Water and Dehydration Study Clarification

A 2008 Equine Veterinary Journal (EVJ) publication examining dehydration in working horses was summarized and printed onTheHorse.com on April 24, 2008, prior to its actual publication in EVJ. It was reviewed and approved by the researcher. The complete publication (in which Dr. Joy Pritchard was first author) was titled "Validity of indicators of dehydration in working horses: a longitudinal study of changes in skin tent duration, mucous membranes, and drinking behavior."

Recently, Dr. Olin Balch, a member of Veterinary Committee of the American Endurance Ride Conference (AERC) contacted Pritchard and TheHorse.com with committee concerns that the summary could be misinterpreted, especially where endurance horses are concerned.

Balch and Pritchard coauthored the following clarification.

Pritchard said, "My research studied changes in physiological parameters in 50 working horses pulling carts or carrying loads at moderate speeds in Lahore, Pakistan, over a period of 5 hours. While these results could be generalized to similar horses in developing countries, the conclusions are not directly transferable to any other horses, endurance or otherwise.

"The results illustrate that--in the sample population studied--neither a set of standardized skin tent tests, nor gingival mucous membrane dryness, nor other clinical parameters including heart rate, respiratory rate, and coat dryness/wetness were associated with hydration status," she continued. "Only volume of water drunk and drinking behavior were significantly associated with hydration status in these animals.

"The EVJ paper states clearly in the discussion and conclusion that working horses may not drink for internal and external reasons. Lack of drinking is not a conclusive sign of normal hydration status in the animals studied."

Balch said, "As a member of the Veterinary and Research Committees of the AERC, I believe that Dr. Pritchard has written a very important paper emphasizing that classic indicators of dehydration in horses (such as skin tent duration and mucous membrane dryness) may be quite misleading. The AERC sanctions approximately 23,000 horse starts in well-organized, veterinarian-supervised endurance rides in the U.S. and Canada annually.

"It is our collective experience, as experienced endurance veterinarians, that it is extremely common for horses in endurance rides to become dehydrated and yet refuse to drink water," continued Balch. "This is especially common early in rides where an excited horse may travel 25 miles without taking a drink. Additionally, we are convinced that exercising horses who refuse to drink water and become extremely dehydrated run the very real risk of suffering metabolic disorders. That is why riders must be attuned to their horses' water intake and take care to assure their horses are drinking adequately during endurance rides. The consequences of severe ileus or other forms of colic following dehydration and electrolyte depletion, although extremely rare at endurance rides, may lead to death.

"Physiologically, this is very understandable," he continued. "The equine thirst response is primarily prompted by an increase in the plasma osmolality (or plasma sodium concentration). Equine sweat has high concentrations of electrolytes (particularly sodium, chloride, and potassium).

"Consequently, horses in strenuous exercise can lose significant fluid and associated electrolytes through appropriate sweating (especially in hot weather)," he said. "Ironically, when horses sweat significantly and lose considerable amount of sodium, their thirst response may be depressed and the horse will not drink adequate fluids to maintain a desirable state of hydration. Subsequent loss of both fluid and ions initially 'defeats' the thirst response even though the horse is clinically dehydrated.

"In our sport, to stimulate drinking and replace electrolytes lost in sweat, endurance riders routinely supplement horses with oral electrolyte pastes during and after the ride."

Balch continued, "Of particular interest, Dr. Pritchard clarifies that the 'gold standard' of dehydration in working horses was the change in osmolality over time (i.e., osmolality could only be used longitudinally and retrospectively, not as a 'snapshot' measure of hydration). As Dr. Pritchard says, this may not apply to endurance horses where the combination of underlying anaemia, hypoproteinaemia, and electrolyte depletion and chronic water deprivation are not such an issue as they are in working equids in developing countries.

"Dr. Pritchard makes a very convincing scientific argument not to oversimplify the clinical evaluation of equine hydration. Her conclusions fit well with the collective judgment of the Veterinary Committee that determination of which physical parameters (or set of physical parameters) indicate pending or clinical dehydration can be very challenging. Determination of the most predictive risk factors for dehydration and subsequent metabolic derangement--resulting in colic--in the endurance horse merits further research."

Important take-home lessons:

1. Drinking water confirms that a horse is dehydrated.

2. Dehydrated horses might not drink spontaneously.

3. Classic indicators of dehydration--such as prolonged skin tent and mucous membrane dryness--can be misleading and are always best interpreted by examining the whole horse.

4. Intensely working/sweating horses exercising for lengthy periods (such as endurance horses) might refuse palatable water despite significant dehydration.

5. With very few exceptions, horses should always be allowed to drink freely and whenever desired.

This clarification was co-authored and reviewed by Joy Pritchard, BVM&S, PhD, CertWEL, MRCVS, University of Bristol, and Olin Balch, DVM, MS, PhD, AERC Veterinary Committee member, AERC Research Committee chair.

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