Dermatophytoses in Dogs and Cats

Author Abstract

Companion Notes

Overview on dermatophytoses in dogs and cats

Dermatophytes are pathogenic fungi

- causing a significant problem in 2 groups of animals

- shelter animals

- household pets with extensive exposure to an outdoor environment

- most common dermatophytes diagnosed in small animals

- Microsporum canis (most common sources are cats)

- in shelter cats infections with other dermatophytes is rare

- Microsporum gypseum (most common source is soil)

- Trichophyton mentagrophytes (most common source is rodents)

- infection is more likely in very young, geriatric, or stressed animals

Clinical Signs

- dermatophytes, because they are keratinophilic, localize to hairs and nails

- when they affect hairs, they lead to folliculitis

- papules and pustules which rapidly rupture, leaving the following:

- epidermal collarettes

- circular areas of alopecia

- crusts

- differential diagnoses for folliculitis

- Staphylococcal infection

- demodicosis, particularly in dogs

- in dogs with signs of folliculitis

- perform cytology to look for intracellular bacteria

- perform deep skin scrapings for demodicosis

- gray skin discoloration may be present with demodicosis

- due to comedone formation

- due to plugging of hair follicles filled with mites

- other causes of pustular disease not oriented to follicles

- such as pemphigus foliaceus

- nodular lesions

- sometimes occur when dermatophytes are inoculated into the dermis (injury)

- causing a severe inflammatory response and a nodular lesion called a kerion

- typically found on bridge of the nose of dogs that like to dig

- inflammatory response can persist even after dermatophytes die

- often require histopathology and culture for diagnosis

- impression smears can be diagnostic however

- dermatophytic mycetoma, also called pseudomycetoma

- manifests as nodules with draining tracts, typically on the back

- primarily occurs on Persian cats

- nail lesions

- nails and footpads can be affected

- infected nails become brittle and deformed

- especially in dogs with M gypseum infection

- differential diagnoses for deformed and brittle nails can include the following:

- systemic lupoid onychodystrophy

- genetically inherited dystrophies that are not immune mediated

- begin working up these cases by collecting samples of affected nails (nail clippings)

- before considering biopsies or other invasive tests

- submit for fungal culture

Diagnostic testing

- recently clinical consensus paper concluded there’s no gold standard for diagnosis

- diagnosis requires complementary methods to confirm a active infection

- direct examination of arthrospores on infected hairs

- fungal culture of hairs collected with a sterile toothbrush

- Wood’s lamp is recommended as a screening tool

- most M canis infections will fluoresce apple green

- reported positivity ranges: 91-100%

- ability to fluoresce develops after 1^st week of infection

- can persist at hair tips after resolution of the infection

- exam should start at the animal’s head

- then moving slowly back while holding the lamp 2-4 cm from the skin

- a false blue fluorescence can be seen with scaling and some topical products

- fungal culture

- procedure simply indicates the presence or absence of spores on the hairs

- results depends partly on sampling technique and the area selected for culture

- toothbrush approach may detect asymptomatic carrier animals

- pressing the toothbrush onto the plate is best way to maximize M canis growth

- and minimize contaminant growth

- direct examination by plucking hairs from margins of an existing lesion

- considered useful for specifically assessing if lesion may be caused by a dermatophyte

- however, hair plucks can still lead to negative results

- slides with 10 to 20 plucked hairs in mineral oil can be examined for arthrospores

- polymerase chain reaction (PCR); positive does not confirm active infection

- results of studies on PCR differ, depending on the PCR studied

- in 1 report of quantitative PCR (qPCR) of toothbrush fungal culture samples

(of lesions on shelter cats)

- found it to be a reliable test for confirming disease

- qPCR and fungal culture matched in 94% of cases

- qPCR correctly identified 2 uninfected cats

- in another study

- one step-PCR

- high accuracy for dogs (area under the curve [AUC] >90)

- moderate accuracy for cats (AUC = 78.6)

- nested PCR

- accurate for cats (AUC = 93.6)

- in dogs

- specificity: 94.1%

- sensitivity: 100%

- in cats

- specificity: 94.4%

- sensitivity: 94.9%

- in another study comparing nested PCR with direct microscopy and culture

- degree of agreement for nested PCR and direct microscopy: 94.4%

- degree of agreement for nested PCR and culture: 83.3%

- recent study reported PCR is less sensitive than previously reported

- but that it is more specific, thus increasing risk for false-negatives

Treatment requires topical treatment, systemic treatment, and environmental disinfection

- topical therapy is essential as dermatophytosis is transmitted by contact with arthrospores

- clipping was once thought a necessary part of treatment

- practice is currently being reconsidered

- whole-body clipping is stressful

- common microtrauma to the skin can worsen infection

- not necessary for short-coated animals

- dips, shampoos, and rinses

- lime sulfur dips still recommended in USA (efficacy shown in several studies)

- twice weekly use more effective than once weekly

- dip has residual activity

- common side effects in the author’s experience

- dryness

- yellow discoloration

- if clients object to the smell, other options available in USA

- combination of miconazole and chlorhexidine twice weekly

- most effective topical treatment

- topical enilconazole is effective but currently not available in USA

- generally well tolerated, but reported to cause adverse events in Persians

- in vitro studies support effectiveness of ketoconazole shampoos

- no clinical trial published

- in vitro studies showed good residual activity of 0.5% climbazole with chlorhexidine

- 1 study on topical terbinafine reported a good response

- systemic therapy

- most effective systemic treatments for both dogs and cats

- oral itraconazole

- long half-life in cats and a propensity to accumulate in hairs and skin

- allowing for use of pulse therapy, decreasing the cost of therapy

- most common dose for itraconazole in dogs and cats: 5 mg/kg sid

- avoid compounded formulations 

- generic itraconazole seems to be better

- therapeutic monitoring is helpful as concentration can be very variable

- with treatment failure, switch to human formulation

- at higher doses, reported to trigger vasculitis in dogs

- liquid formulation available for use in cats

- oral terbinafine; accumulates in hairs

- pulse therapy also possible

- excellent activity against dermatophytes

- shown effective and a suitable cheaper choice for shelter cats

- commonly used dose range for terbinafine: 20-40 mg/kg

- higher doses more effective

- ketoconazole is effective but is typically not well tolerated in cats

- best used for dogs and is typically prescribed at 5 mg/kg PO q12h with food

- fluconazole has poor activity against dermatophytes in vitro

- griseofulvin used historically but there’s safer and more effective choices

- lufenuron; studies have found no efficacy

- vaccines do not prevent development of dermatophytosis

- environmental decontamination

- use minimizes false-positive fungal culture results

- separating animals to minimize contamination has been advocated for decades

- do cautiously as it can be very stressful, particularly for the young

- minimize duration of isolation

- need for extended isolation can be decreased by weekly cleaning

- and topical therapy

- most important part is the actual hard cleaning

- that removes debris and hairs

- over-the-counter household detergents can be used for cleaning

- hard surfaces can be disinfected with 1:100 concentration household bleach

- or accelerated hydrogen peroxide

- for machine washing of soft fabrics use the longest cycle

Monitoring Treatment Success

- clinical cure does not always mean mycological cure

- monitor therapy

- establish a complete cure based on the following:

- resolution of clinical signs

- negative fungal culture

- extent of infection can be monitored with weekly cultures

- PCR can detect dead fungal organisms

- animals can pick up spores from a contaminated environment

- but they may have developed immunity and are not actively infected

- these animals may be a source of infection

- indicating further environmental decontamination may be warranted

- fungal culture can be repeated to make sure results are truly negative

- long-haired cats may be potential carriers

- outbreaks in a multicat household

- every cat should be cultured to find out which are truly negative

- and which are clinically normal but carrying arthrospores on their coats

“Dermatophytosis can represent a significant challenge in dogs of certain breeds, such as Yorkshire terriers. It is very important to go through the list of differential diagnoses and work up each case in a step-by-step logical manner.”