Urticaria is a very common nodular presentation. Edema in the dermis causes a rapid onset of nodules. This condition is often referred to as “feed bumps” or “protein bumps” by the layperson. The pathogenetic mechanism is that of a type I hypersensitivity most often associated with drug administration such as antibiotics, antiinflammatory agents, or vaccines. Drugs may be administered by any route. Other causes of urticaria include allergies to pollens, foods, or insects. For a large number of cases, no specific agent is identified; these cases are then classified as idiopathic. A portion of idiopathic cases actually are likely to be a form of autoimmune disease that results from autoantibodies cross-linking the Fc receptor of mast cells.
Urticaria: Clinical Signs
Clinical signs consist of a sudden onset of localized to generalized wheals. The lesions may or may not be accompanied by pruritus. In some horses the lesions take on a serpiginous or ringlike appearance.
Diagnosis of Urticaria
Diagnosis is usually based on lesion appearance and history of rapid onset. When digital pressure is applied to a lesion, an indentation is made, which supports dermal edema rather than a cellular infiltrate as the cause of the nodule. Biopsy is indicated in cases of recurrent or chronic urticaria and, in severe cases, to rule out vasculitis as a cause for the dermal edema. Occasionally, early dermatophytosis presents with wheal formation. These lesions progress to the more classic dermatophyte lesions in a day or two.
Treatment of Urticaria
Treatment options depend on the cause and severity of the urticaria. Lesions should regress rapidly on their own on termination of exposure to the initiating antigen. Although antihistamines do not cause regression of existing lesions, they prevent further histamine-binding to receptors while the antigen is still present in the tissue and are therefore very helpful in cases of urticaria. Hydroxyzine hydrochloride (1 to 1.5 mg/kg q8-12h) is very effective for this condition. Antiinflammatory doses of corticosteroids (prednisolone 0.5 to 1.0 mg/kg/day) may be indicated in severe or chronic cases. In refractory cases, dexamethasone (at an initial dose of 0.02 to 0.1 mg/kg/day followed by oral maintenance dose of 0.01 to 0.02 mg/kg every 48 to 72 hours) may be of benefit. Lastly, epinephrine may be needed if lesions are associated with systemic signs of anaphylaxis.
In addition to treating the urticarial lesions, identification of the underlying cause is paramount. First, with a careful history, drugs should be ruled out. Insect hypersensitivity can be addressed with good fly control by using 2% permethrin. In chronic recurrent urticaria, food allergy should be investigated by placing the horse on grass hay different from the usual hay. If grain is needed, oats should be added while sweet feed and food supplements are avoided. Some horses with recurrent urticaria have positive skin test results to pollens and molds and may benefit from hyposensitization (see “Atopy”).
Prognosis depends on the underlying cause. When the case can be identified and corrected, prognosis is excellent. Chronic recurrent urticaria is usually idiopathic and therefore has a poor prognosis for cure.