Effect of Cooling Methods in Hot and Humid Environments

Effect of Cooling Methods in Hot and Humid Environments

As we head into summer, riding can present some additional challenges for riders and horses.  There are some excellent sources that you can access to help you when you encounter overheating with your horse.  AAEP has a good paper on heatstroke and how to manage it.  Go to https://aaep.org/horse-owners/horse-health.

As with most things, I believe minimizing the chance of something going wrong is better than having to “treat” the issue.  Emergencies do occur and knowing how to deal with them is crucial.  That is one reason I suggest having your veterinarian on speed dial but knowing what to do until they arrive is critical.  They can often “walk” you through various procedures before they arrive and tell you if they need to come out in person.  DON’T panic and know the normal vital measurements on each of your horses.

In an earlier post we discussed water, electrolytes, and the performance horse.  Here is an excerpt from that posting relative to ambient temperature and humidity.  The horse dissipates this heat load primarily through evaporative cooling.  Sweating and the respiratory tract account for 55% to 60% and 25% of the evaporative cooling, respectively.  Air movement helps with evaporative cooling.  While the horse will still lose heat via evaporative cooling at ambient temperatures greater than body temperature, high humidity severely depresses the evaporative efficiency.  In the case of high temperature and high humidity, conductive cooling plays a more important role.  One way to “visualize” conductive cooling is a dog lying in a cool stream or on a cool floor.  Body heat is transferred from the dog to the water or cooler area.

The comfort index is the sum of the ambient temperature and relative humidity. This is an indicator of the effectiveness of evaporative cooling.  A value of less than 130 has no negative effect on cooling.  Greater than 150, with the humidity contributing 50% of the total (RH over 75%) may compromise cooling efficiency.  When the comfort index is over 150, there is a potential for overheating and extra caution should be observed.  Values over 170 suggest horses be asked for minimal exertion and over 180 there is no heat loss, hence no forced exercise should be done.  The ability to dissipate heat may also be decreased in horses in poor condition, have a heavy hair coat or overly fat.  One paper in the literature reported that horses with a BCS of >6 fatigued sooner than those with a BCS between 4 and 6 due to the inability to dissipate heat.  These same conditions may also increase heat production, adding to a heat loss problem.

There is a very recent article in the Journal of Veterinary Science related to cooling horses post exercise.  This work compared 5 different method of cooling Thoroughbreds in hot and humid conditions.  The methods used included: 1) control (CONT) where the a fan blowing about 4 miles/hour was used post-exercise; 2) cooling with fans ((FAN), fan blowing about 7 miles/hour; 3) cooling with ice water (temp = 50F) poured every 3 minutes to both sides of the horse’s trunk and as much water as possible scraped off (ICW + Scrape); 4) protocol for ICW + Scrape was followed except the water was not scraped off; 5) showering with tap water (SWT) (Photo 1).

Measurements included pulmonary artery temperature (TPA) and rectal temperature (TREC).  The time needed for the TPA to fall below 102F was the primary measurement.  TPA and TREC were measured at 0, 15, and 30 min post-exercise.

All treatments resulted in a decrease in TPA over time.  The times for the TPA to fall below 102F were about 25 minutes, 15 minutes, 12 minutes, 10 minutes, and 2 minutes for the CONT, FAN, ICW + Scraping, ICW, and SWT, respectively.  (Fig. 1).

The amount of water used in SWT was about 950 gallons with a temperature of about 79F.  The ice water had a temperature of about 59F and approximately 5 gallons was used.  The larger volume of water in the SWT horses resulting in lowering the both than TPA and TREC than the other means due to the tremendous amount of water.  Water is a much better conductor of heat than air and the volume of water aided in removing heat.  There was another paper in the literature that reported a drop in body temperature with the use of about 50 gallons applied intermittently.

Photo 1. Horses were washed down continuously for 30 minutes with tap water delivered from 5 hoses attached to a shower.
Fig 1. Time from start of cooling period (post-exercise) until TPA fell to less than 102F.