Horses cough for many of the same fundamental reasons other animals do: something is irritating or obstructing their airways. The cause can be as simple as dusty hay or as serious as a respiratory infection or heart disease. Most coughing in horses falls into a few well-defined categories, and understanding which one you’re dealing with determines whether the problem resolves on its own or needs veterinary attention. A rectal temperature above 101.5°F alongside a cough points toward infection and warrants an immediate call to your vet.
Dust, Mold, and Barn Air Quality
The most common trigger for coughing in stabled horses is the air they breathe indoors. Hay, bedding, and the barn environment produce a cocktail of organic dust particles that includes mold spores, bacterial endotoxins, and fine plant debris. The fungal species most frequently found in stable air are Cladosporium, Alternaria, and Aspergillus, all prolific producers of airborne spores that provoke inflammation in the airways.
Ammonia from urine-soaked bedding compounds the problem. The international guideline for continuous ammonia exposure in animals is 20 parts per million, but poorly ventilated stalls regularly exceed that. Bacterial endotoxins and mold compounds embedded in organic dust are potent inflammatory agents. They drive the same types of airway reactions seen in human asthma and chronic bronchitis: swelling, excess mucus, and the persistent urge to cough.
A horse that coughs mainly when eating hay, being groomed, or standing in its stall is giving you a strong clue that air quality is the issue. Turning the horse out on pasture often produces a noticeable improvement within days.
Equine Asthma
Chronic coughing that doesn’t resolve with basic management changes often falls under the umbrella of equine asthma. This term now covers three related conditions that used to go by separate names: inflammatory airway disease (IAD), recurrent airway obstruction (RAO, historically called “heaves”), and summer pasture-associated airway obstruction. Mild to moderate equine asthma corresponds to what was previously called IAD, while severe equine asthma is the new label for heaves.
Mild to moderate cases typically show up as an intermittent cough, slight nasal discharge, and reduced performance. The horse may seem fine at rest but cough during exercise or take longer to recover after work. Severe equine asthma looks more dramatic: labored breathing at rest, a visible “heave line” along the abdomen from the effort of pushing air out, thick nasal discharge, and frequent coughing. These horses can develop increased respiratory effort even while standing in their stall.
Veterinarians diagnose equine asthma by collecting fluid from the airways and examining it under a microscope. In a healthy horse, fewer than 5% of the cells recovered are neutrophils (a type of white blood cell involved in inflammation). When neutrophils exceed 10%, or when eosinophils or mast cells are elevated, the results confirm an inflammatory process consistent with equine asthma.
Respiratory Infections
Viral and bacterial infections are the leading cause of sudden-onset coughing, especially in younger horses or those exposed to new arrivals at a barn, show, or auction. Two of the most important viral culprits are equine influenza and equine herpesvirus (types 1 and 4, sometimes called rhinopneumonitis). Both produce fever, nasal discharge, depression, throat inflammation, cough, poor appetite, and swollen lymph nodes. Horses with influenza tend to develop a harsh, dry cough that can persist for weeks even after the fever resolves.
Bacterial infections like strangles, caused by Streptococcus equi, produce dramatic swelling of the lymph nodes under the jaw along with thick nasal discharge and coughing. Secondary bacterial infections can also settle in after a viral illness weakens the airway lining, turning a mild cough into something more serious with colored discharge and a second spike in temperature.
Any cough accompanied by fever above 101.5°F should prompt a veterinary call. Infections spread quickly through a barn, so isolating a sick horse early protects the rest of the herd.
Lungworm
Horses that share pasture with donkeys face a specific parasitic risk. The lungworm Dictyocaulus arnfieldi completes its life cycle in donkeys, which typically carry the parasite with few visible symptoms. Adult female worms living in a donkey’s lungs lay eggs that are coughed up, swallowed, and passed out in feces. Once on pasture, the larvae become infective and can survive unless killed by drought or extreme cold. Horses that graze the same fields, even months later, pick up these larvae.
In horses, lungworm infection causes a moderate, persistent cough and slightly faster breathing. Some older horses also lose condition and appear generally unthrifty. A standard fecal exam doesn’t always catch lungworm, so your vet may need a specific test or a trial course of deworming to confirm the diagnosis. If your horse has access to donkey-grazed pasture and develops a cough that doesn’t fit other explanations, lungworm belongs on the list.
Throat and Airway Abnormalities
Some coughs are mechanical rather than inflammatory. The epiglottis, a flap of tissue that acts as a trap door to keep food out of the windpipe during swallowing, can become partially trapped by surrounding tissue. This condition, called epiglottic entrapment, prevents the flap from moving normally. The most common signs are abnormal respiratory noise during exercise and poor performance, though some horses also cough, produce nasal discharge, or shake their heads.
Dorsal displacement of the soft palate is a related issue where the soft tissue at the back of the throat shifts out of position, temporarily blocking the airway. Horses with this problem often make a gurgling sound during fast work and may cough or choke up briefly. Both conditions are most apparent during exercise and are typically diagnosed with an endoscopic exam of the upper airway.
Heart Failure
Coughing caused by heart disease is uncommon in horses, but it does occur. When the left side of the heart fails, blood backs up into the vessels supplying the lungs. This increased pressure forces fluid into the lung tissue, a condition called pulmonary edema. The resulting symptoms include difficulty breathing, a faster respiratory rate, poor performance, and fatigue during even light exercise. Coughing is a recognized sign, particularly in horses with severe degeneration of the mitral valve.
A cardiac cough usually appears alongside other red flags: exercise intolerance that seems disproportionate to the horse’s fitness level, an irregular heartbeat detected on routine exam, or visible distension of the jugular veins. If your horse develops a cough with no obvious respiratory explanation and tires easily, a cardiac workup is worth discussing with your vet.
Reducing Coughing With Management Changes
For the large number of horses whose cough traces back to dust and allergen exposure, environmental management is the single most effective intervention. The goal is straightforward: reduce the amount of irritating particles the horse inhales.
Hay is the biggest source of respirable dust in most barns. Steaming hay in a commercial hay steamer reduces airborne respirable particles, bacteria, and mold by 99%, while preserving the hay’s mineral and nutritional content. Even a simpler setup, like steaming hay for 80 minutes in a sealed container with a heating element, cuts respirable particles by about 95%. Soaking hay in water does reduce dust, but it leaches out valuable minerals and water-soluble carbohydrates while increasing bacterial counts by 1.5 to 5 times, making it a less ideal long-term solution.
Beyond hay, the basics matter: good ventilation (open doors and windows beat closed barns, even in winter), low-dust bedding like shavings or cardboard, mucking out while the horse is outside, and storing hay away from the stable area. Feeding hay on the ground rather than from an elevated rack helps mucus drain naturally from the airways. For horses with confirmed equine asthma, maximizing turnout time and minimizing stall confinement often produces a bigger improvement than any medication.