IN-DEPTH: PREVENTATIVE MEDICINE: BEING PROACTIVE IN PRACTICE


compared to straw, wood shavings are a preferred al- ternative.69 However, two different studies from the same research group have shown that wood shavings were associated with significantly higher airway neutrophilia in healthy horses compared to peat shavings,72 and baled peat was associated with lower airway neutrophil percentages compared to straw pellet or loosely stored peat.73 Other low-dust bed- ding options include shredded paper or cardboard or a rubber mat.74 The potential benefit of choosing low-dust bedding and feed options cannot be over- stated as one study by Woods et al. showed that compared to wood shavings and pelleted feed,man- agement systems utilizing straw bedding and hay generate 30 times more respirable dust in the horse breathing zone.62 Housing and management play significant roles in


organic dust exposure for horses and in clinical signs of EA.64 General recommendations include housing with good ventilation (at least 2 openings for fresh ventilation), grooming the horse outside, not storing dry hay above horse stalls, decreasing barn clutter that accumulates dust, turning horses out while stalls are cleaned and barn aisles are swept and waiting at least 60minutes before bringing them back in, using low-dust bedding such as wood shavings, not using straw for bedding, not feeding dry hay, not using leaf blowers to clean barn aisles, and opting for pasture as much as possible (ideally 12hours or more each day) as breathing zone levels of respirable particulate and endotoxin are significantly less at pasture compared to stabling in a low-dust environment.63,64While pas- ture turnout is optimal for avoiding triggers for horses with barn or hay-dust asthma, horses with severe equine pasture asthma (EPA) experience exacerbation of asthma when grazing during the summer.75 High heat and humidity are additional environmental fac- tors that contribute to triggering exacerbation of EPA.13,76 Indeed, asthma exacerbation due to high heat and humidity has also been demonstrated in horses with previously diagnosed sEA, indicating that sEA and EPA are notmutually exclusive. A retrospec- tive study by Bullone et al. showed significant positive correlations between higher daily heat and humidity, increased respiratory clinical score (worse symptoms), and decreased pulmonary function.76 Clinical signs of asthma improve with hours to days of removing affected horses from pasture into a stall environ- ment,13 which strongly implicates pasture-associated particles as the inciting allergens. A study by Costa et al. identified increased grass pollens and fungal spores as being temporally associated with EPA exacerba- tions.11 High humidity is known to shatter pollen that is otherwisetoo largetoreach thelower airway (>10 mm) into respirable particles (≤5mm).77 Increased dew point temperature is also known to increase release of Nigrospora conidia and basidiospores.78 Fungal par- ticles have also been identified as triggers for mEA and sEA.13


410 2022 / Vol. 68 / AAEP PROCEEDINGS


5. Future Directions for Decreasing Organic-Dust- Induced Lower Airway Inflammation in Horses


gen-specific immunotherapy (AIT) for treating extrin- sic atopic asthma in people is compelling, the evidence for AIT for treating EA is conflicting. In the majority of humans with asthma, the airways become sensi- tized to one or more environmental allergens such as pollen fromtrees, grass, or weeds (seasonal allergens), animal dander, excretions of house dust mites, molds (perennial allergens), insect stings, drugs, or food. Sensitization occurs through increased activation of allergen-specific T lymphocytes that switch to Th2 cells and secrete mediators that cause B lymphocytes to switch immunoglobulin isotype production toward immunoglobulin E (IgE).51Upon subsequent exposure to offended allergens, IgE gets cross-linked and indu- ces immediate release of inflammatory mediators including histamine, leukotriene, and cytokines that cause bronchial constriction and mucus secretion. This immediate Type I hypersensitivity reaction hap- pens within minutes of allergen exposure. Due to recruitment of granulocytes such as eosinophils, baso- phils, and T lymphocytes, this immediate reaction can be followed by a delayed Type IV hypersensitivity reaction about 8hours after the initial reaction.51 Allergen-specific immunotherapy aims to induce


clinical and immunologic tolerance to the offending allergen through administration of select antigen preparations subcutaneously or sublingually over


While the current data are limited, immunomodula- tory therapy is one possible strategy for preventing or minimizing deleterious environmental impacts on equine respiratory health. A study by Nogradi et al. demonstrated thatmEA and sEA horses fed a polyun- saturated fatty acid supplement containing 1.5 to 3 g docosahexaenoic acid for 2months showed signifi- cantly greater improvement in clinical score and air- way neutrophilia compared to horses receiving a low- dust diet alone and placebo.40 Future research could determine whether docosahexaenoic acid supplemen- tation also mitigates lower airway inflammation and/ or respiratory cell “responsiveness” in horses with- out clinical signs of EA. There have also been several studies on the effects of inhaled nanopar- ticle-bound cytosine-phosphate-guanosine immu- notherapy in horses with asthma.79–81 The current hypothesis is that these particles interact with lower airway immune cells through toll-like receptors to activate regulatory T cells and restore balance to the Th1/Th2 response. There is also evidence of an anti-inflammatory effect of this treatment, with a decrease in BAL supernatant interleukin-4, IL-8, and interferon-g in treated horses.80 Future research could evaluate whether this therapy might also work as a prophylactic for horses enter- ing intensive training, such as young racehorses, as training has been associated with changes in re- spiratory immune function and increased airway neutrophilia.82 While the evidence for “allergy testing” and aller-


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