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    Antihistamines for dogs. Which one and when should it be administered?

    Antihistamines for dogs are mainly used as an adjuvant to corticosteroids in the continuous treatment of allergies or to control gastric acid secretion in the case of stomach ulcers.

    Antihistamines are not usually given to dogs, as vets tend to opt for other medical and pharmacological approaches. Here we briefly review the types of antihistamines for dogs and the indications for their use.

    Antihistamines for dogs

    In general, there are three types of drugs for treating the effects of histamine:

    1.     Physiological antagonists. The following compounds, known as sympathomimetic amines, produce physiological effects that oppose histamine activity: adrenaline, ephedrine, xanthine.

    2.     Histamine-release inhibitors, such as ketotifen or sodium cromoglycate, which reduce basophil and mast cell degranulation.

    3.     Competitive pharmacological antagonists or antihistamines. These medicinal products occupy the specific histamine receptors: H1, H2 and H3.

    Antihistamine action in dogs

    Histamine H1-receptors are responsible for vasodilation, vascular permeability and contraction of bronchial and intestinal smooth muscle. Consequently, H1 antagonists are mainly used for anaphylactic shock (combined with physiological antagonists) and allergic reactions. They are also used for pruritus, urticaria, dermatitis, eczema of various origins and insect bites, again in combination with other drugs. The most common H1 antagonists for dogs are chlorpheniramine, diphenhydramine, dimenhydrinate, promethazine and astemizole.

    The histamine H2-receptors regulate the gastric secretion of HCl and induce a certain degree of cardiac stimulation. Antagonists of this receptor are mainly used in the treatment of gastroduodenal ulcers and gastritis, because of their antisecretory effect. These drugs are namely cimetidine and ranitidine.

    Antihistamines should only be administered until the aetiological agents are eliminated, as their action is only symptomatic.

    H2-antagonists: gastroduodenal ulcers in dogs

    Gastric ulcers appear because of an imbalance between stomach acid secretion and mucosal defence mechanisms. Consequently, the main pharmacological treatment is the administration of H2-receptor antagonists (antisecretory drugs) until the protective gastric barrier has been restored.

    Gastroduodenal ulcers can be triggered by stress and the continued use of certain drugs, mainly nonsteroidal anti-inflammatories.

    H2-antagonists: the most common allergies in dogs

    Although 7 out of 10 dog or cat owners have never heard of allergic dermatitis, skin allergies are the second leading cause for veterinary consultations in dogs. Canine atopic dermatitis affects 10% of dogs and is the most common allergy in our four-legged friends. It is clinically diagnosed if the pruritus cannot be linked to parasites or diet. Although the origin of the dermatitis may not be dietary, treatment with specially formulated food forms part of the solution.

    Food allergies are the second-most common allergy in dogs. They occur due to sensitisation to certain proteins from beef, milk, chicken, wheat or lamb. The symptoms of a food allergy are mainly epithelial and gastrointestinal, although up to 25% of cases only show signs of otitis. When there is a clinical suspicion, the diagnosis is confirmed if the clinical signs recede after applying an elimination diet for 6 weeks.

    Dietary treatment of dog allergies

    A dietary approach is essential for the treatment of atopic dermatitis and food allergies or sensitisation in dogs. Patients should receive a hypoallergenic diet especially designed to eliminate the protein residues that cause allergic sensitisation.