Diagnosis

Although “management of chronic disease” is the focus of this book, curable dermatological diseases should be considered in the list of differential diagnosis during the initial evalu­ation and upon follow-up visits for a patient with pruritus.

Traditional lists of differential diagnoses are helpful to ensure that common causes are not overlooked. The author has promulgated the “PAIN 4 ME” mnemonic to assist with recall of differential diagnosis and emphasize the importance of certain etiologies from a dermatology perspective. Each of these dis­eases may directly or indirectly cause a patient to become pruritic. They are grouped together by broad etiological categories including a reminder within the “number 4” category. The reminder emphasizes that lists are inherently incomplete. Clients and patients may benefit from referral to a spe­cialist who can then build upon the previous efforts by the care-giving team.

Differential Diagnosis Grouped by Broad Categories of Etiologic Agents

• P parasitic (demodex, flea, scabies, other

mites)

• A allergic (flea, environmental, food,

contact)

• I immune mediated

• N neoplastic

• 4 four or more potential other causes or

forward the client/patient to a specialist

• M microbial (bacterial, yeast, dermatophyte)

• E endocrine

Although this grouping can be an effective means of remembering potential etiolo­gies, the reality of clinical practice may lead to them being categorized differently. From one point of view, common diseases (e.g., flea allergies, environmental allergies, and infections) should be considered first.

It can be helpful during client discussions to group etiologies according to the following list in conjunction with a discussion of cost. Additionally, more than one etiologic agent is often present at the same time.

Alternative Categorization of Potential Etiologies

• Common: Bacterial and yeast infections, flea-induced pruritus, atopic dermatitis

• Confirmable during exam: Bacterial and yeast infections, flea infestation, demodex, scabies (although possible to confirm, fewer than 50% of patients will have a posi­tive skin scrape)

• Controllable quickly: Scabies, bacterial and yeast infections (but likely to recur due to underlying etiology). Some cases of food allergy and flea bite pruritus

• Curable: Scabies (not always intensely pru­ritic), dermatophytoisis (variably pruritic), demodicosis (variably pruritic)

Some common diseases, such as infections (bacteria or yeast) are quickly identifiable but may be only part of the problem as they are secondary to an underlying primary disease. Management of the pruritic patient often requires the veterinarian and caregiver to remain constantly vigilant for bacterial infec­tions, yeast infections, ectoparasites as well as other factors. For the pruritic patient, treatment of common, confirmable, and/or quickly controllable diseases is often pursued while utilizing palliative antipruritic therapy. Many patients may have more than one dis­ease or factor contributing to pruritus at the same time making this approach a time- and cost-effective option.

Superficial bacterial pyoderma and yeast infections can be identified during an office visit and successfully treated within 3 to 4 weeks. If the infections remain after sys­temic antimicrobials, additional diagnostics or therapeutics should be considered. For bacterial infections, a bacterial culture and sensitivity is appropriate. If topical therapy has not been utilized, it may be reconsidered. For yeast infections, a different systemic anti­fungal agent should be considered along with topical therapeutic options (Lewis 2016). While most infections technically are curable, the improvement is often temporary because they are virtually always secondary to an under­lying primary disease process. For patients in which the infection is the final factor leading to the pruritic threshold being reached, the treatment of secondary infection, without controlling the underlying primary etiology, may be temporarily successful in controlling pruritus but not result in long-term control of the underlying disease. Endocrine dis­eases are seldom pruritic, but from a der­matology perspective, recurrent pyoderma can be the only presenting complaint with hypothyroidism and hyperadrenocorticism. With these patients, the pruritus is not usu­ally as severe and typically totally resolves with appropriate infection control. This scenario emphasizes the need for follow­up appointments and cytologies towards the end of the course of antimic robial therapy. Clients may be reluctant to follow

Table 2.1 Primary, secondary, and coexisting diseases that may be pruritic. (Many patients have more than one concurrently.)

up with their veterinarian when their pet is clinically normal.

Scabies is a relatively rare diagnosis and can be difficult to confirm. Nevertheless, it is extremely important because it is a curable cause of pruritus and can affect both allergic and not allergic patients. Empirical therapy is often necessary to rule out this contagious parasite that can also cause allergic reactions. Some individuals develop a hypersensitivity to scabies mites. This hypersensitivity reaction could in part account for the observation that pruritus may not be observed in all dogs from a household with a scabies positive pet (Lewis 2016; Miller et al. 2013). Some scabicidal agents also provide other benefits, such as flea control and heartworm prevention. These can be excellent choices as part of a complex parasite control plan.

Demodicosis is a relatively common, quickly confirmable diagnosis that in many cases can be successfully treated within a few months. It should be on the differential diagnosis list initially and at virtually each reevaluation of the same patient (cats and dogs) (Lewis 2016; Miller et al. 2013). In dogs, this is particularly true if drugs, such as steroids or oclacitanib, are being utilized for control of pruritus. The author has been referred many patients with undiagnosed demodicosis who were previ­ously well controlled atopic patients receiv­ing long-term medical therapy for pruritus.

Atopic dermatitis is a common cause of pruritus but a definitive diagnosis can be elu­sive (Lewis 2016; Miller et al. 2013; Jackson and Mueller 2012; Favrot et al. 2010; Deboer and Hillier 2001). The presence of a positive allergy test does not confirm the diagnosis, it must be correlated with the history, clinical signs, and response the therapy. The diagno­sis should never be made without a thorough workup and exclusion of other causes. Importantly, confirmation of atopic dermati­tis does not mean that it is the ONLY factor contributing to the current clinical signs. Atopic dermatitis may simply be a coexisting diagnosis that is actually well controlled at the time the patient presents with pruritus caused by another diagnosis. For example, an atopic patient is at least equally as likely to contract an infectious parasitic disease (e.g., scabies) as a non-atopic patient. Due to the pruritic patient's past history and previous workup (including lack of response to scabi­cidal therapy), one might assume that a cur­rent episode of pruritus is associated with an allergy flare due to a change in seasons (aller­gen load). The patient may actually be itching due to scabies recently acquired from another dog. More common than sarcoptes mites, an undetected low level of flea exposure is often the cause for an exacerbation of pruritus in an atopic patient.

A 2010 clinical practice guideline by the International Task Force on Canine Atopic Dermatitis published a succinct table of Favrot's Criteria for canine atopic dermatitis (see Table 2.2) (Favrot et al. 2010).

Adverse reaction to food is a relatively rare, but potentially totally controllable cause of pruritus in some patients. Other symptoms, such as gastrointestinal, may also be noted. The reader is referred elsewhere for a more in-depth discussion of the differences between food allergy and adverse reaction to food.

Table 2.2 Favrot's 2010 criteria for canine atopic dermatitis.

1. Onset of signs under 3 years of age

2. Dog living mostly indoors

3. Glucocorticoid-responsive pruritus

4. Pruritus sine materia at onset (i.e. alesional pruritus)

5. Affected front feet

6. Affected ear pinnae

7. Nonaffected ear margins

8. Nonaffected dorso-lumbar area

A combination of five satisfied criteria has a sensitivity of 85% and a specificity of 79% to differentiate dogs with AD from dogs with chronic or recurrent pruritus without AD. Adding a sixth fulfilled parameter increases the specificity to 89% but decreases the sensitivity to 58%.

Source: Favrot et al. 2010.

more complicated to manage (Lewis 2016; Miller et al. 2013).

Flea-induced pruritus is a commonly over­looked but manageable cause of pruritus. This is especially true when the typical distri­bution pattern of flea allergy is not present. This terminology is used in an attempt to emphasize the importance of strict flea con­trol, even when fleas seem unlikely as a con­tributing factor. For many atopic pets, the mechanical stimuli of a flea bites alone can trigger pruritus without true flea allergy being present (summation of effect and threshold.) These atopic pets as well as flea allergic pets can itch due to flea bites in households with­out any perceived flea infestation (lack of typical flea allergy distribution pattern). This problem can be difficult for many pet owners and even veterinarians to understand and accept. In these situations, the adult flea numbers are so low that they are not seen on pets or humans in the environment. For basic flea control needs of a non-allergic pet, a single product is often sufficient. For an allergic (atopic or flea allergic) pet, a trial 2-4 month or more period of a “Double Down” flea control program may be advisable (see Box 2.1). This protocol typically uses two different products with different active

Box 2.1

“Double Down” Flea Control Benefits

• For the pruritic patient on a hypoallergenic dietary trial, some flea control products can also provide heartworm prevention during a hypoallergenic dietary trial. The use of an unflavored oral product or topical product is necessary in order to provide continuous heartworm preven­tion during the workup of a pruritic patient. Some situations and client preferences lead to diagnostics and therapy for flea bite associated dermatitis, food allergy and scabies concur­rently during a 2-4 month period of time.

• Some flea control products also have scabicidal attributes. Scabies is one of the very few truly curable pruritic diseases and should not be overlooked as a potential diagnosis at the initial assessment or even during follow up visits of a patient that has been diagnosed with other diseases such as atopic dermatitis.

• A few weeks after administration of a flea product, the often-increasing length required for a flea to die becomes important. It may allow undetected flea bites to add to the summation effect with pruritus inducing factors. The resulting increase in pruritus may be noted in the days to weeks preceding the routine repeat application of the single product.

ingredients and benefits. One product is used at the beginning of the month and another product is used in the middle of the month (Gram and Short 2015). It is impor­tant to know the active ingredients of prod­ucts in order to avoid using different brand names but similar active ingredients.

“Double Down” protocol for flea control can be helpful in various pruritic scenarios. For the flea allergic pet, the “Double Down” protocol helps overcome pruritus associated with a reduced speed of kill seen with most flea control products. For virtually all flea control products, the speed of kill is typically faster for the first few weeks after a product is utilized compared to the time period before repeat applications. During the regu­latory approval process/efficacy trials, the percentage of fleas killed is often measured 48 hours after product administration. While this is adequate for flea control in a non- allergic patient, clinically it can be a problem for flea allergic pets. The not-yet-dead fleas may feed several times over a period of many hours before they die. Caregivers often report an increase in pruritus in the days to weeks immediately preceding the due date for repeat administration of a flea product with­out fleas or flea feces being noted. Many studies report the percentage of fleas killed 28 days after product application whereas many months are actually 31 days long. The label instructions on some products may not allow more frequent application. Client com­pliance and delay in product use further exacerbates the slower residual speed of kill issue for the allergic patient. The use of a dif­ferent product at different times of the month overcomes this issue. It also allows the use of products with different attributes in addition to flea control that may be useful during the workup of a pruritic patient, such as scabi­cidal activity or topical heartworm preven­tion during a hypoallergenic dietary trial.

Pruritus Diagnosis Summary

Neither intradermal allergy test nor serum allergy test should be used as a screening test for pruritic patients suspected of having atopic dermatitis (Lewis 2016; Miller et al. 2013; Deboer and Hillier 2001). There is no substi­tute for a veterinarian's clinical judgment cou­pled with a physical examination and complete history (see Box 2.2). With that information, the list of differential diagnoses can be nar­rowed down provided the diagnoses are presenting with their typical characteristics.

Box 2.2

Pruritus Diagnosis Summary (These considerations should not be overlooked during initial and reevaluation appointments).

• Obtain a thorough history and perform complete physical examination in order to narrow down the list of potential differential diagnoses.

• Remember to rule out curable disease and contagious diseases such as dermatophytosis and scabies. A history of negative findings does not mean that these could not be a new problem.

• Ensure that a patient does not have a confirmable or controllable primary disease or secondary factor. Evaluate for parasitic and microbial diseases by performing appropriate cytologies and skin scrapes for dermodicosis, which previously may have been or difficult to confirm.

• Consider food allergy: relatively rare but controllable without medication. Client receptivity and compliance can pose a challenge when performing a strict hypoallergenic dietary trial consisting of novel or hydrolyzed proteins and avoidance of treats and flavored oral medica­tions (such as heartworm and flea control products).

• Consider empirical ectoparasite therapy for scabies and fleas utilizing “Double Down” flea control, even if parasites are not confirmed on examination.

• Consider antipruritic therapy in order to provide comfort for the patient and the caregiver while working to identify and control the primary disease. Educate the caregiver regarding risks and benefits of both short-term and long-term therapy as well as the costs.

• If the work up and treatment for other diseases is negative and medical therapy is necessary for more than 3 months out of the year, consider intradermal allergy testing or serology based allergy testing from a reputable laboratory.

Specialists often see patients with common diseases and an atypical presentation. Within three months of a multifaceted diagnostic and therapeutic approach, most etiological agents can be controlled or ruled out as potential contributing or coexisting factors. In some situations, ruling out one single dif­ferential diagnosis at a time is appropriate. In many situations, however, more than one factor is present concurrently. Following a thorough assessment of a patient's history, physical examination findings, successful treatment of secondary infections, careful modification of a parasite control plan, and a stringent dietary change for several months, the likely diagnosis of atopic dermatitis may be made by exclusion of other causes. Demonstration of IgE antibodies via intra­dermal allergy testing or serum testing is covered in depth by many other publications. They are not pathognomonic for the diagno­sis of atopy. These adjunctive tests should only be used, after a well-orchestrated clinical workup. Tests are performed in order to identify important allergens to be avoided or included in allergen specific immunotherapy. Currently available types of “allergy testing” should not be performed as a screening test for atopic dermatitis. Many clinically normal animals will actually test positive (Lewis 2016; Miller et al. 2013; Jackson and Mueller 2012). Allergen specific immunotherapy is an often overlooked and effective means of providing long-term management of a pru­ritic patient with atopic dermatitis.