Working-up the pruritic dog (Proceedings)

Most cases of pruritus in the dog are caused by:

• Allergies (fleas, atopic dermatitis [environmental allergies])

• Infections (pyoderma, Malassezia [yeast] ; less commonly dermatophytes)

• Ectoparasites (scabies, Demodex)

History

When confronted with the pruritic dog, the veterinarian should ask the following 4 questions:

• What is the duration of the skin disease and is there any seasonality?  

Seasonal pruritus (due to fleas or pollen allergies) typically is more pronounced in the warm weather, whereas allergy due to dust or mold components may be more severe in the winter. 

• What is the progression of the lesions?

The disease may have looked quite different when it first occurred - and it may have been influenced by the owner's attempt to control it, such as bathing the dog an hour before the appointment and washing off certain ‘clues'.

• What medications have already been used?  Did any help (or worsen) the skin condition? 

Ask if the owner has either kept a list or still has all bottles, vials, etc. of previously utilized drugs. Remember that a statement like “antibiotics (or corticosteroids, antihistamines, shampoos, etc.) did not work”, means nothing without knowing what kind, what dosage, how long given, etc. Response of pruritus to corticosteroids does not tell much, but failure of the pruritus to respond to corticosteroids is suspect of scabies, pyoderma, dermatophytes or demodicosis.  Response to antibiotics usually typifies a pyoderma, however, some clients regard all pills as “antibiotics”.  In addition, oftentimes antibiotics are given with corticosteroids and the owner will neglect to mention this fact.

• What is the dog's environment?  Has the dog traveled out of the immediate area? Are other animals (or the owner) affected?  Involvement of other animals or people obviously points to contagious diseases (such as scabies or dermatophytes).  However, lack of disease in in-contact animals does not necessarily rule this out. 

Physical examination

• If the pruritus is primarily directed at the back half of the animal (tail-head, ventral abdomen, caudal thighs) it must be considered due to fleas or flea allergies until proven otherwise.             

• Despite the many effective anti-flea products on the market, many owners do not understand the concept of flea allergy and feel that if they do not see fleas on their dog, fleas are not the problem.

• Another problem is that people misuse the topical spot-ons.

• They apply them too infrequently (often once every 6-8 weeks, instead of every 3 to 4

• They bathe the dog between applications. Both of these practices will seriously decrease the efficacy of spot-on products

• They only use them in the warm weather, or when they see a flea, or when they perceive flea season occurs.

• If the pruritus involves the front half of the body (feet, face, axilla, ears) it is most likely food allergy or atopic dermatitis [environmental allergies]. The latter is rare in dogs that are younger than 1 year or older than 7, but food allergy can occur at any age.

• Food allergy can only be diagnosed by placing the dog on a strict, 8 week ‘hypoallergenic diet. Blood tests for food allergy are a waste of money.

• Atopic dermatitis is a diagnosis of exclusion - other causes of pruritus must be ruled out first. Blood tests for atopic dermatitis [environmental allergies to pollens, grasses, dust] are relatively reliable as a basis for formulating allergens for hyposensitization if the owners want this mode of treatment and don't want to be referred for skin testing. As at least 10% of healthy dogs will have some positive reactions in either skin tests or blood tests, these tests can NOT be used as a diagnostic tool.

• In many allergic dogs, some of the pruritus is due to either a superficial pyoderma (caused by Staphylococcus species, usually S pseudintermedius) or a Malassezia infection, or both.

• Clinical signs of superficial pyoderma are most commonly epidermal collarettes. These circular rings of epithelial tissue, sometimes erythematous are one of the FIRST things the veterinarian should look for. They are caused by Staphylococcus at least 95% of the time and thus are virtually pathognomonic for pyoderma.

• alassezia infection typically present with waxy brown exudate on skin or proximal claws, erythema interdigitally or under tail, or an erythematous , well circumscribed patch on the ventral abdomen. These must be diagnosed by cytology (see below).

• Crusts around the margins of the ears that are associated with pruritus, especially if seen with crust on the elbows, hocks and a papular eruption (rash) on the ventral abdomen are HIGHLY suggestive of scabies.

• Watch out for dogs (especially terriers!) that like to dig in the ground and/or roam outside. These dogs sometimes present with dramatic crusts and erythema on the face, the ears, with progression to the legs. They have a ringworm infection with either Trichophyton mentagrophytes  or Microsprum gypseum. Do NOT be dissuaded by a lack of contagion in other animals or the owners, or even by a negative dermatophyte culture - T mentagrophytes can be difficult to culture. A biopsy, with special stains for fungi, may show the organism quicker than a culture.

• When examining a dog's skin, use a hand lens (an otoscope with the cone removed will work). This will identify small epidermal collarettes that might otherwise be identified as excess scale. It will also help you identify ‘follicular papules' i.e., papules with hairs sticking out of them. This denotes a folliculitis, which in most dogs is either a pyoderma, demodicosis, or dermatophytes.

Diagnostic Tests

• Skin scrapings.   

• The Dermatology Service at UC Davis no longer uses scalpel blades - we use a flat-bladed medical spatula: Fisherbrand* Microspatula with Flat-Ended Blade, catalogue number 21-401-20, Fisher Scientific; http://www.fisherscientific.com.

• Microscopic interpretation - Ectoparasites should be searched for using the 4X and 10X objectives under low lighting so that partially translucent parasites are not missed. 

• Tape technique for cytology

• Take clear [not frosted] ScotchR tape, pushed it against the affected skin several times, placed it on a dry microscope slide, and then inject the area under the tape with the blue dye from a Dif-QuickR stain.

• Alternatively, a few drops of the blue stain could be placed on the slide first, then place the tape over it

• This is then examined under scanning power to identify an area that stained well, then examined under the oil immersion objective.

• Hair plucks

• Affected (seemingly broken or bent) hair should be plucked, preferably with ahemostat with plastic/rubber 'protected' jaws. The hair should be placed in mineral oil, covered with a cover slip, and examined using a microscope.

• Hair may be examined for:

• Demodex mites; if present, these will be found close to or 'clinging' to the proximal (toward the base) of the hair shaft.

• Broken-off distal ends. If the rest of the hair shaft looks normal, this is good evidence that the animal is biting/chewing of the hair (ie, the animal is pruritic).

• Abnormal 'broken log covered with pearls' appearance; suggestive of dermatophytosis (discussed in the lecture on Dermatophytosis).

• Occasionally, lice nits or Cheylletiella mite eggs may be seen attached to the hair shaft.

Treatment Suggestions and New Medications

Antibiotics for Pyoderma

• Cephalexin                                                  20-30 mg/kg q 8-12 h

• Cefpodoxime (Simplicef™)                        5-10 mg/kg q 24 h

• Lincomycin (Lincocin ®)                            20 mg/kg q 12 h

• Ciprofloxacin                                              30mg/kg q24 h

• Enrofloxacin (Baytril®)                              5 - 10 mg/kg q 24 h

• Amoxicillin-Clavulanate (Clavamox®)       13.75 mg/kg q 12 h

• Marbofloxacin (Zeniquin®)                        3- 6 mg/kg q 24 h

• Doxycycline                                                5mg/kg q12 h

• Trimethoprim-sulfa                                     30mg/kg q12 h

Remember that many pharmacologists feel that the quinolones are more effective when given at their total dose ONCE daily.

With the increase in methicillin-resistant S intermedius, S aureus, and S schleiferi, the author now strongly recommends culture of any dog with epidermal collarettes that fails to begin to respond to one of the above antibiotics over a 3-4 week period of time. Epidermal collarettes may be cultured using a dry sterile culturette rolled across the collarettes.

Shampoos may be helpful as adjunct treatment in pyodermas, particularly in superficial pyodermas. There are many good, effective anti-bacterial shampoos available. The author's favorites are and ethyl-lactate containing shampoo (Etiderm®, Virbac), or a 4% chlorhexidine shampoo and spray with Triz-EDTA (Triz-Chlor4®, Dechra).

Anti-yeast

• ketoconazole (5mg/kg q24 h)

• itraconazole (5mg/kg q24 h)

• fluconazole (5 mg/kg q24 h)

Topicals such as Malaseb® (DVM: chlorhexidine and miconazole), Mal-A-Ket® (Dermapet: boric and acetic acids, and ketoconazole) or Resizole® (Virbac: miconazole), are helpful. ‘Wipes'; such as Malaseb®, Malacetic®, or Douxo Chlorhexidine 3% PS Pads are VERY useful in the treatment of interdigital yeast and bacteria infections.

Anti-pruritics

Cyclosporine:

As an antiprutic, this drug has been used with good success in dogs for atopic dermatitis. Cyclosporine is available as Atopica® (Novartis) in 10, 25, 50 and 100 mg capsule sizes.  Ideally, this should be given on an empty stomach, at UC Davis we often start by giving it with food, as up to 20% of dogs may experience GI upset. Keeping the drug frozen until administered will greatly reduce the incidence of GI side-effects. Cerenia® (Pfizer) may also be used to manage vomiting. Because low doses are used for treating atopic dermatitis, usually 5-7 mg/kg/day or less, adverse effects are uncommon. The most common problem is nausea and loss of appetite. Other side effects, though rare, include pyoderma, papillomas, hirsuitism and gingival hyperplasia. The latter has been managed with both reduction of the cyclosporine dose, as well as using azithromycin (9.6 mg/kg q24h). Because of expense, particularly in large breed dogs, administration concurrently of ketaconazole will enable a reduction of Atopica® dosage. (This is due to the body's metabolism of both drugs). In general, with a dose of ketoconazole of 5mg/kg, the author has seen good results using only 50% of the canine dose of cyclosporine (2.5 mg/kg per day instead of 5mg/kg per day). A recent article suggests that as many as >35% of atopic dogs treated with cyclosporine for 4 months may not relapse for as long as 40 days after cessation of the cyclosporine although the author has not had this experience.

Anti-pruritic topicals

Genesis™ Spray (VIRBAC).

The active ingredient is 0.015 % triamcinolone acetonide. While not optimal for long term management, this spray has a low risk of corticosteroid side-effects if used on problem areas (especially the feet) to ‘cool down' the pruritic response.

Allermyl™ Shampoo and Allermyl™ Spray (VIRBAC).

These contain linoleic acid as a potential anti-pruritic, as well as the anti-infectives piroctone olamine and monosaccharides.

Duoxocalm® Shampoo and Duoxocalm®Spray (Sogeval).

These products contain sphingophytosine, a susbstance that stabilizes the stratum corneum, as well as hinokitol, a plant derived substance with anti-infective properties. It is now the widest used non-steroid anti-pruritic shampoo in France, according to the company.

‘New'Anti-Flea Products

Spinosad (ComfortisTM, Elanco)

New oral chewable (beef-flavored) tablet, approved for dogs. Activates nicotinic acetylcholine receptors. Company data supports once monthly administration. Best if given with food. Adverse effects – occasional vomiting. If vomiting occurs within an hour of administration, redose with another full dose. Not to be used if giving daily ivermectin for demodicosis. As with the other oral anti-flea product (Capstar® Novartis, a daily medication), ComfortisTM kills fleas quicker than spot-ons, and is not affected by frequent bathing.

Dinotefuran, pyrproxifen and permethrin (Vectra 3DTM, Summit VetPharm) topical spot-on product.

Dinitrofuran is a new neonicotinoid insecticide, based on acetylcholine structure.

Does: kill adult fleas Does: repel adult fleas (due to permethrin)

Dinotefuran and  pyrproxifen (Vectra TM for cats, Vectra TM for Dogs and puppies) Summit VetPharm) topical spot-on product. Dinitrofuran is a new neonicotinoid insecticide, based on acetylcholine structure.

Does: kill adult fleas Does not: repel adult fleas (due to absence of permethrin)