Small Animal Dermatology

Small Animal DermatologyFungal Skin Diseases

Fungal Skin DiseasesDermatophytosis – Dogs and Cats

Compiled by:

Dr.Kedar Karki

M.V.St.Preventive Veterinary Medicine

A. General Considerations:

1. Dermatophytes invade keratinized structures, e.g. hair, horn, nails, feathers, and cornified epithelium.

2. It is a self-limiting condition:

a. It may take several months or longer to resolve.

b. The host immune status influences the severity and course of the disease.

3. It is a zoonotic disease.

Important Facts

Dermatophytosis is a self-limiting condition but it may take months to resolve spontaneously.

It is a zoonotic disease.

B. Cause:

1. Zoophilic – primarily infect animals. All can be zoonotic. Organisms include: Microsporum canis; Trichophyton equinum; Trichophyton mentagrophytes, Trichophyton verrucosum, Microsporum nanum.

2. Geophilic – inhabit soil and decompose keratinaceous organic debris. Microsporum gypseum.

3. Antropophilic – primarily infect man; rarely infect animals.

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4. Arthrospores on hairs.

a. Endothrix – arthrospores produced inside hair shaft. These are anthropophilic organisms.

b. Ectothrix – arthrospores produced outside hair shaft. These are zoophilic and geophilic organisms. Therefore, the large majority of dermatophytosis cases in animals will be associated with an ectothrix parasitism.

Important Facts

Zoophilic (e.g. Microsporum canis) and geophylic (e.g. Microsporum gypseum) organisms account for most cases of dermatophytosis in cats and dogs.

Most cases of dermatophytosis in animals will be associated with an ectothrix parasitism.

C. Clinical incidence of Dermatophytosis:

1. Dermatophytosis is an overdiagnosed condition (in dogs).

2. Microsporum and Trichophyton genera are responsible for 90% of all clinical cases.

3. The infection is more common in warm, humid tropical areas.

4. The infection is more common when animals are housed in conditions of poor sanitation and overcrowding, e.g. “puppy mills”, pet shops, catteries.

5. Younger animals are more frequently infected.

6. Immunosuppressed animals have higher incidence.

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Important Facts

Dermatophytosis is overdiagnosed. Microsporum canis accounts for at least 80% of the isolates in cats and dogs. Younger and immunosuppressed animals are predisposed to infection.

D. Transmission:

1. Direct contact with infected host, fomite, or contaminated environment. Fungal spores can remain viable in a dry state for years!

2. Animal to man (most common) and man to animal transmission can occur.

3. Asymptomatic carriers (especially cats) can serve as reservoir of infection.

4. Airborne transmission, especially in catteries, is believed to occur.

5. Ectoparasites (fleas, cheyletiella) especially in catteries and multiple cat households.

Important Facts

The main modes of transmission include: direct contact with infected host, fomite, or contaminated environment.

Remember! Fungal spores can remain viable on the environment for years. Remember! Asymptomatic carries (especially cats) can serve as reservoir of

infection.

E. Pathophysiology:

1. The dermatophyte must invade keratin of the stratum corneum and/or hair.

2. The organisms only grow in anagen hairs.

a. Growing hairs contain carbohydrates, nitrogenous substances, and nucleoprotein derivatives in addition to keratin.

3. The fungi grow downward to just above the hair bulb.

4. The hair shaft is weakened and breaks.

5. Infection induces the hair to enter telogen. Then the infection resolves in that hair. But by that time it has spread to a neighboring hair.

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6. Clinical signs associated with dermatophytosis are reflections of the host’s response to the fungus.

a. Well adapted species (e.g. Microsporum canis) will induce minimal inflammation.

b. Less-well adapted species (e.g. Microsporum gypseum, Trichophyton mentagrophytes) will induce more marked inflammation.

7. Inflammation expels fungus from that hair and infection spreads peripherally.

8. Incubation periods vary from 4 to 40 days.

Important Facts

Well adapted species (e.g. Microsporum canis) will induce minimal inflammation. Less-well adapted species (e.g. Microsporum gypseum) will induce marked

inflammation, which will cause more discomfort to the animal but may help reduce the duration of the infection.

Inflammation expels fungus from the hair. Incubation periods vary from 4 to 40 days.

F. Immunology:

1. Cell – mediated immunity is most important.

2. No correlation has been found between circulating antibodies and protection.

Important Facts

Cell – mediated immunity is most important for the animal’s protection against infection.

G. Clinical Signs:

1. Canine Dermatophytosis:

a. Variable clinical signs:

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1. The “classic” lesion is a circular patch of alopecia characterized by broken stubby hair, scaling, and mild erythema. Lesion appears to be spreading outward, often with a central healing.

2. Dermatophyte lesions can be focal, multifocal, or generalized.

3. Generalized conditions warrant search for an underlying cause.

4. May see papules, pustules, and vesicles with increased degrees of inflammation.

5. Pruritus is variable – usually it is NOT pruritic.

6. Major differentials include superficial pyoderma and demodicosis. Remember, dermatophytosis is usually overdiagnosed!

Important Facts

Clinical signs are quite variable. In practice, most cases that look like ringworm will be caused by staphylococcal

pyoderma. Epidermal collarette lesions mimic the classic ringworm lesion (patch of alopecia characterized by broken stubby hair, scaling, and mild erythema).

b. Other Clinical Presentation:

1. Kerions:

a. Nodular dermal reaction with ulceration and draining tracts.

b. Due to extreme inflammatory reaction or hypersensitivity to dermal location of fungus.

c. Most commonly caused by geophilic or other poorly host adapted fungi.

d. Differentials include histiocytoma, deep pyoderma, neoplasia, demodicosis.

Important Facts

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Kerion is a nodular lesion resulting from the severe inflammatory reaction induced by the fungus present in the dermis.

2. Onychomycosis:

a. Infection of keratin at the nail bed. It is rare.

b. Causes abnormal nail growth and brittle nails – it is usually asymmetric. Usually, it is not seen by itself.

c. It is most often caused by Trichophyton mentagrophytes.

d. It can be difficult to diagnosis.

e. It usually requires prolonged systemic therapy. Important Facts

Onychomycosis is a rare condition caused by the dermatophyte infection of the keratin at the nail bed.

It can be difficult to diagnose and also difficult to manage.

Usually other parts of the body are also involved.

2. Feline Dermatophytosis:

a. Microsporum canis is the most common feline dermatophyte (>95%).

b. It can become endemic in some catteries, especially long-haired cats.

c. Clinical Signs:

1. Patches of alopecia, crusting, scaling, especially localized to the face and ears.

2. Miliary dermatitis (multiple, small papules covered with crusts).

3. Asymptomatic carriers – in infected catteries, only the kittens may be clinically affected.

4. Pseudomycetomas:

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a. Subcutaneous nodules with draining tracts and grains.

b. It is caused by dermatophyte infection of the dermis and subcutaneous tissue.

c. Most commonly seen in Persians.

d. It may be resistant to therapy.

5. Check the immune status of cats with the generalized form (FeLV, FIV).

Important Facts

Most cases in cats are associated with alopecia, crusting and scaling localized to the head and ears.

Pruritus can be present but, it is rare. Remember! Cats can be asymptomatic carriers. Pseudomycetomas are characterized by subcutaneous nodules with draining tracts and

grains due to dermatophyte infection of the dermis and subcutaneous tissue. It is most commonly seen in Persians.

H. Diagnosis:

1. Wood’s Lamp Examination:

a. UV light filtered through cobalt or nickel filter.

1. Requires several minutes to warm up.

2. May take 5 minutes for hair to fluoresce.

b. 100 watt Black-Ray lamp is best.

c. The only species of veterinary importance that fluoresce is Microsporum canis. Less than 50% of Microsporum canis strains cause fluorescence.

d. Tryptophan metabolites on infected hairs produced by fungus causes fluorescence (the fluorescence is not present in scales or crusts).

e. Color seen: apple-green on hair shaft. Be sure that the hair root (portion present inside the follicle) also fluoresces.

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f. False positive results can be caused by: medications on hairs; scales. The false positive fluorescence is not apple-green.

g. Negative results do not rule out dermatophytosis.

h. Iodine destroys fluorescence.

i. The wood’s lamp is useful to select suspected hairs for fungal culture.

j. It is not a definitive diagnosis. Important Facts Only 50% of Microsporum canis strains fluoresce under the wood’s lamp. Therefore,

negative results do not rule out dermatophytosis. The wood’s lamp is helpful to select suspected hairs for direct examination and

fungal culture.

2. Direct Examination of Hair and Scales:

a. Wet scalpel blade with water (if using KOH) or apply some mineral oil to the scalpel blade. Scrape the periphery of the lesions or;

b. Pluck hairs (with roots) with a small hemostat. Be sure to select broken hairs.

1. Wood’s lamp can help select hairs.

2. These can be negative but culture positive.

c. Place the material on a slide with a few drops of “clearing” solution.

1. 10% KOH; let stand for 30 min. or gently heat for 15 to 20 seconds.

2. 10% KOH and DMSO (Pitman Moore); do not heat.

3. Chlorphenolac: chloral hydrate 50G; liquid phenol 25 cc; lactic acid 25 cc. Examine immediately; do not heat. It is also good 24 hours later.

4. Samples can also be examined using just mineral oil, if not a lot of scales are present.

d. Examine under low power for “sick” hairs, then examine at 40X.

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1. Use coverslip.

2. The “clearing” preparations are caustic to microscope lens and skin.

e. Ectothrix spores form on outside of hair shaft – they are very refractile balls and are not pigmented.

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f. Hyphae can be found from skin and hair samples.

1. Do not confuse with cotton fibers, thread, fiberglass, etc.

2. They are very difficult to see.

g. Remember, a negative direct examination does not rule out dermatophytosis.

Important Facts

A negative direct examination does not rule out dermatophytosis! Accurate sample collection increases your chances to find the organisms.

Pluck the abnormal hairs (broken) located at the periphery of the lesion. Samples can be visualized using 10% KOH or simply mineral oil (if not a lot of

scales present). Look for spores and/or hyphae parasitizing the outside of the hair shaft (ectothrix).

3. Fungal Culture: It is the only reliable method to make a diagnosis.

a. Specimen Collection:

1. Hair: most commonly cultured specimen.

a. Obtain broken hairs from the periphery of active lesions.

b. Clean gently with 70% alcohol to reduce contamination.

c. Gently pluck hair in the direction of growth to remove the root.

2. Skin:

a. Clean gently with 70% alcohol.

b. Scrape the periphery of active lesions with sterile scalpel.

c. Dermatophytes can usually be cultured, not only from the periphery of the lesion, but also from adjacent normal skin.

3. Nails:

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a. Difficult to culture.

b. Clean with 70% alcohol.

c. Scrape nail bed on most ventral portion of the nail plate.

d. A more successful, but more traumatic technique is to avulse the nail or clip a large portion of the nail as close to the nail bed as possible for culture.

4. Kerions, pseudomycetomas.

a. Cultured by sterile biopsy.

b. Histopathology is important for the diagnosis as well.

5. MacKenzie toothbrush technique: It is used for asymptomatic carries.

a. Brush the animal’s hair coat vigorously using a sterile toothbrush. b. Brush multiple areas of the body.

b. Deposit the sample firmly (do not imbed) on the culture media.

c. If no previous medications, fungal growth may occur in 5 to 14 days.

d. However, it may take 3 weeks if the animal is on therapy.

e. Color change in the media may occur in 4 to 7 days.

f. Keep media at room temperature and examine it daily.

g. Media:

1. Sabouraud’s dextrose agar : used for all fungal cultures.

2. Rapid sporulation Media (RSM ): encourages formation of macroconidia used to identify the fungus. Glucose and bromothynol blue are indicators. Dermatophytes turn medium blue-green early.

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3. Dermatophyte Test Media (DTM ):

a. Sabouraud’s Dextrose Agar.

b. Cyclohexamide – inhibits saprophytic fungal growth.

c. Gentamicin, chlortetracycline – inhibits bacteria growth.

d. Phenol red – pH indicator.

1. Dermatophytes utilize protein first; producing ammonia and an alkaline pH. Therefore, the media turns red at FIRST evidence of colony growth (usually the colony is visible at 7 to 14 days).

2. If dermatophytes stay on plates too long; they deplete the protein and begin utilizing the carbohydrates and the media turns back to yellow.

3. Dermatophyte colonies are white to light tan in color and fluffy or granular in consistency.

4. Contaminant (sapropfhytic) fungi utilize carbohydrate first. So the media will stay yellow at the first sign of colony growth. Some species of Alternaria, Aspergillus and Penicillium will turn the media red after the colony has been growing for a short period of time. Usually the colonies are green to black in color. (Pigmented colony is not a pathogen). Microscopic evaluation of the colony is useful to make the diagnosis.

4. Stained Slide Preparation:

a. Examine all suspect colonies from DTM, Sabouraud’s or RSM.

b. Pick up the surface of the colony with a clear acetate tape. Spread on glass slide flooded with lactophenol cotton blue. Add more lactophenol cotton blue; add a coverslip.

c. Examine for the presence of macroconidia.

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Important Facts

Fungal culture is the most reliable diagnostic test for dermatophytosis.

Use the same technique previously described to collect samples for direct examination. However, do a dry scraping (do not put water or oil on your scalpel blade).

MacKenzie toothbrush technique should be used to collect hair samples from asymptomatic cats.

DTM, Sabouraud’s or RSM media can be used.

Keep your media plate at room temperature and examine it daily for fungal growth.

Change in media color occurs before macroscopic evidence of colony growth (4 to 5 days).

Visible fungal growth occurs in 7 to 14 days if the animal has not been treated.

However, it may take up to 3 weeks before any growth can be seen. So, wait at least 3 weeks before giving a negative result.

Dermatophytic colonies are white or light tan in color, saprophytic colonies are usually pigmented (green, black).

Examine all colonies under the microscope for the characteristic features of the fungal macroconidia.

I. Treatment: Reasons to treat – a) zoonotic disease; b) decrease spread to other animals; c) decrease environmental contamination; d) hasten recovery from infection.

1. Topical:

a. Single Lesions:

1. Spot treatment with topical antifungal creams or lotions.

a. Clip affected area a few inches beyond the edge of the lesion.

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b. Miconazole (Conofite), clotrimazole, cuprimyxin, haloprogin, cyclopirox, povidone-iodine, thiabendazole.

c. Apply 2 times daily.

d. Do not use combination product with steroids.

e. Whole body treatment with topical dip is helpful (Lime-sulfur, enilconazole – not available as dip solution in the US).

f. Never spot treat cats! Remember they can be carriers.

b. Multiple Lesions:

1. Clip affected areas.

2. Treat the entire body every 5 to 7 days.

a. Lime sulfur – diluted 4 oz/gal.

b. Enilconazole (20 mls/liter) – labeled as environmental product – Clinafarm.

1. Dilute to 0.2%.

2. Apply about 100 ml to the cat twice weekly.

c. Chlorhexidine 2% - 25 or 50 ml/liter.

d. Povidone – iodine shampoo.

e. Chlorhexidine shampoo.

f. Nizoral (ketoconazole) shampoo – bath 2 to 3 times per week.

3. Never use solely topical therapy to manage feline dermatophytosis. Important Facts .Never spot treat cats! Remember, they can be health carriers Clip the affected area and surroundings every time you are using topical therapy.

Whole body dip is a good adjuvant therapy for cases associated with multiple lesions. Lime sulfur and enilconazole have been shown to have better effect than the other

topical products

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2. Systemic:

a. Griseofulvin:

1. Fungistatic – effective only against dermatophytes.

2. It is deposited in the epidermal layer of the skin and dermal appendages as they are formed.

3. Adverse reactions:

a. It is teratogenic.

b. With the exception of vomiting (divide BID and give with food), nausea, and diarrhea, signs of toxicity are rare.

c. It can be hepatotoxic and, elevated liver enzymes can be noted during therapy.

d. Cats may be more sensitive:

1. Panleukopenia – may be associated with FIV positive cats or it may be idiosyncratic (CBC every 2 to 3 weeks).

2. Angioedema.

3. Ataxia.

4. Depression, fever.

5. Anorexia, vomiting, diarrhea, lethargy (these are most commonly seen).

6. Side effects may be more common in Persians, Himalayans, Siamese, and Abyssinians.

4. Dose:

a. Fulcin V/F (microsized, Schering): 50 to 150 mg/kg per day divided BID or QD. We usually use 50 mg/kg/day.

b. GrisPEG (ultramicrosized, polyethylene glycol base): 5 to

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10 mg/kg divided BID.

c. Pediatric suspension is available.

d. Give with fat to improve absorption.

e. Do not use in animals less than 6 weeks of age.

b. Ketoconazole:

1. Use it only for failed griseofulvin therapy – not just re-infection.

2. Recommended dose is 10 mg/kg/day.

3. Give with an acidic or acid-producing meal (canned food, tomato juice).

4. Hepatitis, adrenal suppression and decreased testosterone levels have been reported.

c. Itraconazole:

1. 10 mg/kg once daily for cats.

2. 5 mg/kg once daily for dogs.

3. More expensive.

4. Fewer sides effects than ketoconazole:

a. Elevated liver enzymes.

b. Cutaneous vasculitis in dogs.

5. More rapid cure than griseofulvin in cats.

3. Length of Therapy:

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a. Until one negative culture is obtained.

b. Usually a minimum of 7 to 8 weeks of therapy. Therefore, it is important to know what you are treating.

Important Facts

Systemic therapy should always be considered in the management of dermatophytosis.

Griseofulvin, ketoconazole or itraconazole can be selected.

Remember! Cats can develop bone marrow suppression with griseofulvin. FIV cats are more susceptible. Perform a CBC every 2 weeks.

Itraconazole is fairly safe in cats and it is more efficacious than ketoconazole.

Be sure to treat until one negative culture is obtained!

Usually, a minimum of 7 to 8 weeks is necessary to obtain cure.

4. Environment: Remember the environment is an important source of infection. a. Vacuum thoroughly.

b. Steam cleaning will not kill fungal spores unless bleach is added.

c. Clean areas with dilute bleach (1 part of bleach to 10 parts of water) daily.

d. Enilconazole (Clinafarm) is an effective environmental spray – use every 2 weeks.

e. Virkon S – poultry and swine environmental disinfectant.

f. Change heating and air conditioning filters.

g. Clean vents.

Important Facts

Always recommend treating the environment!

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Under ideal environmental condition, fungal spores can survive in the environment for more than 1 year!

Always recommend vacuum cleaning. The environment can be treated with bleach (1:10 dilution) or enilconazole. Change heating and air conditioning filters. Clean vents.

5. Multiple Cat Households/Catteries:

a. Perform toothbrush fungal cultures (Mackenzie toothbrush technique) on all cats in cattery and all animals in the household.

b. Segregate infected from uninfected cats:

1. Reculture negative cats as in most instances they will be positive with Microsporum canis when recultured.

2. Or assume all are culture positive.

c. Isolate cattery.

d. Clip entire coat (especially long-haired cats) including whiskers (remove infected hairs, stimulates new hair growth):

1. Dispose of hair properly.

2. Repeat monthly.

3. Clipping may worsen clinical signs, initially.

e. Topical and systemic therapy.

f. Treat until all cats are culture negative at least once.

g. Treat the environment.

h. Preventive measures:

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1. Isolate and culture all new cats.

2. Pending culture results, dip all new cats weekly.

3. Culture and dip cats returning from cat shows.

6. Fungal Vaccines:

a. It is not effective as preventative vaccines.

b. It does have some efficacy in therapeutics.

c. It is not known if cats truly become culture negative.

d. It is not recommended for pregnant queens or immunosuppressed cats.

e. Major side effect is a sterile abscess at the site of inoculation.

Important Facts

Fungal vaccines do not prevent infection.

They can be used as adjuvant therapy.

It is not known if cats truly become culture negative.

So, be sure to perform a fungal culture after treating a case with fungal vaccine.

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