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Fungal Disease - Dermatophytosis

Synonym: ringworm

Introduction

Dermatophytosis is an infection of hair shafts and stratum corneum caused by keratinophilic fungi.

Predisposed animals

It occurs commonly in dogs and cats, with highest incidences reported in kittens, puppies, immune-compromised animals, and long-haired cats. Persian cats and Yorkshire and Jack Russell terriers appear to be predisposed.

Clinical signs

Skin involvement may be localized, multifocal, or generalized. Pruritus, if present, is usually minimal to mild but occasionally may be intense.

Other symptoms in dogs and cats include erythema, papules, crusts, seborrhea, and paronychia or onychodystrophy of one or more digits.

Rarely, cats present with miliary dermatitis or dermal nodules.

Other cutaneous manifestations in dogs include facial folliculitis and furunculosis resembling nasal pyoderma, kerions (acutely developing, alopecic, and exudative nodules) on the limb or face, and truncal dermal nodules.

Asymptomatic carrier states (subclinical infection) are common in cats, especially among long-haired breeds.

Asymptomatic disease, although rare in dogs, has been reported in Yorkshire terriers.

Lesions usually include areas of circular, irregular, or diffuse alopecia with variable scaling. Remaining hairs may appear stubbled or broken off.

Differential diagnosis

Dogs

Differentials in dogs include demodicosis and superficial pyoderma. If nodular, neoplasia and acaral lick dermatitis should be included.

Cats

Differentials in cats include parasites, allergies, and feline psychogenic alopecia.

Diagnosis

Rule out other differentials.

Ultraviolet (Wood’s lamp) examination: hairs fluoresce yellow-green with some Microsporum canis strains. This is an easy screening test, but falsenegative and false-positive results are common.

Trichogram (hairs or scales in potassium hydroxide preparation): search for hair shafts infiltrated with hyphae and arthrospores. Fungal elements are often difficult to find.

Dermatohistopathology: variable findings may include perifolliculitis, folliculitis, furunculosis, superficial perivascular or interstitial dermatitis, epidermal and follicular orthokeratosis or parakeratosis, or suppurative epidermitis; fungal hyphae and arthrospores in stratum corneum or hair shafts.

Fungal culture: Microsporum or Trichophyton spp.

Polymerase chain reaction (PCR) analysis, where available, may simplify the diagnosis.

Treatment and Prognosis

If the lesion is focal, a wide margin should be clipped around it and topical antifungal medication applied every 12 hours until the lesion resolves. (Some dermatologists believe that clipping spreads lesions on the animal and further contaminates the environment.)

Effective topicals for localized treatment include products that contain the following:

  • » Terbinafine cream
  • » Clotrimazole cream, lotion, or solution
  • » Enilconazole cream
  • » Ketoconazole cream
  • » Miconazole cream, spray, or lotion

If response to localized treatment is poor, the animal should be treated for generalized dermatophytosis.

For generalized disease: Topical antifungal rinse or dip should be applied to the entire body one or two times per week (minimum 4–6 weeks) until follow up fungal culture results are negative. Bathing the animal with a shampoo that contains chlorhexidine and miconazole (or ketoconazole) immediately preceding the antifungal dip may be helpful. Dogs with generalized dermatophytosis may be cured with topical therapy alone, whereas cats almost always require concurrent systemic therapy.

Effective topical antifungal solutions include the following:

  • » Enilconazole 0.2% solution
  • » Lime sulfur 2% to 4% solution

For cats with dermatophytosis and dogs that are unresponsive to topical therapy alone, topical therapy for generalized infection should be combined with long-term systemic antifungal therapy and continued until 3 to 4 weeks beyond negative follow-up fungal culture results. The average duration of therapy is 8 to 12 weeks.

Effective systemic antifungal drugs include the following:

  • » Terbinafine 30–40 mg/kg PO q 24 hours
  • » Ketoconazole 10 mg/kg PO q 24 hours with food (dogs)
  • » Fluconazole 10 mg/kg PO q 24 hours with food
  • » Itraconazole (Sporanox) 5–10 mg/kg PO q 24 hours with food

Less effective systemic antifungal drugs include the following:

  • » Microsized griseofulvin at least 50 mg/kg/day PO with fat-containing meal
  • » Ultramicrosized griseofulvin 5–10 mg/kg/day PO with fat-containing meal

Alternatively, pulse therapy may be almost as effective, and multiple protocols have been published using various drugs. Pulse treatments should be continued until two consecutive follow-up fungal cultures taken 2 to 4 weeks apart are negative.

All infected animals, including asymptomatic carriers, should be identified and treated. Exposed, noninfected cats and dogs should be treated prophylactically with weekly topical antifungal rinse or dip for the duration of treatment of the infected animals.

The environment should be thoroughly cleaned by removing all contaminated materials, and the area should be disinfected with bleach (vacuums may further contaminate the environment).

Lufenuron has not demonstrated consistent efficacy in treating or preventing infection.

Prognosis

The prognosis is generally good, except for endemically infected multicat households and catteries.

Animals with underlying immunosuppressive diseases also have a poorer prognosis for cure.

Dermatophytosis is contagious to other animals and to humans.

Treating Dermatophytosis in Multianimal Homes, Catteries, and Animal Facilities

Culture all animals to determine the extent and location of animal infections.

Culture the environment (cages, counters, furniture, floors, fans, ventilation units, etc) to map infected areas to be disinfected.

Treat all infected animals with systemic antifungals until each animal has two negative fungal cultures taken at least 1 month apart.

Treat all infected and exposed animals with topical 2% to 4% lime sulfur solution every 3 to 7 days to prevent contagion and zoonosis. Continue until all animals have two negative fungal cultures taken at least 1 month apart. Do not clip cats, as this contaminates the clippers and facility and worsens the risk of contagion.

Dispose of all infected material. Remove clutter from animal facilities or other infected areas.

Clean and disinfect all surface areas every 3 days. Continue until all animals have two negative fungal cultures taken at least 1 month apart. Enilconazole is a very effective environmental disinfectant, but it is licensed only for poultry farm use in the United States. Household chlorine laundry bleach (5% sodium hypochlorite) diluted 1:10 in water is an effective, inexpensive environmental disinfectant.


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