The coughing horse can be very frustrating for the owner and a vet to treat. Coughs in horses are mostly due to allergic reactions to stable dust and pollen. There are however many cases of viral infection which also result in long term irritation and produce coughs that do not respond easily to veterinary treatment. Use of antibiotics usually has little effect because the viruses and micro-organisms that cause the problem are not very sensitive to these drugs.
Cough can be associated with a number of different conditions, some minor, others more serious.
Simply coughing is the sudden, noisy expulsion of air from the lungs allowing mucus and other material to be cleared from the airways. In horses it is an involuntary reflex, initiated by sensory cells located in the lining of the trachea and bronchi. These sensory cells are stimulated by irritation, hence they are often referred to as irritant receptors. Once stimulated, these receptors send messages along nerve pathways to the cough centre in the brain which controls the muscles of the larynx and diaphragm as well as the intercostal and abdominal muscles.
Because the horse is an athletic animal, its lungs are extremely large. The volume of air inhaled with each breath is in the order of 6 to 8 litres for an adult horse. The trachea is also a large structure, extending the length of the neck from the larynx to past the first pair of ribs where it divides into the left and right bronchi. Lining the trachea and bronchi (and the smaller lower airways) is a membrane made up of millions of microscopic mucus producing cells with hair-like projections called cilia. This muco-cilliary arrangement means the lining of the airway is always kept moist by a thin layer of mucus which is continuously being pushed forwards by the wave-like motion of the cilia.
There are numerous causes of irritation and hence reasons why a horse starts coughing: Foreign Objects
Occasionally a foreign body such as a stalky piece of hay or straw, finds its way into the larynx or trachea. The horse is usually distressed but the forcefulness of the cough is usually enough to dislodge the object. Viral Infections
This is probably the major cause of coughing in horses. There are several respiratory viruses that can affect horses but in New Zealand the most common are the two members of the Herpes family, known simply as Equine Herpes 1 (EHV1) and Equine Herpes 4 (EHV4). It is important to note that coughing is not always a feature of EHV infections, some horses show few outward signs other than a slight nasal discharge and a loss of appetite.
Other respiratory viruses that can cause coughing include Rhinovirus and Equine Influenza. Bacterial Infections
These often follow a viral infection, in fact the two often overlap. Bacterial infections are usually associated with a thick nasal discharge and a productive cough. The infection may be isolated to the airways, or in more serious cases the infection may develop into pneumonia with involvement of the lung tissue. Two serious bacterial respiratory infections that affect young horses in particular are Strangles and Rhodococcus Bronchopneumonia.
Bacterial infections can involve the guttural pouches causing a persistent cough that does not respond to antibiotic medication. Allergies
These are a common cause of coughing in horses. They are initiated by the inhalation of fungal spores or dust particles and result in a chronic inflammatory reaction in the lower airways. The cough is usually chronic due to the constriction of the small airways. Parasites
Lungworm can cause coughing in horses that are kept with donkeys. The small lungworms live in the lower airways and are a source of irritation. Lungworms only grow to maturity and produce eggs when the donkey is their final host, hence horses with lungworm cannot infect other horses.
Roundworm infections can cause coughing in foals and weanlings due to the larvae migrating through the lung tissue as part of their life-cycle.
Exercise Induced Pulmonary Haemorrhage
Also commonly referred to as "bleeding from the lungs", this is a cause of coughing mainly in galloping horses. It has always been known that chronic lower airway infections play a significant part in the onset of the problem but new research now suggests that the transmission of shock waves from the forelegs through the chest wall and off the rib-cage onto the lungs during galloping, also plays an important part. Any horse that starts coughing soon after intense exercise should be scoped to determine whether they have blood in their trachea. Upper Respiratory Tract Problems
There are several clinical entities involving abnormal function of the upper respiratory tract that can cause coughing. The most common of these are Epiglottic Entrapment and Laryngeal Hemiplegia, both involving the larynx. Horses with these problems usually show other signs besides a cough, in particular abnormal breathing noises during exercise. Diagnosis
To enable effective treatment to be carried out, it is important that the cause of the cough be identified accurately. This involves a detailed veterinary examination beginning with an assessment of the horses general health. The presence of other affected horses in the same stable or paddock maybe significant. Another important fact to consider at this stage is whether the cough is related to feeding or to exercise.
Listening to the chest with a stethoscope while the horse is standing quietly, may reveal abnormal breathing sounds but these can be difficult to detect using this method. Examination of the throat, trachea and bronchi is possible using a flexible fibre-optic endoscope. This is a valuable aid to diagnosis especially when carried out immediately after exercise as it allows direct assessment of all these structures
Another diagnostic test that can be carried out is a tracheal wash. A fine tube is passed through the endoscope into the bronchi and between 50 to 100mls of sterile saline is injected through the tube and allowed to mix with any fluid and cellular debris in the airways. The mixture is then drawn back into the tubing and sent away for laboratory analysis. The presence of bacteria, fungal spores, red blood cells, or inflammatory cells can be assessed.
Routine blood tests can be useful to indicate the presence of infections (white cell count) and inflammation (fibrinogen) although these indicators are not specific for the respiratory system. Treatment
Once a diagnosis has been reached, the appropriate treatment programme can be started. Treatment can involve any of three options; Management,
Good management practices that can be carried out routinely to help avoid coughing related problems include;
Avoid stabling in poorly ventilated stables. Muck out stables regularly. Ammonia from urine soaked bedding is a respiratory irritant.
Avoid feeding dusty or mouldy hay. Hay is best soaked for 15 minutes before feeding.
Feed low to the ground to assist natural drainage of the airways by muco-cilliary clearance.
Vaccinate all horses against EHV1 & EHV4 infections. Vaccines do not always prevent infections but they certainly reduce their severity.
Worm regularly, especially young horses and horses grazing with donkeys.
Keep horses warm after exercise and in cold or wet conditions.
Care needs to be taken during long float trips as this is when horses are most susceptible to respiratory infections. The combined affects of close confinement with other horses, increased body temperature, poor ventilation and the head being raised for prolonged periods, are all significant contributing factors.
There are several products available that can be useful in helping to treat and in some cases prevent coughs and colds. Treatment and Prevention
Avoid Undue Stress.
Treat the Symptoms.