Equine
Asthma: New Term for Old Problem
May is
National Asthma & Allergy Awareness Month
by Drs.
Kathleen Ivester and Laurent Couetil, Purdue University College of Veterinary
Medicine.
For nearly as long as horses have
been domesticated, the relationship between barn confinement and respiratory
disease in the horse has been recognized. This relationship is intuitive,
especially when we consider that deep in the lung, where the blood takes up
oxygen, the barrier between the outside air and the horse’s circulation is as
thin as a couple of cells.
The surface area of this
gas-exchange region of the lung has on average a surface area of 2,500 square
meters (26,900 square feet), equal to nearly half a football field. The
response of the lung’s immune system to inhaled air results in a number of
diseases in both humans and horses. Many of the occupational respiratory
diseases in humans are associated with agriculture due to exposures to organic
dusts. Dusts in agricultural settings, including the horse barn, are rich in
substances such as endotoxin and fungi that can drive inflammation.
Depending on when the conversation
took place, horse owners consulting their veterinarians will have heard many
terms applied to this problem: broken wind, heaves, chronic obstructive
pulmonary disorder (COPD), bronchiolitis, small airway disease, recurrent
airway obstruction (RAO), inflammatory airway disease (IAD), and most recently,
equine asthma. Along the way, distinctions have been drawn between those horses
that develop difficulty breathing at rest (heaves, RAO, severe equine asthma)
and those that do not (IAD, mild equine asthma). While potentially leading to
confusion for the horse owner, the changing terminology actually reflects
improvements in veterinary knowledge.
While the term COPD was applied to
reflect the obstruction or blocked airflow that can occur in severe cases, this
term was discarded due to the differences with the human disease. In humans,
COPD is mainly a consequence of cigarette smoke and is characterized by
structural changes within the lung that are absent in the horse. While the
terms ‘recurrent airway obstruction’ and ‘inflammatory airway disease’ are
descriptively accurate, they are not necessarily terms that immediately help
the horse owner to understand the disease process.
A More Relatable Term
Due to its many similarities with
the human disease, the term ‘equine asthma’ has most recently been adopted.
Like human asthma, equine asthma is triggered by inhalation of dusts that
contain allergens and other irritants, and like human asthma, the cough and
difficulty breathing can be reversed in the short-term by medications, often
delivered by inhalers, or in the long-term by removal from the offending dusts.
Also similar to asthma in humans, the response of the horse’s airway to inhaled
dusts can vary widely. In some highly susceptible horses, inhalation of even
small amounts of dust in the barn environment or airborne allergens (e.g.
pollen, molds) at pasture can cause severe inflammation and difficulty
breathing due to the accumulation of mucus and narrowing of the airways. There
is no known cure for these ‘severely asthmatic’ horses, and they require
special management for the duration of their life span.
In other horses, the inflammation
is milder, with occasional coughing and decreased performance. Respiratory
problems in these ‘mildly asthmatic’ horses often become apparent only when the
horse is asked to perform athletically and may resolve over time. Those horses
with mild asthma do not necessarily go on to become severely asthmatic.
While susceptibility varies widely,
any horse (or person) exposed to enough dust will develop inflammation in the
airway. In the case of stabled horses, dust exposure is mostly due to hay.
Those horses with severe asthma often require that hay be completely removed
from the diet and that the horses be removed from confinement to the barn
altogether.
In the milder cases, decreasing the
dust released from hay by soaking or steaming may improve airway health. As
soaking has a number of draw backs, high temperature steaming is becoming the
preferred method to reduce exposure to the dust from hay. At Purdue University
College of Veterinary Medicine we use a Haygain hay steamer which, has been
shown to reduce respirable dust by 98%.
Article provided by Haygain,
manufacturers of Haygain Hay Steamers and ComfortStall Orthopedic Sealed
Flooring. For more information, visit www.haygain.com.
