Eosinophils of the horse: Part II: Eosinophils in clinical diseases

Abstract

Interpretation of eosinophilia in body fluids or tissues is often not straightforward. Eosinophil counts vary among clinically healthy individuals, and considerable overlap can occur between normal and affected animals in conditions such as allergic airway disease. Parasite exposure is a confounding factor when counts are increased, and in cases where very high counts and dramatic clinical signs make another disease process obvious, the underlying pathology may be uncertain and treatment difficult. Eosinophils are a component of the immune response in many diseases of the horse, but their specific role is often unknown and likely multifactorial. In helminth infections, eosinophils are assumed to be part of the normal host response to a pathogen, whereas in multisystemic eosinophilic epitheliotropic disease (MEED), the predominance of eosinophils likely represents a wildly dysregulated response, or an abnormal response altogether. This distinction is still not clear for other diseases. Understanding the pathways involved in recruitment, activation or suppression of eosinophils is required for more accurate diagnostics, effective therapeutics, and successful strategies for prevention of eosinophil associated diseases. Eosinophils of the horse: Part II reviews published observations on the eosinophil in clinical diseases of the horse. The behaviour of eosinophils in three common and relatively wellstudied conditions is presented first, including gastrointestinal helminth infections, non-infectious respiratory disease, and insect bite hypersensitivity. The less common eosinophilassociated diseases such as eosinophilic disease confined to the intestine (EDCI) and MEED are considered, followed by a brief summary of the eosinophil in phycomycosis and neoplasia. In conclusion, a panoramic view of the equine eosinophil as presented in Parts I and II is placed in the larger context of current eosinophil research, and areas of study are identified that may improve our understanding of eosinophil biology in equine health and clinical disease. Gastrointestinal parasites of the horse Gastrointestinal helminths are a widespread and well-studied problem in the horse. Clinical disease is associated historically with migratory large strongyles (Strongylus vulgaris, S. edentatus, S. equinus), small strongyles (cyathostomes), tapeworms (Anoplocephala perfoliata) and, in foals, ascarids (Parascarus equorum). Early studies sought to define parasite life cycles, but post-infection necropsies also described the eosinophilic response (Duncan and Pirie 1975). Host immunity and anthelmintics were the focus of later works (Bailey et al. 1989; Steuer et al. 2018). Variables exist across studies, including dosing variations, natural versus experimental

Letter to the Editor: Eosinophils of the horse: Part II: Eosinophils in clinical diseases

“…With regard to the recent review on eosinophils in clinical diseases in horses ("Eosinophils of the horse: Part II: Eosinophils in clinical diseases") (Brosnahan, 2020), we would like to highlight some errant omissions in the article particularly in the part about insect bite hypersensitivity (IBH) in horses.…”

The Role of Intestinal Microbial Metabolites in the Immunity of Equine Animals Infected With Horse Botflies

“…Previous studies have shown that cytokine-or leukocyterelated gene expression in the blood can effectively reflect the host immune response in horses (18)(19)(20)(21)(22). But most of the studies on associations between parasite infections and immune responses in equine animals have mainly studied parasite antigen-induced host antibodies (23)(24)(25), and immune cells (26), clinical signs (27-30), and pathophysiology (31) in the host. It is our knowledge that only one study has evaluated gene expression in the whole blood of domestic horses infected with cyathostomin parasites (32).…”

Response to Letter to the Editor from Drs Birkmann and Fettelschoss‐Gabriel

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Thank you for providing the opportunity to respond to comments from Drs. Birkmann and Fettelschoss-Gabriel submitted in reference to 'Eosinophils of the horse: Part II: Eosinophils in clinical diseases' (Brosnahan, 2020a(Brosnahan, , 2020b.The authors begin by noting that the pathogenesis of insect bite hypersensitivity (IBH) in horses may also involve a type IVb hypersensitivity reaction. They further present newer data from their research group that demonstrated a clear increase in both localised and peripheral blood eosinophils, and a correlation of peripheral blood eosinophils with severity of disease.

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“…Thank you for providing the opportunity to respond to comments from Drs. Birkmann and Fettelschoss-Gabriel submitted in reference to 'Eosinophils of the horse: Part II: Eosinophils in clinical diseases' (Brosnahan, 2020a(Brosnahan, , 2020b.…”