Recognition and Emergency Management of Anaphylaxis in Small Animal Patients

Pathophysiology

Anaphylaxis is a severe, systemic hypersensitivity reaction resulting from rapid, IgE-mediated (or occasionally non-IgE-mediated) mast cell and basophil degranulation. Release of histamine, leukotrienes, prostaglandins, and platelet-activating factor produces the clinical syndrome through three principal mechanisms: profound vasodilation and increased vascular permeability, bronchospasm and upper airway edema, and myocardial depression.

In dogs, the primary shock organ in anaphylaxis is the liver—specifically the hepatic sinusoids and portal vasculature, which become engorged and obstructed with degranulating mast cells, producing acute portal hypertension and splanchnic sequestration of blood volume. This differs from cats, in whom bronchospasm and upper airway involvement predominate.

Recognition

Speed of recognition is critical; anaphylaxis can be fatal within minutes. Common triggers include vaccines, drugs (particularly antibiotics and NSAIDs), insect envenomation, blood products, and food antigens.

Dogs typically present with:

• Acute vomiting and defecation, often profuse
• Urticaria, facial swelling, pruritus
• Profound weakness, collapse
• Pale mucous membranes, poor pulse quality
• Hepatomegaly on palpation (acute venous engorgement)

Cats typically present with:

• Respiratory distress (bronchospasm, laryngeal edema)
• Facial pruritus and excoriation
• Hypersalivation, vomiting
• Collapse and coma (rapidly)

Treatment

Epinephrine — First Line, No Delay

Epinephrine is the only life-saving intervention in anaphylaxis and must be administered immediately upon diagnosis. There is no contraindication to epinephrine in confirmed anaphylaxis.

• Dogs: 0.01–0.02 mg/kg epinephrine (1:1000) IM in the epaxial muscles or lateral thigh. Repeat every 5–15 minutes if response is inadequate.
• Cats: 0.01 mg/kg epinephrine IM; use cautiously as cats are sensitive to sympathomimetic effects.

IV epinephrine (1:10,000 dilution at 0.01 mg/kg) is reserved for cardiovascular collapse and must be given slowly with ECG monitoring due to risk of arrhythmia.

Supportive Care

Following epinephrine:

• IV crystalloid boluses (10–20 mL/kg) for hemodynamic support
• Oxygen supplementation; intubate if upper airway edema threatens airway patency
• Diphenhydramine (2 mg/kg IM/IV slowly) — adjunctive H1 blockade; does not replace epinephrine
• Dexamethasone sodium phosphate (0.1–0.2 mg/kg IV) — corticosteroids may reduce biphasic reactions but have no role in acute management

Monitoring

Patients should be observed for a minimum of 4–6 hours after initial stabilization, as biphasic anaphylaxis (recurrence of symptoms after apparent resolution) occurs in a subset of patients.