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There are numerous conditions that cause problems with a cat or dog’s skin, but the most common, by far, is allergies.

Symptoms of allergies
You may correctly recognize allergies in the dog by the symptoms:

In cats the more common symptoms are:

A dog or cat who is allergic to something will show it through skin problems and itching.
Let’s make one point "perfectly clear": a pet who is allergic to something will show it through skin problems and itching (that is, it is pruritis). If a pet is allergic to something he inhales (atopy) like certain pollen grains; something he eats (food allergy) such as beef; something he lays against (allergic contact dermatitis) like a wool carpet; or an insect bite (urticaria or hives); the pet usually just won’t have (in the same order) a runny nose, vomiting, a rash where it touched the suspect substance, or a larger than normal swelling at the site it was bitten. In fact the pet probably won’t have any of these signs. What the pet will have is a mild to severe itching sensation over his body and in dogs, maybe a chronic ear infection.

Allergic dogs will chew on their feet until they are irritated and red (the feet are the only place dogs have sweat glands and these become inflamed with allergies)

Allergic dogs rub their faces on the carpet or couch, scratch their sides and belly, and because the wax glands of the ear overproduce as a response to the allergy, they get ear infections with bacteria and yeast "over growing" on the excessive wax and debris.

The skin lesions of the allergy are usually the ones the dog produces by mutilating his skin through chewing and scratching. Sometimes there is hair loss which can be patchy or inconsistent over the body leaving a mottled appearance. The skin itself may be dry and crusty, reddened or oily depending on the dog. It is very common to get secondary bacterial infections of the skin due to these self-inflicted lesions. Such infections may be treated with antibiotics.

Allergic cats often groom excessively and pull out large amounts of hair. Their skin may appear sensitive, often twitching. The lesions they have on their skin can be very small crusts to large areas of oozing red skin.

Dogs and cats are allergic to:
Pets are commonly allergic to tree, grass and weed pollens; fabrics such as wool or nylon; rubber and plastic materials; foods and food additives such as any of the individual meats, grains or colorings; milk products and benzoic acid; house dust and dust mites; and, of course, flea bites. The molecules the dogs are allergic to are called "allergens".

The body's response to an allergen
The reason that all these allergens cause itchy skin is that, simplistically, when allergens are inhaled, ingested or come in contact with the dog’s body, they cause the immune system to produce a protein referred to as IgE. This protein then fixes itself to cells called "tissue mast cells" that are located in the skin. When these mast cells that are combined with IgE respond to allergens, they cause the release of various irritating chemicals such as histamine. In dogs, these chemical reactions and cell types occur in appreciable amounts only within the skin.

Genetic factors and time influence allergies
Remember that pets must be exposed to the allergen for some time before the allergy develops. Exceptions may occur such as an allergy to insect bites which may develop after only a few exposures. The pet's body must learn to react to the allergen. It is a learned phenomenon of the immune system that is genetically programmed and passed from generation to generation in several breeds. Allergies are especially common in certain terriers such as the Scottish, West Highland White, Cairn, and WireHaired Fox; Lhasa Apsos; and larger breeds such as the English and Irish Setter, Retrievers, and the Dalmatian. Allergies are also well documented in the Pug, Miniature Schnauzer, and English Bulldog. There does not seem to be any breed disposition for allergies in cats.

In pets, allergies usually start to develop between one and three years of age.
In pets, allergies usually start to develop between one and three years of age. They may start as late as age 6 or 8, but over 80% start earlier. To make matters worse, as the animal ages it usually develops allergies to additional things and the response to any one allergen becomes more severe.

Diagnosing allergies
Confronted with a scratching pet and frustrated owner, it is too easy for a veterinarian to miss a diagnosis or at least fail to make the owner fully understand the problem. Sometimes vets take the easy way out and "shotgun" the pet with several different medications hoping that at least one will hit the target and make the signs go away. If the pet stops scratching, the hair starts to grow back and the owner can sleep nights…all may look well but the allergy is not necessarily cured.

Most allergies are the inhalant type and are seasonal (at least at first). The dog or cat may be allergic to a certain tree pollen that is only present in the environment for three weeks out of the year. This case is easy to handle. No matter what you treat the animal with, she will start getting better in four weeks. In cases in which the allergy is mild but the irritated skin has become infected with bacteria, a broad-spectrum antibiotic will eliminate the infecting organism and the skin may return to near normal appearance even though the treatment has in no way been directed toward the allergy.

A definitive diagnosis of an allergy and determination of exactly what the animal is allergic to can only come in one of three ways. They are:

The latter is not as exact but usually shows the quickest improvement for the pet and is easiest on the owner’s pocketbook. An excellent example of this would be the dog or cat that is allergic only to tree pollen. Every year, in the same month, the dog starts chewing his feet, scratching his sides, and rubbing his face on furniture; or the cat starts grooming excessively and gets pinpoint scabs on her head and body. The veterinarian chooses either a tablet and/or single injection that will suppress the allergy for the 3-4 weeks necessary. In two days the animal is back to normal and only has to wait until the following year when he or she will be returned with the same problem. The vet will glance at the file and probably repeat the same treatment. The vet is now two for two, and in the owner’s eyes, has reached the level of genius.

Unfortunately, things just don’t always go that well. A more common scenario in a dog, for instance, would be that the dog is now five years of age. When he was 18 months old he started scratching and hasn’t stopped since. There are large areas of his skin that are 2 to 3 times their normal thickness, it is cracked open and bleeds intermittently and is completely void of hair. The ear canals are painful, infected, and swollen to the point of limiting the dog’s ability to hear. The dog is uncomfortable, rarely able to sleep through a night, waking frequently to scratch and chew on himself. He may even have intermittent fevers from reoccurring bacterial skin infections. He still enjoys being a dog however, chasing squirrels, eating treats, and looking forward to the bus bringing the children home. The owner and dog have by now been in every veterinary clinic within a 100-mile radius and that circle will probably soon grow larger. And to make matters worse, the dog is getting worse. This is an isolated but not rare case and we wanted to tell you about it here so you could appreciate the true difficulty of the diagnosis and treatment of canine allergies. It is extremely important that we find out what allergen(s) the dog is reacting to in order to really help such a dog.

So now you know the basics of canine and feline allergies. If you have more specific questions about the different types of allergies, how to diagnose them or treat them, I encourage you to read the other articles in this section. They provide the newest and most up-to-date information on this very common problem.