Many horses are expected to perform like elite athletes, where peak performance demands an optimally functioning respiratory system. In this article, Dr Deryck Tan from Valley Equine Veterinary Centre gives an overview of the anatomy of the respiratory tract and the most common respiratory conditions that compromise its optimum performance.
Diseases affecting the respiratory tract
There are a number of diseases that affect the respiratory system and the clinical signs associated with respiratory disease are very varied. The causes of respiratory disease are just as numerous, with some having an allergic, parasitic or infectious component.
Veterinarians use many different techniques and tools to ‘work up’ (diagnose) a case of respiratory disease.
These diagnostic techniques and tools range from simple clinical examination to advanced imaging, such as CT scans. Some checks are done with the horse standing still, and others during exercise.
Treatment options for respiratory disease depend on the cause of the disease and, once a diagnosis is made, your veterinarian will be able to explain in greater detail the treatment options.
Although it’s easy to identify, coughing is a very non-specific clinical sign in a horse. In general terms, a dry cough is usually caused by irritation of the respiratory tract and a wet or productive cough is usually caused by mucus or phlegm in the respiratory tract.
The irritation of the respiratory tract may be caused by a dusty environment, a viral or bacterial infection, or inflammatory or reactive airway disease.
Inflammatory Airway Disease (IAD), a non-infectious respiratory disease associated with exercise intolerance occurs in up to 50% of Thoroughbreds and Standardbreds and is a common cause of poor performance. The most common signs are a mucoid/purulent nasal discharge and chronic cough.
Nasal discharge can be unilateral or bilateral, continuous or intermittent, and can be serous, mucoid, purulent, haemorrhagic or feed contaminated.
Serous nasal discharge: A serous (pale yellow and transparent) nasal discharge is usually associated with allergic rhinitis (inflammation of the mucous membranes of the nasal passages) or a viral respiratory infection.
Mucoid, purulent nasal discharge: A mucoid to purulent discharge that contains pus and is yellow or green in colour may point to a bacterial infection. This may be coming from the lower respiratory tract, but in most cases, discharge originating from the lower respiratory tract is swallowed before it has a chance to exit through the nostrils.
Other causes of mucoid/purulent discharge are sinusitis (inflammation of the sinus), either primary or secondary. Discharge from a sinusitis is often unilateral and foul smelling.
Primary sinusitis is caused by defective clearance of mucus from the sinuses, usually as a result of viral or bacterial infection. Early primary sinusitis may respond to antibiotics, but some cases may require trephination - a surgical intervention where a hole is drilled into the sinuses to place a lavage system to wash out the mucus.
Secondary sinusitis may be a result of a tooth root abscess or oroantral fistula (where the hole left by an empty tooth socket has a direct connection into the sinus). When the cause is a tooth root abscess, the offending tooth must be extracted before the sinusitis can be resolved. In the case of an oroantral fistula, the fistula must be closed, which is a procedure fraught with difficulty and requires expert veterinary dental care.
A foul-smelling nasal discharge could also be caused by neoplasia (a tumour).
Guttural pouch empyema is an accumulation of purulent, septic fluid in the guttural pouch. It can present as bilateral purulent nasal discharge. The guttural pouch is a sac of air that expands from the Eustachian - a narrow passage leading from the pharynx to the cavity of the middle ear. In this case, there is a decrease in the clearance of mucus from the guttural pouch and secondary bacterial infection results in pus formation. The pus eventually turns into solid chondroids (tissue resembling cartilage).
Sinus cysts, abnormal fluid filled sacs that form in the sinuses, can also present as a foul-smelling discharge and can also be accompanied by facial swelling and nasal obstruction.
Another cause of nasal discharge and respiratory disease is transport-related pneumonia which is exacerbated during long journeys and when horses are stressed, fatigued or unable to lower their heads below the wither level for long periods.
To read about reducing the risk of transport related pneumonia click here.
Haemorrhagic nasal discharge
A bloody nasal discharge could be due to an ethmoidal haematoma, guttural pouch mycosis or head trauma.
Ethmoidal Haematomas originate from the ethmoidal bones and can be visualised by endoscopy. They can also reside in the maxillary sinuses, in which case radiography or sinuscopy will be needed for diagnosis. Treatment is by formalin injection or surgical removal.
For a more detailed article on ethmoid haematomas click here.
Guttural Pouch Mycosis is a fungal infection in the guttural pouches that surround the pharynx. This is a life- threatening condition because major blood vessels and nerves traverse the pouch. Brown coloured, foul smelling discharge can be a sign of pneumonia or lung abscess.
Fresh blood from one or both nostrils after exercise could be due to Exercise Induced Pulmonary Haemorrhage (EIPH). EIPH, also known as ‘bleeding’ or a ‘bleeding attack’, occurs when horses bleed from the lungs during exercise. Research shows that up to 60% of racehorses bleed from the lungs but only a minority show blood at the nostrils (known as epistaxis).
For a more detailed article on EIPH click here.
Feed contaminated nasal discharge
If a feed contaminated nasal discharge is present in small amounts, the horse could be suffering from dysphagia (swallowing difficulty) or feed aspiration.
This could be a complication of ‘roarer’ or ‘tie back’ surgery, a condition called recurrent laryngeal neuropathy (RLN) or laryngeal hemiplegia which is caused when one or both arytenoid cartilages in the throat become paralysed and partially block the airway.
Feed contaminated nasal discharge could also be caused by an oronasal fistula where there is a direct communication between the oral and nasal cavity.
A cause of copious nasal discharge contaminated with feed is ‘choke’ or oesophageal obstruction. Although not a respiratory disease, it can result in a secondary pneumonia from inhalation of saliva and food material.
Choke is caused by food getting obstructed in the horse’s oesophagus, which is the connection between the mouth and the stomach. The nasal discharge is usually feed- contaminated.
Horses choke due to a variety of reasons. Some bolt down their food without chewing properly due to anxiety from hunger or because they feel threatened by a more dominant horse. Commonly, a horse does not chew properly due to oral or dental pain, hence the importance of proper and regular veterinary dentistry. Horses that are used to being fed a wet feed and are suddenly fed a dry feed may also choke. Most horses affected eventually swallow the obstructed bolus of food by themselves, if left in a quiet place with food withheld.
Offering water to a choked horse is a double-edged sword. Since the horse cannot swallow, attempts at drinking may result in water being inhaled into the lungs and causing a secondary pneumonia. However, water in the oesophagus may help soften the obstruction. If the obstruction is not cleared in one to two hours, veterinary intervention is advised.
For a more detailed article on choke click here.
Horses are obligatory nasal breathers, which means they are unable to breathe through their mouths.
Laryngeal hemiplegia (roaring) is a condition where one-half of the larynx becomes paralysed and decreases the flow of air. When one side is paralysed, the affected flapper is sucked into the airway when the horse inhales, thus obstructing airflow and makes a ‘roaring’ noise when the horse exhales.
Laryngeal hemiplegia usually affects the left arytenoid cartilage, one of several cartilages that allow air to pass into the trachea and protect the airway during swallowing.
Dorsal displacement of the soft palate (DDSP) is another condition that results in a ‘gurgling’ or ‘fluttering’ respiratory noise. The soft palate is an extension of the hard palate that divides the oral cavity and the nasal cavity. During swallowing, the soft palate forms a seal to prevent food from entering the lungs and during exercise, it moves down and opens to allow the maximum amount of air to pass into the lungs. In DDSP, the soft palate ‘flips’ up and partially obstructs the airway.
Other causes of respiratory noise are epiglottic entrapment, subepiglottic, pharyngeal and palatal cysts, arytenoid chondropathy, fourth branchial arch defects and epiglottic flaccidity.
Diagnosing these conditions may have to be done during exercise - on a treadmill or, if available, a dynamic endoscope.
Airway obstruction can be caused by recurrent airway obstruction (RAO), also known as ‘heaves’, by lungworms and/or a collapsed trachea.
RAO varies in clinical signs but, commonly, it is characterised by chronic cough, nasal discharge, exercise intolerance and respiratory difficulty. In contrast with inflammatory airway disease (IAD), horses with RAO are not normal at rest. Affected horses are sensitive to dusts and allergens and are now described as having equine asthma.
Tracheal collapse occurs mostly in Shetland ponies and Miniature horses. It is a congenital condition and, unfortunately, there is no treatment but obesity, exercise and pregnancy can exacerbate the signs. Some affected ponies make a honking respiratory sound.
Infectious respiratory diseases
Strangles is a highly contagious respiratory disease caused by Streptococcus equi subsp equi for which a vaccine is available.
As well as a fever, it causes the lymph nodes near the horse’s jaw to become abscessed and enlarged which may obstruct the airway (hence the name ‘Strangles’) and cause difficulty swallowing. Clinical signs include a high temperature and yellow coloured nasal discharge from both eyes and nostrils.
For a more detailed article about Strangles click here.
Equine Herpesvirus (Rhinopneumonitis) 1, 2 and 4 are also contagious respiratory diseases that can only be diagnosed with blood tests and nasopharyngeal swabs. EHV-2 is thought to cause pharyngeal lymphoid hyperplasia in young horses (meaning pharyngitis or inflammation of the pharynx) which causes difficulty swallowing, a cough and sometimes nasal discharge.
EHV-1 may result in neurological signs and abortion in broodmares. There is no treatment but it can be prevented and controlled with vaccination programs.
Equine influenza virus is another contagious respiratory disease which is not currently present in Australia and New Zealand. It spreads rapidly by direct contact causes a fever, nasal discharge, a dry hacking cough, depression, loss of appetite and weakness, and renders the infected horses susceptible to secondary bacterial infections.
Equine flu is common in other countries and horses are routinely immunised.
Equine viral arteritis is a contagious venereal disease which produces signs of respiratory disease as well as abortion, rhinitis, conjunctivitis, swelling of the limbs and ventral abdomen.
Clinical signs of Hendra virus are mainly respiratory and neurological. It causes a fatal pneumonia in horses and is known to be zoonotic infectious disease (naturally transmitted between animals and humans).
For more information on Hendra virus click here.
Rhodococcus equi and Streptococcus zooepidemicus cause bronchopneumonia in foals. Other organisms are Bordetalla bronchiseptica, Actinobacillus, Pasteurella, Klebsiella, Escherica coli, Aspergillus and Pneumocystis carinii.
Lungworms, Dictyocaulus arnfeldi, are parasites carried by donkeys and donkey crosses. They can spread to horses and cause airway obstruction and cough.
Diagnosing respiratory conditions
To investigate respiratory disease, your veterinarian will need a history of the disease progression. He or she will also conduct a general clinical examination and may perform additional procedures.
The lung fields may be percussed using a large spoon and rubber hammer. Normal lungs will give off a resonant sound.
A rebreathing procedure may be performed using a large bag over the muzzle of the horse. This is useful to work out the borders of the lung, as well as to intensify any abnormal lung sounds.
Endoscopy is a procedure that allows the veterinarian to look inside the body using a long, flexible tube with a lens at one end and a video camera at the other.
The examination can be performed at rest and during exercise usually on a treadmill, although there are remote controlled versions that can be attached to a normal bridle and used under more natural exercise conditions.
To examine the paranasal sinuses, trephination into the sinuses is necessary. This procedure involves drilling a hole (trephination) into the sinuses to allow passage of the endoscope.
Bronchoalveolar lavage is where a tube is inserted into the trachea, sterile fluid is injected and the wash collected. The collected sample is then sent to a laboratory for cytologic examination - to look for viral infections in the cells.
Transtracheal aspiration is a procedure which involves collecting a sample from the trachea using a catheter via a cannula through the trachea.
Thoracocentesis is the collection of a sample from the pleural space between the lungs and the chest wall using a trocar and chest tube. Samples collected are submitted for cytology, gram stain and culture.
The lungs can be imaged using ultrasound or radiography. Nuclear scintigraphy, CT imaging or MRI of the head region can also be performed to identify tooth root abscesses not detected by radiography.
Where neoplasia (abnormal growths of tissue) is suspected, surgical exploration and biopsy is sometimes necessary so the biopsy sample can be sent for histopathology.
Prevention is better...
Prevention of respiratory disease includes vaccination where vaccines are available (for example Strangles, EHV and Hendra virus), and biosecurity/hygiene measures to minimise the risk of picking up an infection from another horse.
When infections are present and/or when a new horse arrives at a property, quarantine measures may be necessary, particularly where the disease is highly contagious.
To learn how to apply practical and effective biosecurity on a horse property click here.
Allowing horses to graze at pasture and keeping their built environments ventilated but as dust and ammonia-free as possible will also help keep your horse’s respiratory system healthy.
An appropriate worming program will help ensure that lungworms will not invade your horse’s respiratory tract.
When horses have to travel long distances it is important to take steps to minimise the risks of transport-related pneumonia. Ensure horses are healthy before they travel, allow and encourage them to lower their heads during the trip, break up the long journeys and allow 24 to 48 hours of recovery, encouraging grazing to clear the airway.
Regular dentistry by a trained equine dental veterinarian will also help identify early, if not prevent, respiratory diseases associated with teeth and sinuses.
Your equine dental vet can also do a health check during his visit and help you clarify any concerns you may have.