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Itching and scratching, along with the appearance of red spots on the skin and swelling, are the most common clinical signs of anaphylactic shock in dogs. This usually occurs within minutes of your dog coming into contact with an allergen.  The most common allergens that cause anaphylactic shock in dogs include insect bites, pollen, mold spores, dust, fleas, and environmental pollutants. The speed and severity of the onset of these symptoms will vary, depending on how much contact your dog has had with an allergen and through what mechanism (e.g., through the skin, through an injection, etc.). The most important thing to look for is the close connection between exposure to an allergen and the appearance of these symptoms. All dogs scratch themselves, but scratching soon after contact with an allergen is more likely to indicate an allergic reaction. After itching and redness, these are the next most common signs of anaphylaxis. If these symptoms appear in your dog as part of an anaphylactic shock, they will also develop very quickly after exposure to an allergen. You may notice your dog urinating more frequently or uncontrollably, as well. Lack of bowel and bladder control is another symptom of anaphylactic shock. The initial symptoms of anaphylaxis (itching, vomiting, etc.) will eventually progress to shallow, rapid breathing, weakness or coldness in the limbs, and general lethargy. Your dog may also begin drooling excessively, even if there’s no food around.  You may also notice irregularities in your dog’s heartbeat; it may have a weak pulse or an elevated heart rate. Some dogs may not show any early signs and may instead progress straight to collapsing.
Notice if your dog starts itching right after being exposed to an allergen. Look out for the rapid onset of diarrhea, vomiting, or defecation. Watch for signs of trouble breathing or moving in your dog.