Dr. Jeff Grognet DVM & Dr. Louise Janes DVM March 2010

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Anatomy of an Allergic Reaction

by Jeff Grognet, DVM, BSc (Agr) and Louise Janes, BSc(Agr), DVM

The immune system’s job is to protect the body from invasion by foreign organisms. However, under certain circumstances, antibodies produced by the immune system react too strongly. This can cause a little itch as we see with hives, or be so severe that it triggers instant death.

 

The most devastating immune reaction a body can undergo is anaphylactic shock. This occurs in children that die from eating a peanut or being stung by a bee. In this case, the antigen (foreign substance) arises from an injection of a drug or a vaccine, ingestion of certain foods, or via insect bites (the most common in dogs).

Anaphylactic reactions begin within seconds of the allergen exposure. A massive amount of histamine is released, causing contractions of the muscle surrounding the airways. This makes it hard to get air in or out of the lungs. Secondarily, blood pools in the lungs, making oxygen transfer difficult.

Blood vessels in the liver and intestine also dilate, diverting blood from the circulation. This creates an instantaneous drop in blood pressure. Combined with the inability to get oxygen into the blood, this can cause death in minutes.

Anaphylactic reactions are, thankfully, rare. However, most veterinarians can tell you of a canine patient that was running happily in the park one minute and dead the next. It likely had a reaction to a bee or wasp sting.

The identical reaction can be seen in veterinary hospitals after injecting a vaccine or drug. In this case, the problem is detected and the animal is immediately put on intravenous fluids to combat hypotension. Injected epinephrine relaxes airways and limits pooling of blood in vessels. With prompt attention, they normally survive.

A milder immune reaction is angioedema. This is a sudden build-up of fluid (edema) in the lips, the membranes of the eyes, and the skin of the face. Allergies to foods are a common reason for this type of reaction, but it can also be seen following insect and drug exposure.

Angioedema takes longer to develop after antigen exposure than anaphylactic shock. What this means is that if a dog’s muzzle is swollen, he is already past the time that a life-threatening anaphylactic reaction will occur. Treatment is directed at reducing the swelling and usually consists of anti-histamines such as diphenhydramine and possibly steroids.

The last type of allergic reaction is hives. It is the least severe, but it is still caused by an over-reaction to allergens. Focal thickenings in the skin (again edema) can be very itchy, but certainly not life-threatening. Oral antihistamines help speed its resolution.

 

You can’t prevent all allergic reactions, but there are ways to avoid some. If you know your dog is allergic to certain food ingredients, avoid them by reading labels carefully. If your dog has had an allergic reaction to any vaccine or medication, tell your veterinarian. And, if your dog reacts to insect bites, ask your veterinarian what antihistamine you can give and the dose that would be appropriate for your dog.

For more information contact Dr. Jeff Grognet or Dr. Louise Janes at

Mid-Isle Veterinary Hospital
5-161 Fern Road West
Qualicum Beach, BC
Tel (250) 752-8969

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