The Common Cough Is Far From Simple

Many horses routinely cough a time or two during warm-up, but if the cough persists it needs to be investigated.

Every once in a while our horses may cough. That’s normal. Sometimes, they just cough once at the beginning of a ride and that’s it. That’s OK, too. Other times, though, the cough persists, and that’s more serious. If the cough persists throughout the course of exercise, or worse, if it consistently occurs at rest, the situation warrants veterinary investigation, rest and therapy. Download a PDF of this article.

What Causes Coughs? Coughs occur when the respiratory tract is inflamed. The respiratory tract consists of several parts that are grouped into either the upper or the lower respiratory tract. The upper respiratory tract begins at the nostrils. It extends through the nasal passages and sinuses and back into the throat where the pharynx acts as the gateway into the lower portion of the tract.

The lower respiratory tract consists of the trachea (or windpipe), the bronchi, and the lungs. Essentially, if any part of the respiratory tract becomes irritated, damaged, or infected, inflammation will result.Inflammation damages tissues, and one of the compensatory mechanisms that results is a cough.

Here is an example: the trachea is lined with a structure called the mucociliary escalator that functions to clear debris and mucus out of the lungs by carrying them up the trachea and into the throat where they are then swallowed. If the horse comes down with an upper respiratory infection (such as a virus), that escalator can be damaged, resulting in a diminished ability to clear out the lungs. Thus, the horse must cough to complete the expulsion of this waste.
Another example is if an infection occurs in the back of the throat. Laryngitis or pharyngitis can occur when the structures in the throat become infected and thus inflamed, and a cough will result.

Common Causes of Coughs
• Upper Respiratory Viruses 
Of all of the causes, viruses are the most common and most difficult to control. Just like in human colds, viruses mutate rapidly, which makes prevention difficult because vaccinations must be remanufactured to keep up with new strains. It’s also why we recommend that you vaccinate for influenza and rhino virus twice yearly. Both rhino virus (herpes) and influenza have many forms.

Beyond them, several other lesser-known upper-respiratory viruses can result in a cough, including adenoviruses, picornaviruses, parainfluenza viruses, and corona viruses. Vaccine companies cannot include every strain of every virus in a single product, which is why vaccines are imperfect. That said, they’re still highly recommended.

• Bacterial infection 
In rare instances, horses can develop a bacterial infection of the upper respiratory or lower respiratory tract. These infections are rare but serious. Horses under four years old, geriatric horses and horses with Cushing’s disease are at a higher risk. In most cases, coughing is persistent and is accompanied by lethargy, fever and sudden weight loss. While strangles (Streptococcus equi) is highly contagious, other bacterial infections (like laryngitis and pneumonia) are not highly contagious.

• Parasites
Some parasites will migrate inside the body before they settle in the digestive tract. The most common parasite in foals and weanlings is the ascarid, aka roundworm. Roundworm larvae will migrate through the lungs leaving a path of inflammation and damaged tissue behind them. A deep, dry cough will result. In the case of some parasites, irritation to the airway is essential to their lifecycle. The host animal must cough up the immature form of the parasite then swallow it, so it can complete its lifecycle in the digestive tract. Threadworms and lungworms can also occur in all ages of horses, and donkeys are particularly susceptible to lungworms.
• Poor Air Quality
If a horse lives in a dusty or moldy environment with poor air quality and circulation, serious damage to the lungs can occur. If bedding isn’t routinely cleared out of a stall, it can wreak havoc in the airway. Many horse owners don’t realize that urine ammonia levels are high in the stall because they don’t get down to ground level to breathe. But that’s where the horse breathes when he’s eating off of the floor or laying down. 

A stuffy, poorly ventilated barn is a primary cause of breathing problems.

Even if air quality and circulation is good five feet up, if the stall isn’t routinely cleaned and air circulation is poor at ground level, airway damage can result. In many cases, airway damage caused by poor air quality is permanent. These horses have a chronic breathing problem called heaves or recurrent airway obstruction (RAO).

• Allergies
Allergies are closely related to poor air quality and can cause coughing.

Inflammation in the lower airway causes air passages to narrow from swelling and also produce mucus, which clogs them. In an attempt to force air through the passages and also clear mucus, the horse coughs. Because the mucus clogs the lower airways, often veterinarians will refer to this condition as RAO.

Horses with this condition often must tense their abdominal muscles in order to force air out of the lungs, thus giving them a characteristic “heave line” along the side of their abdomen. This happens in part because the diaphragm (the muscle that expands and contracts the lungs) isn’t strong enough to create airflow and in part because the elasticity of the lungs is lost due to chronic inflammation and subsequent scarring.

• Coughing Causing Coughing
The problem with a cough is that it can persist for a long period of time and result in permanent airway damage and diminished athletic performance.

Because structures in the airway are inflamed, the act of coughing itself can actually perpetuate and exacerbate a cough. Coughing is a violent action for the structures of the throat and to the lining of the airway. If a turbulent flow of air continues to pass by these vital structures, they will remain inflamed.

• Exercising too soon
Along the same lines as the example above, many horse owners are itching to get back in the saddle too soon after their horse gets a virus or a cough. It takes the respiratory tract a long time (weeks to months!) to recover from damage caused by inflammation.

At rest, the horse may breathe fine following a respiratory infection, but when asked to exercise, a cough can persist. Once the horse coughs, it just sets off inflammation and irritation to the airway all over again, thus forcing the horse back into the vicious cycle described above.

What to do with a coughing horse. Some horses cough once or twice just as they begin exercise. While this can be alarming, it is actually fairly common in horses. Some hypothesize that the sporadic coughs occur at the start of exercise because the horse is expelling air sucked and held in when the girth was tightened. Others believe that some horses have a mild amount of chronic airway changes that could result in the cough. Cough charts (products and ingredients)

Whatever the case may be, if your horse has always coughed once or twice at the beginning of a workout, count yourself among the masses and don’t worry too much. It happens. But, if coughing occurs beyond this scenario, you need to take heed.

If your horse begins coughing persistently out of the blue, one of the most valuable pieces of information that you can obtain is a rectal temperature. Normal resting rectal temperature for a horses ranges from 99° to 100.5° F. If it’s higher, there may be a problem.

In addition, be sure your horse is eating normally, as a depressed appetite may indicate a problem. Is he lethargic (acting tired) in any way? Consider the other horses in the barn. Are any coughing? Finally, does your horse have any increased nasal discharge? If any of the answers to these questions is yes, it’s time for a vet visit. 

Veterinary Intervention. Remember, upper respiratory viruses and the resulting cough are common. They sweep through barns by direct contact between horses, shared water sources, or by fomites (objects that transfer the virus) such as manure carts, flies and human hands.

When these occur, the veterinarian may attempt to quarantine affected horses if facilities and management permit (see July 2013). However, a true quarantine is difficult to achieve and, usually, most of the horses in the barn have been exposed anyway.

Beyond viruses, your vet will work to eliminate the other causes we discussed earlier. This will involve listening to the airway with a stethoscope, palpating the throat area, and attempting to illicit the cough through making the horse rebreathe into a bag or by exercise. Your vet may also run blood tests, ultrasound and/or radiograph the lungs.

If the cause of the cough is due to an allergic condition, parasite infestation, or infection, prepare for a long haul. Treatment will involve medications—some can be costly—and rest. Lots and lots of rest.

Actually, rest is huge for the treatment of any cough. Remember, coughing occurs because the horse tries to clear debris from the airway, and/or because the airway is inflamed. Either way, exercise and athletic performance are not conducive to healing. In fact, they will impede healing because the horse has to constantly use its airway “machinery” and can’t heal. 

Think about it: When you get a cold and subsequent cough, how long does the cough last? Weeks or months for some of us, right? Horses are no different. Sometimes horse owners seem to get a bout of amnesia regarding the vet’s orders! Or maybe it’s impatience. 

Veterinarians understand that it seems counter intuitive to “do nothing.” Or, at least, that’s the perception horse owners get when they rest their horses for many weeks at a time. But rest assured (no pun intended), allowing the airway time to calm down and repair itself is far from doing nothing. In fact, not providing needed healing time could set your horse up for more coughing and potential permanent damage to the airway.

As a rule of thumb, rest the horses (stall rest/hand walk) for a minimum of two weeks. Then, if no coughing is heard during that time, put the horse on a longe line and see how things go. If, during the course of longeing, you hear coughing, then two more weeks of rest are in order.

Repeat as necessary, keeping your vet informed of progress and worsening. Barring any complications, it isn’t uncommon for horses with coughs to have to rest for one to two months. Yes, you read it correctly! 
Bottom Line. Coughs occur because of inflammation and damage to the airway. Ruling out serious causes like infection, infestation or allergic disease is essential. Make sure air quality is good by keeping the stall clean and eliminating sources of allergen in the environment (to whatever extent possible) is also important. While you don’t want drafts in your barn, it’s very important to be sure your barn isn’t shut up too tight and isn’t too warm. As long as horses are out of wet weather and wind, a cold barn is a healthy barn.

Cough Free is our Best Buy cough “supplement.”

Finally, for a horse that is recovering and battling a cough, rest is paramount.

Exercising a horse with a cough will set him up for permanent airway damage that will result in diminished athletic performance.

Our chart includes our favorite cough products that are available over the counter. However, if your horse is coughing, do not attempt to treat it yourself right off the bat. Make sure that your veterinarian rules out any serious illness before you move to using supplements.
Article by Contributing Veterinary Editor Grant Miller DVM. 

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