Vet Notes – Heat Stress

Written by Dr. Jessica Reynolds

2021-2022 Peterson Smith Equine Hospital + Complete Care Ambulatory Intern

What causes heat stress?

  • More common in poorly conditioned horses that are overworked in hot and humid climates (Florida!), in conjunction with anhidrosis (not sweating), dehydration, or exhaustion syndrome.
  • Horses that are confined in poorly ventilated areas (shipping).
  • Healthy horses can also experience heat stress when the sum of temperature (in Fahrenheit) and humidity have a value of >180.
    • If horses are moved from cool to hot climate, several days are needed for the horse to acclimate to the change in temperature.
  • Foals with Rhodococcus equi using a macrolide antibiotic (erythromycin) are also more prone.
  • High fever (up to 106) in horses rarely ever cause heat stroke if due to a viral infection.

How to diagnose heat stroke/stress?

  • History and clinical signs!
    • Increased respiratory rate, lethargy, decreased appetite, shade seeking.

Clinical signs of heat stress:

  • Poor sweating response to anhidrosis (not sweating).
  • Hot, dry skin
  • Increased heart rate and respiratory rate
  • Elevated rectal temperature (106-110).
  • Prolonged capillary refill time, dark mucus membranes (gums).
  • Depression, weakness.

What to do next:

  • Decrease the body temperature:
    • The quicker treatment and more rapid cooling, the better the prognosis.
    • Move horses to a shaded, well-ventilated area with fans if possible.
    • Apply cold water or ice water hydrotherapy (cold hosing) to the entire body, focusing on the jugular groove and chest.
      • Continually scrape the water off the horse and reapply.
    • Can use alcohol over neck/jugular groove/chest area as well.
    • Offer water, can add electrolytes to encourage hydration.
    • Administer anti-pyretic (anti-fever) such as Flunixin Meglumine (Banamine), use the dosage as recommended by your veterinarian.
    • Restore blood volume:
      • Contact your veterinarian to provide IV fluid therapy as they deem necessary.
    • Bloodwork: Chemistry and CBC panel to evaluate organ functioning, and to see if further treatment is needed.

What NOT to do:

  • Do not use any fabric (towels, cooling sheets, etc), they prevent the heat from leaving the body.
  • Do not force the horse to walk, unless its to move into the shade/better ventilated area.
  • Do not withhold water.
  • Do not administer Dormosedan or Xylazine (sedatives), they can cause respiratory distress.



  • Divers, T.J. Orsini, J.A. (2014) ‘Equine Emergencies Treatment and Protocols Fourth Edition’ Elsevier
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