Common causes of allergic reactions include bee stings, vaccination, and spider bites. In many cases, we may never know what your pet has been exposed to that has caused signs of hives, swelling of the face, itchiness and sometimes vomiting and/or diarrhea.
Severe forms of allergic reactions are known as anaphylaxis and cause difficulty breathing, weakness, low blood pressure, and collapse. If your pet is showing any of these severe abnormal signs, bring them to the closest veterinary hospital immediately.
If your pet has had an allergic reaction to a vaccination, your veterinarian may elect not to vaccinate anymore or may pre-treat with an antihistamine before any further vaccinations are administered. Make sure that you remind any new veterinarian that your pet has had a vaccine reaction in the past, to help avoid ongoing problems.