The horse's respiratory system
Respiratory Disorders in Horses
Both the incidence and recognition of respiratory
disease in horses has increased over the past two
decades, and recent research has indicated the need
to pay close attention to the environment, to reduce
the level of exposure to the disease causing agents.
If the horse is suffering from a viral or bacterial
infection of the respiratory tract, or an allergic
response to the presence of inhaled fungal or actinomycete
spores or noxious gases such as ammonia, this can
cause inflammation of the small airways, increased
mucus production and bronchiospasm, and the condition
can be exacerbated by a 'dusty' environment. Indeed
the connection between respiratory tract infection
and allergic respiratory disease is well known in
human medicine, and it appears that horses are more
sensitive to respiratory allergens (agents causing
an allergic response) after infections of the respiratory
tract, since the damage to the airways caused by the
infection allows easier access of airborne allergens
to the sensitive tissues.Thus, not all viral or bacterial
infections will result in the horse becoming sensitized,
but recovery time after respiratory infection is prolonged
in the presence of environmental contaminants. Once
a horse or pony becomes sensitized - and this can
occur over a period of years of being exposed to environmental
pollutants - it will remain so for life, whether that
sensitization was due to exposure to a severe challenge
or as a sequel to respiratory infection.
The major respiratory allergens
include:
- Fungal and other respiratory spores - especially
the thermophilic actinomycetes
- Ammonia and other noxious gases
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The Contaminants
Fungal Spores
Hay is probably
the single most common source of fungal spores, most
commonly caused by baling at too high a moisture content.
A horse eating mouldy hay or stabled on a contaminated
bed may inhale millions of spores with every breath.
Perhaps more importantly, significant levels of fungal
contamination may be seen in hay which looks and smells
'clean'. Thus the challenge to the respiratory system
probably exists to a greater or lesser degree with
all horses that are still fed hay.Even when hay is
soaked, the horse still ingests the spores, along
with any toxins which may be present and as the wet
hay falls to the floor and dries, the spores will
again become airborne and inhaled by the horse, and
will contaminate the bedding. Other feedstuffs, such
as poorly stored or dried oats, can also be a source
of fungal contaminants.
 Bedding
Even the cleanest of straw will contain more fungal
spores than alternative beddings such as wood shavings,
paper or synthetic beddings. Deep litter should be
avoided, as significant moulding can occur, in addition
to the build-up of noxious gases, (ammonia) and the
larvae of gastrointestinal parasites.
Typical photomicrograph of contaminated hay, straw
or plant-based bedding. There are large numbers of
small respirable fungal spores
Ammonia
Ammonia released from deep litter or poorly managed
bedding, where damp patches are left, can seriously
damage the horse's airways and reduce the efficiency
with which the horse copes with other respiratory
allergens such as fungal spores.
Reducing the Respiratory Challenge
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The introduction of semi-wilted
forage in place of hay in the ration will probably
make the single greatest contribution to reducing
the respiratory challenge to all stabled horses
and ponies - since all are being challenged, and
if the challenge continues, horses which are apparently
unaffected may eventually become sensitized.
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Straw bedding, and all forms
of deep litter should be replaced with one of the
materials outlined above, and well managed, as even
shavings or paper can become contaminated if damp
patches remain, releasing ammonia into the horse's
environment.
-
Ventilation is essential to reduce
the environmental pollutant level, thus good stable
construction is essential with an adequate number
of air changes occurring throughout the stable,
without draughts being set up - an extra rug is
definitely preferable to closing the top door. This
may need some adaptations to older buildings.
-
Siting of stables is important,
so that the pollutants from a nearby hay or straw
barn, or dungheap are not carried past the horse's
door. Similarly, there is little point in trying
to control the environment of one horse, if in a
shared airspace there is another horse eating hay
on a straw bed.
-
Feeding high quality, mould-free
feeds is essential and commercial cubes or molassed
feeds usually provide dust- free rations.
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In addition to the management
changes outlined above, in severely affected horses
and ponies, drugs may be necessary to control the
respiratory disease, and veterinary advice should
be sought.
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More recent research has highlighted
the importance of anti-oxidants in lung health.
Recent work has indicated that horses suffering
from RAO may have disturbed oxidant-antioxidant
equilibrium. There are now antioxidant supplements
on the market aimed specifically at horses suffering
with RAO.
Photomicrograph of HorseHage showing negligible fungal
contamination
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