Introduction

When veterinarians are trained in dermatology, a great deal of emphasis is placed upon certain lesions or patterns of lesions being “suggestive” of different diseases. While various types of lesions and patterns do tend to occur more commonly in certain diseases, it is important to realize that such suggestive guidelines are not absolute rules. This discussion attempts to outline those diseases that in the author's clinical experience have mimicked the more classical presentations of dermatoses.

Lesions

Papules or a papular, macular eruption (rash) are often found in various hypersen­sitivity states, particularly flea allergy, food allergy, contact allergy, or scabies. However, papules can on occasion be caused by a folliculitis. The folliculitis is usually bacterial (Staphylococcus intermedius) in origin but may also be caused by demodicosis or dermatophytosis. On close examination, a papular folliculitis will often show a hair follicle associated with each papule; this is not, however, always evident. Any papular eruption that does not respond to conventional therapy (flea control, hypoallergenic diet, scabicidal treatment, corticosteroids, etc) should be biopsied and cultured.

Pustules are most commonly seen in pyodermas. However, pemphigus foliaceus and demodicosis (and very rarely other diseases like sterile eosinophilic pustulosis, subcorneal pustular dermatosis, systemic lupus ery­thematosus, and dermatophytosis) may cause a pustular eruption. Any intact pustules should have their contents stained and examined microscopically. Pustules of pyodermas will usually have variable numbers of neutrophils and bacteria (usually cocci). Finding acantholytic (rounded epidermal) cells, other organisms, or just having pustules that do not respond to standard pyoderma therapy are all indications for biopsy and culture.

Epidermal collarettes are typified by an expanding circular patch of alopecia, erythema and subsequent hyperpigmentation surrounded by a border of peeling stratum corneum. Because of the circular nature of the lesion, they are frequently mistaken for dermatophytes. While in the author's experience, this lesion is usually caused by a superficial pyoderma, any pustule or bulla that ruptures can leave a similar lesion.

Depigmentation of the nasal planum is usually due to one of the autoimmune diseases (discoid or systemic lupus erythematosus, pemphigus, or bullous pemphigoid) or much less commonly nasal solar dermatitis (“Collie nose”). However, the author has seen several dogs with this presentation, which in fact, had a bacterial pyoderma, sterile pyogranuloma, mycosis fungoides, or leishmaniasis. Again, a biopsy of the affected area should provide a diagnosis.

Nodules are not always neoplastic: infectious agents, especially bacteria and dermatophytes, may present as nodules or plaques. Histopathology and/or culture will establish the diagnosis.

Seborrhea, excessive scaling with or without excessive sebum production, may be seen in a number of diseases. Endocrine abnormalities, parasites, allergies or idiopathic, breed-related seborrheas are among the most common causes of seborrhea. However, clinicians should bear in mind that generalized seborrhea may also be seen in pemphigus foliaceus (with or without appreciable mucocutaneous involvement), sebaceous adenitis, Malassezia infections, cutaneous lymphoma, food allergy, and drug eruptions. Histopathology should provide the diagnosis in these diseases.

Distribution of lesions

Traditionally, diseases causing mucocutaneous lesions (ears, lips, footpads, anus, nasal planum) are clinically suspected of being caused by an autoimmune disease (lupus, pemphigus, etc). However, pyodermas, zinc-responsive dermatoses, cutaneous lymphoma, superficial necrolytic dermatitis, drug eruptions, and leishmaniasis may cause very similar signs. Histopathology and culture should dif­ferentiate between these diseases.

Peri-ocular crusts have six main differential diagnoses: pyoderma, dermatophytes, demodicosis, zinc-responsive dermatoses, pemphigus foliaceus, and superficial necrolytic dermatitis.

Tail-head and caudal pruritus and alopecia is most commonly caused by flea infestation or flea allergy. The author has also seen this pattern in a few dogs with inhalant allergy and scabies, in cats with Otodectes (ear mite) infestations, and in dogs and cats with food allergies or Cheyletiella infestation. Skin scrapings, a trial hypoallergenic diet, presumptive ectoparasite treatment, and/or intradermal skin testing should therefore be considered in a dog or cat whose flea allergy dermatitis is not responding to therapy.

Tail alopecia is typically thought of as being due to hypothyroidism (so-called “rat-tail”). However, other endocrinopathies, as well as sebaceous adenitis, may cause this clinical sign.

Trunkal alopecia without pruritus is typically thought of as a sign of endocrine disease, or (more often in cats) dermatophytes. However, sebaceous adenitis or cutaneous lymphoma can also cause this distribution.

Ventral abdominal alopecia in cats is often termed endocrine alopecia or psycho­genic alopecia. While these entities may in fact exist, flea allergy, food allergy or inhalant allergy may cause this hair-loss distribution. In addition, a newly recognized condition termed feline paraneoplastic alopecia has been reported in elderly cats with internal malignancy (usually intestinal adenocarcinoma). When confronted by one of these cases, the clinician should first pluck some hair from the affected area, place the hair on a microscope slide with a drop of mineral oil and a coverslip and examine under low power. If the ends are all thin and tapered, a diagnosis of feline endocrine alopecia or feline paraneoplastic alopecia is more likely. If, on the other hand, many of the hair ends have a “squared-off” ap­pearance then the cat is licking the area and causing the alopecia, probably due to psychogenic causes or an allergy. In most practice areas, the major cause of ventral abdomen alopecia in cats is flea allergy or flea infestation.

An idiopathic seborrhea is well recognized in the Cocker Spaniel, especially the blond variety. Two other diseases may mimic this apparent genetic, difficult to control condition. Both food allergies and hypothyroidism may cause similar lesions. Because these two diseases are essentially curable, it behooves the clinician to place these dogs on a hypoallergenic diet and to evaluate thyroid function.

Finally, there are four diseases, which in the author's experience, may have many different clinical presentations and can mimic many other diseases. These are systemic lupus erythematosus, food allergy, leishmaniasis, and drug eruptions. The clinician should usually include these in the differential of any non-congenital dermatosis.

Reference

1.  White SD: The skin as a sensor of internal medical disorders. In Ettinger SJ, Feldman EC, (eds), Textbook of Veterinary Internal Medicine, 5th ed., WB Saunders Co, Philadelphia, 1999.