Hypersensitivity Reactions & Anaphylaxis in Dogs

In the Literature

Fosset FTJ, Lucas BEG, Wolsic CL, Billhymer AC, Lavergne SN. Retrospective evaluation of hypersensitivity reactions and anaphylaxis in dogs (2003-2014): 86 cases. J Vet Emerg Crit Care (San Antonio). 2023;33(5):577-586. doi:10.1111/vec.13319

The Research …

Type I hypersensitivity reactions, ranging from mild signs to potentially life-threatening anaphylaxis, occur rapidly (within 6 hours) following exposure to an allergen. Immunoglobulin-E–mediated activation of mast cells and basophils leads to degranulation and subsequent release of potent inflammatory cytokines (eg, histamine).¹

The aim of this retrospective study was to characterize type I hypersensitivity reactions and anaphylaxis in dogs and evaluate treatment with an H₁ antagonist (eg, diphenhydramine) alone or in combination with a glucocorticoid. Anaphylaxis was defined as an acute onset of organ dysfunction (ie, respiratory signs, shock, GI signs) with or without cutaneous or mucosal signs after known or potential exposure to an allergen.² Mild hypersensitivity reactions were defined as reactions with only cutaneous or mucosal signs.

Of 86 dogs treated for type I hypersensitivity reactions, 19 met the criteria for anaphylaxis, and 67 had mild cutaneous reactions. Patient age and breed were not reported as risk factors for hypersensitivity reactions, but females were significantly more affected than males. Of the 19 dogs with anaphylaxis, 9 (47.4%) had multiple organ systems affected, with 8 (42.1%) experiencing cardiovascular dysfunction; however, 11 (57.9%) lacked cutaneous signs. Elevations in ALT and gallbladder wall edema have been associated with anaphylaxis and were reported in several dogs in this study³; however, clinically useful conclusions could not be made because this information was not available for most dogs. In dogs with mild reactions, cutaneous and mucosal signs included angioedema, erythema, and urticaria.

In human medicine, epinephrine is the first-line treatment for anaphylaxis because it is the only drug that treats clinical signs while preventing further progression of the reaction.² In this study, epinephrine was only administered in patients with cardiovascular compromise and not as a first-line treatment. Most of the 86 dogs received an H₁ antagonist (ie, diphenhydramine, hydroxyzine), and 48.8% received both an H₁ antagonist and a glucocorticoid (ie, dexamethasone, prednisone). Dogs receiving both drugs did not have an improved outcome, and several dogs with mild hypersensitivity reactions improved without treatment. Overall, prognosis was good (1 dog was euthanized due to severe respiratory signs).

… The Takeaways

Key pearls to put into practice:

• Vaccines and Hymenoptera stings are the most commonly known causes for mild hypersensitivity reactions and anaphylaxis, but the cause is unknown in many cases.

• Anaphylaxis may be difficult to diagnose. In this study, >50% of dogs did not have cutaneous signs. Suspicion for anaphylaxis should be high in patients with acute onset of shock, dyspnea, or GI signs after known or potential exposure to an allergen. Many patients show signs of cardiovascular dysfunction, and epinephrine should be considered. H₁ antagonists and glucocorticoids should be considered as second-line treatments but do not reverse shock.

• Mild hypersensitivity reactions often improve without treatment, but H₁ antagonists may improve patient comfort and accelerate resolution of signs. Administration of glucocorticoids instead of or in addition to H₁ antagonists does not have a known benefit in patients with anaphylaxis or mild hypersensitivity reactions.