Understanding and Managing Equine Heaves/Recurrent Airway Obstruction (RAO)

Lisa Costa

January 2015

 Definition

Equine Heaves is a disease of the equine airways, which has been described as early as 333 B.C. (Morgan & Folch, 2011). It is characterized by a reversible narrowing of the airways via bronchoconstriction through inflammation as well as excessive secretion and accumulation of mucous. This results in a decrease of function of the pulmonary system during episodes of the disease (Morgan & Folch, 2011). The term “heaves” has previously been described and interchangeable with the term Chronic Obstructive Pulmonary Disease (COPD), however, recently the comparison between heaves and COPD has proven that they are not the same disease. Current and recent research is suggesting that heaves is more homologous to asthma than to COPD, as allergenic responses are characteristic when the horse is challenged by the etiological agents. From this switch of theory, heaves is now more acceptably termed Recurrent Airway Obstruction (RAO) (Robinson, 2001). 

Symptoms and Physiology

RAO has a wide range of symptoms and clinical signs, which are as follows (but are not limited to):

-          decrease in performance

-          exercise intolerance

-          cough

-          mucopurulent nasal discharge

-          mucous accumulation in peribranchiolar alveoli

-          mineralized mucous (branchiolar and/or alveolar plug)

-          bronchoconstriction

-          submucosal fibroplasia and fibrosis

-          smooth muscle hyperplasia

-          abnormal lung sounds

-          enlarged lung percussion field

-          abdominal effort

-          “heave line”

-          low blood PaO₂ values at rest   less than 95 mm Hg at sea level

-          high Alveolar-Arterial O₂ gradient immediately and persisting after exercise (more than           7mmhg) (Robinson, 2001)

These symptoms all are due to, or result in, an increase in airway resistance therefore causing an overall decrease in the respiratory system fuction. Due to this decrease, gas exchange between the alveoli (lungs) and the blood cannot occur at sufficient rates to sustain the actions of a horse, especially in exercising horses. From this, it is often seen that the horse will induce extra effort in is breathing, otherwise known as “heaving”. This creates a muscular build of the abdominal muscles involved in the extra exertion resulting in the well-known “heave line” (Morgan & Folch, 2011). RAO Horses that are allowed to reach a severe stage of the disease will become unable to partake in any form/level of exercise and may have to become candidates for euthanasia due to the welfare of their situation.

Etiology and Pathology

RAO is a subject under constant research, as the etiology is not completely defined and universally concluded on. In basic explanation, the airways are narrowed due to inflammation, constriction, and mucous accumulation due to hyper-reactiveness (Morgan & Folch, 2011). There is a range and variety of etiological agents causing this pathology, with the similarity among them being that they are airborne agents which is breathed in.  These agents are mainly found in your general “stable dust”, which is shown to be the cause of the majority of stabled horses presenting RAO (Robinson, 2001; Morgan & Folch, 2011). Summer-Pasture-Associated-Obstructive-Pulmonary-Disease (SPAOPD) is a form of heaves resulting from agents found in the outdoor pasture environment (Robinson, 2001).

Agents found to cause or relate to RAO and SPAOPD are as follows (but are not limited to):

-          moulds

-          endotoxins

-          dust mites, forage mites (and their feces)

-          fungal spores

-          thermophilic actinomycetes

-          organic dusts (β-D-glucans)

-          inorganic dusts

-          pollens

(Robinson, 2001; Morgan & Folch, 2011)

RAO has a similar pathogenesis to asthma, this being that it is understood as an allergic reaction to any of these airborne agents. The horse’s airway walls are hypersensitive to these agents. When these agents contact and bind to the various receptors found on the epithelial wall, a hypersensitive response is triggered initiating an allergic reaction. Through the mobilization of specific cells and signals (Th2 cells, mast cells, neutrophils, eosinophils, cytokines, etc.) inflammation, bronchoconstriction and mucous secretion occurs throughout the airways. This restricts the overall airflow into the lungs/pulmonary system as the horse’s body tries to prevent any more of the agent from entering its system. RAO is also characterized by the reversible nature of these processes once the horse is removed from the challenging environment and agents, allowing the horse relief from these symptoms (Robinson, 2001).

There is no current described relationship between the probability of a horse having heaves and its gender. However, there is evidence suggesting that the condition is heritable, with possible relationships among breeds and lines (Robinson, 2001; Gerber, et al.  2009).

Treatment    

                                              

Treatments are aimed at providing immediate relief of the constricted and inflamed airways as well as reducing the hyper-responsiveness, i.e. the allergic response, of the epithelial tissue in the airways. This is done through β₂-adrenergic suppressors and corticosteroids. The β₂-adrenergic suppressors act as a bronchodilator, suppressing the inflammation of the airways and providing immediate relief. This will reduce the bronchoconstriction and allow for mucociliary clearance. Daily administration of a corticosteroid is aimed at suppressing the hyper-responsiveness of the airways, to help prevent future allergic reactions to the allergenic agents. The best drug therapy shown to have the highest effect with no significant side effects for RAO is the use of aerosolized corticosteroids (Robinson, 2001). However, changing the management practices of the susceptible horses by diminishing the contact with the allergenic agents is critical in preventing the onset of RAO (Ivester & Couëtil, 2014).

Preventative

Along with the daily administration of corticosteroids to the RAO susceptible horse, it is important to try and remove the horse from environments or situations which put the horse in contact of the agents. This is very difficult considering is it commonly the housing conditions which are what presents the etiological agents (i.e. “stable dust”) to the horse. However, there are many easy practices to help diminish the amount of stable dust from coming in contact with your horse and its respiratory system. As well, the feeding management of your horses can result in the onset of RAO due to the various organic dusts, moulds, and mites that can be present in hay and grain (Ivester, et al. 2014).

“Stable Dust” Management:

-          cleaning/dusting/sweeping your stable regularly

-          only cleaning/dusting/sweeping your stable when horses are not present in the building

-          allowing sufficient time for airborne “stable dust” to settle after cleaning before bringing horses back into the environment

-          sprinkling water in your aisles using a watering can before sweeping to prevent airborne dust

Feeding Management:

-          soaking hay to suppress dust

-          steaming hay to suppress dust and fungal spores, kill molds and mites

-          wetting/soaking grain

-          feeding hay on a clean ground free of shavings, dirt and dust

-          not feeding hay out of a manger

o   A manger traps the dusts and fungal spores which accumulate in the air of the manger.

-          not feeding hay out of a hay rack or hay bag

o   By keeping a horse’s head high, is counteracts the natural feeding physiology of a horse. When a horse’s head is down, the neck and airways are aimed, encouraging more effective mucosal removal of captured agents. Also, the act of ripping hay out of a rack or bag will produce airborne dusts and fungal spores

Although a horse may not currently present indications that it is susceptible to RAO, it is important to be preventative towards the unaffected horses as well, as the disease often shows up later in a horse’s life (>5 years) (Leguillette, 2003). Partaking in these management changes may seem simple enough, and maybe even a bit redundant, however can be proven very effective. Most of the etiological agents that cause RAO are microscopic, and although you may not see a significant amount of dust in the air, there will still be microscopic agents airborne, putting the horse at risk (Ivester, et al. 2014). Of course, no practice can achieve perfect environmental status to completely protect a horse, but every small effort made will result in some aid, prevention, and relief of RAO.

References:

Gerber, V., Baleri, D., Klulowska-Rötzler, J., Swunburne, J. E., Dolf, G. (2009). Mixed inheritance of equine recurrent airway obstruction. Journal of Veterinary Internal Medicine/American College of Veterinary Internal Medicine. 23(3), 626-630

Ivester, K. M., Couëtil, L. L. (2014). Management of chronic airway inflammation in the horse: A systematic review. Equine Veterinary Education. 26(12), 647-656

Ivester, K. M., Couëtil, L. L., Zimmerman, N. J. (2014). Investigating the Link between Particulate Exposure and Airway Inflammation in the Horse. Journal of Veterinary Internal Medicine/American College of Veterinary Internal Medicine. 28(6), 1653-1665

Leguillette, R. (2003). Recurrent airway obstruction - heaves. Veterinary Clinics of North America - Equine Practice. 19(1), 63-68

Morgan G., Folch, H. (2011). Recurrent airway obstruction in horses - an allergic inflammation: a review. Veterinarni Medicina. 56 (1), 1-13

Robinson, N. E. (2001). International Workshop on Equine Chronic Airway Disease. Michigan State University 16-18 June 2000. Equine Veterinary Journal. 33(1), 5-19