Just Ask the Expert: Are antihistamines effective in canine atopy?

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Oct 01, 2010

Dr. Bloom welcomes dermatology questions from veterinarians and veterinary technicians.
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Is cetirizine effective in canine allergic inhalant dermatitis, and, if so, what is the dosage?

Tom McCoy, DVM
Harvard Avenue Veterinary Clinic
Tulsa, Okla.


Paul Bloom, DVM, DACVD, DABVP
Using criteria of evidence-based medicine, evidence at this time is insufficient to conclude for or against the efficacy of any antihistamine in treating canine atopic dermatitis (a more appropriate name for what was previously known as canine allergic inhalant dermatitis since most of the antigen is percutaneously absorbed rather than inhaled). That said, strong anecdotal evidence suggests that antihistamines do have some effect, either as a monotherapy or as a corticosteroid-sparing agent.

Limited studies support the efficacy of one antihistamine over another in veterinary patients. An in vitro study using canine cutaneous mast cells and comparing the effects of loratadine, cetirizine, ketotifen, and terfenadine demonstrated that only loratadine caused potent inhibition of histamine release from the mast cells.1 Whether this would be clinically relevant has not been studied. The only study I know of that evaluated cetirizine was a placebo-controlled, blinded study that reported that pruritus was satisfactorily reduced in 4 of 22 (18%) dogs.2 However, only two of the four dogs had an excellent response (76% to 100% reduction), while one of the four continued to receive a low dose of a glucocorticoid during the entire study. The fourth dog had a fair (26% to 50% reduction) response.

Treatment of canine atopic dermatitis was reviewed recently (see the Related Link "New veterinary clinical practice guidelines for canine atopy" below).3 These 2010 clinical practice guidelines recommend that if convincing clinical trial data are unavailable, veterinarians who want to use type 1 antihistamines to treat atopy should use those that have shown demonstrable inhibitory effects after intradermal histamine injections in dogs. At this time only hydroxyzine (2 mg/kg twice daily) and cetirizine (0.5 to 1 mg/kg once a day) have demonstrated these effects.3 It really doesn't matter which of these two antihistamines you use since hydroxyzine is rapidly converted to cetirizine.4 The guidelines also advise that antihistamines be used preventively every day to keep H1 receptors inactive so histamine is not released during immediate allergic reactions.

So cetirizine may be effective in some dogs with atopic dermatitis, but it is not one of the antihistamines that I prescribe for my patients. I don't see an advantage of this antihistamine over the traditional ones (diphenhydramine, hydroxyzine, chlorpheniramine). Because many different antihistamines are available and none is more effective than another, I suggest you choose an antihistamine based on ease of administration (pill vs. capsule, twice vs. three times a day), cost, and side effects. If after 14 days the dog shows no improvement, prescribe another antihistamine until each of the antihistamines has been tried for 14 days or an effective one has been found.

Paul Bloom, DVM, DACVD, DABVP
Allergy, Skin and Ear Clinic for Pets
31205 Five Mile
Livonia, MI 48154

REFERENCES

1. Garcia G, DeMora F, Ferrer L, et al. Effect of H1-antihistamines on histamine release from dispersed canine cutaneous mast cells. Am J Vet Res 1997;58(3):293-297.

2. Cook CP, Scott DW, Miller WH Jr, et al. Treatment of canine atopic dermatitis with cetirizine, a second generation antihistamine: a single-blinded, placebo-controlled study. Can Vet J 2004;45(5):414-417.

3. Olivry T, DeBoer DJ, Favrot C, et al. Treatment of canine atopic dermatitis: 2010 clinical practice guidelines from the International Task Force on Canine Atopic Dermatitis. Vet Dermatol 2010;21(3):233-248.

4. Bizikova P, Papich MG, Olivry T. Hydroxyzine and cetirizine pharmacokinetics and pharmacodynamics after oral and intravenous administration of hydroxyzine to healthy dogs. Vet Dermatol 2008;19:348-357.