Updated: May 8
The successful environmental control of disease involves maintaining the horse's level of exposure to the causative agents below which induce disease. this critical level is called Threshold Limiting Value (TVL). Unfortunately, the TVL for mold spores is unknown. Indeed, it is likely to vary considerably between horses. The best approach lies in minimizing the horse's exposure to such contaminants at all times. There are two main sources of mold spores in the stable: the first is the horse's feed, and the second, it's bedding.
Hay is the single most common source of mold spores for the horse. The soaking of hay is a time-proven method of minimizing the horse's exposure to mold spores. However, feeding soaked, poor-quality hay to horses cannot be condoned. A hay that falls to the floor dries, and the spores can again become airborne to be inhaled by the horse or "seed" clean bedding. Furthermore, even though the spores are not inhaled from soaked hay, they are still ingested along with any toxins present.
Silage and haylage are increasingly being successfully used as alternatives to hay. There have been some deaths due to botulism associated primarily with big-bale silage. In this context, silage that smells of ammonia or contains dirt should be avoided. When opened, bags of silage or haylage can mold quickly, within a matter of days. Opened bags should be used quickly, within two to three days, and damaged bags avoided.
Treated chaffed hay and straw and complete cubed diets also offer alternatives to feeding hay. These products are convenient and usually effective in minimizing respiratory disease. Studies show when cubes are fed in comparison to hay, the dust particles detected within the horse's breathing zone are significantly lower.
Even the cleanest of straws contains significantly more small, respirable fungal spores than alternative beddings, such as wood shavings, paper, peat moss, or the new synthetic beddings. However, in poorly ventilated stables or where deep litter is allowed to accumulate, a significant mold of the plant-based beddings can occur. Deep litter management systems have the added disadvantage of allowing the build-up of noxious gases such as ammonia, infectious bacteria, and the larvae of gastrointestinal parasites.
A well-ventilated stable will help minimize the horse's exposure to a wide range of environmental contaminants. These include fungal spores, bacteria, and noxious gases. However, significant levels of fungal spores can be inhaled in well-ventilated stables when contaminated feeds and bedding are present. The horse is inquisitive; it sniffs at its bedding. When the horse lies down, its nostrils are positioned for maximum uptake of spores released from its bedding. Indeed, a horse given moldy hay in the field ( the ultimate of natural ventilation) can still come down with airway problems.
The importance of ventilation in stalls even where clean feeds or beddings are used cannot be over-emphasized. Ventilation will help overcome condensation, prevent molding of bedding material in situ, and minimize the levels of all airborne contaminants to which the horse may be exposed.
To ensure adequate ventilation when designing new or altering existing horse accommodations, openings must be positioned or mechanical ventilators used so as to allow six to eight air changes per hour. Proper mixing of air throughout the whole building is critical, the latter being particularly important with barns. Sunlight is one useful natural resource often overlooked with horse housing. A skylight or sheet of clear perspex corrugated sheeting in the roof of a stable allows light in the stable, a strong and inexpensive natural killer of bacteria and viruses.