Anaphylactic shock, also called anaphylaxis, is an acute, systemic and extremely serious allergic reaction that, if not treated quickly, can cause rapid cardiovascular decompensation, respiratory distress and death. The symptoms and consequences of anaphylactic shock in dogs differ from those in other mammals. However, anaphylaxis is every bit as much of a life-threatening medical emergency in dogs as it is in people.
Causes of Anaphylaxis in Dogs
There are countless potential causes of anaphylactic shock in dogs, depending upon the particular animal's immune system. The inciting allergen may be saliva from an insect bite, venom from a snake bite or a bee sting, a particular medication, topical parasite treatments, an environmental allergen (mold, pollen, grasses, chemicals, dust, etc.), some component of the dog's diet, a vaccine or virtually anything else. The substance, called the allergen, is perceived as foreign and harmful by the dog's immune system. Initial exposure to the allergen – which can be by physical contact, inhalation or ingestion - produces a cascade of immunological changes that essentially put the dog's body on heightened alert to any subsequent exposure to the same allergen. Upon re-exposure, these mobilized components of the dog's immune system aggressively activate, synthesize and release what are collectively called "inflammatory mediators." Within a matter of minutes, the situation can become critical. The inflammatory mediators cause dilation (stretching beyond normal dimensions) of the blood vessels, increased leakiness (permeability) of the blood vessel walls, dangerously low blood pressure (hypotension), respiratory abnormalities, accumulation of fluid in the throat, excessive secretion of substances into the airways, heart rhythm disturbances, gastrointestinal distress, severe itchiness (pruritis) and widespread pain.
Prevention of Anaphylaxis in Dogs
The best way to prevent anaphylactic shock is to avoid exposing a dog to any substances that it is known to be allergic to. Of course, this is only realistic after the dog has had an allergic reaction to something, and even then only if the offending allergen can actually be identified. Intravenous medications should be administered slowly to reduce the chances of an anaphylactic reaction to the medication.
Once a dog suffers anaphylactic shock from exposure to something in its environment, that allergen usually will continue to cause severe allergic reactions when the dog comes into contact with it again. Occasionally, a dog may go into anaphylactic shock on the very first exposure to an allergen, although it is much more likely to go into anaphylaxis on a second exposure.
For most people, the phrase "anaphylactic shock" conjures up images of a life-threatening allergic reaction to a bee sting or some other trigger that causes rapid throat and bronchial constriction. Anaphylaxsis manifests somewhat differently in dogs than it does in people, but it still is a potentially fatal medical emergency. Anaphylactic reactions in people typically affect the throat, trachea and upper airways, because the reaction normally starts in the lungs. In dogs, anaphylaxis primarily targets the cardiovascular system and the gastrointestinal system, with respiratory signs developing secondarily. Symptoms develop almost immediately (usually within minutes) after exposure to the causative allergen. The dog's immune system mounts an aggressive but inappropriate response to whatever the dog is allergic to, releasing "inflammatory mediators" into the blood stream which in turn trigger a number of other events. The dog's arteries and veins become stretched beyond their normal dimensions and become increasingly leaky, causing blood pressure to drop dangerously. Mucous and other secretions build up in the lungs and throat, causing respiratory distress, and the heart muscle starts pumping irregularly, in abnormal and random rhythms. In addition, affected dogs may develop severe itchiness (called pruritis) and experience extreme pain.
Symptoms of Anaphylactic Shock in Dogs
Anaphylactic reactions occur almost immediately – usually within moments after a dog touches, inhales, ingests or otherwise is exposed to the inciting allergen. The first signs of anaphylactic shock in dogs usually include very rapid onset of one or more of the following:
Development of hives (urticaria)
These symptoms quickly progress to one or more of the following:
Shallow, rapid and difficult breathing (respiratory distress; dyspnea)
Pale gums and other mucous membranes
Elevated heart rate (tachycardia)
Changes in mental clarity (excitement or depression)
Left untreated, the end result of anaphylactic shock almost always is seizures, collapse, coma and death. Dogs suffering anaphylactic episodes normally do not have severe or obvious swelling around their throat or face, unless the allergen came into contact with the dog in that area. Dogs having hypersensitivity reactions to allergens such as bee stings, vaccines or other pharmaceutical injections may develop mild to moderate swelling or bruising around the entry site. If any or even some of these signs appear suddenly in your dog, take him or her to a veterinarian right away. Anaphylactic shock is almost always fatal if not treated immediately.
Dogs At Increased Risk
There do not appear to be any age or gender predispositions to developing anaphylactic shock. Boxers and Pit Bull Terriers are most commonly affected with urticaria – a vascular skin reaction marked by the development of raised red patches on the skin (called wheals) and intense itchiness (called pruritis). Another term for this condition is "hives." Dogs that spend a great deal of time outdoors during the warmer spring and summer months are at an increased risk for insect or snake-bite related anaphylactic reactions.
Anaphylactic shock is not particularly difficult to diagnose. However, it is essential that treatment begin before formal diagnostic efforts are made, because timely treatment is the most important factor in whether affected animals will survive.
How Anaphylactic Shock is Diagnosed
Virtually all cases of anaphylactic shock are diagnosed based only on history and physical examination. Treatment must begin immediately, if not sooner, if a dog experiencing an anaphylactic episode is going to have a fighting chance of recovery. Once the animal is stabilized and hospitalized, most veterinarians will monitor blood pressure regularly, perform a urinalysis and draw blood samples for a complete blood count and serum chemistry panel. The results of these tests are usually unremarkable if anaphylaxis is the only condition affecting the dog. Chest radiographs (x-rays) may be recommended if the dog is having severe difficulty breathing, to assess whether primary pulmonary (lung) disease is causing or contributing to the respiratory distress.
Anaphylaxis should always be suspected if a dog experiences an extremely sudden and severe onset of cardiovascular (heart and blood vessels) and respiratory (lungs) collapse. This condition can cause death within as little as one hour from exposure to the inciting agent.
When an owner notices sudden and dramatic onset of difficulty breathing, severe itchiness and scratching, abnormal drooling, excitement or lethargy in abnormal situations, weakness, hives (raised red patches or bumps on the skin) and/or collapse, he should get his dog to a veterinarian as quickly as humanly possible. Anaphylactic shock in dogs is a real medical emergency that must be treated immediately if the dog is going to survive. This condition cannot be treated by owners at home; intensive veterinary attention and hospitalization are required. At the hospital, dogs in anaphylactic shock will undergo a number of emergency treatments in an attempt to reverse the allergic reaction, treat the signs of shock and stabilize the patient. It is essential that treatment for anaphylaxis begin before any diagnostic testing, because rapid and effective treatment is the main determinant of survival. The goal of treating of anaphylactic shock in dogs is to provide emergency life support. This is accomplished by providing respiratory and cardiovascular support, removing the causative allergen if possible and taking steps to prevent further release of inflammatory mediators into circulation.
In most cases, the first step in treating anaphylactic shock is to place an intravenous catheter and aggressively administer fluids to the dog at shock dosages to restore blood volume and counteract the dangerously low blood pressure caused by peripheral circulatory failure. Intravenous administration of epinephrine usually is the next step in treating anaphylaxis. Epinephrine causes an increase in heart rate and constriction of blood vessels; it also helps to block further release of those compounds in the body which are responsible for perpetuating the anaphylactic reaction (called inflammatory mediators). Other medications may be used by your veterinarian to treat shock, such as corticosteroids, atropine sulfate, dopamine or aminophylline, among others.
The veterinary team will establish an open airway in dogs who are having difficulty breathing or showing other signs of respiratory distress. Several methods are available, including placing a breathing tube through the mouth and down the animal's throat (endotracheal intubation) or making a surgical incision through the neck, directly into the trachea (tracheostomy). Oxygen therapy can be administered if needed. Broad spectrum antibiotics are often given in an attempt to prevent secondary bacterial infections which can develop after an anaphylactic episode.
Frequent in-hospital monitoring of the patient should continue for at least 24 to 48 hours after the allergic reaction has resolved. This includes monitoring of heart and respiratory rate, respiratory effort, pulse rate and quality, blood pressure, mucous membrane color, mentation and body temperature. Blood samples may be taken to assess liver function and the condition of other organs. The large volume of fluids given to combat shock can cause water retention. In many cases, a recovering dog will not be released from the clinic until he or she can urinate without assistance, to be sure that the effects of fluid retention have resolved.
Rapid recognition of anaphylaxis and aggressive veterinary intervention are key to a successful recovery. When this condition is caught early and treated quickly, most dogs recover fully within 24 to 72 hours of onset. It can take less than one hour for this hypersensitivity reaction to be fatal. Owners should keep a keen eye on their canine companions, particularly when they are spending lots of time outdoors where they can become exposed to things not normally found in their home environment.