What is an anaphylactic reaction?
An anaphylactic reaction or anaphylaxis is an immediate, severe, sometimes life-threatening hypersensitivity (allergic) reaction to a foreign substance, especially a foreign protein, known as an allergen or antigen.
What causes an anaphylactic reaction?
Before an anaphylactic reaction can occur, the pet 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 the 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 mast cells and other white blood cells, causing them to release inflammatory chemicals, including histamines. These chemicals cause redness and swelling (hives) 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 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 allergic 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 inflammatory contents of mast cells throughout the body, resulting in systemic anaphylaxis. Localized allergic reactions are common, while systemic anaphylaxis is rare.
"Localized allergic 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, antigens in vaccines, environmental pollutants, and chemicals.
It is important to note that anaphylaxis is an abnormal reaction. The body's immune system overreacts or is hypersensitive to the foreign protein or substance, resulting in an excessive 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 (e.g., mouth, skin, inhalation, injection), the amount of antigen, and the level of the pet's histamine release. When histamines are released in small doses, there are often 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 and may be cyanotic (have a bluish color to the tongue and gums). This occurs because large amounts of histamines cause blood vessels to dilate, leading to hypotension (low blood pressure) and reduced blood flow that is slower to spread oxygenated blood (red) to the extremities.
How is it diagnosed?
Anaphylaxis may be diagnosed by a medical history of recent exposure to an offending substance, physical exam findings, and the presence of characteristic clinical signs. In severe cases, blood tests may reveal elevations in liver enzymes and dehydration (hemoconcentration). Abdominal ultrasound may show an enlarged liver, edema in the gallbladder wall, and fluid around the abdominal organs. Later, 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, such as removing a bee’s stinger. Next, the pet is stabilized to prevent or minimize systemic shock, ensuring the airway is open and blood pressure and circulation are maintained. It is often necessary to provide intravenous fluids, place a breathing tube, and administer emergency drugs such as epinephrine, corticosteroids, atropine, and/or aminophylline. In mild cases, it may be sufficient to administer antihistamines and possibly corticosteroids, followed by close monitoring for 24–48 hours.
What is the prognosis?
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 crisis.
Since anaphylactic reactions often worsen with each episode, the main goal is to prevent subsequent exposure to the offending substance. If you have a pet that experiences anaphylaxis, you may be given emergency medication to keep at home or instructions on what to do if your pet shows any signs of future reactions.
