Why Is My Dog Itching and Scratching?

Allergies In Your Pet - Ron Hines DVM PhD 10/18/03

Next to fleas, allergies are the most common cause of itching and scratching in your pet. Fleas are dealt with in another article in this series.

Once you are certain that no fleas are present, it is likely that allergies are the cause of your pet’s scratching. In human beings, allergies are often expressed through the respiratory system. If you yourself have allergies, you are likely to have a stuffy nose, sneezing and watery eyes. This is because the histamines that humans produce are released primarily in the membranes of the respiratory system and eyes. In dogs, histamines are released primarily in the skin.

How Allergies Occur:

All chronic allergies are based on the body recognizing protein substances as foreign and trying to rid itself of them. Mold, dander, bacteria, pollen, flea saliva along with a host of other substances are allergens made up of proteins foreign to the body.

We will talk about pollen allergies for an example because they are so common. In this case pollen is called an allergen – something the body recognizes as foreign. When a pet’s immune system is exposed to pollen for the first time, some pollen attaches to the linings of the nose and lungs.. The body’s immune system recognizes that pollen is not supposed to be there and, in response, produces antibodies against the pollen called immunoglobulins of the E class (IgE) and G class (allergen-specific IgG). This process is called sensitization. The IgE attaches to cells in the body called mast cells. These cells then leak and release histamines, bradykinins, and proteolytic enzymes, which cause the symptoms of allergy. In people large numbers of mast cells are located in the respiratory system. That is why our allergies usually have respiratory system symptoms. In dogs and cats, more mast cells are present in the skin. That is why dog and cat allergies usually cause signs associated with the skin and ears such as itching, hives and inflammation. This allergic disease is called atopy. Only 15% of pollen allergies in dogs and cats cause respiratory system signs, such as nasal stuffiness and sneezing.

Types of Allergies:

There are two types of allergies that cause dogs and cats to itch. Because susceptibility to allergies is inherited, it is quite common for pets to have both types.

Canine Inhalant Atopy

This is the type of allergy for which we used pollen as an example. It is the most common form of allergy in dogs and cats, making up over ninety percent of the cases I see. About nine percent of the canine population has this disease. Certain breeds are more susceptible to than others. Among the most susceptible are Shar-peis, Fox Terriers, Golden Retrievers, West Highland White Terriers, Scottish Terriers, Shih Tzus, Dalmatians, Lhasa Apsos, Boston Terriers and Labrador Retrievers. Cats of any breed can be affected. Dogs usually show their first signs of itching between 10 months and 4 years. If a dog does not have this condition by the time it is four years of age it is unlikely that it will ever have atopic skin disease. The disease often begins as a seasonal problem but after a number of years (or moving to a southern climate) the itching occurs year round. Some common allergens are flea saliva, tree, grass and shrub pollens, house dust, mold, dust mites, and feathers.

Flea saliva allergy is a special form of Atopy. Fleas survive by sucking blood from your pets. To get this blood they drill their mouthparts into the dog or cat’s skin and inject special anticoagulant saliva into the site to keep blood flowing while they engorge. This saliva is highly irritating to some dogs and cats. These pets do not only itch at the site of the flea bites, they itch all over. The pads of their feet are often itchy, puffy and inflamed. Areas between the toe pads as well as the ears often become infected with bacteria due to the pets constant licking and chewing. Because of the availability of excellent flea-control products such as Advantage and Frontline, I see less flea allergy than I used to.

Food Allergies:

Food allergy cases are less than fifteen percent as common as canine atopy but occur with about the same frequency as atopy in cats. The itching food allergies produce is constant throughout the year. Cocker Spaniels, West Highland White Terriers and Labrador Retrievers are particularly susceptible to food allergies. However, the majority of these breeds that have itching problems do not have food allergies. Dogs and cats can be any age older that a few months when signs first begin. Some common food allergies in dogs are to beef and pork, fish, dairy, eggs, corn, soy, wheat and preservatives and dyes in food and treats. But almost any diet ingredient can be involved. The most common allergens in cats are to fish, beef and chicken.

Symptoms:

Canine Atopy and food allergies are not visually distinguishable because the signs of both diseases are the same. The most common sign of the conditions is constant itching, scratching and licking. Dogs will often rub on objects. The degree of discomfort the pet undergoes is partly dependent on its temperament. High strung breeds and very active pets do severe damage to their skin. The face, lumbar area at the base of the tail (particularly if fleas are the cause) ears and paws show the most damage. In severe cases the skin of the whole body is inflamed and covered in crusts while the hair coat is thin and broken. In mild cases the pets only lick these areas, discoloring light hair coats with salivary stain. When fleas are the problem, small pepper-like granules are present over the neck or in the dorsolumbar area. These specks of digested blood or flea dirt will “bleed” a rust-colored trail if moved to white tissue paper and wetted with water

As the dog or cat scratches, it destroys the outer waxy layer of the skin. This allows bacteria and mold to enter the skin and causes a musty, oily skin condition called seborrhea. Because ears are covered with skin, ear canal redness, odor and inflammation are common in canine atopy. Sometimes ear problems are the only sign present. When this is the case, we treat the ears with specific antibiotic, antifungal, corticosteroid preparations but must treat the whole dog with medications as well.

Cats develop a condition called milliary dermatitis in which the surface of the back and scruff of the neck are covered with bumps and scabs. Allergic cats sometimes develop another condition called eosinophilic granuloma complex in which ulcers form on the lips, tongue and surface of the body. After months and years of scratching, the skin of the inner thighs and flank often becomes thickened and blackish. This condition is called acanthosis nigricans.

Damaged skin due to the pets scratching often leads to superficial staphylococcus bacterial infections (pyoderma) and infection with the yeast, Malassezia. Mild bacterial conjunctivitis or eye infections with greenish matter at the inner corner of the eye are common. It is important to realize that although a bacterial or fungus condition is present and may need treatment, the root of the problem is allergic dermatitis. In very severe cases of skin allergy, the superficial lymph nodes of the body enlarge due to secondary skin infections.

Diagnosis of Allergic Skin Problems:

The clinical diagnosis of allergic dermatitis is one of exclusion of other causes of skin disease. I usually do skin scrapings when pets with this condition are presented to me for the first time. Through skin scrapings, I try to rule out mange as a cause of the pet’s problems and locate bacteria and fungi. If I cannot rule out mange, if the dog’s history suggests it might be mange, or other members of the household are itching, I may put the pet on a test dose of a safe anti mange medicine called ivermectin. I next try to rule out fleas as the source of the problem. Some dogs are so good at grooming that no fleas remain on them when they are examined. Even when I cannot locate fleas on the pet I can often locate flea dirt. In some of these pets the presence of tapeworms is the only indication of fleas or I may find fleas on another household pet, which does not have skin problems.

There are other signs that the problem may be allergic. Typically atopy occurs in dogs between one and three years of age. This makes the diagnosis of allergies in puppies and older dogs less likely. The first two or three years that the condition exists it is often seasonal occurring only in the spring and summer months. Flea-related allergies tend to subside after the first few freezes of the winter. Golden retrievers and arctic breeds with thick oily hair are susceptible to a special form of dermatitis called hot spots or acute pyotraumatic dermatitis. In these cases the dogs suddenly become extremely itchy over a small area. They are so troubled that they pull out the fur of this area and infect it through chewing in a matter of hours. The itching sensation departs rapidly thereafter -with or without treatment. It is not known if they represent a true form of allergy. Another form of the disease is called acral lick dermatitis. In these cases a small area of skin itches over an extended period. Licking leads to a well-defined small area of raw skin, scarring and bacterial infection.

I try to learn if the parents of the dog or cat or its brothers and sisters also have itchy skin disease. Canine atopy as well as food allergies runs in families.

In diagnosing food allergies I look for uniform involvement of the skin over the entire body. I rarely find that diarrhea accompanies food allergies. Often the best way to rule out food allergies is to put the pet on a sixty to ninety day trial eating a hypoallergenic diet exclusively. Many hypoallergenic diets are available. Until recently these diets relied on novel protein sources such as lamb, venison duck or fish. The problem is that with time, pets eventually became allergic to these diets too. Now two brands of hypoallergenic diets are available that use heat and hydrolysis to break food proteins into a size too small to cause allergies (less than 10,000 daltons). The biggest problem with these diets is that some pets won’t eat them.

I will usually examine pets with skin disease in a darkened room with an ultraviolet lamp. Hairs that glow are diagnostic for ringworm. Ringworm is almost never itchy and glowing hairs are not found in cases of allergic skin disease.

I will often encounter Malassezia yeast in skin scrapings of dogs with itchy skin. This fungus is a normal inhabits the skin of dogs. It only becomes a problem when the pet has an underlying problem such as allergic skin disease. Antifungal shampoos are quite helpful in controlling it.

As in humans, allergy testing can be helpful in diagnosing allergic skin disease. Two forms of allergy testing are available. The easiest and most commonly performed test in animal hospitals is called the RAST test. This test, which is performed on a blood sample from your pet, stands for radioallergosorbent test. In the test, the presence of IgE antibodies to specific antigens is determined by a central laboratory. Pets must not have had corticosteroids or antihistamines administered for thirty days prior to testing. The problem with this test is its high number of false positives. If the test determines that a pet is not allergic to an antigen that is probably correct. But positive reactions often turn out to be wrong. If the RAST test is used, the central laboratory will prepare an antigen extract, which can be injected into the pet’s skin periodically to decrease the effects of allergies.

A more reliable test is an intradermal skin test. Before the test is administered, the pet is taken off all corticosteroid, tranquilizing or antihistamine medications for 30-60 days. In this test, small amounts of various allergens are injected into the skin. The entire side of the pet must be shaven to perform this test. Then the injection sites are monitored over an hour’s period to judge the degree of redness and swelling at each site. A list of offending antigens is used to prepare an allergen extract, which is injected periodically into the pet. With time and success, these injections lessen the pet’s allergies to these allergens. Because a high degree of skill is necessary in giving and interpreting the results of this test, veterinary dermatologists usually perform it.

Occasionally, when I am uncertain if the problem is a routine case of allergy, I will take a biopsy specimen of the skin involved and send it to a pathologist for evaluation and diagnosis. This can rule out the presence of other disease such as lupus. Blood tests may detect elevated levels of eosinophiles, which occasionally also occur in allergic skin disease.

Treatment of Allergic Skin Disease:

It is next to impossible to avoid all the environmental allergens that plague allergy-prone dogs and cats. The exceptions are pets that are chiefly bothered by fleas. I enforce strict flea control – even if I am uncertain that fleas are present. Many dogs and cats are cured by this simple task alone.

Approximately one in twenty of my clients elect to undergo skin testing and desensitization of their pets. This figure differs from practice to practice. The procedure takes a bit over a year to show results. Dermatologists argue for the procedure pointing out, correctly so, that even if desensitization does not cure the pets, it may lower the dose of corticosteroids they require later in the disease. Skin testing also identifies the allergens and perhaps some of them can be avoided or eliminated from the pet’s environment. The procedure is extensive, expensive, and requires a life-long commitment to frequent injections. Most owners learn to give these injections themselves.

Medicines of the cortisone class are effective and dramatic in controlling the symptoms of allergic skin disease. The problem is that this class of drugs has substantial and serious side effects when given in high doses or over extended periods of time. My challenge is to treat these pets as long as I can without the use of these steroids. To do so, I use medications that are effective but less dramatically so.

Medicated Shampoos

Medicated shampoos are quite soothing to inflamed skin and lessen itching. I begin using clear tar preparations such as lye tar shampoos. If these are not adequate, I dispense selenium sulfide or benzyl peroxide shampoos. Benzyl peroxide is quite effective in eliminating secondary bacterial infections in these cases. Some pets are helped by oatmeal and antihistamine shampoos. With all these products, the skin should first be cleansed with soapless soaps and then the medicated product massaged in and allowed a long contact period with the skin – the longer the better. Weekly shampooing along with flea control is often sufficient for a number of years. Some pets cannot tolerate weekly bathing as it dries out their skin too much. Use of a skin conditioning rinse helps in some cases. Also, use temperate, not hot water, when you bath your pets.

Antihistamines and tranquilizers

Antihistamines block the release of histamines by mast cells. They are quite helpful to some pets. Unfortunately, the effects are not as dramatic as in humans. Ciproheptadine (0.15-0.5mg/lb twice a day), hydroxyzine (0.25-0.5mg/lb) or even OTC Chlorpheniramine maleate are effective in lessening symptoms in some dogs and cats. Many antihistamines have a calming effect in dogs. I have had cat owners discontinue their use because they did not like personality changes in their cats while on these medications. Some antihistamines can be applied in shampoo form. Some dogs do well when given mild tranquilizers such as acepromazine at 0.25mg/ pound.

Corticosteroids

Many owners are worried when we suggest that their pets receive measured doses of corticosteroids to treat stubborn allergy cases. They should not be. These drugs have nearly miraculous powers in both human and animal medicine. They just should not be over used. Some drugs of the cortisone class are prednisolone, prednisone, triamcinolone, dexamethasone, beclomethasone, and betamethasone. Often I use so little of these drugs that a bit of itching persists but at a tolerable level. I usually give prednisolone or prednisone at 0.38mg per pound of body weight two or three times a week. At this low dose I do not encounter noticeable side effects. There will often be a point, many years into the disease, when higher cortisone doses are required. Combining topical medications, antihistamines, desensitization and cortisone let us keep the cortisone dose as low as possible. I do not feel that I have ever used cortisone to an extent that an allergic pets life was shortened although some have suffered weight gain. I do know that the use of these drugs has greatly improved the quality of many pets’ lives. Whenever possible, the use of long acting corticosteroids such as methylprednisolone acetate should be limited to a single yearly injection of 1.25mg/lb body weight. and oral forms of corticosteroids used instead. You may notice that your pet drinks more and urinates more on these medications. If they are over used a disease called Cushings Syndrome or hyperadrenocorticism can result (see article on that subject).

Antibiotics

Dogs that are presented to me with severe bacterial skin infections subsequent to self- trauma (scratching) need a two-week course of antibiotics to clear up the infection. I like to use a broad-spectrum antibiotic such as one of the fluroquinolones (Batryl) or a potentiated cephalosporin such as Clavamox (Augmentin). I begin antihistamines and medicated shampoos in these cases immediately but I delay any corticosteroids until the skin has healed. If yeasts are an important component of the problem I treat them with ketaconazole shampoos.

Food Supplements

There are a very large number of “neutraceutical” products marketed for allergic skin disease. Many of them have never been scientifically tested. The best that can be said of many of these products is that they can do no harm. When I recommend these products, I usually suggest one that combines omega-3 and omega-6 fatty acids. Many of these products also contain zinc. Some clients find them beneficial.

Your Pet’s Future:

Allergic dermatitis is a life long condition. Unless the problem is solely fleas there are no cures. Luckily it is not a life threatening or a life shortening condition and it can be managed with a minimum of inconvenience. Often, there will be periods of a year or more when the disease is not as severe and needs less or no medications. When a family moves the disease is often left behind. Unfortunately not too much time passes at their new location before the pet becomes allergic to new allergens. Because this is an inherited trait, dogs with this disease should never be bred. The fact that so many dogs suffer from allergic skin disease shows that many AKC and novice breeders disregard this advice.