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Thermoregulation. Base mechanisms and hyperthermia.
Vet Clin North Am Equine Pract 1998 Apr;14(1):45-59
Guthrie AJ, Lund RJ
Equine Research Centre, Faculty of Veterinary Science, University of Pretoria, Onderstepoort, Republic of South Africa.
Metabolic HP is extremely high during exercise in horses. Thermoregulation in horses is primarily dependent on evaporative heat loss from sweating in particular. Under thermoneutral conditions, these mechanisms are sufficient
to allow horses to perform high-intensity exercise for long periods. Under thermally stressful conditions, particularly in high ambient humidity, the efficiency of evaporative heat loss mechanisms is compromised and may result
in horses developing hyperthermia. Early recognition and vigorous treatment are essential to limit the consequences of heat stress in horses. Meticulous planning and management of equestrian events that are held under thermally
stressful conditions are essential to ensure the welfare of competing horses and their riders. The conditioning program of horses expected to compete under thermally stressful conditions must also make adequate provision for
acclimatization to the hot, humid conditions.
Publication Types:
•Review, tutorial
PMID: 9561687, UI: 98222473
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Physiological responses of horses competing at a modified 1 star
3-day-event.
Equine Vet J Suppl 1995 Nov;20:97-104
Kohn CW, Hinchcliff KW, McCutcheon LJ, Geor R, Foreman J, Allen AK, White SL, Maykuth PL,
Williamson LH
Department of Veterinary Clinical Sciences, Ohio State University, Columbus 43210, USA.
The impending 1996 summer Olympic 3-day-event in Atlanta has focused attention on the need to determine
what modifications to the demanding Endurance Test will be required to ensure safety of the horses competing. Three groups of horses participated in a Field Trial held in August of 1994 in northern Georgia to determine the
safety and feasibility of conducting a modified 3-day-event in hot, humid weather. One group (TD) completed a modified 1 Star 3-day-event test, a control group (HT) completed a Horse Trial identical to the modified 1 Star test
except for the omission of Phases B and C and the third group (E), comprised of European horses, completed the modified 1 Star test with a longer, faster Phase C than was used for TD. During the Endurance Test, the ambient
temperature and relative humidity ranged from 24.3 degrees C and 98.9% in the morning to 30.2 degrees C and 51.6% in the afternoon. No horse failed to complete the Trial because of heat stress or fatigue.
There were no
significant (P < 0.05) differences detected in heart rate, rectal temperature, respiratory rate or net weight loss between HT and TD horses at any observation time. The highest rectal temperature recorded at the end of Phase
C was 39.6 degrees C. These findings suggest that the modified 1 Star Endurance Test was as well tolerated by American horses as the control Horse Trial test. Rectal temperature was significantly higher for E than for TD or HT
at the finish of Phase C. European horses had significantly greater decreases in weight than HT and TD at the end of Phases C and D and the next day. These findings probably reflect the faster and longer work effort of E horses
during Phase C. Modification of Phase C and the rest-pause to ensure that recovery and heat dissipation occurred before the start of Phase D resulted in a 3-day-event that was safe for horses. The Field Trial provides a model
for designing a modified Olympic Endurance Test. If the 1996 Olympic 3-day-event is held in hotter and more humid weather than the Field Trial, additional modifications to the Endurance Test (decreased distances, speeds and
numbers of jumping efforts) will probably be required to ensure safety of competing horses.
PMID: 8933091, UI: 97087062
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Hydration effects on physiological strain of horses during
exercise-heat stress.
J Appl Physiol 1998 Jun;84(6):2042-2051
Geor RJ, McCutcheon LJ
Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada N1G 2W1. geor.1@osu.edu
This
study examined the effects of hyperhydration, exercise-induced dehydration, and oral fluid replacement on physiological strain of horses during exercise-heat stress. On three occasions, six horses completed a 90-min exercise
protocol (50% maximal O2 uptake, 34.5 degrees C, 48% relative humidity) divided into two 45-min periods (exercise I and exercise II) with a 15-min recovery between exercise bouts. In random order, horses received no fluid (NF),
10 liters of water (W), or a carbohydrate-electrolyte solution (CE) 2 h before exercise and between exercise bouts. Compared with NF, preexercise hyperhydration (W and CE) did not alter heart rate, cardiac output (Q), stroke
volume (SV), core body temperature, sweating rate (SR), or sweating sensitivity during exercise I. In contrast, after exercise II, exercise-induced dehydration in NF (decrease in body mass: NF, 5.6 +/- 0.8%; W, 1.1 +/- 0.4%;
CE, 1.0 +/- 0.2%) resulted in greater heat storage, with core body temperature approximately 1. 0 degrees C higher compared with W and CE. In exercise II, the greater thermal strain in NF was associated with significant (P <
0. 05) decreases in Q (10 +/- 2%), SV (9 +/- 3%), SR, and sweating sensitivity. We concluded that 1) preexercise hyperhydration provided no thermoregulatory advantage; 2) maintenance of euhydration by oral fluid replacement (
approximately 85% of sweat fluid loss) during exercise in the heat was reflected in higher Q, SV, and SR with decreased heat storage; and 3) W or an isotonic CE solution was equally effective in reducing physiological strain
associated with exercise-induced dehydration and heat stress.
PMID: 9609799, UI: 98278875
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Use of a semi-quantitative sweat test in thoroughbred horses.
J S Afr Vet Assoc 1992 Dec;63(4):162-165
Guthrie AJ, Van den Berg JS, Killeen VM, Nichas E
Equine Research Centre, Faculty of Veterinary Science, University of Pretoria, Onderstepoort, Republic of South
Africa.
A practical test for evaluating the sweating response to various concentrations of the specific beta 2 agonist, salbutamol sulphate, is described. The results of performing this test on horses (n = 54)
considered to be "free sweaters", horses (n = 6) that showed signs of heat stress following exercise, and horses with complete anhidrosis (n = 2) are presented. The results indicate that intradermal injections of 0.1
ml of salbutamol sulphate at dilutions of 10(-7) or less are suitable stimuli to elicit a visually detectable local sweating in horses with a normal sweating response. Horses that only sweated at the sites where salbutamol
solutions with dilutions of between 10(-4) and 10(-6) were injected, probably had a reduced sweating response and could be considered to be suffering from partial anhidrosis. Horses with severe anhidrosis of long duration did
not sweat, even in response to salbutamol solutions with dilutions of 2 x 10(-3).
PMID: 1491421, UI: 93148286
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Environmental effects on thermoregulation and nutrition of horses.
Vet Clin North Am Equine Pract 1990 Aug;6(2):355-372
Cymbaluk NF, Christison GI
University of Saskatchewan, Saskatoon, Canada.
Horses are reared in all types of weather. Temperatures as diverse as -40
degrees C to 40 degrees C are tolerated by horses. The nutrient requirement most influenced by cold weather is energy. In cold weather, feeding good quality hays free-choice is usually sufficient for mature horses in good body
condition. Grain may have to be fed when poor quality hays are used. Hot weather (greater than 30 degrees C) necessitates heat loss to maintain body core temperature. Horses sweat to reduce body heat. Heat stress can be
minimized by feeding diets that reduce the heat increment. Use of grain and fat in the diet, which have a lower heat increment than fibrous feeds such as hays, may benefit horses in hot climates. Wind or precipitation
necessitate protection to minimize chilling in cold weather and discomfort in hot weather.
However, it is a moot point whether horses will use shelters under these circumstances.
Publication Types: •Review, tutorial
PMID: 2202497, UI: 90360308
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Dietary fat affects heat production
and other variables of equine performance, under hot and humid conditions.
Equine Vet J Suppl 1996 Jul;22:24-34
Kronfeld DS
Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061-0306, USA.
Does dietary fat supplementation
during conditioning improve athletic performance, especially in the heat? Fat adaptation has been used to increase energy density, decrease bowel bulk and faecal output and reduce health risks associated with hydrolysable
carbohydrate overload.
It may also reduce spontaneous activity and reactivity (excitability), increase fatty acid oxidation, reduce CO2 production and associated acidosis, enhance metabolic regulation of glycolysis,
improve both aerobic and anaerobic performance and substantially reduce heat production. A thermochemical analysis of ATP generation showed the least heat release during the direct oxidation of long chain fatty acids, which
have a 3% advantage over glucose and 20 to 30% over short chain fatty acids and amino acids. Indirect oxidation via storage as triglyceride increased heat loss during ATP generation by 3% for stearic acid, 65% for glucose and
174% for acetic acid. Meal feeding and nutrient storage, therefore, accentuates the advantage of dietary fat. A calorimetric model was based on initial estimates of net energy for competitive work (10.76 MJ for the Endurance
Test of an Olympic level 3-day-event), other work (14.4 MJ/day) and maintenance (36 MJ), then applied estimates of efficiencies to derive associated heat productions for the utilisation of 3 diets, Diet A: hay (100), Diet B:
hay and oats (50:50) and Diet C: hay, oats and vegetable oil (45:45:10), the difference between the last 2 diets representing fat adaptation. During a 90.5 min speed and stamina test, heat production was estimated as 37, 35.4
and 34.6 MJ for the 3 diets, respectively, an advantage 0.8 MJ less heat load for the fat adapted horse, which would reduce water needed for evaporation by 0.33 kg and reduce body temperature increase by about 0.07 degree C.
Total estimated daily heat production was 105, 93 and 88 MJ for the 3 diets, respectively, suggesting a 5 MJ advantage for the fat adapted horse (Diet C vs. Diet B). Estimated intake energy was 348, 269 and 239 MJ for the 3
diets, respectively, and corresponding daily intakes as fed were 22.2, 16.6 and 12.9 kg, an advantage of 3.7 kg for the fat adapted horse. Water requirement was estimated to decrease by about 6 kg/day in the fat adapted horse:
4 kg less faecal water output and 2 kg less water for evaporation. This model indicated that the fat supplemented diet reduced daily heat load by 5%, feed intake by 22%, faecal output (and bowel ballast) by 31% and water
requirement by 12%. The advantage of fat supplementation over hay and oats was in general about half that gained by hay and oats over hay alone.
Publication Types:
•Review, tutorial
PMID: 8894547, UI: 97049819
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Selective brain cooling in the horse during exercise and
environmental heat stress.
J Appl Physiol 1995 Dec;79(6):1849-1854
McConaghy FF, Hales JR, Rose RJ, Hodgson DR
Department of Animal Health, University of Sydney, New South Wales, Australia.
Five horses were exercised on a
treadmill [to central blood temperature (Tcore) approximately 42.5 degrees C]. Three of those horses were heated at rest in a climate room (53 degrees C, 90% relative humidity) (to Tcore approximately 41.5 degrees C).
Temperatures were measured in the rectum, hypothalamus (Thyp), cerebrum, and cavernous sinus (Tsinus), on the skin of the head and midside, and Tcore. When Tcore increased above 38.5 degrees C, Thyp remained 0.6 +/- 0.1 degree
C (SE) lower during heat exposure and 1 +/- 0.2 degrees C lower during exercise. During heat exposure, Tsinus was 2.2 +/- 0.4 degrees C below Tcore, and during exercise, Tsinus was 5 +/- 0.9 degrees C below Tcore. Upper
respiratory tract bypass during exercise in one horse resulted in substantial reductions in Tcore-Thyp to 0.4 +/- 0.3 degrees C and Tcore-Tsinus to 0.9 +/- 0.2 degrees C. Thus the horse, a species without a carotid rete, can
selectively cool the brain during exercise or heat exposure; this occurs, at least in part, via cool blood within the cavernous sinus, presumably resulting principally from cooling of venous blood within the upper respiratory
tract.
•Comment in: J Appl Physiol 1995 Dec;79(6):1847-8
PMID: 8847243, UI: 96245878
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