heat stress in the paddock despite no continued treatment. Sweating responses were below baseline even 10 days after treatment ended. Hyperthermia in erythromycin-treated foals is due to an anhidrotic effect that reduces sweat gland activity, said Clark. Shade is not totally protective because foals will follow mares out of the shade and into the sun.

Reference:

Pain

Clark described a study in which researchers evaluated the use of tramadol (an opiate agonist) at two different doses—5 mg/kg twice a day for a week versus 10 mg/kg twice a day for a week—in chronic laminitis cases. In the four horses treated, the higher dose reduced weight-shifting and offloading of the feet by 40%, whereas the lower dose only reduced it by 9%. This treatment is not very expensive and can be used with NSAIDs or for those horses that cannot tolerate NSAIDs. The researchers concluded that tramadol should be safe if administered no longer than seven to 10 days, as it was in this study. Clark mentioned that there are reports of horses that don’t tolerate this drug well.

Next, researchers compared the effects of butorphanol when administered both intravenously (IV) and subcutaneously (under the skin) at a high dose of 5 mL for a 500-kg (1,100-lb) adult horse. With IV administration, the drug becomes 100% bioavailable (meaning the horse can absorb and use it), while with subcutaneous administration it was 87% bioavailable. With the subcutaneous form, analgesic (pain-relieving) levels occurred within five minutes, and maximal concentrations occurred at 20 minutes and lasted two hours, which is slightly longer than seen with IV administration of the same dose. Horses also experienced fewer physiological and behavioral side effects with the subcutaneous route than with IV administration. Intramuscular administration yielded only 37% bioavailability. The study authors concluded that subcutaneous butorphanol administration is an acceptable alternative to the intravenous route. Clark noted, however, that administering butorphanol might have negative effects on gastrointestinal motility, resulting in decreased manure output regardless of administration route.

The last study Clark presented involved a universal pain scale for horses. The researchers aimed to develop a face pain scale for horses similar to that for children. They used two pain inducers: ischemic (reduced oxygen) pain achieved with a tourniquet on the forelimb; and neuropathic pain achieved by applying capsaicin on the skin. In horses, facial pain expressions can manifest as:

  • Low or asymmetric ears and outwardly rotated ears;
  • Angled appearance of the eyes;
  • Withdrawn and intense stare;
  • Dilated nostrils; and
  • Tension of certain facial muscles.

This facial pain scale might improve pain recognition in horses and help improve equine welfare.

References:

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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