Equine asthma

The term equine asthma was coined to describe with one name Inflammatory airway disease (IAD) and Reccurrent airway obstruction (RAO). This term enables better comprehension between scientific, veterinarian and medical communities.
This respiratory syndrome is typically caused by a dusty environment, with a high level of organic and inorganic particles. Asthma affects the lower respiratory tract, including the trachea, the bronchial tree (bronchial tubes and bronchioles) and the air sacs or alveoli in the lungs.

Equine asthma includes several different afflictions of varying severity :

Moderate equine asthma (MEA)

Can affect horses of any age, even though it is more frequent in younger horses. It is a very common affliction, and affects around 20 % of the equine population. It is also a frequent cause of counter-performances. A French study brought to light that around 60 % of trotteur horses examined because of counter-performances had signs of MEA (Richard et al., 2010).

Severe equine asthma (SEA)

SEA is also a common equine disease. It is the most frequent chronic pathology for horses kept stabled. It mainly affects horses of over 7 years of age. A study in the United Kingdom showed a 14 % prevalence  ( % of horses affected compared to the total population), the prevalence becoming higher as the horses get older (Hotchkiss et al., 2007).
There is however no established direct link betwen the two illnesses : a horse having been affected by MEA when young will not necessarily develop SEA later on.
Horses affected by SEA associated to being out to pasture (SPAOPD) are allergic to allergens present in the pasture. They will demonstrate signs especially in Spring and Autumn.

Signs of moderate equine asthma

Moderate equine asthma is characterised chronically (the signs are present for more than 4 weeks) by the following :
•    Occasional coughing which is unrelated to exercise
•    Moderate intolerance to effort, which can show up as absence or decrease of performance,  longer recovery time after effort.
•    Occasionally a mucous discharge from both nostrils (accumulation of mucous in the trachea subsequent to the inflammation.
The illness will usually clear up spontaneously or following a change of environment or a treatment. Recurring bouts are rare : a horse having been affected by MEA has no more chance of recontracting the illness than any other horse kept in the same conditions.

Clinical signs of severe equine asthma (SEA)

In the case of severe equine asthma, in the same way as for MEA coughing, intolerance to effort, and in some cases mucous nasal discharge can be observed. These signs are however more marked. SEA is characterised by bronchial hyperreactivity (the bronchial tubes over-respond to a stimulus) which causes bronchoconstriction (decrease in the diameter of the bronchial tubes). This bronchoconstriction associated with an accumulation of mucus renders the passage of air more difficult.
Difficulty in breathing (dyspnea) can be observed at rest, especially during expiration. Expirartion becomes forced instead of passive. The abdominal muscles contract to push the air out, and the appearance of a « heaves line » on the abdomen (diagonally from the tip of the haunch to the lower edge of the ribs) is observed. Breathing frequency can also appear increased at rest (>16 movements per minute) with dilated nostrils. Thses signs will be more or less marked depending on the severity of the illness and the horse’s environment. They can also decrease significantly during periods of remission. If the horse has been affected for a long time, chronic weight loss may become apparent.

Finally, SEA is characterised by a succession of bouts and remissions. There is no cure and it can only worsen with time. In fact, with each bout of the illness, the structure of the  lower respiratory airways is modified irreversibly, the walls of theses airways thicken making the passage of air more and more difficult .

Asthma is a multiple factor syndrome. The main risk factor is subjecting horses to the following :

• Dust, acarids, mould, mainly found in bedding and fodder.
• Endotoxins (consituent of the cell walls of bacteria), fine to ultra-fine particles, micro-organisms, toxic or irritant gases, particularly ammonia from urine when horses are kept stabled.

Some of these molecules can have a synergetic effect. Two molecules have a greater effect when associated than the sum of their effects when they are taken seperately.
Moreover, some studies have shown that SEA could also have a hereditary component, but this data has yet to be confirmed (Jost et al., 2007 ; Schnider et al., 2017).
Horses suffering from SEA associated to pasture usually present clinical symptoms at the beginning of summer and autumn when the pollens from grass, and the spores from mould are the most abundant in the pastures, and when the temperature and humidity increase. Horses with this type of asthma who live out are more affected than those living stabled.

For an older horse suffering from severe asthma, a clinical diagnosis based on the horse’s history (age, living conditions, situations which either exacerbate or diminish the symptoms…) can be enough.
Regarding cases of MEA and SEA at the onset of symptoms, it is recommended to carry out complementary tests :

• An endoscopy of the trachea to observe and quantify the amount of mucus present
• A cytology study following a broncho-alveoli lavage : this examination consists in injecting a sterile liquid into the lungs and drawing it out again with cells which line the mucus of the lower respiratory tract. Cytology then consists in observing and quantifying cells under the microscope, especially any inflammatory cells.  

The presence or absence of dyspnea at rest as well as the intensity of the inflammation shown by these additional examinations will enable to differentiate between the different forms of asthma, and to establish a prognosis.

Changing the horse’s environmental living conditions, sometimes associated to medical treatment aims to cure MEA. On the other hand, SEA is a degenerative condition. Management will alleviate clinical signs and relieve the horse but it cannot be cured.

Environmental measures

The best way to improve the clinical signs of a horse suffering from asthma is to reduce the number of particles present around the nostrils.

Living conditions

Fodder

Other good practices

• Use dust -free materials for arena or indoor school surfaces, or keep them well watered
• Don’t use haynets during transport
• Rest horses suffering from MEA or when having a bout of SEA.

Supplementing with omega-3 fatty acids

Supplementing asthmatic horses with omega-3 fatty acids is recommended due to their anti-inflammatory properties. A study on horses affected by MEA and SEA being supplemented with 1,5g of  docosahexanoic acid (DHA) for 2 months , and associating this supplement to an environment with reduced dust factors, considerably improved the clinical signs and decreased the respiratory tract inflammation, compared to just reducing the impact of a dusty environment (Nogradi et al., 2015). Supplementing with omega- fatty acids (e.g linseed oil) should however be associated to changing the horse’s environment to show any real efficiency .

Medical treatment