EQUINE VETERINARY EDUCATION / AE / MAY 2015
225
Editorial Article Is the time primed for equine asthma?
The complex and evolving terminology that has been used to describe inflammatory syndromes affecting the lower airways of horses has led to confusion for veterinarians and clients alike. Evidence has emerged in recent years that inflammatory airway disease (IAD) and heaves (recurrent airway obstruction, RAO) in horses share striking similarities with human asthma in terms of aetiology, clinical presentation, tissue remodelling alterations and response to therapy. Adopting ‘equine asthma’ to describe chronic, noninfectious lower airway conditions currently known as IAD and heaves or RAO, would not only facilitate the communication with clients, but this terminology would also provide a wide umbrella, which remains appropriate as the diversity of disease manifestations (phenotypes) and underlying immunological processes (endotypes) emerge in the future.
Historical note: numerous efforts to adapt nomenclature to clinical manifestations and pathological alternations
It has been known for centuries that horses may develop a chronic respiratory disease when stabled and fed hay. This condition is reversible by changes in the environment, or by the administration of bronchodilators and corticosteroids. When severe, this syndrome had initially been called ‘heaves’ or ‘broken wind’ by horsemen and veterinarians because of the severe breathing difficulties observed in these horses. It later became known as ‘equine emphysema’ based on the macroscopic appearance of the distended lungs at necropsy (Obel and Schmiterlow 1948), but this was later found to be a misnomer, as hyperinflation due to air trapping, rather than disruption of the alveolar walls, is present in affected lungs. In the early 1980s the development of flexible endoscopy facilitated the study of the cells present in the lower airways of horses. Using bronchoalveolar lavage cytology, it was found that neutrophils were the predominant cells in the airways of horses with heaves during exacerbation. The term chronic obstructive pulmonary disease (COPD) was then proposed, and largely accepted by equine researchers and clinicians, as human COPD was also known to be associated with airway neutrophilia and severe airway obstruction in adult to aged patients. However, this term has been abandoned because, unlike human COPD, this condition is not associated with inhalation of toxic gases (cigarettes, biofuel), affected horses have mostly reversible airway obstruction, and pulmonary emphysema is not a feature of this condition (Robinson 2001). The term RAO was introduced initially to describe the variable airway obstruction detected using lung function in affected horses (Derksen et al. 1985). While it captures the variable degree of airway obstruction commonly observed under field conditions when husbandry varies throughout the year, the term may not be appropriate as the airway obstruction is ‘persistent’ rather than ‘recurrent’ when affected horses are kept in an offending environment (Jean et al. 1999). Bronchoalveolar lavage cytology also revealed that
airway neutrophilia is common in horses with chronic airway conditions of variable severity, and that mast cells and eosinophils are also present in increased numbers in some of
these animals. Conditions associated with milder clinical signs of nonseptic respiratory disease were named ‘small airway disease’, ‘lower airway disease’, ‘inflammatory airway disease’, ‘chronic bronchitis’, ‘chronic bronchiolitis’ and ‘chronic asthmoid bronchiolitis’ (Gerger 1973; Slauson and Hahn 1980; Viel 1983; Moore et al. 1995). The reading of the English literature (not to mention approximate translations in other languages) became very confusing, and a group of clinician-scientists recommended that, for research purposes, ‘heaves’ or ‘RAO’ should be used when affected horses have episodes of laboured breathing at rest, while IAD was preferred for the less severe forms of the syndrome (Robinson 2001). More recently, it was proposed to use ‘tracheal IAD’ and ‘bronchial IAD’ depending on whether abnormalities were identified using tracheal or bronchoalveolar lavage fluid cytology, respectively (Cardwell et al. 2011). To complicate matters even further, although chronic airway diseases occur primarily during stabling and with hay feeding, a similar clinical syndrome also develops in some horses while at pasture and is referred to as ‘summer pasture associated pulmonary disease’, ‘summer heaves’, and ‘summer RAO’. Clearly, while these definitions proved useful for research purposes, it had the perverse effect of suggesting that IAD and RAO were 2 distinct conditions, and left out a number of clinical entities affecting horses, such as SPAOPD and conditions restricted to large airway inflammation. More than 30 years ago, the confusion in terminology for
chronic inflammatory equine airway diseases was already discussed in editorials in both veterinary and human fields (Slauson and Hahn 1980; Hall and Stark 1983). Since then, even more terms have been introduced as our understanding of the syndrome evolved. Initially based on clinical signs (heaves, broken wind), new terms were introduced with refinement in our research techniques (lung function, pathology, cytology, molecular biology), and with the advent of the ‘omics’ (genomics, proteomics, lipidomics, metabolomics etc.) we can only expect new terminology to emerge, which is unsustainable.
Equine asthma: is it too broad?
There are numerous similarities between heaves, IAD, SPAOD and human asthma (Leclere et al. 2011) but many are reluctant to use this term because none of these equine diseases are exactly the same as the classical ‘atopic (human) asthmatic’. However, human asthma is also ‘a heterogeneous disease, characterised by chronic airway inflammation. It is defined by the history of respiratory symptoms such as wheeze, shortness of breath, chest tightness and cough that vary over time and in intensity, together with variable expiratory airflow limitation’ (GINA 2014). Furthermore, while human asthma was initially believed to be primarily a disease associated with allergy, it is now recognised that various factors and immunological events may lead to airway obstruction and inflammation. The use of a broad termunderstood by patients and clinicians has not limited the search for potential different aetiologies and phenotypes such as ‘extrinsic and intrinsic
© 2015 EVJ Ltd
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84
