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Emergency · Workflow hub

Anaphylaxis hub

Workflow for acute anaphylaxis in dogs and cats. Epinephrine is the cornerstone; antihistamines and steroids are adjunctive.

  • Epinephrine first. Always. Antihistamines and glucocorticoids cannot control or prevent anaphylaxis alone. Do not delay epinephrine to give diphenhydramine.
  • Species differences matter. Dogs: GI and hepatic signs predominate (vomiting, abdominal pain, hepatic congestion, gallbladder wall thickening on FAST). Cats: respiratory signs predominate (bronchospasm, airway edema, increased secretions).
  • Biphasic reaction possible. Signs may recur hours after initial resolution. Hospitalize minimum 24–48 hours. Wean epinephrine CRI over 6–12 hours, never abrupt discontinuation.

Diagnostic pattern recognition

Dogs. GI / hepatic pattern

Acute onset vomiting, abdominal pain, diarrhea, collapse. Histamine causes hepatic arterial vasodilation + simultaneous venous congestion → portal hypertension, transudation, decreased venous return. FAST: gallbladder wall edema (halo sign) 93% sensitive, 98% specific. Elevated ALT 85% sensitive. Shock is often mixed: vasodilatory + hypovolemic + cardiogenic.

Cats, respiratory pattern

Acute respiratory distress, open-mouth breathing, bronchospasm, airway edema, increased secretions. GI signs also present. High airway mast cell density in cats makes bronchoconstriction the dominant response. Suspect after recent vaccine, medication, or food exposure. Respiratory compromise may mandate emergency intubation.

Diagnostic criteria (Pashmakova, adapted from human guidelines)

Anaphylaxis is likely if any one of the following:
  • Acute cutaneous signs (urticaria, angioedema, pruritus) AND respiratory compromise OR hypotension/collapse
  • Exposure to likely allergen AND two or more of: cutaneous signs, respiratory compromise, hypotension/collapse, GI signs
  • Hypotension after exposure to a known allergen

Treatment checklist

  1. 1

    Oxygen + IV access

    Flow-by oxygen immediately for all patients. Place IV catheter. Oxygen therapy continues until stabilized; ongoing for any patient with respiratory compromise.

  2. 2

    Epinephrine, first-line, essential

    α₁ activity: vasoconstriction → reverses vasodilatory shock, improves coronary flow, reduces mucosal edema and upper airway obstruction. β₁ activity: positive inotropy and chronotropy → improves cardiac output. β₂ activity: bronchodilation + mast cell stabilisation → halts further mediator release.

    Initial IM dose: 0.01 mg/kg IM (1:1000, 1 mg/mL)
    CRI (preferred): 0.05 µg/kg/min IV, titrate to effect
    Severe hypotension: IV bolus may be indicated before CRI

    CRI must not be interrupted, even briefly during nursing care. Short half-life means any break results in rapid rebound vasodilation. Wean over 6–12 hr once stable; faster if tachyarrhythmia or hypertension develops.

    Open epinephrine CRI calculator →
  3. 3

    Fluid therapy, volume resuscitation

    Balanced isotonic crystalloid. Administer in incremental shock-dose aliquots, reassess after each. Ongoing GI losses may be substantial; vigilant reassessment required for first 12–24 hours.

    Dogs: 30 mL/kg bolus aliquots
    Cats: 10 mL/kg bolus aliquots

    Synthetic colloids or blood products not indicated unless documented hypocoagulability with clinical bleeding (e.g. bee sting envenomation with hemoabdomen). Reassess after each aliquot, avoid overloading.

    Open fluid therapy calculator →
  4. 4

    Diphenhydramine, adjunctive, not first-line

    H1-receptor blocker. Relieves pruritus, lacrimation, and erythema. No evidence it can control or prevent anaphylaxis alone. Give after epinephrine is running, not instead of it.

    Dogs: 1–4 mg/kg IM
    Cats: 0.5–2 mg/kg IM
  5. 5

    Bronchodilation, if respiratory compromise present

    If bronchospasm persists after epinephrine, add:

    • Albuterol: 2 puffs via AeroKat inhaler (cats primarily)
    • Terbutaline: 0.01 mg/kg IM or IV slow (dogs and cats)

    If severe upper airway obstruction unresponsive to epinephrine: emergency intubation. Have tracheotomy equipment ready.

  6. 6

    Dexamethasone, adjunctive, delayed onset

    Onset hours after administration. NOT first-line for acute anaphylaxis. Downregulates late-phase eosinophilic response and blocks arachidonic acid cascade. A 2012 Cochrane review found no evidence supporting glucocorticoids in acute anaphylaxis; not in current human first-line guidelines. Use at anti-inflammatory (not immunosuppressive) dose.

    Dexamethasone: 0.1 mg/kg IV (anti-inflammatory dose)

    A short tapering course of oral prednisone may follow if needed. Do not use immunosuppressive doses for acute anaphylaxis.

  7. 7

    GI mucosal support

    In patients with GI compromise or on vasopressor support, parenteral proton pump inhibitor is reasonable; decreased splanchnic perfusion increases ulceration risk.

    • Pantoprazole: 1 mg/kg IV q12–24h
  8. 8

    Monitoring

    • Continuous blood pressure, vasopressor support as indicated
    • ECG, dysrhythmias can occur; myocardial ischemia possible
    • SpO₂, especially in cats with respiratory signs
    • Serial FAST, hepatic venous congestion, free fluid, gallbladder wall
    • ALT at 24 hr, may dramatically increase; values may require sample dilution
    • Coagulation times (PT/aPTT) if elevated ALT, liver dysfunction, or bleeding
    • Renal parameters 1–2 weeks post-discharge (AKI documented in severe cases)
    • Minimum hospitalisation: 24–48 hr for biphasic reaction surveillance

Sources

  • Pashmakova M. Anaphylaxis. In: Silverstein DC, Hopper K, eds. Small Animal Critical Care Medicine. 3rd ed. St. Louis, MO: Elsevier; 2023. Chapter 141. Primary source for all doses, diagnostic criteria, species-specific presentations, and monitoring recommendations.
  • Simons FE, et al. World Allergy Organization guidelines for the assessment and management of anaphylaxis. World Allergy Organ J. 2011;4(2):13–37. (Cited by Pashmakova as basis for epinephrine as essential drug.)