Anaphylaxis
Anaphylaxis is an acute systemic manifestation of the interaction of an antigen (allergen) binding to IgE antibodies, which are bound to mast cells and basophils. This binding of antigens to cell-bound IgE antibodies triggers the release of chemical substances from the mast cells and basophils. The major biologically active mediators produced by mast cells and basophils include histamine, leukotrienes, the eosinophilic chemotactic factor, platelet activating factor, kinins, serotonins, and proteolytic enzymes. These chemicals directly affect both the vascular system, causing vasodilation and increased vascular permeability, and smooth muscles, causing contraction. Additionally, they result in the migration of eosinophils to the triggering site. The severity of the reaction depends on the type of antigen, the amount of IgE antibodies the animal has produced, and the amount of antigen and route of exposure. Agents that can cause anaphylactic and allergic reactions are numerous and include the venom of stinging and biting insects, vaccines, drugs of any kind, food substances, and blood products.
Clinical signs can be localized or generalized and include restlessness and excitement, itch around the head or site of exposure, facial edema, salivation, lacrimation, vomiting, abdominal pain, diarrhea, dyspnea, cyanosis, shock, incoordination, collapse, convulsions, and death. Dogs differ from other domestic animals in that the major organ affected by shock is the liver, rather than the lung. Signs in dogs are associated with the constriction of hepatic veins, which results in portal hypertension and visceral pooling of blood. Therefore, GI signs rather than respiratory signs are more apt to be seen in dogs. Supportive therapy, in addition to treating respiratory distress, consists of the administration of epinephrine (both locally and systemically as needed), IV fluids for the treatment of shock, antihistamines (systemically for severe acute anaphylaxis or orally as a means to control chronic signs of allergy or more mild allergic signs), and corticosteroids if needed.