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2019), but others show no breed predisposition (Lassaline-Utter et al. 2014). Standardbreds and Quarter Horses are overrepresented among MEED cases (Lindberg et al. 1985; Sweeney et al. 1986; Schumacher et al. 2000; Bosseler et al. 2013).
Peripheral blood and body fluid eosinophilia are variable within and among EADs. In IFEE, eosinophils may be present in peritoneal fluid, but PBEs are typically normal (Scott et al. 1999; Southwood et al. 2000). Increased PBEs have not been reported in EK (Lassaline-Utter et al. 2014). Horses with MEED may (Latimer et al. 1996; Henson et al. 2002; Carmalt 2004) or may not (Nimmo Wilkie et al. 1985) have peripheral blood eosinophilia, and eosinophilia of peritoneal fluid or BALF may occur (Henson et al. 2002; Singh et al. 2006). Increased eosinophil precursors may be seen in the bone marrow in horses with MEED (Pucheu-Haston and Del Piero 2013).
The immune cell infiltrates are distinct in each EAD. Within EDCI, histopathology suggests that IFEE represents an acute, focal exacerbation of DEE. Lesions are found in the submucosa and muscularis where eosinophils and macrophages predominate. The mucosa in these areas harbours predominantly T lymphocytes. Distant from the lesions, macrophages predominate throughout, with increased numbers of eosinophils in the submucosa relative to the mucosa. In horses with DEE, lymphocytes predominate in the mucosa and eosinophils in the submucosa, with macrophages the second most abundant cell type in these regions. Mast cells are notably absent in both conditions, and it is hypothesised that increased eosinophils and macrophages result from a dysfunctional pathway involving T lymphocytes or macrophages (Makinen et al. 2008). Lesions of segmental eosinophilic colitis of the left dorsal colon may include normal to ulcerated mucosa with neutrophilic and eosinophilic infiltrates, and eosinophilic infiltration of the submucosa and sometimes the serosa (Edwards et al. 2000). Lesions in the small colon are most striking in the submucosa and muscularis, with infiltrates that are predominantly eosinophils and macrophages (de Bont et al. 2018). Characteristic EK infiltrates consist primarily of eosinophils
with neutrophils, lymphocytes, mast cells and plasma cells. Conjunctiva may be affected in addition to or independent of the cornea, but surrounding skin has been normal when examined. One or both eyes may be affected (Ramsey et al. 1994; Yamagata et al. 1996; Wolfe et al. 2010; Lassaline-Utter et al. 2014; Gonzalez-Medina 2019). Lesions of MEED vary widely among cases and between
tissues, but can include diffuse or focal infiltration of eosinophils (Pass and Bolton 1982; Breider et al. 1985; Nimmo Wilkie et al. 1985; Platt 1986; Sanford 1989; Singh et al. 2006), eosinophilic granulomas (Pass and Bolton 1982; Nimmo Wilkie et al. 1985; Platt 1986; Gibson and Alders 1987; Sanford 1989; Henson et al. 2002; Singh et al. 2006), and eosinophilic perivasculitis (Nimmo Wilkie et al. 1985; Gibson and Alders 1987; Bosseler et al. 2013). Other cell types present include mononuclear cells (Breider et al. 1985; Nimmo Wilkie et al. 1985; Platt 1986; Gibson and Alders 1987), macrophages (Pass and Bolton 1982; Platt 1986; Singh et al. 2006; Bosseler et al. 2013), mast cells (Breider et al. 1985; Gibson and Alders 1987), and basophils (Gibson and Alders 1987). One proposed
aetiology of MEED is overproliferation of Th2 cells with increased production of eosinophil chemotactic agents (La Perle et al. 1998; Singh et al. 2006).
Response to treatment varies with syndrome. Horses with EDCI have a favourable prognosis. Survival with IFEE ranges from 83 to 100% with or without resection of the lesions (Southwood et al. 2000; Archer et al. 2006; Perez Olmos et al. 2006). Similar outcomes are obtained with resection when lesions are located in the left dorsal or descending colon (Edwards et al. 2000; de Bont et al., 2018). The response to treatment is favourable with EK, but involves long-termtopical corticosteroid therapy (Ramsey et al. 1994; Yamagata et al. 1996; Wolfe et al. 2010). Faster resolution occurs with systemic corticosteroids or plaque removal, and cetirizine can reduce recurrence (Lassaline-Utter et al. 2014). Resolution of localised, idiopathic eosinophilic granulomas and polyps in the trachea has been achieved surgically with or without the use of laser (Lankveld 2001; Collins et al. 2005). By contrast, horses with MEED have a poor prognosis, with most cases subjected to euthanasia (Nimmo Wilkie et al. 1985; Latimer et al. 1996; Henson et al. 2002; Bosseler et al. 2013; Horan et al. 2013). Clinical signs referable to specific body systems are more severe in MEED; for example, weight loss and hypoproteinaemia are features of MEED, but not of EDCI (Schumacher et al. 2000), and fever may be present (Pass and Bolton 1982; Platt 1986; Woods et al. 1993; Bosseler et al. 2013). In cases treated with long-term dexamethasone, horses were alive at follow-up 8–17 months later (McCue et al. 2003; Carmalt 2004).
Eosinophils in other equine diseases
Eosinophils are a prominent feature of phycomycosis Eosinophils are mentioned rarely in equine viral and bacterial diseases (Sells et al. 1976; Allen and Frank 1982; Madigan and Gribble 1987; Kalsow et al. 1993; Wada et al. 2003), but may predominate in phycomycosis (Owens et al. 1985; Souto et al. 2016). The severity of the eosinophil reaction to Pythium insidiosum is unique, with characteristic ‘kunkers’ containing dense eosinophil infiltrates (Martins et al. 2012; Souto et al. 2016). The eosinophilic response does not clear hyphae, and the pathogenesis of pythiosis arises from degranulation of eosinophils and mast cells (Mendoza and Newton 2005). Upon successful vaccination, macrophages and lymphocytes replace eosinophils, suggesting that a Th1 response is curative (Mendoza et al. 2003).
Primary eosinophilic myeloproliferative disease occurs rarely in horses A single case is described in a 10-month-old Standardbred (Morris et al. 1984). Benign and malignant mast cell tumours in foals and adults contain eosinophils (Reppas and Canfield 1996; Brown et al. 2007; Seeliger et al. 2007; Millward et al. 2010; Junginger et al. 2016). Eosinophils are associated with collagen degeneration in benign cutaneous mastocytoma, but their role is unknown (McEntee 1991). Paraneoplastic eosinophilia and MEED are reported in horses with lymphoma (Duckett and Matthews 1997; La Perle et al. 1998).
Conclusions
An immense but fragmented body of literature describes the equine eosinophil. Progress requires integrative studies that © 2020 EVJ Ltd
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