The Importance of Respiratory Health

 

Anouk Frieling, MSc Equine Sciences, BSc (Hons)

 

Horses are hindgut fermenters, therefore high-fibre forage forms the basis of their diet. The most commonly forage fed to horses is field dried hay (Moore-Colyer et al., 2016). High quality and fibrous hay provides a large portion of the daily energy requirements of the horse (Fernandes et al., 2014), with the nutrient quality of hay depending on the stage of growth during harvest and the grass mixture (Earing et al., 2013). Horse owners often assess the quality of hay through a visual assessment and by smelling the hay (Moore-Colyer et al., 2016). Even when hay appears to be of a high quality after such an assessment, it can contain high levels of respirable dust (Intemann et al., 2022). Unhygienic forage can have an effect on the respiratory system and horses can develop chronic respiratory diseases which influences overall health and performance (Pirie, 2014). This article will review the importance of a healthy respiratory system, the effect of forage on respiratory health, forage that supports a healthy respiratory tract and management options that can improve respiratory health and prevent respiratory diseases.

 

Importance of the Respiratory System

The respiratory system of the horse consists of the nose, mouth, pharynx (throat), larynx, trachea (windpipe), bronchi, bronchioles, alveoli and the lungs . (Lekeux et al., 2014). Respiratory muscles, such as the diaphragm, expand and compress the chest wall therefore stimulating breathing motions resulting in inhalation and exhalation of gas (Robinson, 2007). The function of the respiratory system is to remove carbon dioxide from the circulatory system and deliver oxygen to the body (Mazan, 2022). Exchange of gasses occurs in the alveoli which are lined with a pulmonary capillary network (McGorum et al., 2007) (Figure 1). During each breath, the horse inhales approximately 4-5 litres of air, although the total capacity of the lungs is roughly 42 litres (Robinson, 2007). Oxygen is required for energy production within the body, which is an essential component for exercising muscles (Franklin et al., 2012). Exercise increases the energy requirement of the muscles, therefore during exercise ventilation of the lungs increases to accommodate this increasing energy demand (Franklin et al., 2012). Healthy adult horses consume approximately 80 litres of air per minute whilst in rest (Mazan, 2022). When horses exercise this can increase up to 180 litres per minute to deliver enough oxygen to provide energy to the exercising muscles (Mazan, 2022). Walk and trot seem to have little effect on the breathing pattern of the horse, however canter significantly increases the requirement for oxygen in horses (Robinson, 1985)

Besides providing oxygen and removing carbon dioxide from the blood stream, the respiratory system also has an important role in immune responses in the body . A healthy functioning respiratory tract is important for the delivery of oxygen, which is used for energy metabolism, and removal of carbon dioxide but also plays a important role in the innate immune response of the horse. Therefore, if respiratory health decreases due to foreign particles that irritate the respiratory system, it can have an effect on overall health and performance of the horse, making it important to maintain respiratory health through optimal management. (Grzela et al., 2012). The respiratory tract is often exposed to environmental factors, such as bacteria and viruses, that could potentially cause infection or disease (Whitsett & Alenghat, 2015). The respiratory system, specifically the airway epithelium (Parker & Prince, 2011), are the first line of defence against these pathogens and are therefore part of the innate immune system (Martin & Frevert, 2005). The epithelial cells serve as a protective barrier and prevents the entry of pathogens into the alveoli where gas exchange takes place (McGorum et al., 1998; Johnston et al., 2021). When the epithelial cells come in contact with harmful pathogens, it triggers an immune response in the body to inactivate the pathogens before causing an infection (Gilkerson et al., 2015)

Figure 1: The respiratory system of the horse has an important function in exchange of gasses an is a part of the innate immune system. In the alveoli the exchange of gasses takes place, delivering oxygen and removing carbon dioxide from the blood.

 

The effect of Forage on Respiratory Health

When respirable dust or mycotoxins enter the respiratory tract, most particles deposit on the epithelium, from which they are cleared meaning they are unable to cause harm . (Ganesan et al., 2013). However, respirable dust smaller than 5 microns (μm) is able to reach down the alveoli and trigger an immune response (Auger & Moore-Colyer, 2017). If small dust particles enter the respiratory tract they irritate the airway mucosa receptors which trigger a response from the horse causing them to cough or sneeze as a way of clearing unwanted particles from the airway (McGorum, 2002; Robinson et al., 2003). When horses are exposed to an environment containing a high amount of foreign particles, it takes between 3 and 5 hours before neutrophils are present in the respiratory tract, which are a key component of the immune response of the body that is triggered by dust particles (Brazil et al., 2005; Leclere et al., 2011). If horses are exposed to dust for a short period, between 5 and 7 hours, the effect of the inflammation can be reversed within four days if the dust is removed from the environment (Brazil et al., 2005; Leclere et al., 2011). But if dust irritates the respiratory tract for a prolonged period, horses can develop chronic respiratory diseases such as inflammatory airway disease (IAD) also known as equine asthma and recurrent airway obstruction (RAO) (Vandenput et al., 1998; Séguin et al., 2010). Clinical signs of chronic respiratory diseases are increased mucous production, a recurrent cough, increased respiratory effort and poor performance (Ivester et al., 2014)

Figure 2: Dust can irritate the respiratory tract causing the horse to sneeze or cough occasionally. Once the horse is exposed to dust for a prolonged period the horse can develop a chronic respiratory disease.

 

Inflammatory airway disease is one of the most common respiratory diseases in athletic horses caused by respirable dust . (Ivester et al., 2014). Often, IAD is characterised by mild inflammation of the respiratory tract, but even mild inflammation has a negative effect on respiratory health and equine performance (Sánchez et al., 2005). Sánchez et al. (2005) analysed the effect of IAD on the gas exchange in the respiratory tract during high-speed exercise in racehorses. The study suggested that due to the respiratory disease, horses experienced impaired pulmonary gas exchange which is likely to negatively affect the performance of the horses (Sánchez et al., 2005)

Recurrent airway obstruction, previously  known as chronic obstructive pulmonary disease (COPD), is a respiratory disease caused by respirable dust in the environment of the horse, but due to management changes, such as providing a dust free environment, the disease can be managed . (Pirie, 2014). Symptoms following prolonged dust exposure that leads to RAO, often get worse after exercise (Léguillette, 2003; Niedzwiedz et al., 2014). Clinical signs of RAO can also disappear when horses are out on pasture but will reoccur as soon as the horse enters the dusty environment due to irritations of the respiratory tract (Williamson & Davis, 2007; Niedzwiedz et al., 2014). Neutrophilic inflammation in the respiratory tract, due to RAO, can cause bronchospasms and over production of mucous (Davis & Rush, 2002; Gehlen et al., 2008). The severity of the obstruction in the respiratory tract depends on the stage of the disease which is connected to the amount of time spend in a dusty environment (Niedzwiedz et al., 2014). These changes in the respiratory tract can be managed by administering medication or to remove the horse from the dusty environment (Niedzwiedz et al., 2014)

Figure 3. There are other forage options instead of hay which have a lower dust content. Horses are often fed haylage or silage.

 

Forage for Respiratory Health

The digestive tract of the horse is adapted to ferment high-fibre feedstuff into volatile fatty acids (VFA), which can be used as an energy source for various processes in the body . (Intemann et al., 2022). Grass hay is a high-fibre forage which is commonly fed to horses, but in countries such as Sweden and Finland hay is often replaced for haylage (Besier et al., 2013). Reasons given for this switch were difficulties with storing hay in a dry place to prevent moulding and horses displaying symptoms of respiratory diseases after feeding hay (Müller, 2018). Feeding hay to stabled horses has been associated with a tenfold increase of respirable dust in the environment of the horse (McGorum et al., 1998)

Haylage is made from grass that is cut earlier than grass used for hay production . (Müller, 2018). After cutting and drying of the grass till it reaches the correct dry matter percentage, haylage is wrapped in plastic (Clements & Pirie, 2007). Due to wrapping the haylage a fermenting process occurs, preserving nutrients of the forage and resulting in lower dust content in the forage (Clements & Pirie, 2007). Replacing hay with haylage can result in a reduction of 60 to 70% of the respirable dust in the environment of the horse (Clements & Pirie, 2007). Olave et al. (2021) studied the effect of feeding haylage instead of hay on pro-inflammatory markers in racehorses. Results concluded that haylage reduces exposure to respirable dust and therefore irritation of the respiratory tract, and resulted in the suppression of airway neutrophilia, which indicates a decreased inflammatory response in the body (Olave et al., 2021)

Another forage option, which is being fed more often nowadays to reduce exposure to environmental irritants, is silage suggested that silage aids the maintenance of a stable respiratory system in horses diagnosed with COPD, also known as equine asthma. Thus, indicating that silage is a suitable forage to feed to support respiratory health. (Müller, 2018). Silage is also a forage which is harvested earlier than hay and contains a higher moisture content, therefore decreasing the respirable dust in the forage and the environment of the horse (Olave et al., 2021). The median dry matter percentage of high-quality silage measured in a study on horse farms in Switzerland, was 65.8% (Wichert et al., 2008). Therefore, suggesting that high-quality silage can be used as a suitable hay replacement. Results from a study carried out by Vandenput et al. (1998)

Even though silage is a suitable forage to increase and maintain respiratory health there are a few considerations that should be taken into account before switching to silage. Silage is higher in energy therefore feeding a smaller portion, compared to hay, will meet the energy requirements of the horse . (Müller et al., 2007). Although the energy content is higher, the amount of fibre is lower (Müller et al., 2007). This type of forage is wrapped and stored, but once the seal breaks and air is able to enter the bale, the amount of toxins increases and it moulds, making it unsuitable to feed to horses (McNamara et al., 2002; Müller et al., 2007)

Figure 3: There are other forage options instead of hay which have a lower dust content. Horses are often fed haylage or silage.

 

Management of Respiratory Health and Prevention

Horses with severe respiratory diseases require long-term management . (van Erck-Westergren et al., 2013). Respiratory diseases can mainly be managed by providing an environment that is as dust free as possible (see Industry update: Biosecurity in equine premises) or by providing medication prescribed by veterinarians suitable for management of respiratory diseases. To support the respiratory tract horses can also undergo steam inhalation therapy as an alternative treatment (Cha & Costa, 2017)

Respiratory diseases can be avoided by ensuring the environment contains a low dust percentage. This can be achieved by ensuring that forage that is fed contains a low dust content, by using low-dust bedding materials and providing ventilation in the stables, and by allowing as much time out of the stables as possible. As mentioned earlier, there are also other forage options which contain less dust and will therefore support a healthy respiratory system.

Soaking hay is a commonly used method to lower the dust content in hay. It is suggested that soaking hay for 5 to 10 minutes decreases respirable dust in hay but maintains hay quality . (Clarke & Madelin, 1987). Even though this method has been shown to reduce content, it also decreases the amount of valuable minerals and increases the bacterial concentration (Moore-Colyer, 1996). Warr & Petch (1993) stated that soaking hay for a 12-hour period significantly decreases the water-soluble carbohydrate content in hay. The dry matter content decreased by 2 to 4% whilst crude protein decreased approximately 1.5 to 2%. Therefore, soaking hay for a longer period degrades the quality of the hay (Moore-Colyer et al., 2016)

Another more recently developed method is steaming hay to lower the dust content using a commercial steamer. Steaming hay is a method which results in decreased airborne respirable particles whilst maintaining mineral and protein content and therefore not decreasing forage quality . In conclusion, hay-steaming provides a long-term solution to increase hygienic quality of hay and prevent or manage respiratory diseases in horses. (Moore-Colyer et al., 2016). Humer et al. (2019) concluded that hay steaming decreases yeast and mould especially in hay that has been hygienically compromised, therefore improving the quality. Moore-Colyer et al. (2016) showed that, in comparison to soaking hay for a long period, steaming hay for 50 minutes on a 100 degrees Celsius reduces dust content but does not have a negative effect on water-soluble carbohydrates or minerals such as calcium, sodium, magnesium, zinc and copper. Humer et al. (2019) did suggest that steaming hay at a 100 degrees Celsius decreases the dry matter content of hay and therefore adds more moisture, making the hay more susceptible to moulds and yeast when stored. But after storing steamed hay for 24 hours the quality of the hay was not negatively affected (Humer et al., 2019)

Conclusion

The respiratory system of the horse has an important function in gas exchanges and overall health and performance. Therefore, if the respiratory tract is irritated or compromised this influences the health of the horse and the performance. Respirable dust in the horse’s environment originating from forage can cause an inflammatory response in the respiratory tract. If the respiratory system is irritated for a prolonged period, horses can develop a chronic respiratory disease such as equine asthma, also known as inflammatory airway disease, or recurrent airway obstruction. Clinical signs of respiratory diseases are increased production of mucous, developing a recurrent cough and poor performance. As respirable dust often originates from hay, it is an option to provide different types of forages such as haylage and silage which usually contain a lower dust content but still provide the required nutrients. Horses that develop airway disease require suitable long-term management through for example, medication and removing the horse from the dusty environment. Over recent years, steaming has been adopted as a suitable method to reduce the dust content in hay whilst maintaining nutrient quality. In contrast, soaking hay has been shown to reduce nutrient content and hygienic quality of the forage.  

 

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