HEAT STRESS: TOO HOT TO TROT?
by Dr. Jenifer Nadeau, Equine Extension Specialist – University of
heat and humidity can be very hard on horses, particularly working
ones. This is an an article written by Dr. Jenifer
Nadeau, Equine Extension Specialist from the University of
Connecticut. Many cities around the world that have commercial
horse-drawn carriage rides have regulations that often do not factor in
humidity. This article discusses combining the temperature with
humidity as a guideline for working horses.
This is a summary of the article below:
the horse cools itself by sweating. Heat is lost and the body cools as
sweat evaporates from the skin's surface. Less moisture evaporates in
times of high humidity, however, and this causes the cooling mechanism
to become less efficient. Prolonged exposure to high temperatures
results in dilation of surface blood vessels. When dilation occurs
without an increase in blood volume, circulatory collapse may also
occur. The cooling mechanism of the horse is most effective when the
sum of the ambient (outside) temperature and relative humidity is less
than 130. Efficiency of cooling decreases between 130 and 150. When the
sum of the ambient temperature and relative humidity is greater than
150, the horse's ability to cool itself is greatly reduced. When the
sum is greater than 180, horse owners need to be cautious, since these
conditions could be fatal if the horse is stressed. Some horses are
anhydrotic, meaning they have little or no ability to produce sweat.
These horses are prime candidates for heat stress.”
Department of Animal Science, University of Connecticut
Effective Horse Management – Horse Health Series
Heat Stress: Too Hot to Trot?
Jenifer Nadeau, M.S., Ph.D
Associate Professor, Equine Extension Specialist
Department of Animal Science
hot summer weather, heat should be a concern for horse owners.
Horse owners need to provide extra care during hot weather to decrease
stress and maintain the health and well being of their horses.
the horse cools itself by sweating. Heat is lost and the body
cools as sweat evaporates from the skin’s surface. Less moisture
evaporation occurs in times of high humidity, causing the cooling
mechanism to become less efficient. Some horses are anhydrotic,
meaning they have little or no ability to produce sweat. These
horses are prime candidates for heat stress. Below is a table
that will help horse owners determine when it is too hot to work a
horse. If humidity is more than 75%, heat stress is a likely result due
to inability to sweat regardless of ambient temperature.
Ambient (outside temperature) + relative humidity (◦ F) Horse's efficiency of cooling
Less than 130 Most effective
Greater than 150 Greatly reduced
Greater than 180 Conditions could be fatal if horse is stressed
terms for horse overheating include hyperthermia, heat exhaustion, heat
cramps, heatstroke or sunstroke. Explanations of the signs
associated with each are listed below:
Hyperthermia or overheating in the horse is due to a disturbance in the
heat regulating mechanism of the horse’s body that can result from hot
weather, high humidity, poor stable ventilation, prolonged exposure to
direct sunlight, excessive work, transportation, or obesity. Some
signs of overheating include muscle tremors, profuse sweating,
collapse, dark urine, dull expression and behavior, dark mucous
membranes, and slow capillary refill.
Heat exhaustion will cause the horse’s temperature to rise to 105-109°
F, the pulse rate to rise to 50 to 100 beats per minute, and the
respiration rate to rise to more than 30 breaths per minute.
Normal vital signs of the horse include a temperature of 99.5 – 101.5°
F, pulse rate of 28-44 beats per minute, and a respiration rate of 8-20
breaths per minute.
Heat cramps are most commonly found in horses that are sweating
profusely while doing hard work in intense heat. Signs of heat
cramps include spasms of the abdomen and/or legs, muscle twitching and
cramping of the muscles. These occur due to a loss of
electrolytes (i.e. severe salt loss).
Heatstroke or sunstroke is more serious. Horses undergoing
prolonged hard or fast work during hot weather, horses exposed to
direct sunlight without shade, young, poorly conditioned horses, and
horses with long hair coats are susceptible to heatstroke or
sunstroke. Signs of heatstroke or sunstroke are rapid breathing,
weakness, in coordination, and refusal to work. Body temperature
can increase to 106–110° F, sweating stops, and the skin dries.
Prolonged exposure to high temperatures also results in dilation of
surface blood vessels. When dilation occurs without an increase
in blood volume, circulatory collapse, delirium, and convulsions may
also occur. Death can occur within a few hours if the horse is
not cooled and does not receive emergency veterinary care.
Treatments for these conditions include:
For heat exhaustion and heatstroke, the horse should be sprayed with
cool water and moved to a shady area or cool, well-ventilated
For heatstroke, ice packs should be placed on the horse’s head and
large blood vessels on the inside of its legs.
· For heat cramps, the horse should be cooled, rubbed down, and given electrolytes.
overheating the horse owner should use fans, allow the horse to have a
few swallows of cool, clean, fresh water every few minutes, and call a
veterinarian because the horse may need to receive intravenous
administration of fluids.
Ideally, overheating would not occur in the first place. Prevention of overheating includes:
· removing blankets or sheets from stabled horses during extreme heat
· clipping horses with long hair
· providing adequate ventilation in stables
· using fans in barns or stalls
· sprinkling the aisle of the barn with water to aid in cooling in areas of low humidity
· providing shade for all outside horses
consider feeding fat as an energy source. One study examined the
effect of feeding 10% corn oil (a fat source) and 3% soybean meal
instead of 13% of the cracked corn ration in the control diet.
Using fat as an energy source produces less metabolic heat for the
energy produced when compared to carbohydrate as the energy
source. This results in the horse having less heat to
replacing electrolytes after physical exertion, since sodium,
potassium, calcium and chlorine are lost in the urine and sweat.
Loss of electrolytes can lead to metabolic problems, a decrease in the
thirst response, and loss of interest in eating and drinking. A
simple source of these electrolytes is feeding a 2-ounce mixture of 3
parts lite salt (potassium chloride), and 1 part limestone on a daily
basis. This is also a good source of sodium.
providing plenty of clean, fresh water. At 0° F, the horse will
drink about 1 pint of water per pound of dry feed consumed and this
increases to 1 gallon of water per pound of dry feed consumed at 100° F
· properly conditioning horses
limiting strenuous riding to late evenings or early mornings when the
temperature is lower and following the guidelines above
walking a hot horse which protects against placing a horse in an area
void of airflow where sweat will not undergo convection and evaporation
· transporting horses at cool times of the day and providing them with adequate ventilation in the trailer
using the skin pinch test to determine if a horse is properly
hydrated. To do this, a section of skin on the neck or shoulder
is pinched. The skin should recoil in 1-2 seconds in a normally
hydrated horse. A delay indicates some dehydration.
common myths have also been found in previous equine
publications. Each of these misconceptions prevents the
overheated horse from cooling off properly. They are:
“Never let a horse drink more than one or two swallows of water at a
time. ” This comes from the belief that allowing a hot horse to
consume unrestricted amounts of water may lead to problems such as
colic due to hyperdistension of the stomach. However, a horse’s
stomach can hold between 2 and 4 gallons of fluid without being
distended, so one or two sips of water at a time is overly restrictive
when the hot horse is rapidly losing water and trying to keep itself
cool. The horse should therefore be allowed to have a few
swallows of cool, clean, fresh water every few minutes.
“Never give ice-cold water to a hot horse either inside or out.” The
second myth has been the source of controversy over the years because
people believed that ice cold water placed on a hot horse’s body will
“shock” the horse’s thermoregulatory system into shutting down blood
flow to the skin. This belief has been found to be incorrect
based on extensive research conducted during 1995 at the University of
Illinois and University of Guelph, and at the 1996 Olympic Summer Games
in Atlanta. Researchers proved conclusively that horses working
under hot and humid conditions were better able to maintain, or even
reduce, core body temperature. Furthermore, after intense
exercise, ice water baths caused a decline in heart rate during rest
periods when used. Horses were also observed trotting more freely
after the baths. Application of cold water to the overheated
horses helped to dissipate heat by providing more water to evaporate
from the skin, and by direct conduction of the horse’s body heat into
the water that runs off the horse, carrying excess heat with
“Never let a hot horse cool out without a blanket or sheet.” Many
people cool out the horse by placing a sheet or blanket on the horse
while walking it. Inhibiting the evaporation of water from the
skin by using a blanket or sheet is not recommended in hot and humid
“Never let a hot horse cool out in a drafty area.” This comes
from the belief that a draft will make the horse susceptible to a
“chill”. However, restricting access to moving air during hot and
humid conditions makes little sense. Fans work to increase
evaporation and dissipate heat by the cooling process known as
convection. Misting fans take advantage of the additional cooling
property of blowing water onto the horse when its sweat is changing
phase from liquid to gas.
Horse owners are capable
of preventing overheating. Know how to recognize the signs of
overheating and what to do when overheating occur. Be aware of
the myths regarding overheating treatments and share correct
information with others.
American Medical Equestrian Association. August 1993. News. 3(3):online
Bonner, Laurie. July 2002. Water works. Equus. p. 45-54.
Cirelli, Albert A. 1993. Avoiding hyperthermia in the horse. Horse Industry Handbook, p. 410-1-410-2.
Gallagher, Juliana M. 2001. Developments in the reduction of heat
stress in the endurance horse. Veterinary Education and
Information Network. Online at
Kline, Kevin H. 2002. Managing heat stress in horses. Illini Horse Net.
Stotts, Donald. 2002. Symptoms Give Warning of Heat Stress in
Horses. Online at http://www.mrhorse.com/articoli/Art024.htm.
am grateful for the reviews by Dr. Krishona Martinson of the University
of Minnesota, Equine Extension Specialist and Dr. Debra Hagstrom of the
University of Illinois, Horse Extension Specialist.
Note: The articles and/or
excerpts listed are for information only.
The beliefs and opinions expressed and promoted by this website
are not necessarily held by the articles’ authors.
Use Notice: This website may contain copyrighted material whose
use has not been specifically authorized by the copyright owners.
We believe that this not-for -profit, eduction use on the Web
constitutes a fair use of the copyrighted material (as provided for in
Section 107 of the US Copyright Law.) If you wish to use this
copyrighted material for purposes of your own that go beyond fair use,
you must obtain permission of the copyright owner.