Each year, researchers publish hundreds of studies in the field of equine medicine. Steve Reed, DVM, Dipl. ACVIM, an associate at Rood & Riddle Equine Hospital in Lexington, Ky., had the unique (and behemoth) task of deciding which to feature to a veterinarian audience during the Kester News Hour at the 2011 American Association of Equine Practitioners Convention, held Nov. 18-22 in San Antonio, Texas.  Reed described the studies he found most important, interesting, or applicable to present day equine medicine during the session that is a perennial favorite among veterinarians.

Metabolic and Endocrine Disorders

Reed began by discussing a handful of studies on diagnosing the single most commonly detected endocrine disorder in aging horses—pituitary pars intermedia dysfunction (PPID, or equine Cushing's disease), which is characterized by enlargement of the pars intermedia region of the pituitary gland). Diagnosing PPID, he noted, has been a challenge in the past, and usually PPID horses are identified through a combination of age and physical signs of the disease.

In one study Reed referenced researchers evaluated which breeds are most prone to PPID diagnosis and what—if any—effect geographic area had on PPID diagnosis. About 36% of all horses are diagnosed with PPID, Reed noted, but the disease is most commonly found in pony breeds and Morgan horses.

Further, the researchers found that levels of two hormones released by the pars intermedia—adrenocorticotropic hormone (ACTH) and α-melanocyte-stimulating hormone (α–MSH)—increase in all horses in the fall. And while fall begins earlier in the North then in the South, horses in the South display a greater pars intermedia response to the change in season.

The study is a step in fulfilling certain diagnostic needs, he noted: "Seasonal and photoperiod (daylight's effect on plants and animals) reference ranges are needed when interpreting pars intermedia hormones in horses and may be helpful for early recognition of PPID in horses."

Additionally, Reed discussed a third study in which a research team evaluated ACTH levels in more than 900 horses with PPID and another 900 without PPID. The researchers found that during the fall months (August to October) the horses with PPID had an average ACTH concentration of 120 pg/mL compared to November to July when the levels were only 60 pg/mL. While levels fluctuated throughout the year in the horses without PPID as well, the average thresholds for ACTH were significantly lower year round in normal horses.

Reed concluded that because of the variation in hormone concentrations year-round, the fall could be the best time to test as a result of the observed spike in ACTH levels in PPID horses.

Funk RA, Stewart AJ, Woodridge AA, et al. Seasonal changes in plasma adrenocorticotrophic hormone and α-melanocyte-stimulating hormone in response to thyrotropin-releasing hormone in normal aged horses. J Vet Intern Med 2011; 25:579-585.
McFarlane D, Paradis MR, Zimmel D, et al. The effect of geographic location, breed, pituitary dysfunction on seasonal adrenocorticotrophic hormone and α-melanocyte-stimulating hormone plasma concentrations in horses. J Vet Intern Med 2011; 25:872-881.
Copas S, Durham AE, et al. Published ahead of print. Equine Vet J 2011.

Respiratory and Cardiovascular

Changing gears, Reed mentioned an adage in human medicine, "one airway, one disease," and how it might apply in veterinary medicine. Traditionally, doctors considered asthma and allergic rhinitis as distinct lower and upper airway conditions, respectively, but are now shifting to a theory that these are better described as a continuum of inflammation involving one common airway. He described a study in which researchers performed endoscopic evaluations on 128 horses to score the presence of pharyngitis, pharyngeal mucus, tracheal mucus, tracheal septum thickness, and bronchial mucus. The investigators determined that lower airway scores were significantly worse in horses with evidence of severe inflammation in bronchoalveolar lavage fluid (BALF, the fluid used to “wash" the lungs that is later analyzed under a microscope after collection).

"Lower airway endoscopy scores are reflective of lower airway inflammation; however, upper and lower airways are independent in terms of severity of inflammation," the researchers explained in the study. "Therefore, observing upper airway inflammation is not an indication to test for lower airway inflammation." Basically, the "one airway, one disease" concept does not appear to apply to horses.

Reed also discussed a study in which researchers examined causes of sudden deaths in Thoroughbred racehorses by reviewing postmortem examination records of 268 horses in six racing jurisdictions worldwide. A definitive cause of death was determined in only 53% of these animals, and in 25% of the remaining horses, a presumptive cause of death was suggested. In the remaining 23% no cause of death could be determined, the researchers relayed.

"To me, this was a very valuable study because it demonstrates the similarities of … exercise-related sudden death from different racing jurisdictions," Reed said, "and because it emphasizes the need for careful physical examinations—including cardiac and cardiopulmonary assessment—by veterinarians at the racetrack."

Reed described a third study in which scientists evaluated the effect of furosemide (commonly known as Salix) on treating exercise-induced pulmonary hemorrhage (“bleeders”). They found that administering 250 mg of Salix four hours prior to exercise (they tested the horses at 80% of maximum energy output) did not alter the horses' exercise performance, and it did not impact the way capillaries in the lungs regulated red blood cell volume, despite fact it decreased cardiac output of blood from the heart. Reed noted that researchers are still working to understand exactly where in the lungs the bleeding originates, and that it's possible a higher dose of Salix could have yielded different results.

Veterinarians often treat another common respiratory disorder, recurrent airway obstruction (RAO, or heaves) with systemic corticosteroids. Although this method of treatment is often effective, prolonged use can have adverse side effects (including a suppressed immune system, a predisposition to infection, and an increased risk of founder) in some horses. Reed discussed a study in which a team of researchers examined the effects of using an inhaled corticosteroid to treat RAO as opposed to the traditional method of administration (via injection). The team assessed effects on the two arms of the immune systems, cell-mediated (protecting against intracellular organisms) and humoral immunity (antibodies in the serum that protect the horse from infection), and noted no adverse reactions. Further, the team found that prolonged administration yielded no undesirable results.

Finally, Reed discussed a study in which scientists evaluated horse owners' ability to detect lung inflammation in horses. The team evaluated 167 horses that were either healthy or had evidence of lower airway inflammation, then asked owners to use a questionnaire to evaluate which horses were healthy and which horses were affected by lower airway disease. They also asked the owners to record whether they thought the airway disease was mild, moderate, or severe. Reed explained that owners were able to detect horses at risk or affected by lower airway inflammation by using the questionnaire, but they were unable to differentiate between mild to moderate and severe inflammation.

There's a benefit to veterinarians using an RSQ themselves as a screening tool, Reed said, but a BAL is necessary to definitively diagnose lower airway inflammation.

Koblinger K, Nicol J, McDonald K, et al. Endoscopic assessment of airway inflammation in horses. J Vet Intern Med 2011; 25(5): 1118-1126.
Lyle CH, Uzal FA, McGorum BC, et al. Sudden death in Thoroughbred horses: An international multicentre study of post mortem findings. Equine Vet J 2011; 43(3): 324-331.
Vengus F, Kerr C, Staempfli HR, et al. Effect of furosemide on transvascular fluid fluxes across the lung in exercising horses. Equine Vet J 2011; 43(4): 451-459.
Dauvillier J, Felippe MJB, Lunn DP, et al. Effect of long-term fluticasone treatment on immune function in horses with heaves. J Vet Intern Med 2011; 25: 549-557.
Wasko AJ, Barkema HW, Nicol J, et al. Evaluation of a risk screening questionnaire to detect equine lung inflammation: Results of a large field study. Equine Vet J 2011; 43(2): 145-152.

Diagnostic Techniques

Veterinarians are continually finding new ways to detect various equine medical conditions, along with improving on the old. Reed delved into a few studies regarding new or updated diagnostics.

Historically, renal (kidney) biopsies were often contraindicated due to the relatively high complication rate. However, Reed discussed a recent study in which veterinarians evaluated over 150 biopsies and showed that renal biopsies—when performed properly—can actually yield a low complication rate. Additionally, the researchers noted that when postmortem findings were available (i.e., when horses that underwent renal biopsy before being examined on necropsy), there was a "good correlation" between gross findings and the ones obtained via renal biopsy.

"The one disappointment from this study was the fact that the authors were unable to determine the usefulness of the biopsy in altering the management of the patient," Reed added. In other words, they could successfully perform a biopsy and confirm kidney disease but weren’t able to apply that information to treatment of the patient.

Next, he discussed a study in which investigators determined that dehydration in horses led to abnormalities evident on echocardiographic examination (an ultrasound of the heart).  The most significant changes were observed in dehydrated horses’ left ventricle (the lower chamber of the heart that pumps oxygenated blood throughout the body) and left atrium (the upper chamber that transports the oxygenated blood from the pulmonary veins into the left ventricle), both of which appeared decreased in size. The thickness of some wall structures within the heart also increased.

"The authors warn that (dehydration) could result in alteration of variables often applied to predict athletic potential in a horse (via an echocardiogram)," Reed said.

Veterinarians are diagnosing performance-limiting cervical (neck vertebrae) problems in equine athletes on a more frequent basis. While clinical assessment and cervical vertebral radiographs are the usual diagnostic tools veterinarians use, Reed discussed a study in which researchers examined the use of electromyography (EMG, which measures electrical activity of muscles) for diagnosis. Commonly used in human medicine for diagnosing movement disorders, the researchers tested the technique in horses and found it useful for evaluating horses with suspected cervical lesions.

"In this paper, (the researchers) seek to establish normal values to use for the diagnosis of horses with neurological problems involving the cervical vertebrae," Reed said. This should help future researchers to then decipher what is abnormal once normal parameters are determined.

Finally, he described the development of a new technique for the early detection of botulism, an often fatal disease known for impacting foals. Early detection of the disease (in foals) can allow for additional treatment time and potentially a more favorable outcome, Reed said.

"(The test) may not be ready for prime time, but it will be very important," he added.

Tyner GA, Nolen-Waltson RD, Hall T, et al. A multicenter retrospective study of 151 renal biopsies in horses. J Vet Intern Med 2011; 25: 532-539
Underwood C, et al. Echocardiographic changes in heart size in hypohydrated horses. J Vet Intern Med 2011; 25: 563-569.
Wijnberg ID, et al. Quantitative motor unit action potential analysis in 2 paraspinal neck muscles in adult royal Dutch sport horses. J Vet intern Med 2011; 25: 592-597.
Aleman M, Williams DC, Jorge NE, et al. Repetitive stimulation of the common peroneal nerve as a diagnostic aid for botulism in foals. J Vet Intern Med 2011; 25: 365-372.

Gastrointestinal Tract Medicine

Reed discussed several studies on the equine gastrointestinal (GI) tract. The first study he discussed involved a review of non-steroidal anti-inflammatory drugs’ (NSAIDs) effect on equine intestines; he noted veterinarians have observed side effects including gastric ulceration and right dorsal colitis, especially when using these drugs to treat endotoxemia. Through their research, the team confirmed that extended use of NSAIDs causes damage to the intestinal mucosal barrier, which leads to a loss of normal barrier function (against some common antigens and pathogens).

Another study Reed discussed involved the association between certain GI issues and neurologic clinical signs.

"The recognition of neurological signs in horses presented (for veterinary assessment) for diarrhea, colic, colitis (inflammation of the colon), or enteritis (inflammation of the intestinal tract) should not be a surprise to practitioners examining horses with these problems," he said.

The researchers found that horses with the aforementioned clinical signs typically have increased blood ammonia concentrations, which are believed to cause the neurologic signs.

"(Conversely) this can be very important as some horses with clinical signs indicative of diffuse cerebral disease may have a primary gastrointestinal problem," he concluded, adding that the prognosis for these horses is typically guarded to poor.

Reed then shifted gears to discuss a study focusing on Lawsonia intracellularis, an organism that causes protein loss and GI issues in weanling foals.

He stressed that infection with the bacterium causes a thickening of the intestinal walls, which leads to malabsorption of protein and a low daily weight gain, and veterinarians are diagnosing an increasing number of cases. Further, he noted, the treatment of choice, oxytetracycline, does not seem to be as effective as it used to be; some cases simply do not respond optimally.

Marshall JF, Blickslager AT. The effect of nonsteroidal anti-inflammatory drugs on the equine intestine. Equine Vet J 2011; 43: 140-144.
Dunkel B, Chaney KP, Dallap-Schaer BL, et al.  Putative intestinal hyperammonaemia in horses: 36 cases. Equine Vet J 2011; 43(2): 133-140.
Page, A et al. Adaptation and validation of a bacteria-specific ELISA for determination of farm-specific Lawsonia intracellularis seroprevalence in Kentucky Thoroughbreds. Equine Vet J 43; (Suppl. 40) :25-31.

Ocular Health

Reed closed by discussing a study in which veterinarians evaluated the presence of ophthalmic lesions in neonatal foals admitted to a referral hospital for unrelated reasons. More than half the foals (56%) had at least one ophthalmic disease (potentially acquired as a result of another health problem), and many of those problems have the potential to be vision-limiting.

Reed cautioned practitioners, "Don't forget to look at the entire foal. If you treat them early on, you might find things that could save their vision."

Labelle AL, Hamer RE, Townsend WM, et al. Ophthalmic lesions in neonatal foals evaluated for nonophthalmic disease at referral hospitals. J Am Vet Med Assoc 239; (4): 486.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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