Products & Articles

Anhidrosis in the Horse; Thoughts and Considerations

Anhidrosis is a common problem, impacting upwards of 10% of horses, which are usually subjected to high heat and humidity for prolonged periods of time.  It is an important condition in the horse as its presence can impact stamina, recovery, and overall ability to exercise.  Sweat production is vital for the body, not only to aid in detoxification, but also in thermoregulation.  When your horse cannot sweat they can quickly overheat.  Solutions for anhidrosis are sporadic with some of these therapies helping a few horses while others do not benefit.  Hopefully, if you take a deeper look into this condition you can arrive at some potential options to improve management.

Horse Sweat and Anhidrosis
Horse Sweat and Anhidrosis

Anhidrosis in the horse is a reduced ability to sweat.  The pathophysiology behind the condition is still rather confusing and even with more in-depth knowledge, we still do not know exactly why it occurs. Sweating is of benefit to the body, not only as a means of detoxification, but also to thermoregulate and maintain body temperature.  During exercise or when exposed to heat for prolonged period, the body should sweat as the body temperature rises.  This is a normal and vital response.  If your horse’s body cannot regulate the body temperature then bad things can happen.

One of the most obvious clinical signs associated with anhidrosis is simply a lack of sweat or decreased amount of sweat produced given a certain level of exercise and considering the environmental conditions.   One of the most critical factors needed is an elevation of body temperature which is produced secondary to a certain level of work by the body.  The level of exercise is important and in many cases of a light workout we wouldn’t necessarily expect sweat production, however, with more intense work, especially on a hot and humid day, we would expect it.  However, there are different levels of fitness between horses, which can also dictate the need for sweat production.  So, aside from lack of obvious sweat, you can look for other signals that there is a problem.  These signs generally revolve around an inability to cool down post-exercise and to recover properly.  Signs would include a persistently elevated heart rate, respiratory rate, and distress. Many of these horses cannot be worked during prime times of the day, resorting to early morning or evening hours due to cooler environmental temperatures.

Anhidrosis has been noted in equine literature for quite awhile.  I believe one of the first notations of the condition was in Thoroughbreds shipped from the United Kingdom to more tropical islands.  Thus, at least initially, the condition was believed to only impact Thoroughbreds.  Obviously, looking at today’s equine society we know this isn’t true.

Diagnosis of Anhidrosis in the Horse:

Diagnosis of anhidrosis is done by evaluating clinicals signs, such as those mentioned above, but can also be ruled in or out via an intra-dermal injection of epinephrine or terbutaline, which stimulates the sweat glands.  A lack of response to the injection can be suspect while obvious sweat production usually rules out the presence of anhidrosis.

Current Therapies for Anhidrosis in the Horse:

Current therapies utilized for anhidrosis are purely supportive in nature and include daily use of electrolytes, magnesium, and some amino acids which have shown benefit.  However, some horses respond to these therapies, while in others there is no clinical benefit.  In some cases, a horse may respond to the therapy this year, but then next year there is a reduced or no response to that same therapy.  Overall, there is no proven therapy for anhidrosis, but more or less, we approach the condition in a prophylactic manner, trying to keep the patient cool and hydrated.  Again, we are back to working during less prime times of the day, using cold water baths post exercise and making sure electrolytes and fluids are balanced.

What Do We Know About Equine Anhidrosis?

Quite honestly, not a lot.  Looking at research that is available, there is really only about 1 1/2 pages listed on PubMed, which is not much.  Not sure why, really?  Lack of interest or maybe a complete stumbling block?

What we do know is that the sweat glands are under stimulation of epinephrine to produce sweat.  Much like other cellular processes, each cell has receptors on its surface, which when stimulated, produce a certain function.  In this case, epinephrine is the stimulant and reacts with epinephrine receptors on the surface of sweat gland cells, which then triggers sweat production. Plain and simple, really.  The sweat glands, like other cells in the body, are also dependent on energy production (ATP) to do their job. Calcium is also involved in the sweat gland normal function, demonstrating a rise in intracellular calcium at the time the cell is producing sweat.  So, we have a few factors involved in sweat production; epinephrine, energy (ATP), and calcium. Likely, there are other factors involved on different levels.

When the body temperature increases, we also see an increase of blood circulation to the skin.  This process is also under hormonal control, in addition to body temperature rise and likely heart rate. This is obvious to any horse owner, seeing a horse post work, full of sweat and having numerous blood vessels at the level of the skin noticeable. These blood vessels are really delivering blood and fluid to the sweat glands, acting as a thermoregulatory mechanism themselves. The closer the blood vessels or circulation is to the surface, the more easily it can dissipate heat.

Overall, research seems to tell us that anhidrosis is a result of desensitization of the sweat glands to epinephrine.  Basically, this implies that due to prolonged stimulation by epinephrine and prolonged exposure to high temperatures and humidity, the sweat glands simply fail to respond any longer. In some cases, there is actually a decreased production of cellular receptors, thus reducing the ability of those cells to respond. Essentially, it is like crying wolf constantly and eventually, the body fails to respond appropriately.  In some points of view, we could see this as exhaustion on some level, which may be important.

One of the most interesting things about anhidrosis is that it generally impacts horses exposed to high temperatures and humidity for prolonged periods of time.  Many of the horses impacted are in the deep south, but this is not always true as high temps and humidity impact other areas of the United States which are generally perceived as cooler. So, we know that heat and humidity are a factor, but the exposure apparently needs to be prolonged in order to generate the condition. Another interesting notation is that many of these patients, if moved to a cooler and less humid climate, they tend to resume sweating, meaning they appear to return to normal.  This is not always the case, which will be discussed.

Now, I will admit, before going further, that as a veterinarian, I did not and have not encountered this condition often in my 20 years of practice, in my patients.  Maybe it is because of our location, being in the foothills of mountains or maybe it is our general clientele.  In either case, I do not have much direct experience with anhidrosis, but do assist owners outside of our area and overall, ‘feel your pain’ when it comes to finding a solution.  What I have to say, is purely opinion, theory really.  The reason is that in order to find solutions, we have to start with theories, and mainly, the research that has been done before us really has little to offer.

Concepts & Theories Regarding Anhidrosis in the Horse:

There are many theories that I have regarding this condition of anhidrosis.  In humans, when exposed to high temperatures and work, we expect sweat and if we are outside and suddenly fail to sweat, this is a medical emergency, viewed potentially as a heat stroke.  We know, as people, that when exposed to high temperatures and humidity for prolonged periods, it drains us of vital energy.  We feel depleted and in reality, this energy depletion may lead us from one extreme of producing sweat one minute to not producing sweat the next, heat exhaustion.  Of course, we also have a dehydration factor involved, and if we oversweat, we can become dehydrated, and if body fluids are low, then circulation to the skin for sweat production will be low likewise.

In many of the horses that I have assisted, dehydration is a factor, but not necessarily a cause…more a consequence. Taking into consideration that many of the affected horses are high level performance athletes, we then have to take into consideration their level of work, time frame, level of health/fitness and likewise energy production.  A horse that is exposed to high temperatures and humidity for years, in addition to high levels of energy output, may actually lose the ability to sweat.  Sort of makes sense in my head.  Energy is lost, not only in battling or enduring ongoing heat and humidity, but also through high levelf of training or exercise, especially over time. If more energy is utilized than is being produced in that body, then it has a reduced ability to adapt or respond.

Now, some would then respond that a horse in New Jersey or Massachusetts has anhidrosis, but is not worked and only in pasture.  This is actually common, but again, we have to look at that patient.  In many of these cases, that horse has concurrent health problems, whether if they are noted by the owner or not.  These health ailments can be metabolic syndrome, laminitis, allergies, or respiratory problems, including EIPH.  To me, this is potentially noteworthy, for if we take into consideration that chronic health ailment, we can then see that energy depletion on behalf of recovery, may prove to be a factor.

To be honest, stress, whether if that is physical or mental, is an energy drain on the body, especially if ongoing.  We can all attest to this fact, as we often feel depleted after a heavy workout or exhausted after weeks of stress at the office or in our family life.

In most of the horses that I consult with, I view the presence of anhidrosis as being the ‘icing on the cake’, more or less a result of progression in the health failure in that patient.  I view anhidrosis as a sign.  A sign that we have some major health problems and an inability of that patient to adapt or respond. Again, many of these patients also have ongoing or chronic health ailments, which have to be taken into consideration, seeing those health conditions as ‘drains’ on the system.  In many, those other health ailments have been present for a few years, not managed properly, and then this year the owner notes that the horse is not sweating as well.  I view this as condition progression, or really health deterioration, over time.  The owner may only be interested in the fact that the horse does not sweat, while I am more interested in the other health condition(s), what they mean and how to better manage them.

Now, with all of that being said, we have had luck traditionally with a few approaches in these patients. In many, my goal is to manage inflammation and support the body through nutrients and secondarily, energy production.  Many of the herbs we use, also help to ‘cool’ the body and reduce heat, which is really an anti-inflammatory type of action.  The main formula that we have relied on in these patients is the Cur-OST® EQ Total Support.  In addition, one of my goals is to balance the stress response in that patient, helping to manage cortisol.  Here, we will also use our Cur-OST® Adapt & Calm formula. If we think these patients are not under stress, think again.  

This approach has benefited many patients, but is not the solution for all.  I think that those that do respond are the horses that have not progressed too far, their problem is more easily remedied, while others are more deep rooted.  It is still a good first line approach and often a baseline approach to guide us moving into future options.

Energy and lack of production, within the body, is one theory.  When we look back at how the sweat glands function, this theory is not far removed, as energy or ATP is directly involved.  If the body reaches a point of exhaustion, usually due to a combination of factors, including prolonged high temperature exposure, intense training and possible co-existing health problems, then sweat production is often one of the last things on the body’s mind.  It wants to conserve energy, focus it to vital organs to ensure their function. All of these put a tremendous drain on the body, depleting vital energy.

Another factor that is interesting to me is the hindgut and possible bacterial derrangements in this area.  In humans and pets, we know that heat exhaustion can and does influence gut health, not only influencing shifts in bacterial populations, but also leading to ‘leakiness’ or changes in gut wall permeability.  These changes can have dire consequences for the patient and at high extremes, contributing to major health problems and toxemia on various levels.

This concept of bacterial dysbiosis or imbalance is one we have discussed in depth, noting in research (ours and others) that in many equine health conditions, there is a noted bacterial imbalance, often with overgrowth of lactic acid producing bacteria.  We have also discussed how this influences health, digestion, stamina, immune health and inflammation production.  There is a direct connection.

In almost all anhidrotic equine patients that we have cultured, as part of our ongoing research, they have all demonstrated a bacterial imbalance to varying degrees, often with tremendous overgrowths of lactic acid producers.  What are the implications? Is this imbalance a cause or an effect?  What causes this imbalance?  These are all questions that we do not know the answers to, but in preliminary studies, it seems to be both cause and effect, being pushed along by many contributors from diet to stress.

One thing that I can say, in making observations, is that a high percentage of anhidrotic horses have a complicated supplement and feed regimen, often utilizing high levels of processed grains and numerous synthetic based supplements.  The correlation is that in our studies, these two habits are often correlated with more bacterial changes and potential damage to the hindgut.  Could we actually be contributing to the problem?

Stepping back from anhidrosis, we have been evaluating this ‘energy depletion’ and bacterial imbalance in our research herd and rehabilitation patients. We are not dealing with anhidrotic patients in our group, but we are dealing with chronic ailments from tendon injuries, sore feet, laminitis and metabolic conditions.  Again, these chronic conditions, if not resolved, can be a constant drain of energy on the body, which then can impact healing.  In a high percentage of these patients, we are also seeing a bacterial imbalance in the feces, which is reflective of a problem on a different level, potentially contributing to inflammation and poor overall recovery, including energy.

In these patients, we have been using two new supplements that target energy production and also aid in balancing hindgut bacterial populations.  These formulas are the Cur-OST EQ Revive and the Cur-OST EQ Rejuvenate. What I am seeing in these patients, in less than 2 weeks, is not only an improvement in energy in that patient, but improved clinical condition (soundness) and a more balanced bacterial population in the hindgut or feces.  Is it possible that these patients, much like potentially anhidrotic patients, were actually just energy depleted?  Is it possible that due to this energy drain, exhaustion, that their healing potential was reduced?

Anything is possible, and again, when we look at the circumstances surrounding most anhidrotic patients, this theory is not far fetched.  It may also explain why some respond to simple electrolyte remedies, which do improve cellular function and energy production on a basic level, but may also explain why more advanced cases fail to respond…as they have gone beyond those simple approaches, their condition has progressed further.

Another approach we have been using is via herbs to directly ‘cool’ the body down, which include artichoke, blueberry, and aloe extracts.  These can be found in the Cur-OST EQ Cool Down formula. Interestingly enough, not only do these herbs have a ‘cooling’ energy to them, but pack a punch regarding nutrients and antioxidants, which helps to promote and protect cellular health and may reduce long term damage associated with anhidrosis.  All 3 of these herbs also possess potent pre-biotic properties additionally, which helps to support healthy digestion and microbial balance.  One more area that is likely tied closely to these cases.

At this time, I have no plans to investigate these theories in anhidrotic patients, as they are just not accessible in my region, however our current approaches will likely be available pending more data gathering in the following weeks.  Considering the apparent high demand for solutions for anhidrotic patients and the lack of efficacy in current therapies, I do think that we need to take a different approach.

Looking back at those horses that respond or improve when moved to a cooler climate, is this due to decreased stress put upon the body by the temperatures, giving the body a break, a chance to recover?  Or is it also that many of these horses are actually rested when moved back north, which again, may allow for a break, reduction in stress, improvement in energy production and thus recovery?

We are all looking for a one step solution, a quick fix, but in reality, I believe anhidrosis goes much deeper and is a sign that a deeper level of therapy is needed for that patient.  It is likely not a one-step fix in any patient, but more so different approaches are taken to support that horse and reduce the pathways that have led to the creation. 


Recommended Article:  The Hot Horse, Health, and Cooling them Down.


Author:  Tom Schell, D.V.M, CVCH, CHN





Leave a Comment

Your email address will not be published. Required fields are marked *