Immunizations (or vaccines) are intended to stimulate the immune system so as to protect the animal from a specific infectious agent. However, this stimulation may cause some minor symptoms. Your cat may react to immunizations in ways that range from soreness at the site of injection to mild fever to allergic reactions, which can range from mild to severe.
- Mild - Mild reactions include fever, sluggishness and loss of appetite. Mild vaccination reactions usually require no treatment. However, if the symptoms persist for more than 24 hours, call your veterinarian.
- Moderate - Urticaria is a moderate vascular reaction of the skin marked by hives or wheals and rapid swelling and redness of the lips, around the eyes, and in the neck region. It is usually extremely itchy. Urticaria occurs soon after vaccination, often shortly after the animal gets home. Your veterinarian will recommend immediate return to the hospital for treatment. An oral antihistamine such as diphenhydramine (BenadrylÂ®) may be recommended to initiate treatment. Urticaria is usually treated successfully with injectable corticosteroids like dexamethasone or prednisone. Antihistamines do little to help with acute allergic reactions but may be given by injection to help prevent recurrence of symptoms after steroids wear off. Urticaria may progress to anaphylaxis, which is considered life-threatening. Urticaria is the most common reaction in cats.
- Severe - The most severe reaction is anaphylaxis, a sudden, severe allergic response that produces breathing difficulties, collapse and possible death. Symptoms usually include sudden onset of vomiting, diarrhea, staggering, rapid drop in blood pressure, swelling of the larynx leading to airway obstruction (and inability to breathe), seizures and cardiovascular collapse or death. This reaction is life-threatening for your cat. Anaphylaxis is an extreme emergency. Your veterinarian will begin immediate emergency life support including establishing an open airway, oxygen administration, intravenous fluids to increase blood pressure and drugs such as epinephrine, diphenhydramine and corticosteroids. Cats that survive the first few minutes usually return to normal health. Anaphylaxis usually occurs soon after vaccination, often while the cat is still in the veterinary clinic.
Both anaphylaxis and urticaria are reactions that are triggered by antibodies that the immune system has made to some portion of the vaccine and usually requires at least one previous exposure to the vaccine. The antibodies cause inflammatory cells like basophils and mast cells to release substances that cause the allergic reaction. The impact on the cat may be life threatening but if treated successfully the prognosis for long-term health is good. Mild reactions usually resolve without treatment.
There is no diagnostic test for anaphylaxis or urticaria, but a quick physical exam will show common signs of an allergic reaction and the history will reveal recent vaccination.
Be sure to schedule vaccination appointments when you will be available to monitor your cat after the vaccine is administered. Be sure to call your veterinarian with any questions or concerns.
The good news is that severe vaccination reactions are rare. The risk of anaphylaxis and urticaria are much more rare than the benefit of the vaccine in most cases. You can limit vaccines to those that prevent diseases to which your cat may be exposed. Along with your vet, you are the best judge of what vaccines are needed to protect against the diseases in your area.