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Basics ⬇

Outline

BASICS

Overview!!navigator!!

  • Anhidrosis (also known as dry coat disease, nonsweaters, or “dry puffers”) is the inability to sweat effectively. The current theory is that overstimulation of sweat gland β2 receptors downregulates sweating at first temporarily and then permanently
  • Systems affected—skin/exocrine

Signalment!!navigator!!

No coat color, age, or sex predilections. Warmbloods and Thoroughbreds are more commonly affected. Rare in Arabians. Up to 20% of horses may be affected when exercising in a hot, humid climate.

Signs!!navigator!!

  • Tachypnea at rest in warm conditions
  • Extended tachypnea and hyperthermia after exercise
  • Reduction or absence of sweating during or after exercise
  • Brief bouts of excessive sweating may precede anhidrosis
  • Dry and flaky skin with alopecia, especially on the face and neck, in chronic cases
  • Lethargy and altered (decreased or increased) water intake in severe cases

Causes and Risk Factors!!navigator!!

  • Heat-stressed horses may have increased circulating epinephrine, an adrenergic agonist, that may overstimulate the sweat gland β2 receptors, resulting in downregulation or degradation
  • Horses maintained in hot, humid climates are susceptible. Poor acclimatization after introduction from a cool climate, intense exercise during hot, humid conditions, water/salt restriction, and feeding a high-carbohydrate diet are suspected risk factors
  • Treatment of foals with macrolide antibiotics causes temporary anhidrosis

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

Respiratory diseases that cause an increase in respiratory rate, especially heaves. Infectious diseases that cause an increase in rectal temperature.

CBC/Biochemistry/Urinalysis!!navigator!!

Hyponatremia and hypochloremia in chronic severe cases. Decreased renal fractional excretion of chloride.

Diagnostic Procedures!!navigator!!

Intradermal injections, 0.1 mL/site, in the neck below the mane, of a drug with β2-agonist activity (e.g. terbutaline sulfate, albuterol (salbutamol), epinephrine) using serial dilutions (0.001–1000 μg/mL) and a control (sterile saline)—read the results at 30 min. Sweat can be collected into absorbent pads taped over injection sites for more accurate results. Normal horses sweat in response to all dilutions. Anhidrotic horses have a decreased response to some or all.

Pathologic Findings!!navigator!!

Thickened basal laminae, thin myoepithelial and fundic epithelial cells, thickened connective tissues, and marked reduction of vesicles in the secretory cells. Luminal microvilli often are absent and the lumen of the duct is clogged with cellular debris.

Treatment ⬆ ⬇

TREATMENT

  • Environmental management is the only reliable treatment option at present
  • Horses exhibiting signs of heat stress should be immediately taken to a shaded environment, and cooled with cold water and fans
  • Restrict to a stall with a fan and/or mister during hot periods of the day
  • Minimize the feeding of carbohydrates
  • Provide free-choice cool, fresh water and salt supplementation
  • Inform clients that these horses will be prone to poor performance and will only improve once effective sweating has returned
  • Discontinue use of β2-agonists such as clenbuterol
  • Keep foals that are treated with macrolides out of direct sunlight during and for a week after treatment

Medications ⬆ ⬇

MEDICATIONS

Drug(s) of Choice

  • Supplemental electrolytes, especially sodium salts
  • Anecdotally, iodinated casein (10–15 g/day for 4–8 days) and with 1000–3000 IU PO of vitamin E (α-tocopherol) daily for 1 month, or amino acid supplements containing tyrosine

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Normally, a horse can reduce its body temperature to within normal limits approximately 30 min after exercise. Monitor respiration and rectal temperature post exercise.

Prevention/Avoidance!!navigator!!

  • Carefully acclimatize horses to exercise in hot/humid conditions
  • Keep foals treated with macrolides out of direct sunlight during and for a week after treatment
  • Exercise during the cooler periods of the day
  • Stall the horse in a cool environment (e.g. fan, water mister, air-conditioning) during the hotter periods of the day
  • Anhidrosis is usually a lifelong problem, consider relocating the horse to a more temperate climate
  • Avoid administration of β2-agonists

Possible Complications!!navigator!!

Heat stroke may occur if horses are exercised during the hotter periods of the day.

Expected Course and Prognosis!!navigator!!

  • Most horses respond to a change in environment and begin to sweat normally after a few weeks
  • Horses that have previously suffered from the disease will usually become anhidrotic if exposed to hot, humid conditions again

Miscellaneous ⬆ ⬇

MISCELLANEOUS

Associated Conditions

Skin lesions—dry, flaky skin and alopecia, especially around the eyes and shoulders.

Suggested Reading ⬆ ⬇

Hubert JD, Norwood G, Beadle RM. Equine anhidrosis. Vet Clin North Am Equine Pract 2002;18:355–369.

Author(s) ⬆

Author: Robert J. MacKay

Consulting Editors: Michel Lévy and Heidi Banse

Acknowledgment: The author and editors acknowledge the prior contribution of Jeremy D. Hubert and Ralph E. Beadle.