The Effect of Management Regime on Airborne Respirable Dust Concentrations in Two Different Types of Horse Stable Design

Highlights

• Steamed hay + shavings = lowest dust in the breathing zone (BZ) and stable zone (SZ).

• Steaming hay reduces dust in BZ.

• Shavings as bedding produces less dust than straw in the general SZ.

• Dry hay + straw regimes is particularly dusty in American barn systems.

Abstract

Airborne respirable dust (ARD) (<5 μm) in the stable environment is strongly implicated in equine asthma. Bedding, forage, ventilation rate, and stable management activities all contribute to ARD. This study investigated the relationship between ARD in the breathing zone (BZ) and the stable zone (SZ) in eight American barns (ABs) and eight stable complexes with individual stables under four different management regimes. Airborne respirable dust was measured in eight replicate stables per regime in ABs (n = 32) and eight per regime in single stables (n = 32). Regimes were as follows: (1) steamed hay and shavings; (2) dry hay and shavings; (3) haylage and straw; and (4) dry hay and straw. Samples were taken in quite periods between 3 PM and 4 PM hours. Airborne respirable dust levels were analyzed using Split-Plot analysis of variance and Wilcoxon matched-pairs test (Genstat 15) with P < .05 as significant. The lowest ARD (<360 RP/L air) was found for shavings and steamed hay in AB and single stables in both BZ and SZ. Straw and dry hay in ABs produced ARD of 6,250 (SZ), 5,079 RP/L air (BZ) which was greater (P < .05) than the 2,901 (SZ) and 942 (BZ) RP/L air in single stables. Straw and haylage produced more ARD across both zones in single stables compared with ABs. Shavings and dry hay = more dust in the BZ than in the SZ, whereas straw and haylage = higher ARD in the SZ versus the BZ across both stable designs. Using dry hay and/or straw cannot be recommended as a suitable management regime for stabled horses.

Introduction

It is widely acknowledged that airborne respirable dust (ARD) of <5 μm in size found in the stable environment has a major negative impact on respiratory health in horses and can cause the debilitating allergic condition known as equine asthma [1], [2], [3], [4], [5]. The economic impact of respiratory problems in racehorses can be considerable and are linked to poor performance in young Thoroughbreds [6]. Respiratory problems as a whole are the second biggest cause of days lost to training in the Thoroughbred industry [7], [8].

Airborne dust in the stable environment is composed of different sized particles of plant fragments, mites, bacteria and mould spores, yeasts, and endotoxins [9] with the level and composition being influenced by the choice of bedding [10], forage [11], and horse care activities, such as mucking out and sweeping [12]. The persistence of dust in the air and the probability of inhalation by the horse into the different regions of the respiratory tract are influenced by release rate from bedding and forage, particle size [13], and the speed of removal by ventilation rate which can vary greatly between stables [14], [15]. In humans, dust entering the nose and throat (inhalable dust) is composed of particles of <100 μm, the extrathoracic portion that reaches the larynx is 10–100 μm, whereas tracheobronchial fraction (potentially caught on the mucociliary boarder) is 10 μm [16], although May et al [17] have suggested that a small percentage of these particles can reach the lower airways. However, it is widely accepted that the potentially allergenic dust to horses, which can reach the noncilia protected alveoli, is similar to in humans at <5 μm in size [13], [18], [19] and includes some of the fungal spores, notably Aspergillus spp (2–4 μm) which are of optimal size for alveolar deposition. Thermophilic actinomycetes such as Thermoactinomyces vulgaris and Saccharopolyspora rectivirgula are small enough at <1 μm to flow freely with inspired and expired air, but repeated exposure increases the chances of particles being caught in the alveoli and so these actinomycetes are also implicated in the allergic respiratory disorder equine asthma [20].

Airborne particles obey Stokes Law [21] and sediment at velocities proportional to the square of their radii, and this can result in particles remaining in the air for extended periods, increasing the likelihood of inspiration by the horse. The best way of reducing the animal: dust interaction when horses are housed is to ensure adequate ventilation and to reduce contamination of the air from sources such as bedding and forage.

Many horses are now kept in loose boxes within an American barn (AB) system and thus share airspace. Jones et al [22] reported serious negative consequences in terms of respiratory health, when seven horses shared airspace of approx. 39 m², particularly during calm conditions when the ventilation rate was low at only 6.6 changes of air/hr. Increasing the ventilation rate in an AB system by leaving windows and doors open was reported [23] to significantly decrease particulate matter in the air; however, horse owners often cite inclement weather as a reason for closing doors and windows. Art et al [13] suggested that the benefits of a low-dust regime in one stable can be lost; unless, the adjacent stables are also on low-dust regimes and this cross-contamination could be more problematic if ventilation rate is compromised by closing windows and doors.

In single stables, commonly 3.6 × 4.2 m, the airspace available to the horse is restricted by roof height, and air movement can be negligible particularly if back or side windows are kept closed. However, the horse only inhales dust created from its own bedding and forage and does not have to be subjected to the dust created from neighboring stables or from horse management activities such as yard-sweeping, filling hay nets, and grooming, which can negatively impact on animals in ABs.

Previous work [17] has shown that time of day, stable construction, and ventilation can influence ARD in the general stable zone (SZ) in TB stables. However, the study did not compare the effect that different types of bedding and forage can have on the dust levels in the breathing zone (BZ), compared with the SZ in different stable designs.

The aim of this study was to investigate the contribution of bedding and forage on ARD in the BZ and general SZ when horses were kept on different management regimes. The contribution of different management regimes to ARD within single stables or when sharing air space as in an AB was also determined with the objective of identifying suitable management regimes for each housing system.