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Equine Asthma, Heaves in Horses
Equine Asthma, Heaves in Horses
Chronic Obstructive Pulmonary Disease (COPD)
Recurrent Obstructive Airway Disease (ROAD)
Summer Pasture Associated Obstructive Pulmonary Disease (SPAOPD)
Introduction
It may start as a transient cough when the horse begins exercising but over time with repeated episodes equine asthma, or heaves, can become a severely debilitating respiratory condition where the horse fights for every breath it takes. Heaves is the traditional name but through the decades the scientific community has put a number of acronyms on it COPD (chronic obstructive pulmonary disease) RAO (respiratory airway obstruction) and then a number of recombinations of the above. Also added are horses with heaves but with the unusual history of worsening on summer pasture, SPAOPD (summer pasture associated obstructive pulmonary disease!). Is was with some relief that the veterinary community has settled on the catch-all term "equine asthma" (EA) for this chronic non-infectious lower airway disease.
Equine asthma is caused by repeated exposure to inhaled allergens. It begins as a transient cough that usually improves with exercise and worsens with stabling. As chronic changes predominant the cough worsens and difficult breathing predominates. This article discusses causes, diagnosis, treatment, prognosis, and prevention of this potentially debilitating disease.
Causes
Allergens: mold spores, pollens(?)
EA occurs throughout the world with most cases developing in response to repeated exposure to hay with high levels of mold spores in it. In some parts of the world there are reports of a form of EA that is associated with being out on summer pasture and has been called summer pasture associated obstructive pulmonary disease (SPAOPD) or now equine pasture asthma (EPA). The cause has not been identified and it may turn out to be pollens or molds associated with certain summer grasses. Found mostly in the south eastern USA it has been reported in Oregon and the United Kingdom during hot summers. Most commonly high humidity seems to potentiate EPS but there are reports of a UK form that seems exacerbated by dry hot weather.
It seems probable that molds are not the only cause of COPD, but other causes have not been proven. When critical testing is done on COPD horses almost 100% of COPD horses, respond adversely to mold spores while the controls do not. There is no doubt that other environmental factors play important but usually secondary roles in EA and may because of their effect on molds and sporulation.
Genetics
A often asked question is why some horses develop COPD and others do not while under identical management conditions. Reviews of equine family histories has found a strong genetic predisposition for developing chronic respiratory allergies. In one case a autosomal dominant form of inheritance is implicated. This has important implications for management to prevent the onset of horses predisposed and may be a considered factor in developing a breeding program.
Respiratory Viral Infections
There are two aspects of the association of virus's and EA. It has been seen where horses develop EA following a viral infection. It has been conjectured that the viral infection may have attenuated the lungs immune system making it hyper sensitive to allergens so the horse goes on to develop EA. The second aspect is Equine Herpes Virus 5 (EHV-5). EHV-5 is a chronic lower respiratory virus that causes a progressive fibrosis of the lungs. In its early stages it can resemble EA and even has a similar cytological appearance on a broncho-alveolar lavage. The biggest difference clinically in the two diseases is that EHV-5 does not respond as well to steroids and not at all to environmental management.
Clinical Signs
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