Clinical Knowledge Insight created by Andrew Lowe, DVM, MSc, DACVD
- Infection of hair, stratum corneum, and claw caused by protein-consuming fungi of the genuses Microsporum and Trichophyton, most commonly M. canis, M. gypseum, and T. mentagrophytes.
- Furunculosis and subsequent deep dermal and subcutaneous infection is an uncommon presentation.
- M. canis is the most frequent cause of dermatophytosis in the dog and cat. Some individuals, particularly cats, are asymptomatic carriers.
- Dermatophytosis is obtained via contact with infected hair or scale from infected/carrier animals, fomites, or spores in contaminated soils or indoor environments. Infective spores may remain viable in the environment for up to 18 months.
- M. canis is a zoophilic dermatophyte; the primary hosts are cats and dogs. It is of particular concern in shelter environments and multi-pet households.
- M. gypseum is a geophilic dermatophyte, inhabiting soil.
- T. mentagrophytes is transmitted via contact with reservoir hosts [typically rodents] or their environments.
- Incidence of dermatophytosis appears to have geographic variability.
- Immunocompromised, geriatric, and young individuals are at greater risk for infection.
- Genetics may play a role as Persian cats and Yorkshire terriers are over-represented, and some individuals in other breeds appear to be predisposed to recalcitrant or repeat infections.
- Concurrent overgrowth of Staphylococcus species is common.
- Zoonosis is possible with all three species, but most often with M. canis.
Dermatophyte test medium [DTM] [right] and enhanced sporulation agar [left] are innoculated with plucked hair and scale.
Positive culture shows color change and buff - colored colonies in the DTM [right] after 8 days of incubation. Color change occurred within 24 hrs. of colony growth.
Positive culture demonstrates color change and buff - colored colonies in the DTM after 14 days of incubation. Color change occurred within 24 hrs. of colony growth.
Macroconidia of M. canis from a positive culture. Note that each spore has more than 6 divisions.
Macroconidia of M. gypseum from a positive culture. Note that each spore has 6 or fewer divisions.
Microconidia of T. mentagrophytes from a positive culture.
Feline patient with multifocal alopecia, erythema and scale due to M. canis infection.
Positive Wood's light examination in a feline patient with multi focal alopecia and scale due to M. canis. Note blue/green color change associated with the hair shafts.
Canine patient with nodular dermatophytosis ["kerion"] lesion due to T. mentagrophytes infection.
Canine patient with alopecia, erythema, and crusting due to T. mentagrophytes infection mimicking pemphigus foliaceus.
Canine patient with alopecia, erythema, and crusting due to T. mentagrophytes infection.
Canine patient with alopecia, erythema, and crusting due to T. mentagrophytes infection.
Canine patient with single-limb alopecia, erythema, and crusting due to T. mentagrophytes infection.
- Presentations may be focal or generalized and may involve the trunk, limbs, tail or facial areas including the pinnae.
- Pruritus is typically mild, but varies from absent to severe.
- Lesions may not have a ring-like appearance as is classic in human "ringworm."
- Dermatophytosis is over-diagnosed in dogs, and it is often stated for this species, "If it looks like ringworm, it's probably staphylococcal pyoderma."
- Treatment involves a multi-pronged approach: topical/and or systemic therapy, environmental management, and in the case of M. canis, assessment for household canine and feline carriers.
- Recheck with repeat culture should be performed 1-3 weeks after initiation of therapy and every 1-3 weeks thereafter. Treatment should be continued until 2-3 negative cultures are obtained.
- Treatment duration is variable and may take from 14 days to 6 months.
- Spontaneous resolution may occur within three months for otherwise healthy patients.
ENVIRONMENTAL CONTROL
- M. canis spores may be viable for up to 18 months. Environmental control may reduce reinfection of the patient, humans, and other household animals.
- Lack of control may lead to treatment failure or relapses.
- Consider the Following Measures:
- Culture positive animals should be isolated from negative animals, preferably in an easily-cleaned room devoid of clutter with minimal upholstery
- Treat non-porus surfaces with 1:10 household bleach or enilconazole twice weekly
- Vacuum areas inhabited by positive animals daily, and floors/walls cleaned with electrostatic, microfiber cloths
- Upholstery may be treated with Lysol spray
- Remove/discard bedding and rugs that are difficult to disinfect
- Cleaning of ducts/vents and replacement of furnace filters
- Of note, steam cleaning alone is not effective as temperatures achieved and sustained are not high enough to kill spores.
- Culture of the environment [household or cattery] may be helpful to determine if control measures are effective. Electrostatic cloths or gauze can be wiped on surfaces and then touched to the surface of culture medium three times to inoculate.
- For patients with T. mentagrophytes, reduced exposure to heavily-populated rodent habitats or rodent control is recommended. If rodents are kept as household pets, they may be screened via the MacKensie toothbrush technique
- Causes of treatment failure are inadequate treatment duration, failure of environmental control and immunocompromised status, such as FIV or FeLV infection in cats and hyperadrenocorticism in dogs.
- Prognosis is good, with the exception of heavily contaminated environments such as catteries, shelters, and homes with multiple infected animals. Read the suggested references for additional tips on dealing with these situations.
- Instruct clients or staff members in contact with positive patients to consult a physician for diagnosis and treatment should they develop skin lesions.
A discrete swelling containing purulent material, typically in the subcutis
Perianal abscess in a dog
Absence of hair from areas where it is normally present; may be due to folliculitis, abnormal follicle cycling, or self-trauma
Extensive alopecia secondary to cutaneous epitheliotropic lymphoma
well-circumscribed, circular, patchy to coalescing alopecia, often associated with folliculitis
Moth-eaten alopecia secondary to superficial bacterial folliculitis
Regional subcutaneous edema
Angioedema due to cutaneous drug eruption
Ring-like arrangement of lesions
Annular lesions in a dog with erythema multiforme
Thinning of the skin or other tissues
Cutaneous atrophy due to glucocorticoids
Fluid-filled elevation of epidermis, >1cm
Bullae in a dog with bullous pemphigoid
Blood-filled elevation of epidermis, >1cm
Interdigital hemorrhagic bulla in a dog with deep pyoderma and furunculosis
dilated hair follicle filled with keratin, sebum
Comedones on the ventral abdomen of a dog with hypercortisolism
Dried exudate and keratinous debris on skin surface
Multifocal crusts due to pemphigus foliaceus
Nodule that is epithelial-lined and contains fluid or solid material.
Epidermal inclusion cyst
Extensive loss of pigment
Depigmentation of planum nasale in dog with vitiligo.
Patches due to hemorrhage >1cm
Ecchymoses of a dogs leg due to vasculitis
Circular scale or crust with erythema, associated with folliculitis or ruptured pustules or vesicles
Epidermal collarettes in a dog with Staphylococcus superficial bacterial folliculitis
Defect in epidermis that does not penetrate basement membrane. Histopathology may be needed to differentiate from ulcer.
Erosions in a dog with vasculitis
Red appearance of skin due to inflammation, capillary congestion
Erythema in a dog with cutaneous drug eruption
Thick crust often related to necrosis, trauma, or thermal/chemical burn
Eschar from physical trauma
Erosions and/or ulcerations due to self-trauma
Excoriations in a cat with atopic dermatitis
Excessive stratum corneum, confirmed via histopathology. This term is often used to describe the nasal planum and footpads.
Fissures of the footpads in a dog with superficial necrolytic dermatitis
Ulcer on skin surface that originates from and is contiguous with tracts extending into deeper, typically subcutaneous tissues
Perianal fistulas in a dog
Accumulation of scale adherent to hair shaft
Follicular casts surrounding hairs from a dog with hypothyroidism
Excessive stratum corneum, confirmed via histopathology. This term is often used to describe the nasal planum and footpads.
Idiopathic hyperkeratosis of the nasal planum [left] and footpads [right]
Increased melanin in skin, often secondary to inflammation
Inflammatory lesions [left] resulting in post-inflammatory hyperpigmentation [right]
Partial pigment loss
Idiopathic hypopigmentation of planum nasale
Lack of hair due to genetic factors or defects in embryogenesis.
Congenital hypotrichosis in chocolate Labrador puppies.
Lack of cutaneous pigment
Macular leukoderma in a dog
Loss of hair pigment
Progressive leukotrichia in patient with vitiligo.
Thickening of the epidermis, often due to chronic inflammation resulting in exaggerated texture
Lichenification of skin in a dog with chronic atopic dermatitis and Malassezia dermatitis
Flat lesion associated with color change 1cm
Nodules on nose of dog with cutaneous histiocytosis.
Abnormal nail morphology due to nail bed infection, inflammation, or trauma; may include: Onychogryphosis, Onychomadesis, Onychorrhexis, Onychoschizia
Onychodystrophy in dog with chronic allergies
Abnormal claw curvature; secondary to nail bed inflammation or trauma
Onychogryphosis in a dog with symmetric lupoid onychodystrophy
Claw sloughing due to nail bed inflammation or trauma
Onychomadesis in a dog with symmetric lupoid onychodystrophy
Claw fragmentation due to nail bed inflammation or trauma
Onychorrhexis in a dog with symmetric lupoid onychodystrophy
Claw splitting due to nail bed inflammation or trauma
Onychoschizia in a dog with symmetric lupoid onychodystrophy
Solid elevation in skin 1cm
Papules on a dog with superficial bacterial folliculitis
Solid elevation in skin 1cm
Papules on a dog with superficial bacterial folliculitis
Inflammation of the nail fold
Paronychia in a dog with symmetric lupoid onychodystrophy
Flat lesion associated with color change >1cm
Hypopigmented patch [left], erythematous patch [right]
Small erythematous or violaceous lesions due to dermal bleeding
Petechiae in a dog with cutaneous vasculitis
Venous dilation; most commonly associated with hypercortisolism
Phlebectasia and cutaneous atrophy due to hypercortisolism in a dog
Flat-topped elevation >1cm formed of coalescing papules or dermal infiltration
Plaques in a cat with cutaneous lymphoma
Raised epidermal infiltration of pus
Pustules on the abdomen of a dog with superficial staphylococcal pyoderma.
Net-like arrangement of lesions
Reticulated leukotrichia on the back of a horse
Accumulation of loose fragments of stratum corneum
Loose, large scales due to ichthyosis in a Golden Retriever
Fibrous tissue replacing damaged cutaneous and/or subcutaneous tissues
Scarring [right] following the healing of an ulcer [left] in a dog with sterile nodular dermatitis
Undulating, serpentine [snake-like] arrangement of lesions
Serpiginous urticarial lesions on a horse
Permanent enlargement of vessels resulting in a red or violet lesion [rare]
Telangiectasia in a dog with angiomatosis
A defect in epidermis that penetrates the basement membrane. Histopathology may be needed to differentiate from an erosion.
Ulcerations of the skin of a dog with vasculitis.
Wheals [steep-walled, circumscribed elevation in the skin due to edema ] due to hypersensitivity reaction
Urticaria in a horse
Fluid-filled elevation of epidermis,