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Differential Diagnoses
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CHAPTER
1
Differential Diagnoses
1
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CHAPTER 1 Differential Diagnoses
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Cellulitis Dogs Deep pyoderma Actinomycosis Nocardiosis Opportunistic mycobacteriosis Protothecosis Pythiosis Zygomycoses Juvenile cellulitis Anal sac disease Neoplasia
Cats l-form infection Actinomycosis Nocardiosis Opportunistic mycobacteriosis Plague Phaeohyphomycosis Pythiosis Zygomycoses Sporotrichosis Neoplasia
Nodular Diseases Dogs Subcutaneous abscess Botryomycosis Actinomycosis Nocardiosis Opportunistic mycobacteriosis Tuberculosis Dermatophytosis Eumycotic mycetoma Phaeohyphomycosis Protothecosis Pythiosis Zygomycoses Sporotrichosis Blastomycosis Coccidioidomycosis Histoplasmosis Ixodid ticks Cuterebra Dracunculiasis Viral papillomatosis Leishmaniasis Systemic lupus erythematosus Sterile nodular panniculitis Idiopathic sterile granuloma and pyogranuloma Tail gland hyperplasia Acral lick dermatitis Callus Hygroma
Eosinophilic granuloma Canine truncal solar dermatosis Neoplasia
Cats Subcutaneous abscess Botryomycosis Actinomycosis Nocardiosis Opportunistic mycobacteriosis Feline leprosy Plague Tuberculosis Dermatophytosis Eumycotic mycetoma Phaeohyphomycosis Protothecosis Pythiosis Zygomycoses Sporotrichosis Blastomycosis Coccidioidomycosis Cryptococcosis Histoplasmosis Ixodid ticks Cuterebra Dracunculiasis Feline cowpox Viral papillomatosis Leishmaniasis Sterile nodular panniculitis Eosinophilic granuloma Neoplasia
Diseases Primarily Limited to the Face Dogs Mucocutaneous pyoderma Chin pyoderma Nasal pyoderma Eosinophilic furunculosis of the face Pemphigus erythematosus Pemphigus foliaceus (early) Discoid lupus erythematosus Uveodermatologic syndrome Juvenile cellulitis Familial canine dermatomyositis (early) Nasal depigmentation Canine nasal solar dermatosis
Cats Mosquito bite hypersensitivity Pemphigus foliaceus (early) Pemphigus erythematosus Discoid lupus erythematosus Feline acne Idiopathic facial dermatitis of Persian cats
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CHAPTER 1 Differential Diagnoses
Indolent ulcer Feline solar dermatosis
Pododermatitis Dogs Bacterial pododermatitis Dermatophytosis Malasseziasis Demodicosis Trombiculiasis Hookworm dermatitis Pelodera dermatitis Atopy Food hypersensitivity Contact dermatitis Pemphigus foliaceus Pemphigus vulgaris Bullous pemphigoid Systemic lupus erythematosus Superficial necrolytic migratory erythema Neoplasia
Cats Bacterial pododermatitis Dermatophytosis Trombiculiasis Pemphigus foliaceus Pemphigus vulgaris Systemic lupus erythematosus Superficial necrolytic migratory erythema Plasma cell pododermatitis Neoplasia
Seborrheic Diseases Dogs Superficial pyoderma Dermatophytosis Malasseziasis Demodicosis Canine scabies Cheyletiellosis Pediculosis Leishmaniasis Food hypersensitivity Contact dermatitis Pemphigus foliaceus Pemphigus erythematosus Systemic lupus erythematosus Cutaneous drug reaction Hyperadrenocorticism Hypothyroidism Sex hormone imbalances Canine primary seborrhea Vitamin A–responsive dermatosis
3
Ichthyosis Epidermal dysplasia of West Highland White terriers Sebaceous adenitis Zinc-responsive dermatosis Superficial necrolytic migratory erythema Canine ear margin seborrhea Neoplasia
Cats Superficial pyoderma Dermatophytosis Malasseziasis Demodicosis Feline scabies Cheyletiellosis Cat fur mite Pediculosis Pemphigus foliaceus Pemphigus erythematosus Systemic lupus erythematosus Cutaneous drug reaction Superficial necrolytic migratory erythema Tail gland hyperplasia Idiopathic facial dermatitis of Persian cats Neoplasia
Nonpruritic Alopecic Diseases Dogs Superficial pyoderma Dermatophytosis Demodicosis Alopecia areata Hyperadrenocorticism Hypothyroidism Sex hormone imbalance Alopecia X Recurrent flank alopecia Pituitary dwarfism Congenital hypotrichosis Color dilution alopecia Black hair follicular dysplasia Canine pattern baldness Idiopathic bald thigh syndrome of Greyhounds Anagen and telogen defluxion Postclipping alopecia Traction alopecia Injection reaction
Cats Dermatophytosis Demodicosis Alopecia areata Hyperadrenocorticism Congenital hypotrichosis Feline preauricular and pinnal alopecia
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CHAPTER 1 Differential Diagnoses
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Anagen and telogen defluxion Injection reaction
Vesicular and Pustular Diseases Dogs Impetigo Superficial pyoderma Chin pyoderma Demodicosis Contact dermatitis Pemphigus foliaceus Pemphigus erythematosus Pemphigus vulgaris Bullous pemphigoid Systemic lupus erythematosus Cutaneous drug reaction Epidermolysis bullosa Canine familial dermatomyositis Canine mucinosis Subcorneal pustular dermatosis Sterile eosinophilic pustulosis
Cats Impetigo Superficial pyoderma Contact dermatitis Pemphigus foliaceus Pemphigus erythematosus Pemphigus vulgaris Cutaneous drug reaction Epidermolysis bullosa
Erosive and Ulcerative Diseases Dogs Mucocutaneous pyoderma Pyotraumatic dermatitis Candidiasis Cryptococcosis Fly bite dermatitis Rocky Mountain spotted fever (RMSF) Leishmaniasis Pemphigus vulgaris Bullous pemphigoid Systemic lupus erythematosus Vasculitis Erythema multiforme/Toxic epidermal necrolysis Cutaneous drug reaction Epidermolysis bullosa Canine familial dermatomyositis Neoplasia
Cats Pyotraumatic dermatitis Candidiasis Feline calicivirus
Feline rhinotracheitis virus Leishmaniasis Pemphigus vulgaris Vasculitis Erythema multiforme/Toxic epidermal necrolysis Cutaneous drug reaction Epidermolysis bullosa Eosinophilic plaque Indolent ulcer Plasma cell pododermatitis Idiopathic ulcerative dermatosis Neoplasia
Generalized Pruritic Diseases Dogs Superficial pyodermatosis Dermatophytosis Malasseziasis Canine scabies Cheyletiellosis Ear mites Trombiculiasis Pediculosis Hookworm dermatitis Pelodera dermatitis Atopy Food hypersensitivity Flea allergy dermatitis Contact dermatitis Acral lick dermatitis Subcorneal pustular dermatosis Sterile eosinophilic pustulosis Epitheliotrophic lymphoma
Cats Superficial pyoderma Malasseziasis Feline scabies Cheyletiellosis Ear mites Trombiculiasis Pediculosis Atopy Food hypersensitivity Flea allergy dermatitis Contact dermatitis Psychogenic alopecia Eosinophilic plaque Idiopathic ulcerative dermatosis Feline paraneoplastic alopecia
Nasodigital Hyperkeratosis Dogs Canine distemper Leishmaniasis
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CHAPTER 1 Differential Diagnoses
Familial footpad hyperkeratosis Zinc-responsive dermatosis Superficial necrolytic migratory erythema Idiopathic nasodigital hyperkeratosis Pemphigus foliaceus
Cats Mosquito bite hypersensitivity Pemphigus foliaceus Systemic lupus erythematosus Plasma cell pododermatitis Cutaneous horn
Miliary Dermatitis Cats Superficial pyoderma Dermatophytosis Demodicosis Cheyletiellosis Ear mites Cat fur mite
Pediculosis Atopy Food hypersensitivity Flea allergy dermatitis Pemphigus foliaceus Cutaneous drug reaction
Diseases of Nasal Depigmentation Dogs Contact dermatitis Pemphigus erythematosus Pemphigus foliaceus Pemphigus vulgaris Bullous pemphigoid Discoid lupus erythematosus Systemic lupus erythematosus Uveodermatologic syndrome Vitiligo Nasal depigmentation Canine nasal solar dermatosis Neoplasia
5
Hypothyroidism and hyperadrenocorticism can cause a relative immunosuppression that predisposes the dog to secondary infections. The secondary infections can be pruritic, mimicking allergy; however, if the pruritus resolves when the infections are controlled, allergy is less likely.
Pododermatitis Bacteria, Malassezia
Yeast Dermatitis
Food allergy is the one allergic disease that can be nonpruritic. Food allergy can change the normal function of the skin, predisposing it to secondary infections that are pruritic. If the pruritus resolves when the infections are treated, endocrine disease or food allergy is most likely. Atopy and scabies are almost always pruritic.
Consider food allergy
Autoimmune skin diseases are usually not considered pruritic; however, the cutaneous inflammation and crusts can cause mild to moderate pruritus.
Lesions on the nasal planum, ear pinnae, footpads
Primary keratinization defects (sebaceous adenitis, primary seborrhea, ichthyosis, epidermal dysplasia) change the normal skin defense functions, predisposing the dog to secondary infections that may be pruritic.
Follicular casts, entire skin surface affected
Go to page 2
Pruritus persists without secondary infections?
(steroid therapy will make it difficult to evaluate the pruritus and its association with the secondary infections)
Pruritus resolved without secondary infections?
Consider endocrine diseases
Otitis
Aggressively treat all secondary infections for 30 days since they complicate case evaluation.
Pyoderma, Demodex, Dermatophytosis
Folliculitis
Identify and treat all secondary infections
6
Pruritic Dog
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CHAPTER 1 Differential Diagnoses
These are most consistent with atopy (environmental allergies). Flea exposure can mimic the seasonal nature of atopy and should be ruled out.
FIGURE 1-1
These are most typical of scabies but could be features of food allergy. Skin scrapes and response to aggressive scabicidal therapy will confirm or rule out scabies. Skin scrapes are only 20% accurate.
Intense uncontrollable pruritus
More than 1 dog affected
Algorithm for working up the pruritic dog.
1-5 years of age
These are suggestive of a food allergy. A 10-12 week food trial with a novel protein source and extremely limited diet with subsequent food challenge should confirm or eliminate food allergy.
These are nonspecific symptoms that could be caused by atopy, food allergy, or scabies. Consider treating for scabies, performing a food trial, and finally consider allergy testing if the patient's pruritus persists.
No ear margin/elbow lesions
No GI symptoms
No perianal dermatitis
Nonseasonal pruritus
Pruritus on the face, feet, and ventrum
GI symptoms
Older than 5 years
Less than 1 year old
Extremely responsive to steroid therapy
Less than 1 year old
Elbow lesions
Started between 1-3 years of age
Perianal dermatitis
Ear margin lesions
Flea allergy dermatitis is the most common cause of lumbar dermatitis. With flea allergy dermatitis, the majority of the lesions will be caudal to the rib cage. Additionally, foot licking is not typically associated with flea allergies.
Lumbar dermatitis
Seasonal symptoms
Autoimmune skin diseases are usually not considered pruritic; however, the cutaneous inflammation and crusts can cause mild to moderate pruritus.
Lesions on the nasal planum, ear pinnae, footpads
Pruritus persists without secondary infections?
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CHAPTER 1 Differential Diagnoses
7
Biopsy the skin lesion to confirm possible pemphigus. Pemphigus in cats often causes an erosive dermatitis around the nipples and nail beds (paronychia).
Acantholytic cells and neutrophils identified.
Bacteria or Malassezia identified. Treat these secondary infections with topical and systemic treatments. Identify the primary/underlying disease (diabetes, hyperthyroidism, allergies, ectoparasites, etc).
Cytology of lesions
Cytology of lesions
Treat the parasitic infestation with appropriate therapy for sufficient duration to eliminate the parasites.
Skin scrapes positive
Dermatophyte is the most common skin infection in cats and is zoonotic, making it an important differential.
DTM fungal cultures, trichogram, Wood's lamp
Skin scrapes negative
Alopecia with or without apparent cutaneous inflammation is common in cats with allergies or ectoparasites.
This is the most common clinical presentation in cats and can be caused by numerous conditions.
Rule out Dermatophytosis
"Allergic" alopecia
Miliary dermatitis
Go to page 2
Eosinophils
Cytology of lesions
Eosinophilic plaques, eosinophilic granulomas, linear granulomas, indolent ulcers, fat chin syndrome are all manifestations of eosinophilic dermatitis associated with numerous etiologies.
Eosinophilic granuloma syndrome
8
Feline Dermatitis
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CHAPTER 1 Differential Diagnoses
Food allergy is one of the more common causes of feline dermatitis. Feeding a novel protein source in an extreme limitedingredient diet for 10-12 weeks will confirm or eliminate this differential.
Ectoparasites such as Demodex gatoi, Cheyletiella, and Otodectes can cause dermatitis and be difficult to find on skin scrapes. Weekly dips with lime sulfur for 4-6 weeks should resolve any infections. Treatment with avermectins will kill Otodectes and Cheyletiella but are ineffective for D. gatoi.
Biopsy
Environmental allergies including insect hypersensitivity can occur in cats but are less common than flea and food allergies. Allergy testing of cats is difficult due to the lack of specific serum testing and the poor reactivity of skin tests.
Allergy testing
Old cat with sudden onset of dermatitis
Paraneoplastic dermatitis caused by hepatic and pancreatic adenocarcinoma or cutaneous lymphoma can mimic allergies but occurs in old cats that usually present with weight loss and other evidence of metabolic disease.
Biopsy, consider medical workup and abdominal ultrasound.
FIGURE 1-2
Algorithm for working up the pruritic cat.
This will identify the etiology or at least be able to suggest an infection (folliculitis or diffuse pyogranulomatous dermatitis), hypersensitivity reaction (eosinophils and mast cells), autoimmune skin disease (interface dermatitis), neoplasia, or psychogenic (complete absence of inflammation).
Food trial
Lime sulfur dip trial
Young to middle-age cat
Flea allergy dermatitis is the most common cause of feline dermatitis. Since fleas may not be apparent, aggressive flea control on all contact pets to eliminate the flea population is the primary of confirming this diagnosis.
Aggressive flea control
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CHAPTER 1 Differential Diagnoses
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CHAPTER
1
Differential Diagnoses
1
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CHAPTER 1 Differential Diagnoses
2
Cellulitis Dogs Deep pyoderma Actinomycosis Nocardiosis Opportunistic mycobacteriosis Protothecosis Pythiosis Zygomycoses Juvenile cellulitis Anal sac disease Neoplasia
Cats l-form infection Actinomycosis Nocardiosis Opportunistic mycobacteriosis Plague Phaeohyphomycosis Pythiosis Zygomycoses Sporotrichosis Neoplasia
Nodular Diseases Dogs Subcutaneous abscess Botryomycosis Actinomycosis Nocardiosis Opportunistic mycobacteriosis Tuberculosis Dermatophytosis Eumycotic mycetoma Phaeohyphomycosis Protothecosis Pythiosis Zygomycoses Sporotrichosis Blastomycosis Coccidioidomycosis Histoplasmosis Ixodid ticks Cuterebra Dracunculiasis Viral papillomatosis Leishmaniasis Systemic lupus erythematosus Sterile nodular panniculitis Idiopathic sterile granuloma and pyogranuloma Tail gland hyperplasia Acral lick dermatitis Callus Hygroma
Eosinophilic granuloma Canine truncal solar dermatosis Neoplasia
Cats Subcutaneous abscess Botryomycosis Actinomycosis Nocardiosis Opportunistic mycobacteriosis Feline leprosy Plague Tuberculosis Dermatophytosis Eumycotic mycetoma Phaeohyphomycosis Protothecosis Pythiosis Zygomycoses Sporotrichosis Blastomycosis Coccidioidomycosis Cryptococcosis Histoplasmosis Ixodid ticks Cuterebra Dracunculiasis Feline cowpox Viral papillomatosis Leishmaniasis Sterile nodular panniculitis Eosinophilic granuloma Neoplasia
Diseases Primarily Limited to the Face Dogs Mucocutaneous pyoderma Chin pyoderma Nasal pyoderma Eosinophilic furunculosis of the face Pemphigus erythematosus Pemphigus foliaceus (early) Discoid lupus erythematosus Uveodermatologic syndrome Juvenile cellulitis Familial canine dermatomyositis (early) Nasal depigmentation Canine nasal solar dermatosis
Cats Mosquito bite hypersensitivity Pemphigus foliaceus (early) Pemphigus erythematosus Discoid lupus erythematosus Feline acne Idiopathic facial dermatitis of Persian cats
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CHAPTER 1 Differential Diagnoses
Indolent ulcer Feline solar dermatosis
Pododermatitis Dogs Bacterial pododermatitis Dermatophytosis Malasseziasis Demodicosis Trombiculiasis Hookworm dermatitis Pelodera dermatitis Atopy Food hypersensitivity Contact dermatitis Pemphigus foliaceus Pemphigus vulgaris Bullous pemphigoid Systemic lupus erythematosus Superficial necrolytic migratory erythema Neoplasia
Cats Bacterial pododermatitis Dermatophytosis Trombiculiasis Pemphigus foliaceus Pemphigus vulgaris Systemic lupus erythematosus Superficial necrolytic migratory erythema Plasma cell pododermatitis Neoplasia
Seborrheic Diseases Dogs Superficial pyoderma Dermatophytosis Malasseziasis Demodicosis Canine scabies Cheyletiellosis Pediculosis Leishmaniasis Food hypersensitivity Contact dermatitis Pemphigus foliaceus Pemphigus erythematosus Systemic lupus erythematosus Cutaneous drug reaction Hyperadrenocorticism Hypothyroidism Sex hormone imbalances Canine primary seborrhea Vitamin A–responsive dermatosis
3
Ichthyosis Epidermal dysplasia of West Highland White terriers Sebaceous adenitis Zinc-responsive dermatosis Superficial necrolytic migratory erythema Canine ear margin seborrhea Neoplasia
Cats Superficial pyoderma Dermatophytosis Malasseziasis Demodicosis Feline scabies Cheyletiellosis Cat fur mite Pediculosis Pemphigus foliaceus Pemphigus erythematosus Systemic lupus erythematosus Cutaneous drug reaction Superficial necrolytic migratory erythema Tail gland hyperplasia Idiopathic facial dermatitis of Persian cats Neoplasia
Nonpruritic Alopecic Diseases Dogs Superficial pyoderma Dermatophytosis Demodicosis Alopecia areata Hyperadrenocorticism Hypothyroidism Sex hormone imbalance Alopecia X Recurrent flank alopecia Pituitary dwarfism Congenital hypotrichosis Color dilution alopecia Black hair follicular dysplasia Canine pattern baldness Idiopathic bald thigh syndrome of Greyhounds Anagen and telogen defluxion Postclipping alopecia Traction alopecia Injection reaction
Cats Dermatophytosis Demodicosis Alopecia areata Hyperadrenocorticism Congenital hypotrichosis Feline preauricular and pinnal alopecia
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CHAPTER 1 Differential Diagnoses
4
Anagen and telogen defluxion Injection reaction
Vesicular and Pustular Diseases Dogs Impetigo Superficial pyoderma Chin pyoderma Demodicosis Contact dermatitis Pemphigus foliaceus Pemphigus erythematosus Pemphigus vulgaris Bullous pemphigoid Systemic lupus erythematosus Cutaneous drug reaction Epidermolysis bullosa Canine familial dermatomyositis Canine mucinosis Subcorneal pustular dermatosis Sterile eosinophilic pustulosis
Cats Impetigo Superficial pyoderma Contact dermatitis Pemphigus foliaceus Pemphigus erythematosus Pemphigus vulgaris Cutaneous drug reaction Epidermolysis bullosa
Erosive and Ulcerative Diseases Dogs Mucocutaneous pyoderma Pyotraumatic dermatitis Candidiasis Cryptococcosis Fly bite dermatitis Rocky Mountain spotted fever (RMSF) Leishmaniasis Pemphigus vulgaris Bullous pemphigoid Systemic lupus erythematosus Vasculitis Erythema multiforme/Toxic epidermal necrolysis Cutaneous drug reaction Epidermolysis bullosa Canine familial dermatomyositis Neoplasia
Cats Pyotraumatic dermatitis Candidiasis Feline calicivirus
Feline rhinotracheitis virus Leishmaniasis Pemphigus vulgaris Vasculitis Erythema multiforme/Toxic epidermal necrolysis Cutaneous drug reaction Epidermolysis bullosa Eosinophilic plaque Indolent ulcer Plasma cell pododermatitis Idiopathic ulcerative dermatosis Neoplasia
Generalized Pruritic Diseases Dogs Superficial pyodermatosis Dermatophytosis Malasseziasis Canine scabies Cheyletiellosis Ear mites Trombiculiasis Pediculosis Hookworm dermatitis Pelodera dermatitis Atopy Food hypersensitivity Flea allergy dermatitis Contact dermatitis Acral lick dermatitis Subcorneal pustular dermatosis Sterile eosinophilic pustulosis Epitheliotrophic lymphoma
Cats Superficial pyoderma Malasseziasis Feline scabies Cheyletiellosis Ear mites Trombiculiasis Pediculosis Atopy Food hypersensitivity Flea allergy dermatitis Contact dermatitis Psychogenic alopecia Eosinophilic plaque Idiopathic ulcerative dermatosis Feline paraneoplastic alopecia
Nasodigital Hyperkeratosis Dogs Canine distemper Leishmaniasis
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CHAPTER 1 Differential Diagnoses
Familial footpad hyperkeratosis Zinc-responsive dermatosis Superficial necrolytic migratory erythema Idiopathic nasodigital hyperkeratosis Pemphigus foliaceus
Cats Mosquito bite hypersensitivity Pemphigus foliaceus Systemic lupus erythematosus Plasma cell pododermatitis Cutaneous horn
Miliary Dermatitis Cats Superficial pyoderma Dermatophytosis Demodicosis Cheyletiellosis Ear mites Cat fur mite
Pediculosis Atopy Food hypersensitivity Flea allergy dermatitis Pemphigus foliaceus Cutaneous drug reaction
Diseases of Nasal Depigmentation Dogs Contact dermatitis Pemphigus erythematosus Pemphigus foliaceus Pemphigus vulgaris Bullous pemphigoid Discoid lupus erythematosus Systemic lupus erythematosus Uveodermatologic syndrome Vitiligo Nasal depigmentation Canine nasal solar dermatosis Neoplasia
5
Hypothyroidism and hyperadrenocorticism can cause a relative immunosuppression that predisposes the dog to secondary infections. The secondary infections can be pruritic, mimicking allergy; however, if the pruritus resolves when the infections are controlled, allergy is less likely.
Pododermatitis Bacteria, Malassezia
Yeast Dermatitis
Food allergy is the one allergic disease that can be nonpruritic. Food allergy can change the normal function of the skin, predisposing it to secondary infections that are pruritic. If the pruritus resolves when the infections are treated, endocrine disease or food allergy is most likely. Atopy and scabies are almost always pruritic.
Consider food allergy
Autoimmune skin diseases are usually not considered pruritic; however, the cutaneous inflammation and crusts can cause mild to moderate pruritus.
Lesions on the nasal planum, ear pinnae, footpads
Primary keratinization defects (sebaceous adenitis, primary seborrhea, ichthyosis, epidermal dysplasia) change the normal skin defense functions, predisposing the dog to secondary infections that may be pruritic.
Follicular casts, entire skin surface affected
Go to page 2
Pruritus persists without secondary infections?
(steroid therapy will make it difficult to evaluate the pruritus and its association with the secondary infections)
Pruritus resolved without secondary infections?
Consider endocrine diseases
Otitis
Aggressively treat all secondary infections for 30 days since they complicate case evaluation.
Pyoderma, Demodex, Dermatophytosis
Folliculitis
Identify and treat all secondary infections
6
Pruritic Dog
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CHAPTER 1 Differential Diagnoses
These are most consistent with atopy (environmental allergies). Flea exposure can mimic the seasonal nature of atopy and should be ruled out.
FIGURE 1-1
These are most typical of scabies but could be features of food allergy. Skin scrapes and response to aggressive scabicidal therapy will confirm or rule out scabies. Skin scrapes are only 20% accurate.
Intense uncontrollable pruritus
More than 1 dog affected
Algorithm for working up the pruritic dog.
1-5 years of age
These are suggestive of a food allergy. A 10-12 week food trial with a novel protein source and extremely limited diet with subsequent food challenge should confirm or eliminate food allergy.
These are nonspecific symptoms that could be caused by atopy, food allergy, or scabies. Consider treating for scabies, performing a food trial, and finally consider allergy testing if the patient's pruritus persists.
No ear margin/elbow lesions
No GI symptoms
No perianal dermatitis
Nonseasonal pruritus
Pruritus on the face, feet, and ventrum
GI symptoms
Older than 5 years
Less than 1 year old
Extremely responsive to steroid therapy
Less than 1 year old
Elbow lesions
Started between 1-3 years of age
Perianal dermatitis
Ear margin lesions
Flea allergy dermatitis is the most common cause of lumbar dermatitis. With flea allergy dermatitis, the majority of the lesions will be caudal to the rib cage. Additionally, foot licking is not typically associated with flea allergies.
Lumbar dermatitis
Seasonal symptoms
Autoimmune skin diseases are usually not considered pruritic; however, the cutaneous inflammation and crusts can cause mild to moderate pruritus.
Lesions on the nasal planum, ear pinnae, footpads
Pruritus persists without secondary infections?
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CHAPTER 1 Differential Diagnoses
7
Biopsy the skin lesion to confirm possible pemphigus. Pemphigus in cats often causes an erosive dermatitis around the nipples and nail beds (paronychia).
Acantholytic cells and neutrophils identified.
Bacteria or Malassezia identified. Treat these secondary infections with topical and systemic treatments. Identify the primary/underlying disease (diabetes, hyperthyroidism, allergies, ectoparasites, etc).
Cytology of lesions
Cytology of lesions
Treat the parasitic infestation with appropriate therapy for sufficient duration to eliminate the parasites.
Skin scrapes positive
Dermatophyte is the most common skin infection in cats and is zoonotic, making it an important differential.
DTM fungal cultures, trichogram, Wood's lamp
Skin scrapes negative
Alopecia with or without apparent cutaneous inflammation is common in cats with allergies or ectoparasites.
This is the most common clinical presentation in cats and can be caused by numerous conditions.
Rule out Dermatophytosis
"Allergic" alopecia
Miliary dermatitis
Go to page 2
Eosinophils
Cytology of lesions
Eosinophilic plaques, eosinophilic granulomas, linear granulomas, indolent ulcers, fat chin syndrome are all manifestations of eosinophilic dermatitis associated with numerous etiologies.
Eosinophilic granuloma syndrome
8
Feline Dermatitis
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CHAPTER 1 Differential Diagnoses
Food allergy is one of the more common causes of feline dermatitis. Feeding a novel protein source in an extreme limitedingredient diet for 10-12 weeks will confirm or eliminate this differential.
Ectoparasites such as Demodex gatoi, Cheyletiella, and Otodectes can cause dermatitis and be difficult to find on skin scrapes. Weekly dips with lime sulfur for 4-6 weeks should resolve any infections. Treatment with avermectins will kill Otodectes and Cheyletiella but are ineffective for D. gatoi.
Biopsy
Environmental allergies including insect hypersensitivity can occur in cats but are less common than flea and food allergies. Allergy testing of cats is difficult due to the lack of specific serum testing and the poor reactivity of skin tests.
Allergy testing
Old cat with sudden onset of dermatitis
Paraneoplastic dermatitis caused by hepatic and pancreatic adenocarcinoma or cutaneous lymphoma can mimic allergies but occurs in old cats that usually present with weight loss and other evidence of metabolic disease.
Biopsy, consider medical workup and abdominal ultrasound.
FIGURE 1-2
Algorithm for working up the pruritic cat.
This will identify the etiology or at least be able to suggest an infection (folliculitis or diffuse pyogranulomatous dermatitis), hypersensitivity reaction (eosinophils and mast cells), autoimmune skin disease (interface dermatitis), neoplasia, or psychogenic (complete absence of inflammation).
Food trial
Lime sulfur dip trial
Young to middle-age cat
Flea allergy dermatitis is the most common cause of feline dermatitis. Since fleas may not be apparent, aggressive flea control on all contact pets to eliminate the flea population is the primary of confirming this diagnosis.
Aggressive flea control
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CHAPTER 1 Differential Diagnoses
9