Written by Becky James BSc MSc
Non-infectious inflammatory lower airway diseases of horses share marked similarities to human asthma in terms of causes, clinical presentation and response to therapy. For these reasons the medical term “Equine asthma” has recently been agreed to describe these problems. Severe Equine Asthma (also known as COPD, RAO and heaves) affects horses predominantly over 8 years old and according to an owner-based survey it is estimated the prevalence is between 10-17% in the UK.
In contrast, mild to moderate equine asthma also known as Inflammatory Airway Disease (IAD) can affect horses of all ages and clinical signs are usually subtle, including poor performance and occasional coughing but with normal breathing at rest. A study by Dauvillier and Erck found the prevalence of IAD was 84% in a population of 482 horses. This is supported by previously published research including Gerber et al (2003) who analyzed two age groups of asymptomatic well-performing sport horses and found that although clinically healthy, all of the examined horses housed in a conventional stable environment showed evidence of inflammation in their airways.
The aims of medical therapy of equine asthma are to control airway inflammation and relieve airflow obstruction thereby controlling the clinical signs of the disease. However, it is important to understand the causes so you can treat the source of the problem to manage or even avoid the disease not just ease clinical signs in horses already suffering with Equine Asthma.
High dust concentrations are known to be found within the stable environment. The respirable fraction can contain a variety of organic and inorganic particles including fungi, moulds, endotoxin, beta‐D‐glucan, ultrafine particles (<100 nm in diameter), microorganisms, mite debris, vegetative material, inorganic dusts, and noxious gases.
It is well documented that the main source of respirable dust in the stable environment comes from the bedding and forage making them instrumental in the development of this disease.
More than 50 different species of microoganisms have been identified in stable air and large numbers of these potentially harmful tiny particles are breathed in by the stabled horse every day.
The choice of bedding and whether you muck out each day or do deep-litter influences the level of dust in the general environment but the major influence in terms of what the horse is inhaling (its breathing zone) is the forage. 
Dry hay can generate 932,090 Airborne Respirable Dust (ARD) particles per litre of air, the average horse has a mean tidal volume of 5 litres so a horse could be inhaling 4.6 million ARD particles in every breath while their nose is in the hay! Although haylage is lower in ARD than dry hay, it can still contain 4500- 8800 ARD per litre of air.
The traditional treatment of soaking to reduce the dust, while it does make the dust less airborne, is counter-productive on several levels. Soaking leaches nutrients, increases bacteria, produces a polluting post-soak liquor, uses vast amounts of water and is messy and laborious.
High temperature steaming in a Haygain has been shown to not only reduce respirable dust in the breathing zone by up to 99% but also kill harmful bacteria, mould and fungal spores (Moore-Colyer et al 2015).
Key management tips:
- Plenty of turnout (unless horse has summer-pasture induced equine asthma)
- Good ventilation in the stable – open a window, clean/add vents, aim for 5 changes of air per hour
- Choose a low-dust bedding option
- Make sure stable activities such as mucking out/sweeping take place while the horse is out of the stable
- Store dry hay (and straw) in a separate building to the horses
- Hoover cobwebs, keep window ledges, door frames clean
- Regular disinfecting
References:
ACVIM June 2016 “The Prevalence of Fungi in Respiratory Samples of Horses with Inflammatory Disease” by Dr. J Dauvillier and Dr. E Westergren.
Couëtil LL, Cardwell JM, Gerber V, Lavoie J ‐P., Léguillette R, Richard EA. Inflammatory Airway Disease of Horses—Revised Consensus Statement. Journal of Veterinary Internal Medicine. 2016;30(2):503-515.
Gerber V, Robinson NE, Luethi S, et al. Airway inflammation and mucus in two age groups of asymptomatic well-performing sport horses. Equine Vet J 2003; 35:491–495.
Moore-Colyer, M.J.S. Taylor, J. and James, R (2015). The effect of steaming and soaking on the respirable particle, bacteria, mould and nutrient content in hay for horses. Journal of Equine Veterinary Science. Aug 2015
