Horses are very athletic animals. They are in an intense exercise routine to keep their bodies in the best condition. These exercises produce a huge amount of heat, and therefore if the heat-reducing mechanisms get impaired, that becomes a significant health issue for them. Usually, we can see horses pouring sweat from their abdominal area and a foamy lather on either side of the neck and in between the hind limbs caused by sweat after the end of an exercise session.
Various thermoregulatory mechanisms are there to regulate the core body temperature; heat produced in the body is transported to the body surface and dissipated to the environment to maintain a stable body temperature. These mechanisms could be either voluntary or involuntary. Some involuntary mechanisms that help reduce the body temperature are sweating, peripheral blood vessel dilation (in the skin), panting, and a decrease of basic metabolic rate. However, in horses, the main mechanism of heat loss is sweating, which is about 65%-70% of the total body heat. Once the sweat comes out of the sweat glands to the body surface, the sweat is evaporated using energy in the form of heat. That way, heat is lost, and the body temperature comes back to a normal level. Horses can move into shady areas if there is a sweltering climate. It is a voluntary mechanism.
In the dermis of horse skin, sweat glands are densely packed as about 810 glands/ cm3. The sweat glands are simple coiled tubular glands that open to the skin surface close to hair follicles. Unlike sebaceous glands, sweat glands don’t open to the skin surface along with a hair follicle.
As we discussed so far, sweating is essential for horses to keep their body temperature at an average level. But some horses have impaired sweating. This is called anhidrosis. Anhydrotic horses are also known as “non-sweaters” and “dry-coated.” The poor and dry-looking condition of the coat has given anhidrosis another name as “Dry coat syndrome.”
Anhidrosis limits the performance of the horses. This has a big adverse effect on the owners, especially when it comes to performance horses. Anhidrosis can also lead to hyperthermia (Abnormally high body temperature) and dangerous situations such as heat strokes, which can sometimes be fatal if not treated immediately. Anhidrosis is common among horses during the summer time in Florida and the gulf coast states where the nighttime temperature doesn’t go below 27C. According to a study done on anhidrosis, out of 834 thoroughbred horses in farms of Central Florida, 2%-6% were affected. This study could also come up with supporting facts to link anhidrosis and the activeness of the horse. More horses diagnosed with anhidrosis were racehorses and non-pregnant broodmares, while young horses were less frequently affected. A more recent study based on 4600 horses in Florida stated that 2% had anhidrosis. Generally, southeast Asian countries have hot and humid environment. This climate does not help to remove the heat from the horse body to the outside. It predispose to develop anhidrosis in horses.
Although these horses are commonly called non-sweaters, the degree of the problem varies among the animals. Some may have completely stopped sweating, making them show severe signs of hyperthermia. Some may sweat, but not as much as a healthy horse. These horses may show only subtle clinical signs of hyperthermia. There are also cases in which the sweating is impaired only in certain areas while the rest of the skin sweats at the normal level. As horses are financially high-cost animals, horse owners should identify the condition as early as possible to avoid unwanted losses to their wallets. Now let’s look at how to identify this condition.
How to identify one
Apart from impaired sweating, the other clinical signs of anhidrosis are elevated respiratory rate, increased rectal temperature that doesn’t come back to a normal level even after 30 minutes from the end of an exercise session. The normal body temperature of a horse is 37.5C - 38.3C. After exercising, this value gets increased significantly. But usually, it comes back to the normal range within 30 minutes. So when it takes longer time than 30 minutes, there is something wrong with the animal’s thermoregulation. That is why this becomes a clinical sign of anhidrosis.
Incomplete or partial anhidrosis is the most common form of anhidrosis that can be seen. Although this is common in performance horses, it can be seen in non-performance horses as well. However, the horses with dark skin seem more prone to this problem than those with light-colored skin despite the activeness.
Apart from lack of sweating, there are differences in the clinical signs of acute and chronic anhidrosis. Signs of acute anhidrosis include:
• Labored breathing
• Flared nostrils
• Increased heart rate
• Increased body temperature
• Fatigue
• Collapse
• Onset of heatstroke
Chronic cases are characterized by:
• Increased drinking of water
• Increased urination
• Poor appetite
• Impaired performance
• Dryness of the skin coat
• Scaling
• Regional balding
• Itching
It is important for a horse owner to clearly understand these signs to suspect the condition in the initial stage. If anhidrosis is not cared for properly by a veterinarian, the condition will become severe, and the animal will undergo the risk of developing a heat stroke. Heatstroke is also known as hyperthermia. This condition occurs when a horse cannot lose its’ body heat after performing a high workload like exercises or horse races in hot, humid conditions.
Diagnosis
A horse owner should consider contacting a veterinarian if they understand that the horse’s performance is declined during the hot and humid times of the year. The increased humidity in the air further declines the efficiency of evaporation of sweat. It will be better to get the horse checked for anhidrosis by a veterinarian with equine practice if the signs as leaving the herd to rest in the shade and increased respiratory rate are observed.
A veterinarian can diagnose the condition by injecting a series of injections of diluted terbutaline. Terbutaline can stimulate the sweat glands of the injected region to sweat. This way, the horses with anhidrosis can be diagnosed, and the level of sweating gives information on the severity of the horse’s condition. (https://onlinelibrary.wiley.com/doi/abs/10.1002/9781118939956.ch73)
Also, the vet can perform a skin biopsy which allows microscopic examination of sweat glands and look out for any decrease in the number of sweat glands. This method is less frequently performed compared to the former method.
Treatment
Through proper management, medications recommended by a veterinarian with equine practice and supplements, treating the symptoms that arise with the disease manages the condition.
Various management strategies help the animal to deal with the health issues that come out with anhidrosis. Some of them will be discussed further.
• Creating a cooler environment for the horses during the warm and humid months of the year helps in great deed.
• Providing a shaded area with fans or an accommodation allowing movement of fresh air, introducing water misters, cold water hosing, and air conditioning are some of the procedures that can be done for this.
• Misting the coat and spray the body, neck, and legs help to keep the horse cool in extreme heat.
• As for the coat, the hair should be clipped regularly.
• When grooming the horse, rubbing the skin aggressively and vigorously stimulates blood flow to the skin. When this happens, heat dissipated to the environment through radiation from the peripheral blood vessels helping the body to cool down.
• The exercises can be done in the early morning or late evening when the ambient temperature is lower. Soon after the exercise, the horse should be given enough time to have a proper rest. During this resting time, one should monitor the respiratory rate and check whether it is normal or not.
• Sending horses to pasture for turnout during the nighttime or late evening is also a good practice. At this time of the day, the environment is cooler, and more air movement is present, which causes less sweating.
• Providing plenty of fresh, cool water throughout the day is also one of the key steps to lower the horse’s body temperature. The most common modes of delivering water are the automatic water feeder and a water bucket. Buckets are the best way to determine the water intake. If you can see there is a clear reduction in the water intake, you can monitor the urine color of your horse. If the color is abnormal, meet your veterinarian as soon as possible.
Although the main constituent of sweat is water, it also includes various metabolic excretory products, glycoproteins, surfactants, and a significant amount of electrolytes as Na, K, and Cl. Out of them, the main electrolyte is K. The concentration of electrolytes in the sweat is higher than that in blood. This helps create a friendly environment for the commensal flora of the skin and defense against the invading microbes.
. Blood work, including an electrolyte analysis, helps to develop a treatment plan. Well qualified nutritional specialists should develop this plan, and it should be performed under a qualified veterinarian. The salt solutions are administered through ingestion. This is an electrolyte therapy that includes all of the important electrolytes. In this case, especially sodium is released into the gastrointestinal tract slowly, allowing for sustained absorption.
Electrolyte supplementation gives a quick and remarkable improvement in the anhidrosis condition for some horses. This is an integral part of the treatment plan.
Some researchers also believe that supplementation with Vitamin E is also helpful for anhidrotic horses. This supplement will be more effective if the vitamin comes from a natural source rather than synthetic products.
According to a study done at the University of Florida, Acupuncture and herbal medicine may have an effect in improving sweating. However, this study needs further investigation to be fully validated since the effect lasted less than four weeks from discontinuing the treatment.
If the horse does not respond to any therapy done and does not improve the condition, the horse should be moved to a less hot and humid environment. This will make it comfortable for the horse and help in the restoration of sweating. It is better if measures are taken beforehand to minimize the chance of getting anhidrosis during the summer season.
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