Urticaria is the result of degranulation of mast cells and the subsequent release of histamine into the dermis. Histamine release causes vasodilation and dermal edema. The edema is often multifocal, leading to 0.5–3.0 centimeter diameter wheals; although, it may be diffuse, affecting large areas of dermis or subcutis. Diffuse edema (angioedema) causes gross swelling of entire body regions such as the limb, face, or ventrum.
Urticaria is generally an acute immune response, often occurring within minutes of the stimulus. Urticaria may resolve without treatment, generally within 12–48 hours. It may be frequently recurrent depending on the underlying cause, and it may be chronic in rare cases.
Many causes of urticaria exist. Most commonly, urticaria occurs as a result of an inhaled or contact allergen, vaccination, or insect bite (such as bees, wasps, hornets, mosquitoes, black flies, ants, spiders, and certain caterpillars).
Other causes include: plants (such as stinging nettle), transfusion reactions, heat or cold, exercise, pressure, dermatographism, psychologic stress, viruses, bacteria, fungi, and parasites. Additionally, exposure to topical medications or chemicals such as carbolic acid, turpentine, carbon disulfide, or crude oil may induce urticaria. Foods, food additives, and systemically administered drugs (such as penicillin, ampicillin, tetracycline, vitamin K, propylthiouracil, amitraz, ivermectin, moxidectin, and doxorubicin), and estrus may also cause urticaria.
A unique form of urticaria in cattle has been described chiefly in the Channel Island breeds (Jersey, Guernsey), which become sensitized to the casein in their own milk (“milk allergy”); it occurs in cases of milk retention or unusual engorgement of the udder with milk.
Urticaria may occur in any mammal but is most often recognized in dogs and horses. Common lesions include acute-onset, haired, dome-shaped wheals. Occasionally, urticarial lesions will become very large or take on a linear or even serpiginous shape. Angioedema may occur in severe cases.
Urticaria can affect any area of the skin, but some areas are more commonly affected than others. In dogs, the head, limbs, trunk, and ventrum are commonly affected. In horses, wheals are seen mainly on the back, flanks, neck, eyelids, and legs. In sheep, lesions are usually seen only on the udder and hairless parts of the abdomen. In pigs, eruptions are seen around the eyes, between the hindlegs, and on the snout, abdomen, and back. Urticaria may or may not be pruritic. Large urticarial lesions and angioedema will pit when digital pressure is applied.
The history typically describes an acute onset of lesions. Physical examination reveals wheals and/or angioedema that pit when digital pressure is applied. Wheals are haired, multifocal, and crusting; exudates are generally not concurrent signs. Lesions subside quickly when a glucocorticoid is administered. Wheals that are chronic or accompanied by other clinical signs should be biopsied.
Mild cases of urticaria may spontaneously respond within 12–48 hours. Treatment is generally initiated, however, because it is difficult to predict whether untreated urticaria will resolve or worsen over time. Generally, urticaria and angioedema respond quickly to treatment with glucocorticoids and antihistamines.
For dogs with urticaria, 1–4 mg/kg diphenhydramine, an antihistamine, may be given IM or PO. Additional treatment with a glucocorticoid will result in quicker resolution of urticaria compared with antihistamine alone. Glucocorticoid treatment options are as follows: dexamethasone sodium phosphate may be administered at a dosage of 1–4 mg/kg, IV; methylprednisolone sodium succinate at a dosage of 30 mg/kg, IV; or prednisone sodium succinate at a dosage of 10–25 mg/kg, IV. Oral administration of prednisone, 0.5–1.0 mg/kg, is generally sufficient for milder cases. Although urticaria generally responds quickly to treatment, it may recur as the antihistamine and glucocorticoid are metabolized. Administration of diphenhydramine every 8 hours for 3–5 days after initial treatment helps to prevent acute recurrence.
For horses with urticaria, dexamethasone is administered at a dosage of 0.01–0.04 mg/kg, IV, and can be repeated 2 or 3 times over several days if necessary.
Uncommonly, urticaria is a precursor to anaphylaxis. Anaphylaxis is a life-threatening condition characterized by systemic signs and inadequate response to glucocorticoids. Epinephrine at a dosage of 0.01 mg/kg of a 1:1,000 (1 mg/mL) solution, IM, (maximum dose of 0.3 mg in patients <40 kg and 0.5 mg in patients >40 kg) is recommended for the initial treatment of anaphylaxis. Fluid therapy and other supportive care is essential for patients experiencing anaphylactic shock. In general, the prognosis is favorable.
Fatalities are rare and are probably due to anaphylaxis or associated angioedema involving the respiratory passages.
Prevention is dependent upon avoidance of or desensitization to the underlying cause of the urticaria.
For insect bite-induced urticaria, several actions can be taken. An insect repellent should be applied to the affected animal. Permethrin products offer reasonable insect repellency and are available for dogs and horses. Permethrin products can be fatal to cats. Protective clothing such as fly masks for horses can be helpful. Horses and other animals spending time outside should be kept away from environments that favor biting insects. Biting insects tend to favor low-lying areas with standing water and poor air circulation. Additionally, sheltered outdoor areas tend to favor spiders.
Environmental allergens such as pollens and molds can be detected through allergy testing. Allergen-specific immunotherapy can desensitize animals to the things they are allergic to.
Daily administration of an antihistamine such as diphenhydramine 2 mg/kg, PO, twice daily, or cetirizine 0.5 mg/kg, PO, once to twice daily, may be necessary for those animals that cannot avoid allergens or insects that induce urticaria.