Utility of Glucocorticoids in Treating Canine Allergic Reactions

In the Literature

Helgeson ME, Pigott AM, Kierski KR. Retrospective review of diphenhydramine versus diphenhydramine plus glucocorticoid for treatment of uncomplicated allergic reaction in dogs. J Vet Emerg Crit Care (San Antonio). 2021;31(3):380-386.

The Research …

According to this study, ≈1.2% of emergency veterinary visits are for mild allergic reactions. Type I hypersensitivity reactions occur when immunoglobulin E antibodies on the surface of mast cells and basophils (formed during initial exposure to an allergen) crosslink and trigger degranulation after re-exposure to the allergen. Exposure can occur via direct contact with or ingestion, inhalation, or injection of an allergen. Mast cell and basophil degranulation releases histamine, leukotrienes, and prostaglandins. Histamine activates H₁ receptors on the endothelium, vascular smooth muscle, hepatocytes, and lymphocytes. 

Clinical signs of a mild allergic reaction include urticaria, angioedema, and pruritus. Anaphylaxis is a systemic type I hypersensitivity reaction that can progress to anaphylactic shock, which involves multiorgan dysfunction with cardiovascular collapse. In humans, only 1% of allergic reactions are classified as anaphylactic.¹

Glucocorticoids block inflammation generated by the arachidonic acid cascade; corticosteroids require 4 to 6 hours to downregulate this late-phase response and turn off proinflammatory gene transcription. Diphenhydramine is a rapidly acting H₁-histamine–receptor blocker that minimizes the effects of histamine on target organs.

This study retrospectively compared outcomes in dogs with mild allergic reactions given an initial treatment of diphenhydramine alone versus diphenhydramine and a glucocorticoid. A control group in which no treatment was given was not included. 

All 880 dogs in the study received diphenhydramine IM; dogs given glucocorticoids (n = 581) also received variable amounts of dexamethasone sodium phosphate IV (range, 0.02-2 mg/kg; median, 0.19 mg/kg). Most dogs (72%) had a clinical response after initial treatment. Adding a glucocorticoid to the treatment regimen for a mild allergic reaction did not improve the response rate to initial therapy, need for additional care after discharge, or persistence of signs at follow-up.

…The Takeaways

Key pearls to put into practice:

• Diphenhydramine (2 mg/kg IM once)² can treat uncomplicated allergic reactions.

• Oral diphenhydramine (2 mg/kg PO every 8 hours for 2-3 days) can be administered to prevent recurrence of clinical signs in patients with possible persistent antigen exposure.

• Corticosteroids are not needed during initial treatment, as they do not improve outcomes and may be associated with adverse effects (eg, GI upset, increased risk for infection).³

Editor's note: This article was originally published in February 2022 as "Treatment of Allergic Reactions in Dogs: Are Glucocorticoids Necessary?"