Scratching
There are many reasons why dogs scratch. Some of these are completely curable conditions while others are frustrating long term disease processes, which we hope to manage long term. This discussion will touch on the causes of pruritis (itchiness), the ways we diagnose the causes and ultimately the treatment, for the most common causes.
Causes
- Flea infestation
- Mange (sarcoptic)
- Atopy (Food hypersensitivity, inhalant allergies, contact allergies)
- Pyoderma (bacterial skin infection)
- Yeast dermatitis
- Neuritis
Flea infestation
Dogs scratch from fleas not because fleas walk around on the surface of the dog's skin, but more commonly because of a reaction to the flea's bite. When a flea bites a dog it injects a tiny amount of saliva into the skin. The dog's skin has a hypersensitivity reaction to that saliva which causes a release of histamine at the bite site. Histamines cause the dog to become pruritic (itchy) at that site. We diagnose flea infestations by looking for evidence of fleas on the dog's skin. Sometimes we can find live fleas; sometimes we only find flea dirt (flea feces). Flea feces look like tiny reddish, black flecks of dirt on the dog's skin. It helps to use a flea comb to find this evidence. If fleas are found, the diagnosis is made and the dog can be treated with medication to kill the fleas. There are many safe products on the market that effectively kill fleas and prevent further infestation. The dog may also need to be treated for the hypersensitivity reaction at the bite sites. This is usually done with the use of antihistamines or a short course of cortisone.
Mange
The term mange refers to the infestation with a mite called Sarcoptes scabei. This is a microscopic bug that burrows into the skin causing and intense hypersensitivity reaction. Dogs that suffer from this type of mange are intensely pruritic (itchy), and tend to self traumatize their skin creating wounds. This mite likes to inhabit the skin on the limbs, face and ears. We diagnose this disorder by the use of skin scrapings. The veterinarian uses a blade and scrapes the surface of the skin. The scraped cells are then placed on a slide and looked at under a microscope. This mite can be quite elusive and multiple skin scraping may need to be performed in order to find the mite. If the mite is found, the diagnosis is made, and the dog is treated with a drug that kills the mite. There are two approved drugs on the market to treat this mite. The veterinarian will choose the right one depending on the general health status of the dog.
Atopy
This is the most common cause of pruritis(itchy skin) in dogs, and is also the most frustrating. This disease is difficult, because it is very hard to identify all of the things that a specific dog is allergic to. For this discussion I will break down this discussion further to differentiate between the types we look at.
Food hypersensitivity
This is a very common type of allergic condition. In theory, the easiest to treat, but truly very hard to identify the specific food that creates the hypersensitivity reaction. The most common food sources that dogs are allergic to are beef, poultry, corn, wheat, soy, lamb and fish. As you can see from this list it's hard to avoid these food constituents in a diet. In order to diagnosis this type of allergy we must use elimination diets. Elimination diets are diets that are lacking a specific ingredient that we think a dog is allergic to. To do a diet trial the dog is to be fed a specific diet and only that diet for at least 6 weeks. An allergic reaction to a specific constituent in a diet may not subside for up to 6 weeks. In theory, each trial diet may need to last 6 weeks in order to see improvement in the dog. Thankfully there are a few diets on the market that are considered truly hypoallergenic. These are called hydrolyzed diets (Hills Z/D, Purina HA). These diets have had their protein sources broken down (hydrolyzed) so small that the body cannot recognize it as an allergen. We use these diets as a diagnostic tool. If we feed these diets for 6 weeks and the dog improves, then that means food hypersensitivity is a component of this dog's allergic disease. Once we have diagnosed a food allergy then we can try different elimination diets (diets which have unique protein sources) to find a diet that works for each dog. Hydrolyzed diets can be used long term but can be expensive. Most dogs with food hypersensitivity have significant pruritis (itchiness). You can understand this since, in theory, they are coming in contact with their allergen every day. The good thing is we should be able to identify this disease and find a diet that will work for the dog.
Inhalant allergies
These are probably the most difficult allergy to identify and control. This is an extremely common type of hypersensitivity and does have some genetic basis. Dogs can be allergic to almost anything they inhale including pollens from trees, grasses, molds, fungi and dander from other animals. There is still controversy in the veterinary field which is the most accurate way to identify what an animal is allergic to. At this point skin testing is considered to be the gold standard with blood testing as a second choice. With skin testing a small amount of an allergen is injected under the skin and a dermatologist evaluates the size of the skin reaction to that allergen. The dermatologist tends to do this type of testing because experience and standardization is important in the assessment of the skin reaction. The dermatologist also chooses which allergens and how many allergens to test for based on a detailed history and the area of the country that the dog resides in. This type of test is an accurate assessment as long as the tester has experience. Blood tests look for the amount of antibodies in the blood stream to a specific allergen. Some feel that this is not as accurate since an antibody can last in the blood stream for a long time. If an animal has antibodies to a specific allergen it doesn't necessarily mean that this is the allergen that is currently causing the dog to itch. Once we have identified the suspected allergens we can attempt to remove some of the allergens from the dog's environment and a serum can be made to begin immunotherapy. Immunotherapy involves injecting a tiny amount of an allergen into a dog in the hopes of desensitizing the pet's immune system to that allergen. This is a tried and true therapy but it takes a very long time and not all dogs respond completely to this therapy. Many dogs that suffer from inhalant allergies must be maintained on medical therapy. There are many drugs on the market to help allergic dogs, these include antihistamines, cortisone drugs (prednisone) and immunosuppressants (cyclosporin). Since allergic skin disease changes the surface of a dog's skin these dogs very commonly suffer from secondary skin infections (most commonly yeast +/- bacteria). If they have secondary infections, they will require therapy for that as well. Inhalant allergies are a lifelong disease process. We never cure an allergy, we simply hope to manage these reactions with the best protocol that has the least negative side effects for the dog.
Contact allergy
This is the last type and not as common as food and inhalant allergies. Since dogs have fur their skin is usually protected from most contact problems. We see these type of reactions in the thinly haired areas on the body (ventral chest, inner thighs). The only other time we see these reactions is when we use certain products on the skin which the dog can have an inflammatory reaction to (shampoos, topical medications). This type of reaction is easy to diagnose since they occur just after coming in contact with their allergen. The treatment of this reaction is to wash off the skin and then treat with a short course of cortisone. We can cure this type of reaction as long as we control the dog's exposure to the allergen.
Pyoderma ( bacterial skin infection)
This causes an intense inflammatory response which causes dogs to scratch. The majority of skin infections are secondary to other diseases which change the dog's skin immunity, but some can occur as a primary condition. The most common bacteria causing pyoderma are Staphylococcus sp..There are many different bacteria which can invade the skin. The diagnosis of pyoderma is based on physical exam and the presence of pustules, scabs, bald spots with crusts and pruritis (itchiness). In order to identify the specific bacteria a culture is performed. The culture is collected from an active pustule (pimple). Sometimes a veterinarian may not culture pustules on a first time pyoderma, but will collect a sample if the infection does not respond to therapy or is a recurrent infection (one that comes back after treatment is discontinued). The treatment consists of the use of antibiotics and topical therapy (shampoos). Most skin infections need to be treated for 3-6 weeks or longer. If the dog has concurrent allergic skin disease or other skin disorder, that disease must be treated or managed effectively so that the pyoderma does not return.
Yeast Dermatitis
This is the infection of the skin most commonly caused by Malasezzia sp. This infection causes intense pruritis (itchiness) and usually large flakes on the surface of the dogs skin. This is also a very common cause of foot licking dogs. Yeast is a type of organism that thrives in a moist environment, therefore body parts that have sustained moisture issues tend to be open to this type of infection. These areas include ears, feet, skin folds (around the vulva, face folds, in between the toes). The diagnosis is based in skin cytology (skin scrapings/hair plucks placed on a slide, stained and looked at under a microscope) and/or culture. Treatment is usually topical therapy, shampoos and creams that kill the organism. Some severe infections will need to be treated with systemic anti-fungal medications. Since this infection causes intense pruritis anti-inflammatory drugs (cortisone) may be needed short term to prevent self trauma. The last component of therapy involves controlling moisture in the area that is infected. For some deep skin folds that may even involve surgical intervention to remove the skin folds. Many yeast infections can be cured, but since this is an organism that thrives in moisture it tends to be a recurrent problem.
Neuritis
This is the inflammation of nerve endings. It causes a tingling sensation or "pins and needles" that a dog responds to by chewing on the area affected. This is very difficult to diagnose since there is no specific diagnostic test. The diagnosis is really the attempt to find and underlying problem in the tissue below the skin. These lesions on dogs tend to become a thickened, chronically open wound on the dogs skin. These are sometimes called acral lick granulomas. Neuritis is not the only cause of acral lick granulomas but does play a role in the pathogenesis. The treatment of this condition involves the use of intra-lesion injections of cortisone drugs and sometimes the use of pain medications to control the nerve pain. If there is an underlying deep tissue injury (old fracture/ tendon injury/tumor) it must be addressed. There has also been some research on the use of prozac to help control the obsessive compulsive component to a deep itch or tingling sensation.
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Dr. Ellen Leonhardt
Animal General
East Norwich
(516) 624-7500
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