Horse Health Article - Equine Asthma
Everyone knows that humans can get asthma, but did you know that horses can get it too?
Over the years, the condition experienced by horses experiencing signs of lower respiratory disease such as coughing, nasal discharge, and poor performance has had several name changes, from “Heaves” to “Recurrent Airway Obstruction” to “Inflammatory Airway Disease,” to finally being colloquially known as “Equine Asthma.”
Above photo: Arnd Bronkhorst Photography
What are the signs of Equine Asthma?
Like many diseases, the signs of equine asthma can vary from horse to horse depending on a multitude of factors, including severity, seasonality and contributing factors.
In horses with mild to moderate equine asthma, common signs may include the following:
Occasional coughing, at rest or during work
Poor performance
Increased respiratory rate, particularly after work
In horses with severe equine asthma, common signs include the following:
Persistent or chronic cough, usually at rest
Poor performance
Increased respiratory rate at rest
Increased respiratory effort, sometimes with development of “heave line” due to recruitment of abdominal muscles to assist with respiration
Mucoid nasal discharge, likely due to increased tracheal-bronchial mucous.
What is the cause of Equine Asthma?
This is a million-dollar question, and one that doesn’t have a straightforward answer. There are different molecular endotypes of asthma that have been observed in humans and horses, and these different types have different responses to treatment.
For example, there is both T helper 2 cytokine (T2) asthma, and non-T2 asthma. The former type is more “allergic” based, with higher IgE levels and eosinophils present. This type responds well to inhaled corticosteroids, whereas non-T2 asthma doesn’t respond as well.
Th17 type cells have been found in severe asthmatics with high numbers of neutrophils within the airways, and this type is also known to have severe steroid-unresponsiveness. In horses with pasture-associated asthma, there is a high association with neutrophilic inflammation within the airways, similar to Th17-mediated disease.
In general, we know that the condition is heritable, as horses with known parents having allergic airway disease are statistically much more likely to present with the condition themselves.
How do we diagnose Equine Asthma?
Horses with asthma are often presented due to the clinical signs observed by their owners at home. Important aspects of a workup of a horse with asthma include:
Obtaining an accurate and detailed clinical history: including what clinical signs are observed and when, the duration of clinical signs, as well as any other conditions the horse may have (particularly EMS due to the risk factors associated with treatment)
Thorough airway auscultation (listening to the horse’s airways during breathing)
Rebreathing examination: this involves placing a rebreathing bag over the horse’s nose for roughly 1 minute to make the horse rebreathe expired air, which causes them to take deeper breaths in order to try to inhale more oxygen (as the CO2 levels build up within the bag). This allows for a greater period of inhalation which allows for detailed auscultation of the airways, as well as observation of any physical signs of difficulty in breathing
Bronchalveolar lavage (BAL): this is the most important step in figuring out what type of Asthma your horse is affected by (mild, mild-moderate, or severe).
What does a Bronchoalveolar Lavage (BAL) involve?
A bronchoalveolar lavage involves passing a tube down into the horse’s lower airways (bronchi) and lavaging a set amount of saline (60-500mL depending on practitioner preference). This fluid is then sucked back up the tube and collected into sample pots, which are sent for cytological analysis.
While some fluid may remain in the airways, this is quickly absorbed by the horse and is not harmful.
Horses can cough quite a lot during this procedure, so they are always sedated to make sure they are as comfortable as possible, and to reduce the amount of coughing.
Analysis of the lavage fluid by a pathologist will provide a differential cell count (percentage of cell population) as well as morphological assessment (appearance of the cells), which allows us to categorise your horse’s asthma into mild, mild-moderate or severe asthma.
Accurately assessing the horse’s severity of asthma is important, as treatment can vary depending on the severity of disease present.
Forage is the biggest contributor to organic dust particles inhaled by horses.
How do we treat Equine Asthma?
As discussed above, assessing the severity of disease is important to ensuring that your horse receives the most appropriate treatment for their condition.
Treatment is usually via the systemic (i.e. oral/intravenous/intramuscular) administration of corticosteroids + bronchodilators, or via inhaled corticosteroids +/- bronchodilators.
Systemic medications include oral or injected dexamethasone, as well as oral bronchodilators such as Clenbuterol (e.g. Airway Gel, Ventipulmin.) It is worth noting that systemic administration of corticosteroids has been found to be less effective than inhaled administration.
Furthermore, the use of bronchodilators alone is not likely to be of any benefit in treating equine asthma, and conversely can risk exacerbating disease due to increased delivery of organic particles (dust, pollens etc) in horses that have not been removed from these environments.
Until recently, veterinarians were recommending use of asthma puffers (an adaptation of human puffers, featuring a “cup” which was placed over the nostril, and an attachment to the cartridge containing the relevant medicines – usually a combination of a corticosteroid and a bronchodilator.)
These were clinically observed to be of relatively poor efficacy, due to the difficulty in getting a good seal over the nose, as well as the difficulty in timing each “puff” with inhalation to ensure optimum uptake of the drug.
Recently, there have been exciting new developments in this space.
Aservo® Equihaler® – a new device manufactured by Boehringer Ingelheim, the Aservo® comes pre-loaded with ciclesonide and is inserted into the horse’s nose (up the nasal passage) and the medication is delivered as a soft mist. The Breathview® breath indicator allows for accurate timing for the medication to be delivered.
Flexineb® E3 Nebuliser – these function as an over-muzzle mask and deliver aerosolised medications to the horse through an enclosed, sealed system – allowing for maximal inhalation of the medications.
Nutraceuticals may be of benefit in treating horses with Equine Asthma. The supplementation of asthmatic horses with Omega-3’s in their biologically active forms (EPA & DHA) has been observed by researchers to decrease the number of neutrophils present in bronchoalveolar lavage fluid (BALF) after 8 weeks when compared to the placebo group.
Pasture-induced asthma provides the biggest challenge in both treatment and management, due to the relationship between pollen exposure and increased resistance to treatment over time.
What environmental management is needed for asthmatic horses?
In all horses with asthma, environmental management is key. Organic dust particles play a key role in the pathogenesis of asthma, and as such environmental dust should be reduced as much as possible. In horses that are stabled, this may mean a change in bedding to a low-dust alternative (or removal of bedding and replacement with dense foam matting).
However, though a change of bedding can decrease the inhaled dust by up to 50%, the highest contributor to inhaled dust is actually the horse’s forage.
Any hay fed should be at least immersed in water (30% dust reduction) or soaked for 30 minutes (90% dust reduction.) Unsoaked hay should not be fed in a haynet or ad-lib (i.e. access to a hayroll). Dust and mould are far more likely to be an issue in round bales than in soaked hay from square bales, and it is always my recommendation that horses with asthma are not fed from a round bale.
Anecdotally, late-harvest, second cut hay stored in a dry environment is considered to be the lowest risk for dust and mould/fungi content.
What about horses with pasture-induced asthma?
This “subcategory” of equine asthma is important to address on its own. As mentioned previously, horses with pasture-induced asthma exhibit a seasonal pattern to their asthma. In the US, this is known as “summer pasture associated asthma,” as it is commonly observed to occur in the summer months.
Conversely, data from Australia shows that horses with this particular type of asthma tend to present in Spring or “Hayfever season.” This likely has a relationship to the production of pollens by the grasses, however may be exacerbated due to increased dust from harvesting (such as cutting hay.)
A link to the Australian Society of Clinical Immunology and Allergy (ASCIA) Pollen Calendar can be found here, and may provide an indicator of potential allergens affecting your horse depending on the months they are affected: https://www.allergy.org.au/patients/allergic-rhinitis-hay-fever-and-sinusitis/guide-to-common-allergenic-pollen
The exposure of horses to pollens leads to the generation of Th2-type asthma as discussed previously, however if this exposure is repeated over time, the body starts to produce Th17 responses, leading to neutrophilic airway inflammation and decreased response to corticosteroids over time.
Therefore, wherever possible, it is essential that horses experiencing pasture-associated asthma are segregated from the inciting grass pastures during inflammatory periods. The difficulty in achieving this when grass pollens can affect a horse up to 144 kilometres away from the source (Costa et al., 2006) is not easy – however it is worth noting that this type of disease is progressive and will become more resistant to treatment with repeated exposure.
Some clients do trial spelling their horses in distant areas to “trial” if there is significant improvement in clinical signs, and may opt to rehome their horses to areas where they are not affected by asthma to ensure a better quality of life.
If you think your horse may have Equine Asthma, please contact the clinic to book an appointment to have your horse investigated. You can email us at hello@capeequine.com.au or contact us by phone at 0467 485 976.
Reference List:
Couetil, L. et al. (2020) ‘Equine asthma: Current understanding and future directions’, Frontiers in Veterinary Science, 7. doi:10.3389/fvets.2020.00450.
Lavoie, J. ‐P. et al. (2019) ‘Effect of different doses of inhaled ciclesonide on lung function, clinical signs related to airflow limitation and serum cortisol levels in horses with experimentally induced mild to severe airway obstruction’, Equine Veterinary Journal, 51(6), pp. 779–786. doi:10.1111/evj.13093.
Renaud, L. (2021) ‘Treatment and Management of Mild/Moderate and Severe Equine Asthma’, in AAEP Proceedings - In Depth: Respiratory Diseases In Horses. AAEP, pp. 469–478.
Calzetta, L. et al. (2018) ‘Clinical effect of corticosteroids in asthma‐affected horses: A quantitative synthesis’, Equine Veterinary Journal, 50(5), pp. 594–601. doi:10.1111/evj.12815.
Couëtil, L.L. et al. (2016) ‘Inflammatory airway disease of horses—revised consensus statement’, Journal of Veterinary Internal Medicine, 30(2), pp. 503–515. doi:10.1111/jvim.13824.