Merck Manual

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Professional Version

Topical Antihistamines and Mast Cell Stabilizers in Animals


Nick Whelan

, BVSc, DACVCP, DACVO, Animal Eye Clinic of Waterloo Region

Reviewed/Revised Oct 2021 | Modified Nov 2022

Antihistamines and mast cell stabilizers are used to treat allergic conjunctivitis. They slow or minimize the inflammatory response and provide relief of symptoms, signs, and complications of chronic inflammation. Topical antihistamines can be single or dual activity. Antazoline and pheniramine are single activity first-generation topical histamine-1 (H1) receptor antagonists used in combination with vasoconstrictors. Cetirizine 0.24%, a second-generation antihistamine, is also available in drop form. Antazoline, pheniramine, and cetirizine all act only as antihistamines, and with the shorter duration of activity, may need to be used frequently and in conjunction with other agents (mast cell stabilizers and steroidal or nonsteroidal anti-inflammatory agents).

Mast cell stabilizers include 2% nedocromil sodium, 4% sodium cromoglycate, and 0.1% lodoxamide. They inhibit the increase in cutaneous vascular permeability associated with IgE/antigen-mediated reactions (itching, irritation, watery eyes, photophobia). Their mechanism of action is unknown; however, they may prevent Ca2+ influx into mast cells upon antigen stimulation. Lodoxamide is significantly more potent than sodium cromolyn. Mast cell stabilizers work best when used prophylactically, and they require a loading period. With the availability of dual action agents, they are not commonly used singly.

Dual activity agents are more effective than antihistamines or mast cells stabilizers alone. They act like both, and some can also inhibit mast cell chemotaxis, eosinophil activation, and formation of other inflammatory mediators (leukotrienes and PAF). Available drugs include olopatadine (0.1%, 0.2%, 0.7%), 0.25% ketotifen, 0.05% epinastine, 0.25% alcaftadine, 0.05% azelastine, and more recently 1.5% bepotastine. They are recommended as first-line treatment for allergic conjunctivitis.

Vasoconstrictors (naphazoline, tetrahydrozoline) are also used to reduce the hyperemia associated with allergic conjunctivitis. Their use often results in a rapid and short-term onset of drug response, and rebound vasodilation when discontinued. Because more efficacious drops are available, they are not recommended for use in allergic conjunctivitis in animals.

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