COMPROMISE of the respiratory system is one of the main reasons for poor performance in the horse (Buechne-Maxwell et al, 1996). Recurrent Airway Obstruction (RAO) is an allergic, inflammatory and obstructive airway disease that is often associated with older horses.

The condition is like that of human asthma. It is mostly associated with horses that are stabled and fed on hay. It can result from the inhalation of dusts, such as those associated with hay and bedding materials. These dusts contain allergens, endotoxins, moulds and other small particles that can start the severe inflammation of the lungs associated with RAO. This condition affects between 10-15% of adult horses living in temperate climates (Hotchkiss, Reid and Christley, 2007).

Respirable dust particles are generally considered to be particles which are between 0.5 and 5 µm in diameter (Clements and Pirie, 2007b).

These particles are thought to be small enough to penetrate the small airways. The number of these particles present in an environment and particularly in the breathing zone (in the region of the nostrils) is considered to be a good index of the health hazard posed by airborne dust to humans and horses.

There are many management practices recommended for reducing horses’ exposure to dust in the stable. Consideration should be given to management practices such as choice of bedding and forage type, location of forage and bedding stores and mucking out and yard sweeping routines.

The conclusion of a study by Elfman et al. (2009) on the influence of the horse stable environment on human airways was that improved management of the stable climate would be beneficial for the health of both stable workers and horses. A range of dust masks are on the market for use by humans to minimise the exposure of their airways to inhalation of dust particles.

At CAFRE, as part of a student project, the dust levels in the Equitation Unit are currently being investigated. Dust levels at forage feeding time are being measured in stables in different locations within the American Barn block when horses are being fed hay and haylage.

Some findings from studies into dust levels and control systems are outlined in this article.

HAY AND HAYLAGE

Hay should be avoided, especially round baled hay (Camargo et al, 2008).

If hay is fed it should be soaked or steamed. Work by Moore-Colyer (1996) indicated that soaking hay in a net for 30 minutes reduced the number of respirable particles by 90%. There was no significant further reduction obtained by soaking the hay for 12 hours.

Prolonged soaking of hay reduces the nutritive content of the hay due to leaching of soluble nutrients. An additional down side of soaking hay is that the liquid produced as part of this process has a high Biological Oxygen Demand (BOD) making it a dangerous environmental pollutant. This liquid should be disposed of appropriately.

Work by Clements and Pirie (2007b) found that feeding immersed hay (hay immersed in a bucket of water until completely submerged and then fed immediately) and feeding soaked hay (hay which had been immersed in a bucket for 16 hours prior to feeding) resulted in 60% and 71% reductions in mean Respirable Dust Concentration (RDC) respectively compared to feeding dry hay.

There was no significant difference in mean RDC between immersed and soaked hay. Feeding immersed hay resulted in a 53% reduction in maximum RDC compared to feeding dry hay. It is important not to let hay that has been immersed or soaked dry out as this would allow the airborne release of particles to recur (Robinson et al., 2001).

Vandenput et al., (1997) investigated in the laboratory quantities of dust particles in the respirable particle size range from a range of feedstuffs and bedding materials. Haylages (with dry matters of 78% and 50%) and alfalfa pellets generated low levels of dust compared to good hay (84% dry matter).

Rolled grains produced high levels of respirable particles compared to whole grains and molassed concentrates. Rolled grains generated more dust than the good hay. Flax straw produced lower levels of dust than good straw (the type of straw was not specified) which was less dusty than wood shavings. All of the bedding materials generated less respirable dust than the good hay.

BEDDING

Bedding material has an important effect on stable air quality (Fleming, Hessel and Van den Weghe, 2008). These researchers studied airborne particle formation when wheat straw, wood shavings and wheat straw pellets were used as bedding materials for horses.

Airborne particle concentration was lowest with straw pellets followed by the wood shavings and the wheat straw. Based on their laboratory studies they stated that bedding such as hemp and wood shavings often considered to be particle reducing alternatives to straw did not fulfil these requirements.

In their yard based study high average levels of dust were observed with the wheat straw. Wheat straw was added to the stable on a daily basis whereas wood shavings and straw pellets were not topped up during the experimental period. They also suggested that more dust was generated as a result of horses moving in the straw bedding material when compared to them moving in the other materials used.

Work by Kirschvink et al, (2002) found that fewer particles were generated when shredded cardboard was used compared to straw. Chopped paper is a bedding type considered by many to produce low amounts of dust.

FORAGE/BEDDING

COMBINATIONS

Work by Clements and Pirie (2007a) looked at RDC in ponies’ stables under four different management regimes: Fed on hay and bedded on wood shavings; fed on hay and bedded on straw; fed on haylage and bedded on wood shavings and fed on haylage and bedded on straw.

Their findings were that the wood shavings/haylage environment had significantly lower mean RDC in the breathing zone than the straw/hay environment.

The straw/haylage environment had significantly lower RDC than the wood shavings/hay environment and the straw/hay environment. The wood shavings/hay environment had a significantly lower RDC than the straw/hay environment.

They concluded that feed had a greater influence on mean and maximum RDC than bedding. Changing horses’ feed from hay to haylage can result in a reduction of approximately 60-70% in mean RDC and a reduction of 76-93% in maximum RDC depending on bedding type.

Changing bedding from straw to wood shavings resulted only in a reduction of approximately 23-32% in mean RDC and a 19-78% reduction in maximum RDC depending on feed type. Feeding haylage is preferable to feeding hay when considering equine respiratory health.

Clements and Pirie (2017b) also concluded that optimising the management system in one stable resulted in a significant reduction in the background mean and maximum RDC in a second stable within a common airspace.

They stated that this finding supports findings by Art, McGorum and Lekeux (2002) that the beneficial effect of careful management in one stable can be totally lost by hay or straw storage on close proximity or by failure to address the environment in adjacent stables.

HOUSING

Ideally horses which suffer from RAO should be managed at pasture (Camargo et al., 2008). Pelleted food should be provided to supplement the diet at grass when there is limited grass available as opposed to hay.

If circumstances necessitate stabling affected horses should be moved to a place in the stable block or yard where exposure to airborne particles and allergens is reduced. This should be away from areas where hay and straw are stored and used and away from any manure/muck heaps (Mair and Derksen,2000).

Hay and straw should not be stored in loft areas above stables.

MANAGEMENT

Clarke (1987) suggests that removing horses from stables during periods of increased activity, such as mucking out, will minimise their exposure to peak levels of fungal spores.

Clements and Pirie (2007b) found that 81% of all maximum RDCs were recorded within 35 minutes of peak stable activity, such as removing the horse from the stable and mucking out.

They looked at dust levels in two adjacent stables - stable one (housed a pony) and stable two (no pony, feed or bedding) which were within one building with one common entrance and a common airspace. Some 81% of the maximum RDCs recorded in stable two occurred while stable one was being mucked out. They found a 19-fold increase in RDC during mucking out of stable one and a nine-fold increase in RDC in stable two while stable one was being mucked out.

Clarke, Madelin and Allpress (1987) reported increases of two to 16 times when racehorse stables (bedded on deep litter wood shavings) were mucked out and that mucking out in a poorly ventilated barn that housed 16 horses resulted in a 45-fold increase in RDC.

MUCKING OUT

The practice of removing all horses within a common airspace during mucking out and grooming should be considered (Clements and Pirie, 2007b). This is not always possible but might be workable if horses are being turned out daily to fields or turnout paddocks, if they are being put on to a horse walker or if they are out for exercise as a group for example in racing yards.

Provision of adequate ventilation helps to remove dust particles from the environment (Leguillette, 2003). Mair and Derksen (2000) stated that eight to 10 air changes per hour were required to ensure adequate ventilation in a stable.

CONCLUSION

Dust levels in the stable environment can contribute to respiratory problems in humans and horses. Feeding hay is considered to present the biggest problem in relation to generating respirable dust particles.

Fodder is considered to present a greater dust risk than bedding materials as horses spend time with their nostrils in the fodder when they are ingesting the forage thus being exposed to any dust present in the fodder. Implementing systems to reduce levels of dust present in the stable environment such as feeding haylage as an alternative to hay can enhance the respiratory health of horses.