Approach to the pruritic dog (Proceedings)

Pruritus is the most common symptom of skin disease in the dog, and having a methodical approach to itchy dog cases is critical in ensuring successful management. A thorough history, observant physical examination, and appropriate diagnostic tests are the fundamentals that will lead to effective therapy.

Causes of Pruritus

The most common causes of pruritus in the dog include the allergic diseases (atopic dermatitis, flea allergy, adverse food reactions, contact allergy) and parasitic infections (scabies, cheyletiella, demodex when secondarily infected). All of these diseases can result in secondary bacterial or yeast infections, which also contributes to the itch level. Although less common, keratinization defects, dermatophyte infections, autoimmune skin diseases, and neoplastic skin diseases can also cause variable levels of pruritus.

History

A detailed and accurate history is vital to establishing the cause of dermatological disease. Specific questions to ask include, but are not limited to:

      1. How pruritic is the patient on a scale of 1-10? This can aid in the assessment of the case as treatment progresses and also help with a differential list. A dog that is 10/10 itchy is much more likely to have sarcoptic mange, for example, then a dog that is a 3/10 itchy.

      2. Where is the pruritus and where was the initial body location? Many of the pruritic diseases have typical presentations-such as dorsal lumbar for flea allergy and ear margins/elbows/hocks for sarcoptic mange.

      3. Is the pruritus seasonal or year round? Purely seasonal disease typically indicates atopic dermatitis, flea allergic or some other insect hypersensitivity.

      4. What is the age of onset? Young age of onset, less than 6 months of age would be more indicative of adverse food reaction or parasitic conditions, whereas atopic dermatitis classically starts between 1-3 years of age.

      5. Are other pets/humans affected? This can clue the practitioner into a contagious or zoonotic condition.

      6. Is the condition worse indoors/outdoors? This is helpful when evaluating atopic patients.

      7. Is there a history of vomiting/diarrhea/increased bowel movements that may indicate a concurrent gastrointestinal history? This is important when evaluating adverse food reaction patients.

Physical Exam

Physical examination of the pruritic dog can indicate whether primary or secondary lesions are present and can give clues as to the distribution pattern. Secondary lesions consistent with scratching and self-trauma are common, but may not help distinguish the underlying cause. These include excoriations, erosions, alopecia, lichenification and hyperpigmentation. Primary lesions such as papules, macules, pustules may also be present depending upon the underlying disease process. The location or distribution of the lesions should correlate with sites of pruritus and can aid the differential list. For example, atopic patients often display secondary lesions of self trauma on the face, muzzle, periocular, pinnae, antebrachial, pedal, axilla, groin and perineal regions. Scabies cases often have pinnae, elbow and hock lesions, and many will have a positive pinnal-pedal reflex. The physical examination can also indicate if it appears that secondary bacterial or Malassezia infections are present, warranting skin cytology samples.

Diagnostic Tests

Skin scrapings: one of the easiest and most important tests in dermatology. The author routinely scrapes virtually every dog presented to her, since many of the parasitic conditions can mimic other conditions. It is most commonly used to verify or rule out the diagnosis of demodectic mange, but is also used to establish the diagnosis of sarcoptic mange (positive about 25% of the time), cheyletiella infestations and other ectoparasitic diseases. It is imperative to remember that only a few sarcoptic mange mites are required to cause a severe reaction, so a negative scraping for sarcoptic mange does NOT eliminate it as a differential. Scrapings are most commonly performed to evaluate for demodectic and sarcoptic mites, and these skin scrapings are not performed in the same way. For evaluating demodectic mange, where the mites reside in the hair follicles, multiple deep scrapings from new lesions should be obtained until capillary bleeding is produced. Canine sarcoptic mites reside within the superficial epidermis, and multiple superficial scrapings are indicated with emphasis on the pinnal margins, elbows and hocks. However, because small numbers of mites are usually present, they are difficult to find, and many cases of sarcoptic mange are ultimately diagnosed by response to therapy.

Scotch tape preps: Clear scotch tape preps can be helpful in picking up ectoparasites, especially cheyletiella. The tape is impressed onto the affected surface of the hair and skin and then placed on a glass slide and examined for parasites.

Hair coat collections: Combing the coat and collecting dander, scale and hair with a flea comb and examining on a glass slide with mineral oil is an effective method for identifying many parasites. It has been shown to be the most successful method of cheyletiellosis identification, and can be helpful in flea and flea feces detection.

Cytology: Cytology is perhaps the single most important test in dermatology. It is a rapid tool to assess the presence of pyoderma or bacterial overgrowth, infections organisms, inflammatory cells, acantholytic cells and neoplastic conditions in the skin. A very good review article that can aid improvement of cytology skills is:

Mendelsohn, C.L., W.S. Rosenkrantz, and C. Griffin, Practical Cytology For Inflammatory Skin Diseases. Clin Tech Small Anim Pract 2006. 21: p. 117-127.

Dermatophyte test media (DTM): The DTM is one of the most accurate ways of identifying a dermatophyte infection. Dermatophytes utilize protein and produce an alkaline by-product that produces a red color change. However, after all the carbohydrates are utilized, saprophytic contaminants can utilize the protein and turn the media red as well over time. For this reason DTM cultures should be inspected daily, and any suspected fungal growth should be lifted with clear plastic tape and stained with lactophenol cotton blue to look for characteristic macroconidia and fungal identification. Speciation of the dermatophytes should always be performed to determine the source of infection to help prevent future re-infection.

Flea control trials: The response to intense flea control can be of diagnostic value to support flea allergy when noticeable clinical improvement occurs without concurrent long-term anti-pruritic therapy to mask symptoms. Products that contain rapid killing systemic therapies such as spinosad (Comfortis®, Lilly) and nitenpyram (Capstar ®, Novartis) or adulticides with permethrin such as Dinotefuran (Vectra 3 D®, Summit VetPharm) and Imidacloprid (Advantix®, Bayer) are very effective products. Other impressive products for flea control include metaflumizone (ProMeris®, Fort Dodge), imidacloprid (Advantage®, Bayer), fipronil (Frontline® and Frontline plus®, Merial) and selamectin (Revolution®, Pfizer). These products are often applied more frequently than manufacturer recommendation in flea allergic dogs. Environmental flea control is occasionally recommended, depending on the flea burden in the indoor and outdoor environment of the pet. The author will occasionally recommend the use of indoor and outdoor premise spraying in multiple pet households or densely populated pet areas. Insect growth hormone regulators (methoprene or pyriproxyfen) are very effective, as are the borate-based products.

Food elimination diets: To evaluate for food allergy in a dog, an elimination food diet should be exclusively fed for an 8-12 week period. Switching from one commercial food to another is not acceptable since many of these contain similar ingredients. The author prefers an eight to twelve week course consisting of home-cooked or limited protein based commercial diets, although hydrolyzed diets can be effective in some food allergic animals. Examples include: Royal Canin® (Innovative Veterinary Diets IVD) duck, rabbit, venison or lamb and potato, Eukanuba® Kangaroo and Oat or Fish and Potato diet, and Hills® canine Z/D low allergen, a hydrolyzed chicken and chicken liver diet. When the owner is willing or eager to home cook, they can be directed to www.balanceit.com, where they can purchase recipes and supplements formulated by the UC Davis nutrition department. At the end of the 8-12 week period, the dog is re-challenged with the original diet and observed for exacerbation of clinical signs. If evaluating food allergy in young, growing dogs, the nutritional needs of that breed need to be considered before switching them to an elimination diet which may be lower in protein/calories then what they need.

It is imperative that all other foodstuffs containing potential allergens be eliminated from the diet including treats, rawhides, chew toys, flavored chewable heartworm preventatives, additive-containing vitamin and mineral supplements, flavored medications and toothpastes. The author allows carrots and apples in her canine food trial patients for treats.

Allergy testing: Atopic dermatitis in the dog is diagnosed based on history, physical findings and ruling out all other pruritic diseases. Allergy testing can be of some diagnostic value, but does not replace a good history, physical findings and ruling out other differentials. It should be emphasized that allergy testing is NOT used to diagnose atopic dermatitis, but rather is used to determine what specific allergens the patient is allergic to after the diagnosis has been made to help formulate allergen specific immunotherapy. The two methods of allergy testing are intradermal skin testing (IDST) and serum in vitro testing (IVT), with IDST considered the gold standard. Immunotherapy has about a 75% success rate, but owners need to be told that it can take at least 3-4 months and up to 12-18 months to see the response to therapy.

Skin biopsies: Histopathology can be very useful in the diagnosis of pruritic skin diseases in the dog. The pattern of inflammation and cell types can be very helpful helping to establish a definitive diagnosis or disease category. Many specific infectious, parasitic, autoimmune and neoplastic diseases will be diagnosed with skin biopsies. Biopsies are also indicated when there is a lack of response to what is perceived to be appropriate therapy. Multiple samples should always be taken and samples should be sent to a dermatopathologist who has expertise or special interest in skin disease.

General Therapeutic Approach

Treat secondary infections: It cannot be overemphasized that secondary bacterial folliculitis and Malassezia dermatitis contribute to pruritus, and without their proper identification and treatment the patient may not respond to appropriate therapy for the primary cause of pruritus. Appropriate duration and dosing is critical for skin infections, and if both yeast and bacterial infections are identified, both must be addressed.

Ectoparasites: Ruling out ectoparasites and fleas is critical in any pruritic dog. Flea trials were discussed above. Trial therapy with selamectin (Revolution®, Pfizer ) every two weeks for at least three treatments is the preferred way to rule out scabies, otodectes or cheyletiella. All in contact dogs should be treated if scabies is to be ruled out, and all in contact cats and rabbits must also be treated if cheyletiella is to be ruled out. Scabies should be ruled out anytime that it appears contagious pruritus may be present, and the author will typically rule it out in an 10/10 pruritic dog before beginning an allergy work up.

Biopsy: consider biopsy and histopathology early on in the workup if the patient doesn't appear to be a typical allergic dog and you suspect a more unusual cause of pruritus, such as an autoimmune disease or neoplastic condition.

Flea allergy: When flea allergy is suspected, a flea control trial should be performed to rule out this exceedingly common cause of pruritus. Most flea allergic dogs will be pruritic in the caudal ⅓ of their body, focusing on their dorsal lumbar region. See previous section of the notes for information about flea control trials.

Food trial: If all other causes of pruritus are eliminated and you are down to food adverse food reaction and atopic dermatitis, it is then time to start the elimination food trial. See previous section of the notes for information about elimination food trials.

Allergy testing: should be pursued in cases where all other causes of pruritus have been eliminated and the owner wants to pursue allergen specific immunotherapy to ameliorate symptoms. This is considered the safest long-term treatment for moderate-severe atopic patients that aren't otherwise controlled with benign therapies like bathing or antihistamines.

Topical Therapeutic Trials: There are many excellent topical shampoos and rinses that can be used to minimize and in some situations control pruritus. The author prefers shampoos and rinses that contain specific antipruritic agents (colloidal oatmeal, pramoxine, hydrocortisone and phytosphingosine). These products can be used adjunctively in the majority of cases.

Antihistamines and Fatty Acid Trials: These are typically only useful in dogs with mild atopic dermatitis, and, then, only if ectoparasites and skin infections have been controlled first. The author will typically have owners try 2-3 different antihistamines, one at a time, for 1-2 weeks at a time to see if one is effective.

Cyclosporine and glucocorticoid therapy: These therapies are typically utilized for short term symptomatic control when working up an allergic dog, or for longterm treatment in patients with severe symptoms that don't respond to other treatments or when other treatments can't or won't be utilized. Cyclosporine, or Atopica (Novartis®) is considered safer longterm that glucocorticoids, but glucocorticoids are often utilized since they are more cost effective. When glucocorticoids are utilized, the author only uses them orally at every other day-every third dosing, never giving long-term repositol steroid injections as they have unpredictable release and therefore control of symptoms are associated with more deleterious side effects. When cyclosporine or Atopica® is utilized, the author begins with a dose of 5mg/kg/day for 4-6 weeks. If no improvement after 6 weeks, treatment is discontinued. If treatment is effective it is attempted to lower the dose or frequency. Regardless of which treatment is utilized, it is recommended to perform baseline bloodwork, repeat bloodwork a month later to evaluate for any idiosyncratic reactions, and then every 6 months thereafter.