Overview of Allergies – Part 3 of a 3 part series by Dr. Jean Hofve, DVM


In the first two installments of our 3-part Allergy Series, we’ve covered food allergies and inhalant allergies (atopy). This time, we’ll go into a little more detail about what allergies are, and how to prevent and deal with them; and we’ll reveal the most common allergy of all!

Allergy = Immune Hypersensitivity

An allergy is an over-reaction of the immune system to an allergen (usually a protein). There are four major types of hypersensitivity reaction:

  • Type I or “immediate” hypersensitivity, also known as anaphylaxis. An example is the potentially life-threatening reaction to vaccines in sensitive animals. The type of reaction usually occurs within 30 minutes, but always within 12 hours. The problem usually occurs in tissue that has direct contact with the outside world, such as the skin, respiratory tract, and gastrointestinal system. This is the most common type of reaction seen in pets. Food allergies, atopy (inhalant allergies), and fleabite allergy usually fall into this category.
  • Type II or cytotoxic hypersensitivity is what we think of as an autoimmune reaction, where antibodies attach to the body’s own tissues, causing inflammation and tissue destruction. Transfusion reactions and vaccine-induced autoimmune hemolytic anemia are examples. The reaction begins within 3-10 hours.
  • Type III or immune complex hypersensitivity occurs within 3-10 hours of exposure to the allergen. While relatively rare in animals, systemic lupus erythematosis in dogs is an example.
  • Type IV or delayed hypersensitivity is the reaction we commonly think of as a “contact” allergy, such as a reaction to poison ivy. For instance, a dog may develop an allergy to the stuffing in, or the detergent used to wash, his dog bed, and develop a rash on his belly and paws (which have the least amount of fur and are thus in direct contact with the bed). This type of allergy typically takes days to develop, and is relatively uncommon in pets.

Diagnosing allergies

Diagnosing allergies can be difficult. First, your veterinarian needs to rule out other diseases or problems that cause those symptoms. For skin reactions, other causes include parasites, autoimmune disease, and skin infections. Diagnosis may include skin scrapings to look for mites (several species commonly infect dogs and cats); fungal culture for ringworm; or even biopsy to look for skin and gland abnormalities. In the case of gastrointestinal reactions, there are dozens of other potential causes, such as parasites; viral, bacterial, or fungal infections; toxins; liver or pancreas disorders; neurological problems; and cancer. The pet’s history may also provide clues: atopy and fleabite allergy are more seasonal, while food allergies tend to be constant.

There are two major tests specifically for allergies manifesting as skin problems: intradermal skin testing, and blood tests.

  • Intradermal skin testing involves injecting dozens of allergens into the skin to assess the degree of reactivity. The animal must be anesthetized for this process.
  • Blood tests check for antibodies to a variety of allergens.

These tests are not 100% accurate, but they may help narrow down the list of suspects so that treatment can be targeted more efficiently. These tests are best reserved for dogs who will be getting immunotherapy (hyposensitization), which involves giving frequent injections of a combination of allergens in order to minimize the immune system reaction.

Food trials are also a way of diagnosing allergies, since symptoms of food allergy may involve either the skin or the gastrointestinal tract. It is worth keeping in mind that food allergies are far less common than other allergies, but food can still contribute to symptoms. The trial food should contain ingredients the pet has not been exposed to before, and should be fed exclusively for 8-12 weeks! Digestive symptoms typically resolve sooner than skin symptoms.

This “FAD” is the Top Allergy

FAD, or Flea Allergy Dermatitis, is the most common allergy of dogs and cats. The usual suspect is the cat flea, Ctenocephalides felis, which is just as likely to infest dogs as their namesake cats. Fleas inject their saliva as they feed on the pet’s blood; and that saliva contains histamine-like compounds and other proteins. These components make fleabites extremely itchy on their own; but in some animals, they cause an allergy that is itchy to the extreme.

The most typical sites for FAD to show up are the lower back, base of the tail, inner thighs, and lower belly; although a severely allergic pet may be itchy all over.. The fur in affected areas may be stained brown from the pet’s licking, and the skin can become hairless, crusty, thickened, or even blackened from chronic irritation. Hot spots (areas of moist, reddened skin) can also be the result of FAD. Secondary infections of the skin with yeast or bacteria are common.

Most people, confronted with a potentially flea-allergic pet, will adamantly deny that there is a flea problem in their homes—and most of them are probably correct. But you don’t have to see fleas to have a flea problem. Fleas may be lurking in the yard, on the beach, or in the dog park. If your pet has a flea allergy, it only takes a single fleabite to produce a severe and long-lasting reaction.

In addition to causing itching and allergies, fleas can transmit tapeworms, roundworms, and the bacteria that cause bubonic plague, cat scratch disease, typhus, and Lyme disease. Many of these diseases can also be transmitted to humans. It is important to stay vigilant if you are in a flea-prone area.

For more information on fleas and how to combat them naturally, please see these articles:

Common Flea Myths

Click here to read the second article in our allergy series on Inhalant Allergies (Atopy)

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