5 Summer Horse Health Risks

Help keep your horse healthy and get the most out of the coming months by increasing your understanding of—and watching for—five common health conditions seen during the summer.
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horse wearing fly sheet
Understanding what bugs summer brings to your area and monitoring your horse for signs of bites is important. | Photo: iStock

5 conditions that can impact your horse’s health this summer

Summer provides ample opportunity for trail riding, horse showing, and simply moseying around the stable. But with summer comes its trappings: hot temperatures, high humidity, hungry bugs, and strong UV rays. Help keep your equine partner healthy and get the most out of the coming months by increasing your understanding of—and watching for—five common health conditions seen during the summer.

Rebecca McConnico, DVM, PhD, Dipl. ACVIM, a professor at Louisiana Tech University, in Ruston, and Tresha Robinson, DVM, CVA, a veterinarian at Arizona Equine, in Gilbert, share what you need to know about dehydration, sunburn and photosensitization, pasture-associated asthma, insect bite hypersensitivity, and heat stress.


Condition and causes Horses’ risk for dehydration—loss of body water at a rate greater than the body can replace it—increases with rising temperatures. McConnico lists potential causes such as overexertion, overwork, anhidrosis (the inability to sweat), lack of water intake, and any type of illness.

Signs and management If you suspect your horse isn’t getting enough water, check his gums.

“We really assess how well a horse is hydrated based on how slick and moist their mucous membranes are,” Robinson says, adding that dehydrated horses will have dry, tacky gums.

Hydrated gums, on the other hand, are moist and pink. “If you press on the gum, and the color doesn’t come back in less than two seconds, then that usually indicates that they’ve got a clinical dehydration problem where the fluid that’s supposed to be circulating as part of their blood has gotten to the point where it’s too low,” McConnico says. 

It’s important to note that not all dehydration cases are the same; some stem from underlying health conditions rather than summer heat. Either way, if your horse is lethargic, refusing food and water, or has tacky mucous membranes, both McConnico and Robinson recommend calling your veterinarian.

checking a horse
If you suspect your horse isn’t getting enough water, check his gums. A hydrated horse's gums will be moist and pink. | Alexandra Beckstett/The Horse

Prevention You can take a variety of simple steps to prevent dehydration in your horse. Robinson advocates soaking hay or pellets in water to increase water intake and says unlimited access to fresh, clean water is a must.

Alternatively, she adds that electrolyte pastes and powders, table salt and salt blocks, and mineral licks can help stave off dehydration.

For owners adding electrolytes directly to their horse’s water, Robinson suggests providing a second bucket containing just fresh water.

For healthy horses working during the summer, McConnico says adding a salt containing potassium chloride—which horses lose more often when they get dehydrated—to the feed will encourage water consumption and help recover elements lost during ­sweating.

She adds that owners should know how much their horse drinks normally and consult the heat index before riding. If adding the humidity percentage to the outside temperature value (in degrees Fahrenheit) results in a figure above 180, she advises skipping your ride.

On summer days where a workout is appropriate, however, Robinson says a good cool-down can go a long way toward preventing excess sweat loss.

horse with sunburned nose,
Sunburn typically occurs in lightly pigmented areas, and chronic cases can lead to skin cancer. | iStock

Sunburn and Photosensitization

Condition and causes Both of these problems involve your horse’s skin, but the causes are vastly different. Sunburns, of course, are caused by prolonged ultraviolet light exposure and typically occur in vulnerable places on the body, such as lightly pigmented areas. Chronic sun damage can lead to skin cancers, says Robinson, as it does in humans.

Photosensitization, on the other hand, takes place when toxins in the horse’s skin interact with UV rays, causing severe dermatitis, or skin inflammation. McConnico notes that ingesting something toxic (typically a plant) can cause primary photosensitization, while a malfunctioning liver causes secondary photosensitization (in which case the organ is unable to excrete phylloerythrin—which is what chlorophyll is broken down into in the body—so the compound reacts with UV light).

Signs and management Problem spots for sunburn include the muzzle and the eyes, says Robinson, who sees a lot of cases in Arizona due to the intensity of the sun. “The skin becomes inflamed, swollen, raised, and bunched up kind of like an accordion,” she says.

If your horse has sustained a mild sunburn, McConnico recommends cleaning it with a gentle soap. If it appears serious, call your veterinarian to rule out further treatment needs and photosensitization, as well as assess skin cancer risk. Robinson reminds owners to keep the horse in the shade to prevent further damage.

Photosensitization plays a more aggressive role than a one-time sunburn. “It’s almost like a chemical burn injury,” McConnico says. She describes clinical signs including redness, itching, crusting, and sloughing of the skin.

“If a horse has never shown a sensitivity to sunlight and has a sudden or severe reaction or sensitivity, call your veterinarian,” Robinson says. Your vet can do bloodwork to rule out liver problems and recommend treatment, she explains.

Prevention To protect horses from sun damage, McConnico and Robinson advise horse owners to keep at-risk animals—those with white markings/pink skin on the face or the rest of the body—out of the sun, especially during the hottest parts of the day. They add that flymasks, fly sheets, and sunblock help protect especially sensitive areas from damaging UV rays.

Regarding photosensitization, both veterinarians suggest monitoring your pasture for problematic plants such as alsike or red clover, St. John’s wort, ragwort, perennial ryegrass, and buckwheat. McConnico adds that a yearly blood test is a good way to screen for liver problems in susceptible horses.

Pasture-Associated Asthma

Condition and causes If your horse struggles to breathe in the pasture on warm summer days, pasture-associated asthma (previously called pasture-­associated heaves) could be at play.

“Pasture-associated asthma in horses occurs secondary to an allergic response to something in the pasture” that triggers airway constriction, McConnico says. She lists molds and dust as two of the likely culprits, though researchers have not definitively pinpointed the agent(s) to blame.

What causes this condition can vary from state to state. In humid North Louisiana, McConnico says mold shows up in all the wrong places during the ­summer—including in the pasture. Arizona, by contrast, features an abundance of dirt turnout paddocks due to a lack of moisture. Robinson says dust in these enclosures can cause conventional equine asthma—which is treated ­differently than pasture-associated asthma and seen primarily in “horses stalled in poorly ventilated areas.” 

Signs and management The signs of pasture-associated asthma include a distressing cough, wheezing, a creamy-white nasal discharge, and exercise intolerance. “It can be pretty profound, the coughing that happens,” says McConnico.

In the acute case, where the horse is having a hard time breathing while housed on pasture in hot, humid conditions, move the horse to an indoor environment that’s as distant from the pasture as possible. Confirm the horse doesn’t have a fever, hose him off, and call your veterinarian, who will check that your horse doesn’t have other conditions that cause respiratory distress.

In chronic cases Robinson describes what’s called a heave line along the horse’s lower abdomen. It appears when the horse develops specific muscles to support chronic labored breathing.

Management consists of removing the horse from the pasture and calling your veterinarian for more serious cases. For horses that are truly struggling to breathe, your veterinarian can recommend therapies such as corticosteroids and bronchodilators, says Robinson.

Prevention Preventing pasture-­associated asthma signs requires keen observation skills.

“The most important part is recognizing when it starts and figuring out if it’s in the pasture,” McConnico says. You can determine this by keeping the horse indoors, away from inciting allergens. 

She recommends owners invest in a stethoscope and ask their veterinarians to show them how to use it. “Get used to hearing what your (healthy) horse sounds like normally,” she says, so you can recognize when there’s a serious issue.

Alternatively, owners can use a daily scoring system to recognize horses having mild signs of asthma and remove them from the inciting pasture prior to a severe attack. Your veterinarian might be able to help you learn this system.

Additionally, McConnico advises watching for moist, humid weather conditions that can cause mold growth in pastures, keeping alternative feed sources on hand if your hay develops mold, and keeping your hay as dust-free as possible. She says owners can dunk their hay in water briefly to help minimize airborne particulates.

You can also use low-dust shavings or pelleted or paper bedding—anything to help control dust in the pasture-­associated-asthmatic horse’s ­environment.

Insect Bite Hypersensitivity

Condition and causes Insect bite hypersensitivity is an allergic reaction to bug saliva, McConnico says. The flying insects that cause problems for your horse might vary according to your climate.

McConnico says the Culicoides fly (also called no-see-ums or midges) is the main culprit in humid Louisiana. In the dry heat of Arizona, Robinson sees a variety of problem insects, including gnats, mosquitoes, black flies, stable flies, horn flies, and no-see-ums.

Understanding Insect Bite Hypersensitivity

Signs and management Understanding what bugs summer brings to your area and monitoring your horse for signs of bites is important. McConnico says patches of broken hair, damaged skin, thickened skin, and a crusty mane, all caused by excessive rubbing due to the itchiness of bites, are common with this condition. She recommends monitoring the poll, ears, mane, withers, chest, hind end, and tail for these signs.

McConnico advises owners in her area to move horses struggling with no-see-ums to a barn with a fan when the bugs are active (dawn and dusk). Additional recommendations include lining your horse’s stall with bug netting and ­applying fly repellent to your horse’s body. If your horse has rubbed himself to the point where broken skin and sores are infected, call your veterinarian for further treatment, she says.

In her region Robinson advises watching for intense itching that can cause the skin damage described, in addition to hair loss. She commonly observes horses scratching the neck, underside of the belly, legs, and tail area. 

Reach out to your veterinarian promptly, she says, to determine via allergy tests which bug is causing your horse’s hypersensitivity. Your vet can assist with further treatment if needed and give insect control recommendations.

Prevention McConnico suggests setting up/installing barn fans so they prevent bugs from being able to fly near your horse. She also recommends feeding your horse supplements containing omega-3 fatty acids (which lessen inflammatory response), having your vet perform acupuncture, and avoiding Culicoides breeding grounds (e.g., wooded areas, ponds, and any standing water) when possible. 

In the Southwest, where different bugs attack throughout the day and night, Robinson suggests reaching out to your veterinarian as soon as you see signs of severe itching. “Early detection (and mitigation) has a better outcome for the rest of the summer,” she says.

She also recommends developing a comprehensive insect control plan that targets the environment (fans, fly traps), topical solutions (insect spray, sheets, masks), and even ingestible options.

cooling out horse after cross country ride
A horse’s temperature rises during exercise, but a healthy horse’s temperature should return to normal within 20 minutes of cooling out, says McConnico. When it doesn’t, heat stress becomes a concern. | Stephanie L. Church/The Horse

Heat Stress

Condition and causes Take care when working with your horse on hot summer days, as he could be at an increased risk for overheating. “Heat stress occurs when the body cannot get rid of excess heat,” Robinson says.

She lists several causes, including hot or humid weather, exercise when the horse is unable to cool off, inadequate shade during hot days, insufficient water access, dehydration, anhidrosis, and poorly ventilated spaces.

Signs and management McConnico says to watch for nostril flaring, difficulty breathing, fever (rectal temperature above 101 degrees Fahrenheit), and an elevated heart rate in potentially affected horses.

A horse’s temperature rises during exercise, but a healthy horse’s temperature should return to normal within 20 minutes of cooling out, says McConnico. When it doesn’t, heat stress becomes a concern. If the horse’s temperature reaches 106 degrees Fahrenheit, she notes, permanent brain damage can occur.

If you are concerned that your horse is starting to overheat, take immediate steps to help him cool off. “The best places to start with are on the insides of the legs of the horse,” Robinson says. The blood vessels are closer to the surface in those locations, she explains, so run cool water there. Be sure to use a sweat scraper or towel to remove the excess water so it doesn’t trap heat on the horse.

Robinson also suggests several other methods, including pouring rubbing alcohol over the horse’s topline, offering water in moderation, and putting the horse in front of fans or misters.

Because this condition can be life-threatening, if your horse is not cooling down and you suspect heat stress, McConnico advises calling your veterinarian right away. Be ready to give the vet as much information about your horse’s condition as possible, she says, from vitals to detailed observations you’ve made about his clinical signs.

Prevention Do you know what your horse’s resting temperature is? Both ­veterinarians say knowing this, along with his typical temperature after a workout, can help you determine if heat stress is a concern in the future.

McConnico advocates checking the heat index before riding, and developing a solid cool-down plan for your horse.

“You may have to adjust your routines as the temperature rises during the hot months,” Robinson says. This could mean choosing to ride in the mornings and evenings when the temperature is lower or changing the length or intensity of your workouts, she explains.

Take-Home Message

Summer offers many chances to get outside with your horse. Owners who understand their horses’ normal behaviors and vital signs will be better equipped to identify when something isn’t quite right. Additionally, a good working relationship with a dependable veterinarian is important. He or she will be your No. 1 resource if summer conditions take a bad turn.


Written by:

Kayli Hanley is a freelance writer for KH Creative Solutions LLC, in Mesa, Arizona. She spends most of her free time in one of two places: at a desk writing about horses or in the great outdoors riding them.

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