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Allergic Reaction from an Unknown Cause in Dogs

By: PetPlace Veterinarians

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Allergic reactions can vary from mild to severe. The most severe reaction is anaphylaxis, an immediate reaction that leads to life-threatening symptoms that include vomiting, diarrhea, staggering, rapid drop in blood pressure, swelling of the larynx leading to airway obstruction, and cardiovascular collapse. Urticaria is rapid swelling and redness of the lips, around the eyes, and in the neck region that can be very itchy. Hives may also occur. Urticaria may progress to anaphylaxis.

Both anaphylaxis and urticaria are reactions that are triggered by antibodies that the immune system has made. The antibodies cause inflammatory cells like basophils and mast cells to release substances that cause the allergic reaction.

Allergic reactions may occur in animals of any age, breed, or sex. The impact on the dog may be life threatening for both urticaria and anaphylaxis. However, if treated successfully the prognosis for long-term health is good.

What to Watch For

  • Vomiting
  • Diarrhea
  • Staggering and weakness
  • Collapse
  • Difficulty breathing
  • Rapid swelling and redness around the lips, eyes and/or neck
  • Itching
  • Hives


    There is no diagnostic test for anaphylaxis or urticaria, but a quick physical exam is diagnostic.


    Anaphylaxis is an extreme emergency. Your veterinarian will begin immediate emergency life support including establishing an open airway, administering oxygen, intravenous fluids to increase blood pressure, and drugs such as epinephrine and corticosteroids. Animals that survive the first few minutes usually return to normal health. Anaphylaxis usually occurs soon after exposure to the allergic substance.

    Urticaria also occurs soon after exposure. Your veterinarian may recommend immediate examination and treatment and an oral antihistamine such as diphenhydramine (Benadryl) to initiate treatment. Urticaria is usually treated successfully with injectable corticosteroids like dexamethasone or prednisolone. Antihistamines do little to help with acute allergic reactions but may be given by injection to help prevent recurrence of symptoms after the steroids wear off.

    Home Care and Prevention

    Since the cause of the allergic reaction is unknown, you may not realize your dog is developing an allergic reaction until it is far beyond home care. Call your veterinarian immediately for advise regarding administering diphenhydramine.

    Severe allergic reactions are rare. Since the cause of the allergic reaction is unknown, prevention is difficult.

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