An anaphylactic reaction or anaphylaxis is an immediate hypersensitivity (allergic) reaction to a foreign substance, especially a foreign protein.
What causes an anaphylactic reaction?
Before an anaphylactic reaction can occur, the pet must have a previous
exposure to the offending substance. A common example is a dog that is
stung by a bee and later develops a hypersensitivity to bee stings. The first
time the dog is stung, a short-term localized reaction called a humoral
response occurs. This response causes the immune system to produce a
substance called Immunoglobulin E (IgE) that binds to the surface of
mast cells. Mast cells are responsible for the redness and swelling (hives) that
you observe at the site of the sting. At this point, the patient is said to be sensitized to the bee's toxin. The second time the dog is stung, the sensitized mast cells will recognize the foreign protein (bee toxin) and release their contents in a process known as degranulation and activation. In milder cases of anaphylaxis, a localized hypersensitivity reaction such as extreme swelling at the site of the bee sting will occur. In severe reactions, the localized reaction causes further release of the contents of mast cells throughout the body, resulting in systemic anaphylaxis. Localized hypersensitivity reactions are common while systemic anaphylaxis is rare.
Theoretically, any foreign substance can produce an anaphylactic reaction. The most common substances to cause anaphylaxis are food proteins, insect bites, medications, vaccines, environmental pollutants and chemicals. It is important to note that this is an abnormal reaction. The body's immune system over-reacts to the foreign protein or substance resulting in the reaction. Most cases of anaphylaxis are thought to have a hereditary or familial basis.
What are the clinical signs of anaphylaxis?
Clinical signs are dependent on the route of exposure (mouth, skin, inhalation, injection, etc.), the amount of antigen and the level of the pet's IgE response.
The most common clinical signs of anaphylaxis include itching, red skin swellings called wheals or hives, a swollen face or muzzle, excessive salivation or drooling, vomiting, and diarrhea. In systemic anaphylaxis, the patient will have difficulty breathing and may be cyanotic (have a bluish color to the tongue and gums).
How is it diagnosed?
Anaphylaxis is diagnosed by a medical history of recent exposure to an offending substance and the characteristic clinical signs. Intradermal skin testing and IgE blood tests may be performed to identify specific allergens.
How is anaphylaxis treated?
An anaphylactic reaction is a medical emergency and immediate
treatment is required. The first step is removal of the inciting substance
whenever possible. Next, the pet is stabilized to prevent or minimize
systemic shock, ensuring that the airway is open and that blood pressure
and circulation are maintained. Intravenous fluids, placement of a
breathing tube and administration of emergency drugs such as
epinephrine, corticosteroids, atropine and/or aminophylline can be
necessary. In mild cases, it may be sufficient to administer antihistamines
and perhaps corticosteroids, followed by close monitoring for twenty-four to forty-eight hours.
What is the prognosis?
The initial prognosis is always guarded. It is impossible to know if a localized or minor reaction will progress to a major crisis.
Since anaphylactic reactions often worsen with each episode, the main goal is to prevent subsequent exposure to the offending substance.
This client information sheet is based on material written by: Ernest Ward, DVM