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parasite clearing (MacDonald et al. 2002). A role for eosinophils in the tissue injury response is hypothesised, and evidence from the horse supports this. Upon oral inoculation with S. vulgaris larvae, eosinophils predominate in the ileo- caeco-colic artery following challenge, and arterial structure remains intact. Parenteral inoculation with larval homogenates is followed by lymphocytic–plasmacytic infiltration with severe disruption of the arterial architecture (Monahan et al. 1994). Redistribution or activation of eosinophils also is observed after nonparasitic gastrointestinal tract injury (Kooreman et al. 1998; Rotting et al. 2003; McConnico et al. 2005; Chiavaccini et al. 2011; Hopster- Iversen et al. 2011; Grosche et al. 2011a,b).
Non-infectious respiratory disease
Eosinophils are increased in peripheral blood, respiratory secretions and lung tissue in many species with noninfectious respiratory conditions such as asthma and COPD (Shibly et al. 2014; McBrien and Menzies-Gow 2017; McDonald 2017). Although their pathophysiological role is unclear, in some respiratory patients with eosinophilia disease exacerbation occurs more commonly than in those without (McDonald 2017). While known functions of eosinophils may account for clinical signs such as hyperresponsive airways, tissue damage and airway remodelling (McBrien and Menzies-Gow 2017), delineation of specific mechanisms has been hindered by insufficient laboratory models (Rothenberg and Hogan 2006; Blanchard and Rothenberg 2009).
Non-infectious respiratory diseases of the horse Noninfectious respiratory disease is diagnosed frequently in horses as recurrent airway obstruction (ROA) or inflammatory airway disease (IAD), conditions recently renamed as severe and mild-to-moderate equine asthma, respectively (Couetil et al. 2016). Historically, noninfectious airway disease was diagnosed as ‘heaves’, with acknowledgement that this clinical syndrome could be a manifestation of multiple disease processes (Lowell 1964). Early studies sought to define the immune cell populations in respiratory secretions of normal horses and to correlate changes with clinical observations. Currently, the two forms of asthma are considered to be separate entities rather than a continuum, with increased eosinophils and mast cells in bronchoalveolar lavage fluid (BALF) usually (Couetil et al. 2007; Couetil et al. 2016; Pirie et al. 2016), but not always (Couetil et al. 2001) associated with IAD. A smaller body of work evaluates the pulmonary function of horses with airway eosinophilia. Challenges exist when drawing conclusions from these studies, including variations in study populations, clinical presentations, terminology, cases and control definitions, and criteria for diagnosing IAD, and techniques. Eosinophil counts may be affected by sample collection and processing methods. Inter-rater reliability was poor for eosinophil counts in BALF using two different counting methods (Fernandez et al. 2013), and significant differences in BALF eosinophil counts were noted using two different slide preparation methods (Pickles et al. 2002a). Spurious changes in eosinophil counts were noted with increasing BALF storage temperature (Pickles et al. 2002b). Variable correlation has been found between eosinophils in BALF and transtracheal aspirates across studies (Derksen et al. 1989; Hughes et al. 2003). Considering this, observations on eosinophils in horses with
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healthy and affected respiratory tracts are summarised below.
Peripheral blood eosinophils may be increased in horses with non-infectious respiratory disease and may decrease upon exacerbation. In Standardbreds with poor performance, clinical signs of airway disease and >5% eosinophils in BALF (Hare and Viel 1998), and in horses with severe equine asthma studied in remission (Fairbairn et al. 1996), investigators found that PBEs were greater than in control horses. Two studies found a decrease in PBEs during acute exacerbation of allergic airway disease, with a rebound eosinophilia occurring in one. This decrease was attributed to late recruitment of eosinophils to the lungs, or to the immunosuppressive effects of corticosteroid hormones produced in response to the physiological stress of exacerbated respiratory disease (Lowell 1964; Fairbairn et al. 1993).
Eosinophils are variably present in the respiratory secretions of clinically normal horses of all ages. Eosinophil counts in respiratory secretions are generally higher in foals than adults. Tracheobronchial aspirates (TBA) showed that about one- third of clinically normal foals aged 1 to 6 months had >5% eosinophils on initial examination, with nearly half having eosinophilia on subsequent examinations, in some cases exceeding 25% (Crane et al. 1989). In another study, clinically normal foals aged 6 months had significantly higher eosinophil counts (2.2%) than foals aged 1 week or 1 month (0%), and yearlings (1.5%) had a higher percentage of eosinophils than adults (0.3%) (Hostetter et al. 2017). Eosinophils counts are low in respiratory secretions of adults regardless of demographics, sampling techniques including TTA or BAL, or quantification method, with differential counts consistently under 3% (Mair et al. 1987; Derksen et al. 1989; Sweeney et al. 1992; Dixon et al. 1995; Moore et al. 1995; Hare and Viel 1998; Christley et al. 1999; Couetil et al. 2001; Depecker et al. 2014; Wysocka and Klucinski 2015), or subjectively few (Whitwell and Greet 1984; Pacheco et al. 2014). Up to 44% of horses have no eosinophils (Beech 1975; Whitwell and Greet 1984; Sweeney et al. 1992). Rarely, a clinically normal horse has eosinophils exceeding 10%. Counts this high often are attributed to parasite exposure (Bracher et al. 1991; Sweeney et al. 1992). Eosinophils are rare in nasal passages and guttural pouches (Mair et al. 1987; Chiesa et al. 1999).
Eosinophils are variably present in the respiratory secretions of horses with clinical signs of non-infectious respiratory disease. Across studies, 5 to 27% of horses with coughing, poor performance or IAD have eosinophils in their respiratory secretions; therefore, upwards of three-quarters of affected animals may not have eosinophils (Fig 2) (Beech 1975; Whitwell and Greet 1984; Hughes et al. 2003; Hughes et al. 2011). When present, eosinophil counts are often still below 3% (Couetil et al. 2001; Richard et al. 2009; Hughes et al. 2011; Wysocka and Klucinski 2015) or subjectively scored as few (Winder et al. 1989), but mean eosinophil counts of 3.8 to 11.8% and individual counts up to 54% are reported in horses with poor performance or exercise intolerance (Moore et al. 1995; Hare and Viel 1998; Hughes et al. 2003). When horses are categorised as IAD based upon BALF neutrophils or mast cells, eosinophils are found more frequently and at higher percentages in controls (Hughes et al. 2011).
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