Allergic Reactions

What is anaphylaxis?

Anaphylaxis, or anaphylactic reaction, is an immediate, severe, sometime life-threatening allergic reaction triggered by a foreign substance, especially a foreign protein, known as an allergen or antigen. It is caused by the immune system overreacting to the presence of certain compounds. Anaphylaxis can negatively affect multiple organ systems, including the heart, lungs, stomach and intestines, and the skin.

What causes an anaphylactic reaction?

Several allergies can cause anaphylaxis in dogs. A recent study showed that almost 50 percent of cases resulted from insect stings, 4 percent were drug side effects, while the other 46 percent were from unknown causes. Theoretically, any foreign substance can produce an anaphylactic reaction. The most common substances to cause anaphylaxis are food proteins, insect bites, medications, antigens in vaccines, environmental pollutants and chemicals.

Ants, wasps, bees and fire ants have venom that contains toxins and compounds that can cause allergic reactions in some dogs. There may also be other chemicals in insect venom that can stimulate the immune system or cause tissue damage.

Before an anaphylactic reaction can occur, the dog usually has had a previous exposure to the offending substance. A common example is a dog stung by a bee that later develops an allergy to bee stings.

The first time a dog is stung, a short-term, localized reaction occurs. The sting causes the dog’s immune system to produce a substance called Immunoglobulin E (IgE), which binds to the surface of the mast cells and other white blood cells, causing the release of inflammatory chemicals, including histamines. These chemicals cause redness, swelling and hives at the site of the sting. At this point, the dog is said to be sensitized to the insect’s toxin.

The second time a dog is stung, the sensitized mast cells recognize the foreign protein, which is the insect toxin, and release their contents in a process known as degranulation and activation. In milder cases of anaphylaxis, a localized allergic reaction such as extreme swelling at the site of the insect sting will occur. In severe reactions, the localized reaction causes further release of the inflammatory contents of mast cells throughout the body, resulting in systemic anaphylaxis. Localized allergic reactions are common, while systemic anaphylaxis is rare.

What dogs are more likely to suffer from anaphylactic reactions?

Dogs of any age can have an anaphylactic reaction. It was thought that puppies are more prone to these allergic reactions, but this is not the case. Puppies present to the emergency hospital more often than adult dogs because they are more playful and inquisitive, putting themselves in contact with insects more often.

When dogs have anaphylactic reactions later in life, the subsequent reactions may be worse than the initial reaction. However, some dogs may only have one reaction and then never experience it again in their lifetime.

There is some evidence that Dachshunds seem particularly susceptible to anaphylaxis from insect stings. In my experience, Dachshunds and Pugs have the highest incidence of vaccine reactions. Some cases of anaphylaxis are thought to have a hereditary or familial basis. This is information you should share with your veterinarian when vaccinating puppies from parents with a history of anaphylactic reactions.

What are the signs of an anaphylactic reaction?

The symptoms of anaphylaxis depend on the route of exposure (e.g., mouth, skin, inhalation, injection), the amount of antigen in the exposure, and the level of the dog’s histamine release. When histamines are released in small doses, there are signs of itching, red skin swellings called wheals or hives, or a swollen face or muzzle. When histamines are released in larger amounts all over the body, dogs may experience excessive salivation or drooling, vomiting and diarrhea.

In severe cases, dogs will have difficulty breathing. They may be cyanotic, which means they are not getting enough oxygen and their gums and tongue will be a bluish color. This occurs because large amounts of histamines cause blood vessels to dilate, leading to low blood pressure and reduced blood flow that is slower to spread oxygenated red blood to the extremities.

How is anaphylaxis diagnosed?

Anaphylaxis is diagnosed by a medical history of a recent exposure to an offending substance, physical examination findings, and the presence of the characteristic signs. In severe cases, blood tests may reveal elevations in liver enzymes and dehydration. An ultrasound of the abdomen may show an enlarged liver, swelling of the wall of the gallbladder, and fluid around the abdominal organs.

Once the episode is resolved, skin testing and IgE blood tests may be performed to identify specific allergens. These tests are best conducted by a veterinary dermatologist.

What should I do if I think my dog is having an anaphylactic reaction?

Anaphylaxis in dogs is a medical emergency. Symptoms can occur quickly and severely, so rapid veterinary treatment is important, especially if you think your dog is in shock or is having trouble breathing. Anaphylaxis usually occurs within the first 30 minutes after exposure to the allergen or insect sting and progressively worsens.

If the reaction is caused by an insect sting, use tweezers to remove the stinger and place the stinger into a container. Bring the stinger with you to the veterinarian. This is important for identification of the insect.

Flat-faced dogs, such as Bulldogs, need to be kept calm and cool while traveling. They are at higher risk of breathing difficulties and over-heating. Turn on the air conditioning in your car to maintain a cool environmental temperature as you make your way to the veterinarian.

At the veterinarian, your dog will be treated to minimize shock, ensure the airway is open, and blood pressure and circulation are stabilized. It is often necessary to administer intravenous fluids, place a breathing tube, and inject emergency drugs such as epinephrine, corticosteroids, atropine and aminophylline.

Dogs that are having trouble breathing may need to be intubated to maintain an open airway. Oxygen at a high flow rate can be administered with a face mask, nasal cannula, endotracheal tube or oxygen cage.

In mild cases, it may be sufficient to give antihistamines and possibly corticosteroids to stop the reaction and reduce the swelling. The patient should be closely monitored for the next 24 to 48 hours in case the reaction recurs.

What is the prognosis for a dog that has an anaphylactic reaction?

The initial prognosis is always guarded and depends on the severity of the reaction and how quickly it is treated. It is impossible to know if a localized or minor reaction will progress to a major health crisis.

Since anaphylactic reactions often worsen with each episode, the main goal is to prevent exposure to the offending substance. If you have a dog that has had an anaphylactic reaction, you may be given emergency medication, such as diphenhydramine (Benadryl) and prednisone, to keep at home and instructions on what to do if your dog shows any signs of future reactions.

In some cases, an epi-pen may be prescribed for owners to use during a future anaphylaxis episode, but the effectiveness of this treatment has not been established.