Introduction and Background
Equine anhidrosis describes a condition where horses develop a reduced capacity to sweat. The term comes from the Greek “An” for absence and “Hidros” for sweat. Strictly speaking, this is an inaccurate term as affected horses do not usually lose all ability to sweat, and in the early stages of developing the condition, the reduced sweat production is also abnormal in terms of sweat release over time compared with unaffected horses. So, technically, the condition should be called dyshidrosis, which describes abnormal sweating. This is similar to the terminology in cardiology, where abnormal rhythm is dysrhythmia, whilst arrhythmia means the absence of rhythm. The condition primarily affects horses living in hot, dry, or hot and humid environments. It is often less well recognised in hot, dry environments as sweat evaporates rapidly. However, it is easier to identify horses with a reduced ability to sweat in hot and humid environments where sweat evaporates more slowly.
Importance of Sweating to Horses
Horses rely heavily on sweating to dissipate heat, particularly heat produced during intense exercise, and horses can produce sweat faster than any other animal on the planet. For example, a 500kg horse has around 5m2 of skin surface, and almost all of that surface is covered in sweat glands. Horses have a type of sweat gland known as apocrine sweat glands, where there is a sweat gland at the base of every hair. In contrast, humans have eccrine sweat glands. These may be found both on their own or associated with hairs. In contrast, a human weighing 80kg would have around 2m2 of skin surface. So, although the horse is over six times heavier, it only has 2.5 times more surface area. And as heat is mainly lost at the body surface, the horse is at a disadvantage. Large animal lose heat slowly, small animal lose heat quickly. So, the horse has developed a phenomenal capacity to sweat. Whilst a human can lose 4 litres of sweat per hour, equivalent to 2 litres per m2 of body surface, a horse could lose 30 litres per hour, equivalent to 6 litres per m2 of body surface – that equates to being able to sweat at least three times faster than a person, even when allowing for the difference in size!
Differences Between Horses and Human Sweat
Horse sweat is also different to human sweat in two ways. Firstly, horse sweat is hypertonic. This means the concentration of the major electrolytes (sodium, potassium and chloride) in horse sweat is greater than the concentration in plasma from which the sweat was formed. So, the horse’s sweat glands actually concentrate electrolytes from the plasma in producing sweat, although it’s not entirely clear what advantage this might be. In contrast, humans produce hypotonic sweat – that is, whilst there are electrolytes in human sweat (we know sweat tastes “salty”), there are fewer electrolytes per litre of sweat than there are electrolytes per litre of plasma. Horse sweat also has one other very unusual and unique feature; it contains a protein called latherin. Latherin is a surfactant (think washing up liquid) and helps the sweat spread out along the hairs, increasing the area available for evaporation (as evaporation actually cools the skin) and increasing the amount of evaporation and, therefore, the capacity to cool.
What Triggers Sweating In Horses?
Horses produce sweat due to adrenaline (a beta-2 agonist) reaching the sweat glands in the circulation. Adrenaline is a hormone produced and released into the blood (circulation) by the adrenal glands, which sit on top of the kidneys. Sweat can also be produced by direct heating of the skin. Finally, sweating in certain areas, such as the neck, appears to be under nervous control, although an intact nerve supply is not necessary for sweating as skin taken from a horse post-mortem and placed on a heat plate will be seen to sweat, so neither blood flow nor nerve innervation are required for sweating. Sweating can be studied by using an intravenous infusion of adrenaline or by placing a horse in a heated room. However, interestingly, neither of these approaches produces the sweat rates seen during exercise when both heat and adrenaline are present.
Sweat production was measured continuously on the neck and gluteal region in resting horses in response to increasing concentrations of adrenaline infusion. Mean sweating rate on the neck (SRN) and gluteal region (SRG, g.m2.min-1) during the infusion of 8 different concentrations of adrenaline and for 20 min after the end of infusion in 6 horses. From Scott et al (2001).
Development and Diagnosis of Anhidrosis
When hot and when the environmental conditions are hot, horses rely on sweat evaporation to dissipate around 85% of their body heat, so it’s easy to see how any condition that reduces sweat production could seriously compromise health and welfare.
Anhidrosis may develop slowly or suddenly. Affected horses can be observed not to sweat the same as unaffected horses in the same environmental conditions. In severe cases, reduced sweating may only be seen under the mane, between the legs and on the neck. Other symptoms associated with the development of anhidrosis include panting, reduced appetite, weight loss, decreased thirst, hair loss, poor coat and skin condition and depression. A practical field-based test for anhidrosis involves the injection of different dilutions of a drug called terbutaline (which has the same action as adrenaline). Whereas unaffected horses will be seen to produce sweat at the injection site at low concentrations of terbutaline, while anhidrotic horses require much higher terbutaline concentrations to induce sweating.
From: MacKay, R. J.; Mallicote, M.; Hernandez, J. A.; Craft, W. F.; Conway, J. A. . (2015). A review of anhidrosis in horses. Equine Veterinary Education, 27(4), 192–199. doi:10.1111/eve.12220
An anhidrotic horse shows minimal sweating, mainly only on the neck after intravenous adrenaline infusion. Marlin et al. (1999). Sweating on the gluteal region is depressed much more, often to the extent of no production at all, compared with sweating on the neck (as seen above). Interestingly, the composition of the reduced amount of sweat that is produced by anhidrotic horses following intravenous adrenaline is similar in terms of sodium, potassium, chloride, calcium, magnesium and protein concentrations.
An anhidrotic horse shows minimal residual sweating between the hind legs. Marlin et al. (1999)
What Causes Anhidrosis?
Anhidrotic horses appear to have normal adrenaline levels, and in fact, one study found the levels were higher than in unaffected horses. Furthermore, there is no suggestion that the adrenaline does not reach the sweat glands. However, it appears there is evidence that the receptors that bind adrenaline in the sweat glands become desensitised. Another way to look at this is that with repeated high stimulation, they become “exhausted”. Another feature of the sweat glands of anhidrotic horses have less aquaporin-5; a water channel protein critical for sweat production. However, at this time, whilst it is clear that sweat gland function deteriorates and the glands can atrophy (waste away), what causes a change from reversible into permanent sweating dysfunction is currently not understood.
A large number of factors have been suggested to increase the risk of anhidrosis, many of them purely hypothetical and not proven, including nervous temperament, arsenic based tonics, electrolyte deficiency, low plasma chloride, potassium deficiency, thyroid dysfunction, adrenal dysfunction, vitamin deficiency and infection.
Which Horses are Affected by Anhidrosis?
In hot or particularly in hot and humid climates, it has been estimated that 2-6% of horses may be affected, although on racetracks, it may be as high as 20%. Interestingly, anhidrosis is very rare in Arabian horses; horses that originate from hot climates. Some studies have suggested that Thoroughbreds and Warmbloods are more likely to be affected by anhidrosis. But horses in work are also more likely to be affected. Recently moving to a hot or hot and humid climate from a cooler climate has also been identified as a risk factor. In the author’s personal experience, a previous episode of heat exhaustion may be a predisposing factor. One study reported that the risk of anhidrosis was 5 times higher in showing and 15 times higher in riding schools compared with on ranches. There is also good evidence that predisposition to anhidrosis may be genetic.
Treatment of Anhidrosis
At this time, there is no effective treatment for anhidrosis. Affected horses may begin to sweat more in the cooler months of the year, but their sweating response is always abnormal and never fully recover to be normal. The only real effective management is to maintain them in air-conditioned stables or to move them to a cooler region. If these options are not possible they should only be exercised in the cooler times of the day or times of the year and sprayed down frequently to supply them with “artificial sweat” i.e. water. Lots of treatments have been proposed, but these are none are currently considered to have any evidence of efficacy and include anti-histamines, vitamins, oral or IV electrolytes, thyroid supplements, prostaglandins, alpha-methyl dopa, herbal medication and acupuncture.
Risk of Anhidrosis
Anhidrotic horses are at a very high risk of heatstroke. This can lead to multi-organ failure, shock and death. They, therefore, require very careful management in the hotter times of the year.
Summary
- Anhidrosis is a condition where horses have a reduced ability to sweat and sweat abnormally.
- It can affect 2-6% of horses in hot, humid or hot dry climates but can affect up to 20% of certain populations.
- It appears to be an inherited condition and more common in Thoroughbreds and Warmbloods imported into hot dry or hot and humid climates.
- It is very uncommon in Arabian horses
- Horses in regular and or hard work are usually more at risk
- Diagnosis is usually made with intradermal adrenaline or terbutaline injection, which stimulates sweating
- There is no known cure at this time
- Affected horses must be moved to cooler climates or managed carefully in hot weather, including not exercising them in the heat and frequently hosing/spraying them down.
References
- Jenkinson, D.M., Elder, H.Y. and Bovell, D.L. (2006) Equine sweating and anhidrosis – Part 1- equine sweating. Vet. Dermatol. 17, 361-392.
- Jenkinson, D.M., Elder, H.Y. and Bovell, D.L. (2007) Equine sweating and anhidrosis Part 2: anhidrosis. Vet. Dermatol. 18, 2-11.
- Jenkinson, D.M., Montgomery, I., Elder, H.Y., Mason, D.K., Collins, E.A. and Snow, D.H. (1985) Ultrastructural variations in the sweat glands of anhidrotic horses. Equine Vet. J. 17, 287- 291.
- Marlin, D.J., Schroter, R.C., Scott, C.M., White, S., Nyrop, K.A., Maykuth, P.L. and Harris, P.A. (1999) Sweating and skin temperature responses of normal and anhidrotic horses to intravenous adrenaline. Equine Vet. J. 31, Suppl. 30, 362-369.
- Scott, C.M., Marlin, D.J. and Schroter, R.C. (2001) Quantification of the response of equine apocrine sweat glands to beta2-adrenergic stimulation. Equine Vet. J. 33, 605-612.
- Scott, C.M., Marlin, D.J. and Schroter, R.C. (1997) Relationship between sweating rate and skin temperature responses during incremental exercise in the horse. In: Proc. 1997 Symposium on Thermal Physiology, Nielsen Johannsen, B. and Nielsen, R. (Eds). August Krogh Institute, Copenhagen, Dennmark, 45-48.
- Scott, C.M., Marlin, D.J. and Schroter, R.C. (1996) A modified ventilated capsule for the measurement of sweating rate in the exercising horse. Equine Vet. J. Suppl. 22, 48-53.
- Sneddon, J., Ritruechai, P., Marlin, D., Riggs, C., Sidlow, G., Osbourne, C., & Lloyd, W. (2006, April). Physiological responses of seasonally anhidrotic horses to exercise. In Comparative Biochemistry And Physiology A-Molecular & Integrative Physiology (Vol. 143, No. 4, Pp. S77-S77).
