A 4 year 11-month-old, male neutered, mixed breed dog presents for a 6-month history of hair loss and skin lesions mainly affecting the muzzle, front limbs, and ears.
Dr. Matthew Levinson, DVM, DACVD
Pet Derm, Chicago Illinois
History:
The owner reports the clinical signs were first seen during the summer while visiting relatives in a rural area in Missouri. When the dog was initially taken to be assessed by his primary veterinarian, there were concerns about a possible cutaneous vasculitis based on the distribution of the hair loss and associated erythema to the affected skin. Dermatophytosis is prevalent in infected animals, and specific diagnostic tests are essential for accurate diagnosis. Several treatments we tried which includes, prednisone, doxycycline, niacinamide, pentoxifylline, and oclacitinib.
At initial presentation, the dog was currently receiving prednisone 20mg (0.6mg/kg) daily. Owner mentioned that the prednisone provided partial relief and improvement at first but then the lesions began to spread and worsen.
Exam:
T: 102.5F
P: 90
R: 16
mm: pink, moist
CRT: 1-2 seconds
On physical exam, the patient had marked alopecia on the muzzle and bridge of the nose with erythema and crusting appreciated in those affected areas. Nasal planum was unaffected. The pinnae were erythematous with adhered white scale and marked crusting seen on the pinnal margins. Alopecia, erythema, and scale also noted around the declaws, medio-dorsal aspect of the right front paw, carpal, and tarsal regions.
Image above shows alopecia, erythema, and crusting on the muzzle extending up the bridge of the nose. Crusting on the pinnal margins. Hypotrichosis and erythema on the medial dorsal aspect of the left front paw and 2nd digit.
Diagnostics:
Initial diagnostics included skin cytology and ringworm polymerase chain reaction (PCR) assay and fungal culture (dermatophyte test medium, DTM). Achieving a negative fungal culture is crucial to confirm complete cure, as clinical cure does not always indicate mycological cure.
Cytology of the affected skin revealed corneocytes, scarce acantholytic cells, nuclear streaming, neutrophils, 0-2 cocci, and 4-10 fungal spores per oil immersion field.
Assessment:
Dermatophytosis (Trichophyton spp.)
Treatment plan:
This patient was started on a systemic therapy with an oral dose of itraconazole 6.5mg/kg daily, a twice weekly topical shampoo containing 2% chlorhexidine/2% miconazole, and a daily application of a topical mousse containing 2% chlorhexidine/2% miconazole. There was a discussion with the owner that dermatophytosis is zoonotic and the importance of environmental decontamination to limit fomite potential and re-infection.
Follow up:
Dermatophyte PCR and fungal culture confirmed Trichophyton mentagrophytes. and results were relayed to owner. The dog presented 4 weeks after initial presentation and the owner reported at that time marked reduction in the crustiness and erythema of the affected lesions. Exam findings at that time showed while the affected areas were still alopecic, the affected skin was no longer erythematous, and no crusting appreciated. DTM was submitted to see to confirm whether there was still positive or negative growth of T. mentagrophytes.
Environmental decontamination is crucial in preventing reinfection, and the efficacy of disinfectants against infective spores plays a significant role in this process.
Six weeks after initial presentation, fungal culture results were negative for fungal growth. The itraconazole dosing was tapered to 3 days a week (Mondays, Wednesdays, Fridays) and advised to continue with 2% chlorhexidine/2% miconazole shampoo twice weekly. The 2% chlorhexidine/2% miconazole topical mousse had been discontinued by the owner at that time.
The dog returned 10 weeks after initial presentation, marked hair regrowth was appreciated at the previous affected sites. Another DTM was submitted for a second negative culture.
Twelve weeks after initial presentation DTM results were negative. Given the dog has had two negative cultures and lesions have fully healed with no evidence of recurrence, the itraconazole was discontinued at that time.
Discussion:
Trichophyton mentangrophytes, is a dermatophyte fungus that commonly affects dogs, causing dermatophytosis (ringworm). This fungal infection is zoonotic, meaning it can be transmitted between animals and humans. The primary source of infection for T. mentagrophytes is often rodents, rabbits, hedgehogs. Most Trichophyton infections are suspected to be due to contact with infected rodents or their nest which was likely the case with this dog. Canine and feline dermatology plays a crucial role in diagnosing and treating dermatophyte infections in small animals, particularly cats.
The clinical presentation of dermatophytosis can include hair loss, papules, scale, crusts, erythema, hyperpigmentation and changes in nail growth/appearance. Lesion may be annular or asymmetrical with pruritus often being variable. Nodular lesions may also develop in some cases. In many cases of T. mentagrophytes infections, the lesions may first develop on the muzzle, paws, and forelimbs as dogs are often coming into contact by digging or placing their muzzle in infected rodent nests/burrows. T. mentagrophytes infections often result in a much more dramatic scaling, crusting, and even scarring of the affected areas on the skin. Cytokines play a significant role in the immune response to canis infection, influencing the body’s ability to control fungal growth and inflammation.
When it comes to diagnosing dermatophytosis there is not one diagnostic test that is considered as the gold standard. There are several ways to help diagnosis dermatophytosis which includes:
Microscopic examination which involves examining hair plucks or superficial impression smears of the affected areas on the skin can help identify arthrospores and hyphae. Wood’s lamp examination is not useful in the case of T. mentagrophytes as it does not fluoresce. Fungal cultures using dermatophyte test medium (DTM) or Sabouraud Dextrose Agar and cultivating samples for 1-3 weeks can help identify characteristic growth pattens for T. mentagrophytes. PCR (Polymerase chain reaction) assays can help detect fungal DNA providing a more rapid diagnosis. Lastly, histopathology via skin biopsy samples of the affected areas can help identify fungal elements using special stains (periodic acid Schiff or Grocott methamine silver). Cellular immune responses are crucial in preventing uncontrolled fungal growth and susceptibility to dermatophyte infection.
Treatments include both topical and systemic antifungals. Topical treatments of a twice weekly application of lime sulfur, enilconazole, or miconazole/chlorhexidine shampoo ca be effective in the treatment of generalized dermatophytosis. As for systemic antifungal therapies, itraconazole and terbinafine were found to be the most effective for resolving dermatophytosis. The diagnosis and treatment of microsporum canis infection in cats involve similar approaches, with a focus on effective antifungal therapies and immune response management.
Environmental decontamination should also be considered for the primary purpose of preventing fomite contamination and potential false positive fungal culture results. Infection from the environment alone is rare and minimizing contamination can be accomplished through clipping hair of affected lesions, topical therapies, and routine cleanings. Dermatophytosis, as a skin disease, significantly impacts small animals, necessitating thorough diagnosis, treatment, and preventive measures.
References:
- Moriello, K.A., Coyner, K., Paterson, S., Mignon, B. (2017). Diagnosis and treatment of dermatophytosis in dogs and cats: Clinical Consensus Guidelines of the World Association for Veterinary Dermatology. Veterinary Dermatology. Volume 28, Issue 3, Pages 266-e68.
- Scott, D.W., Miller Jr, W.H., & Griffin, C.E. (2013). Muller and Kirk’s Small Animal Dermatology (7th). Elsevier.
- Moriello, K.A. (2015). Dermatophytosis (Ringworm) and dermatomycoses in dogs and cats. In Infectious Diseases of the Dog and Cat (4th).
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