Equine Asthma: New Term for Old Problem
May is National Asthma & Allergy Awareness Month
By Drs. Kathleen Ivester and
Laurent Couel, 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.
EQUINE Health
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%.
44 www.EliteEquestrianMagazine.com
Will Coleman and TKS Cooley
/www.EliteEquestrianMagazine.com