SAEVA Congress 2018 Proceedings | 12-15 February 2018 | ATKV Goudini Spa
Despite publication of these consensus statements, a universally accepted definition
of IAD does not exist. This partially stems from the fact that the current
understanding of the etiology, pathogenesis, clinical signs, diagnosis and effect on
performance of what we call IAD is still incomplete. As a consequence, a number of
terms have been used, at times interchangeably, to describe the presence of airway
inflammation in horses with reduced exercise tolerance but which also appear
normal at rest. These include small airway inflammatory disease (SAID), lower
respiratory tract inflammation, small airway disease (SAD), non-infectious
inflammatory airway disease and bronchiolitis. This range of names is perhaps the
clearest indication that much remains to be determined about the syndrome currently
referred to as IAD and it is likely that a variety of clinical entities will be included
within this one syndrome until it is better defined.
Etiology
The etiology is not definite, although a variety of noninfectious causes have been
implicated, such as environmental factors, allergy, dietary components, and toxic
metabolites. An unknown number of animals may suffer from a hypersensitivity-like
condition in which they are either extrinsically allergic to a specific allergen or
intrinsically hyperreactive to nonspecific stimuli such as airway desiccation and
airway cooling. This may be a particular problem in some racehorses.
Among racehorses the condition is recognized most commonly in the younger age
groups, being most prevalent in 2 yr-olds. However, horses of any age can develop
IAD and in non-athletes clinical signs can be present without detection for months or
years. The term IAD has no implications with regard to the etiology, the site of
inflammation (large versus small airways) or the type of inflammatory cell (neutrophil,
eosinophil or mast cell) involved. All of these alternatives can exist in horses.
High-intensity exercise, transportation, co-mingling with other horses, exercise-
induced pulmonary hemorrhage (EIPH), housing in stables, and age have been
proposed as risk factors contributing to the development of lower airway
inflammation although there have been only a few epidemiological studies to analyze
the influence of each of these factors on populations of horses.
There has been considerable debate regarding the pathogenesis of IAD and the
exact etiological agents contributing to its development. A temporal relationship may
exist between infectious (e.g., equine influenza virus, equine herpesvirus type 4 and
less commonly type 1, and equine rhinitis A and B viruses, Streptococcus
zooepidemicus, S. pneumoniae, Actinobacillus spp and Bordetella bronchiseptica)
and non-infectious causes in the pathogenesis of some cases of IAD, and both may
be necessary for development of clinically significant disease. However, at the time
IAD is diagnosed, the patient should not be showing signs of respiratory tract
infection. If it is, the diagnosis should reflect the existence of infection (e.g.,
bronchopneumonia). Effective prevention and treatment of this syndrome might be
difficult until the underlying causes and their interrelationships are elucidated.
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