Inflammatory disorders of the airway are common in horses. Until recently, the terminology for inflammatory lower airway diseases was vague, making comparison of data difficult. The distinction between lower airway inflammatory disease of young horses and chronic airway inflammation was not clear and often called chronic obstructive pulmonary disease (COPD). However, this is also the name used in human medicine to describe inflammatory lung disease in smokers, a syndrome which has a completely different pathophysiology and clinical evolution.¹ Clinicians now make the distinction between the syndrome in young horses called inflammatory airway disease (IAD) and a chronic, reversible, inflammatory airway obstruction in mature horses called either ‘‘heaves’’ or recurrent airway obstruction (RAO), also differentiating these from infectious diseases that cause airway obstruction.

RAO is a naturally-occurring environmental condition more frequently encountered in equine populations of the Northern Hemisphere, where horses are housed indoors part of the year and where adequate drying of hay may be more difficult to achieve.² In any case, RAO is the most common chronic respiratory disease of housed mature horses. Conversely, summer pasture–associated obstructive pulmonary disease, which is clinically in-distinguishable from RAO, is mostly found in southern regions, where horses are kept on pasture throughout the year^3,4 Horses older than five years of age are most frequently affected, and the prevalence does appear to increase with age. There does not seem to be a predisposition for development of this condition related to gender, but breed and heredity seem to factor into occurrence.⁵

Researchers know that certain equine family lines seem more likely to develop heaves when placed in environmental conditions that favor it. And within these lines, horses that have one parent with heaves are more likely to develop the condition, and horses with both parents affected are significantly more likely to have heaves.⁶

Heaves is part of a chronic inflammatory process involving the lung’s mucosal lining. This inflammatory component is characterized by neutrophils entering the lungs, mucus production that plugs the airways, and constriction of the bronchial tubes.⁵ Obvious clinical signs of heaves include constant coughing, labored breathing as the horse exhales, and wheezing, which may be more noticeable when the horse eats hay or exerts effort under physical stress.

Dexamethasone

The primary treatment for control of RAO and IAD in horses is corticosteroid therapy.⁷ These are often used in conjunction with any antimicrobial agents being administered. Veterinarians have long employed throat sprays to treat a variety of airway issues in horses, including airway inflammation, nasopharyngeal cicatrix (scar tissue), lymphoid hyperplasia (an inflammatory condition of the throat), and arytenoid chondritis (inflammation of the arytenoid cartilage). The most common administration method for corticosteroids to treat airway disorders is via nasopharyngeal catheter, although horses that resist this method can receive the spray directly through their mouths.⁸

Dexamethasone is a potent anti-inflammatory with therapeutic action that is used in many species. Typically, it is used in the management of various rheumatic, dermatologic, allergic and other diseases known to be responsive to anti-inflammatory corticosteroids.

Dexamethasone is often used in horses with acute musculoskeletal inflammations. It also provides supportive therapy in a wide variety of cases, such as heat exhaustion, influenza, laminitis, and retained placenta, provided that the primary cause is diagnosed and corrected.⁹ Its anti-inflammatory properties have made it a useful medication in preventing and treating the effects of RAO and IAD in horses.

Dexamethasone is a glucocorticoid (corticosteroid) that is typically available in injectable, oral, and ophthalmic forms. It is long acting, and 30 times more potent than hydrocortisone with no mineralocorticoid activity.¹⁰ In one study, dexamethasone 0.05 mg/kg IM every 24 hours was as effective as inhaled fluticasone in reducing clinical signs, airway inflammatory cells, and bronchoprovocative histamine response in horses.¹¹

For horses, it should be noted that the Association of Racing Commissioners International’s [ARCI] Uniform Classification Guidelines for Foreign Substances [UCGFS] classifies dexamethasone as a CLASS 4 Drug.

General dosages of dexamethasone for horses (per Plumb’s Veterinary Drugs) are as follows:

Anti-inflammatory; glucocorticoid agent (Labeled dosage): Dexamethasone 2.5 – 5 mg (total dose) IV or IM (Adapted from Package Insert; Azium®); dexamethasone sodium phosphate: 2.5 – 5 mg (total dose) IV (Adapted from Package Insert; Azium® SP)

Adjunctive treatment of recurrent airway obstruction (extra-label):

a) For a 500-kg horse, dexamethasone 40 mg (total dose; NOT mg/kg) IM once every other day for 3 treatments, followed by 35 mg (total dose; NOT mg/kg) IM once every other day for 3 treatments, followed by 30 mg (total dose; NOT mg/kg) IM once every other day for 3 treatments; continue tapering until horse is weaned off this therapy.

b) For short-term treatment with environmental control: In a study, dexamethasone sodium phosphate was dosed at 0.1 mg/kg IM once daily for 4 days, 0.075 mg/kg IM once daily for 4 days, and 0.05 mg/kg IM for 4 days. Except for bronchoalveolar lavage cytology results, PO prednisolone (1 mg/kg PO for 4 days, 0.75 mg/kg PO for 4 days, 0.5 mg/kg PO for 4 days) was shown to be as effective as IM dexamethasone.

Adverse effects of dexamethasone can include weight gain, depression and digestive issues, but these appear to be restricted to small animals, such as dogs and cats.⁹ When administering a regimen of corticosteroids, the goal is to find the lowest dose possible and use it for the shortest period of time. Horses receiving dexamethasone should remain under close observation. Therapy with dexamethasone, as with any other potent corticosteroid, should be individualized according to the severity of the condition, anticipated duration of steroid therapy, and the animal's tolerance for steroid excess. Long term therapy involving dexamethasone may play a role in the development of laminitis in horses, but this is thought to occur only rarely.⁹

Except for emergency therapy, dexamethasone should not be used in horses with nephritis or Cushing's Syndrome, congestive heart failure, or viral infections, unless usage is determined to be appropriate by a veterinarian.⁹ Dexamethasone should be withdrawn at least 14 days prior to intradermal skin testing in horses.¹²

Dimethyl Sulfoxide (DMSO) and Glycerin

Dimethyl Sulfoxide (DMSO) is Free radical scavenger that has anti-inflammatory, cryopreservative, anti-ischemic and radioprotective effects.¹⁰ The reported and purported uses for DMSO are numerous, but the only FDA-approved veterinary indication for DMSO is: “…as a topical application to reduce acute swelling due to trauma.”⁹ Other possible indications for DMSO include: adjunctive treatment in transient ischemic conditions, CNS trauma and cerebral edema, calcinosis cutis, endometritis, skin ulcers, wounds, burns, adjunctive therapy in intestinal surgeries, analgesia for postoperative or intractable pain, amyloidosis in dogs, reduction of mammary engorgement in the nursing bitch, enhancement of antibiotic penetration in mastitis in cattle, and limitation of tissue damage following extravasation injuries secondary to chemotherapeutic agents. DMSO’s attributes as a potential carrier of therapeutic agents across the skin and into the systemic circulation and its synergistic effects with other agents have long been reported in human and animal medicine.

The pharmacologic effects of DMSO are diverse. DMSO traps free radical hydroxide and its metabolite, dimethyl sulfide (DMS), traps free radical oxygen. It appears that these actions help to explain some of the anti-inflammatory, cryopreservative, anti-ischemic, and radioprotective qualities of DMSO.⁹

DMSO easily penetrates the skin. It serves as a carrier agent in promoting the percutaneous absorption of other compounds (including drugs and toxins) that normally would not penetrate. Drugs such as insulin, heparin, phenylbutazone, and sulfonamides may all be absorbed systemically when mixed with DMSO and applied to the skin. DMSO has been shown to increase the rate of oral absorption of the triazine antiprotozoal drugs, diclazuril, and toltrazuril in horses.¹³

The anti-inflammatory/analgesic properties of DMSO have been thoroughly investigated. DMSO appears to be more effective as an anti-inflammatory agent when used for acute inflammation versus chronic inflammatory conditions. The analgesic effects of DMSO have been compared to that produced by narcotic analgesics.⁹ These attributes, combined with the practical uncertainty relative to the dispersal of oral and nasopharyngeal medications, make it an ideal synergist to dexamethasone for the equine throat wash. Glycerin is known for its active properties to relieve the symptoms of sore throat and cough by aiding in fostering a normal coating over irritated mucous membranes.

Where to buy Dexamethasone + Glycerin + DMSO

Dexamethasone + Glycerin + DMSO is typically available in the U.S. through veterinary custom compounding companies.

NexGen’s Equine Throat Wash (DMSO, Glycerin, Dexamethasone 4mg/Ml) suspension is a superior solution for the treatment of inflammatory disorders of the airway in horses. It aids in the reduction or elimination of upper and lower respiratory infections, helps to control allergies and reduce mucous congestion and reduce irritation from pharyngitis and swollen tissues due to inflammation.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

¹Le ́guillette, R., DVM. Recurrent airway obstruction—heaves. In: Vet Clin Equine 19 (2003) 63–86.

²Traub-Dargatz JL, Salman MD, Voss JL. Medical problems of adult horses, as ranked by equine practitioners. JAVMA 1991;198:1745–7.

³Costa LR, Seahorn TL, Moore RM, et al. Correlation of clinical score, intrapleural pressure, cytologic findings of bronchoalveolar fluid, and histopathologic lesions of pulmonary tissue in horses with summer pasture-associated obstructive pulmonary disease. Am J Vet Res 2000;61:167–73.[10]

⁴Seahorn TL, Beadle RE. Summer pasture-associated obstructive pulmonary disease in horses: 21 cases (1983–1991). JAVMA 1993;202:779–82.

⁵McPherson EA, Lawson GH, Murphy JR, et al. Chronic obstructive pulmonary disease (COPD): identification of affected horses. Equine Vet J 1978; 10:47–53.

⁶TheHorse.com.

⁷Bond, S. L., Timsit, E., Workentine, M., Alexander, T., & Léguillette, R. (2017). Upper and lower respiratory tract microbiota in horses: bacterial communities associated with health and mild asthma (inflammatory airway disease) and effects of dexamethasone. BMC microbiology, 17(1), 184. https://doi.org/10.1186/s12866-017-1092-5

⁸TheHorse.com.

⁹EquiMed.com.

¹⁰Plumb’s Veterinary Drugs.

¹¹Leguillette R, Tohver T, Bond SL, Nicol JA, McDonald KJ. Effect of dexamethasone and fluticasone on airway hyperresponsiveness in horses with inflammatory airway disease. J Vet Intern Med. 2017;31(4):1193-1201.

¹²Petersen A, Schott HC. Effects of dexamethasone and hydroxyzine treatment on intradermal testing and allergen-specific IgE serum testing results in horses. Vet Dermatol. 2009;20(5-6):615-622.

¹³Dirikolu L, et al. Current therapeutic approaches to equine protozoal myeloencephalitis. Javma-Journal of the American Veterinary Medical Association. 2013;242(4):482-491.