Superficial Mycoses

By | November 15, 2015

These are fungal infections that involve the skin, hair and nails.

Dermatophytosis (ringworm)

  • dermatophytosis is the term used for infection with species of Microsporum, Trichophyton or Epidermophyton. Dermatomycosis is the term used for infection with non-dermatophytes
  • almost all cases of dermatophytosis in the dog or cat are caused by fungi of the genera Microsporum or Trichophyton
  • the fungi attack the keratin of the hair only in the anagen phase of the growth cycle

Clinical features

  • dermatophytosis in the dog is rare in the United Kingdom
  • prevalence () in the UK is:
  1. canis 65%
  2. mentagrophytes (mouse or vole) 23%
  3. mentagrophytes var. erinacei (hedgehogs) 5.2%
  4. persicolor (mouse or vole) 4%
  5. verrucosum, T. terrestre, M. gypseum (soil) 1.6%
  • infection is by direct contact with an infected animal or contaminated material from the environment
  • in healthy animals dermatophytosis is a self-limiting disease; young animals which lack acquired immunity or any animal with compromised immunity (particularly cell-mediated) are susceptible
  • lesions are variable and include circular patchy alopecia (), erythematous plaques, folliculitis, scales and crusts. The lesions may be localized or diffuse, and occasionally involve large parts of the body — especially with T. mentagrophytes (). Infection may involve the nails (onychomycosis), which break off and subsequently grow back deformed. Other lesions resemble a histiocytoma, with severe inflammation and secondary infection (). Secondary infection may lead to hair loss and scarring

Diagnosis

  • Wood’s lamp examination: a useful screening test. Of the important pathogenic fungi, fluorescence is only seen in certain strains of M. canis
  • the fluorescence is due to a tryptophan metabolite produced by the fungus
  • it is important to warm the lamp up for 5 minutes to achieve the correct wavelength, and to examine the animal in a darkened room
  • affected hairs fluoresce bright green. False fluorescence may be caused by certain ointments, soaps and powders containing, for example, oxytetracycline
  • a negative test does not rule out dermatophytosis
  • potassium hydroxide preparation: suspect infected hairs are mounted in io% KOH or KOH-DMSO, and examined for fungal hyphae or arthrospores, which are seen as small, spherical, refractile bodies in chains or in a mosaic sheath around the hair. Staining with lactophenol cotton blue or blue-black ink may facilitate identification of the arthrospores
  • fungal culture: ideally fluorescent hairs or hairs with broken roots should be selected at the periphery of the lesion. In addition skin scrapings can be made from the centre of the lesion. Where there are no lesions material can be collected with a sterile toothbrush and transferred to the culture medium (Mackenzie brush technique)
  • the medium employed for culture is Sabouraud’s dextrose agar. Dermatophytes are identified by their colony morphology, and by characteristic macro-aleurospores, which are usually produced after approximately three weeks’ incubation at room temperature. Their identification is a specialized task and the details are not discussed in this text. In the hands of a competent veterinary mycologist, fungal culture is the most reliable means of diagnosing dermatophytosis in the dog and cat
  • dermatophyte test medium (DTM) is based on Sabouraud’s dextrose agar with phenol red added as an indicator. Pathogenic dermatophytes metabolize protein, producing alkaline metabolites which induce a red coloration in the medium. This change in colour occurs normally at between 3 and 7 days (occasionally up to 14 days). Saprophytic fungi metabolize carbohydrate first, and produce alkaline metabolites much later. Thus any colour change will occur after 14 days
  • DTM cultures should be examined daily and considered negative if there is no colour change before 14 days. They are best used in conjunction with plain Sabouraud’s dextrose agar, since used alone they do not allow identification of the dermatophyte species
  • biopsy: staining with periodic acid Schiff (PAS) will often reveal septate fungal hyphae and spores, either on hairs or within the hair follicle or stratum corneum

Differential diagnosis

  • superficial pyoderma
  • demodicosis
  • seborrhoea
  • contact dermatitis
  • histiocytoma
  • acral lick dermatitis
  • mast cell tumour
  • mycosis fungoides
  • pemphigus complex

Treatment

  • many cases will undergo spontaneous resolution
  • griseofulvin (Grisovin, Coopers Pitman-Moore) is the drug of choice for severe, widespread or chronic cases
  • the dose suggested is 20—100 mg/kg daily, with severe cases requiring the higher dose
  • the drug is given with a high-fat meal or with vegetable oil to facilitate absorption
  • do not administer griseofulvin to pregnant animals as it is teratogenic
  • clipping the coat, especially in long-haired cats, is essential (burn the clippings)
  • topical treatment with 0.2% enilconazole (Imaverol, Jannsen) in dogs or the sodium salt of benzuldazic acid (Defungit, Hoechst) in the cat is a useful adjunct to therapy
  • ketoconazole (Nizoral, Jannsen) is an alternative to griseofulvin, and has been used at a dose of 15 mg/kg. It does not have a product licence in the dog and cat, and side-effects (hepatotoxicity, vomiting and pruritus) have been reported

Control of dermatophyte infection in kennels and catteries

  • this is an arduous and time-consuming task
  • good communication with, and co-operation from, the owner is most important
  • all in-contact animals should be screened with the Wood’s lamp and by culture, using the Mackenzie brush technique
  • infected animals are treated with griseofulvin until negative on two consecutive cultural examinations one month apart
  • negative in-contacts are treated topically as above for two weeks
  • burn as much contaminated material in the premises as possible
  • effective disinfectants include formalin, 0.2% enilconazole, sodium hypochlorite and steam
  • paint woodwork to seal in infection
  • spores may remain viable for a year or more; thus environmental treatment should be as thorough as realistically possible

Candidiasis

  • infection of the mucous membranes and skin caused by Candida albicans
  • very rare
  • mainly dogs

Clinical features

  • ulceration (white-grey plaques) of the mucous lining of the cheeks and ventral surface of the tongue; there may be a foul-smelling discharge
  • vaginal discharge
  • otitis externa
  • acute moist dermatitis near mucocutaneous junctions
  • paronychia

Diagnosis

  • direct smear: stain with lactophenol cotton blue
  • culture: Sabouraud’s dextrose agar
  • biopsy: the organism may be seen in the stratum corneum after routine staining with haematoxylin and eosin (H and E) or with special stains such as periodic acid Schiff (PAS)

Differential diagnosis

  • stomatitis
  • acute moist dermatitis
  • causes of otitis externa
  • fold pyoderma
  • pemphigus vulgaris, bullous pemphigoid
  • dermatophytosis

Underlying factors

  • prolonged administration of antibacterial agents
  • immunosuppression or deficiency
  • concurrent debilitating diseases
  • persistent moisture

Treatment

  • correct underlying cause
  • topical, for example miconazole (Conaderm, C-Vet) or enilconazole (Imaverol, Janssen)
  • systemic, for example ketoconazole (Nizoral) orally (possible side-effects are vomiting, hepatotoxicity and pruritus) or amphotericin B (danger of nephrotoxicity; see systemic mycoses)
  • neither of the above drugs is licensed for use in the dog

Malassezia pachydermatitis (Pityrosporum canis)

  • M. pachydermatitis, a yeast, is a normal commensal of the ears which occasionally is associated with otitis externa
  • rarely a more generalized dermatosis may occur and Scott and Muller () have reported on a series of young West Highland white terriers which presented with a severe pruritic seborrhoeic disorder from which pure growth of the yeast was obtained on skin culture
  • it is suggested that these dogs have a genetically determined epidermal dysplasia predisposing to M. pachydermatitis

Diagnosis

  • aural swabs or skin scrapings followed by staining (Diff-quik, methylene blue) or culture in Sabouraud’s dextrose agar
  • biopsy may demonstrate the yeast in the epidermis

Treatment

  • many aural preparations contain antifungal agents
  • it is more important to identify and treat the underlying cause of the otitis
  • failure to treat the underlying cause will inevitably mean recurrence of pathogenic numbers of yeast organisms following apparently successful symptomatic treatment
  • oral ketoconazole (Nizoral, Janssen) was used successfully by Scott and Muller () in the generalized seborrhoeic form at a dose of 10 mg/kg. Cessation of therapy was invariably followed by relapse.

 

Selections from the book: “Skin Diseases in the Dog and Cat”. D. I. Grant, BVetMed (1991)