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Feline dermatophytic mycetoma (pseudomycetoma)

An uncommon condition, feline dematophytic mycetoma is typically caused by Microsporum canis and is most often described in Persian cats

17 April 2018, at 11:50a.m.

Feline dermatophytic mycetomas (pseudomycetoma) are uncommon deep dermal and subcutaneous fungal infections caused by dermatophytes (Gross et al., 1992). Mycetomas are defined by a characteristic triad of nodular swelling, draining sinuses, and grains in tissue. The grains are micro-aggregates of the causative organism. Causative agents in mycetomas may be bacterial or fungal. In true fungal mycetomas (eumycetomas), the organisms are saprophytic fungi in the environment, introduced into the dermis by trauma such as a thorn. 

The disease as seen in cats is almost exclusively caused by Microsporum canis, a unique form of dermatophytosis, with the majority of cases described in Persian cats, suggesting a genetically programmed selective immunodeficiency (Gross et al., 1992). In addition, Persian cats are known to be predisposed to the more common superficial dermatophytosis, (Paterson, 2008). 

There are differences in granule formation of the micro aggregates of M. canis when compared to eumycetomas (Miller et al., 2013). This was first noted following examination of a 25-year-old woman with an abscess in her arm, which yielded M. canis (Ajello et al., 1980). As a result of this case, and examination of previous case material, it was suggested that mycelial aggregates formed by dermatophytes are best referred to as ‘pseudogranules’ and that the term ‘pseudomycetoma’ should be applied to such dermatophyte infections. Clinically, however, the feline disease as described here demonstrates the triad of lesions associated with other mycetomas, and causes similar problems in diagnosis and especially treatment.

FIGURE 1 Swelling with draining sinuses in the gluteal region of a four-year-old Persian male cat. The cat had not previously responded to a long course of cephalexin before diagnosis
FIGURE 1 Swelling with draining sinuses in the gluteal region of a four-year-old Persian male cat. The cat had not previously responded to a long course of cephalexin before diagnosis

Clinical features 

Firm irregular nodules develop, gradually enlarging to coalesce in the dermis and underlying subcutaneous tissue; the nodules are not pruritic or painful. Commonly, if untreated, the larger nodules may ulcerate (Figures 1 and 2) and discharge a seropurulent to granular material, which may contain yellow grains. Liquefaction of the panniculus may impart an oily appearance to the exudate (Figure 2). The affected area is frequently, but not invariably, the dorso-lumbar region (Figure 1), leading to the speculation that traumatic implantation of M. canis resulting from cat fights could initiate lesions (Gross et al., 1992). 

More conventional lesions of dermatophytosis may be evident elsewhere, such as patchy alopecia, circular areas of broken hairs, and scaling. Some affected cats may be inapparent carriers of the dermatophyte (Gross et al., 1992). Systemic signs are uncommon. There is a zoonotic risk, which can persist for months due to the protracted treatment required (Paterson, 2008).

Differential diagnosis

  •  Other opportunistic fungal saprophytic infections. These are either uncommon or not present in the UK (Miller, 2010) 
  • Bacterial infection (botryomycosis) 
  • Deep pyoderma 
  • Neoplasia 

Diagnosis 

The history and clinical signs are suggestive, particularly in Persian cats. Steps to diagnosing the condition include: 

  • Wood’s lamp examination of entire body 
  • Examination of broken hairs microscopically 
  • PCR examination of hairs 
  • Cytological examination of fine needle aspirate or impression smears of exudate. Pyogranulomatous inflammation is seen, which may reveal fungal hyphae and grains 
  • Cytological examination of crushed grains may reveal hyphae
  • Fungal culture from lesion sites elsewhere if present, or McKenzie brush technique (sterile toothbrush whole body) in the absence of obvious lesions
FIGURE 2 Close-up of draining sinuses from the same cat as Figure 1. The discharge is seropurulent with an oily appearance, suggesting that the infection involves the panniculus
FIGURE 2 Close-up of draining sinuses from the same cat as Figure 1. The discharge is seropurulent with an oily appearance, suggesting that the infection involves the panniculus

For a definitive diagnosis, histopathological examination and tissue culture are required. Some colonisation of hair shafts with dermatophytic spores and hyphae may be seen. The main diagnostic lesions are found in the sub-follicular dermis and panniculus where there is a nodular or diffuse pyogranulomatous dermatitis. Large amorphous aggregates of fungal hyphae with thick-walled dilatations resembling spores are frequently present. Identification of these structures is facilitated by periodic acid-schiff or Grucott methenamine silver stains. The fungal aggregates are embedded in amorphous eosinophilic material (Splendore-Hoeppli reaction) representing an antigenantibody reaction to infectious agents (Gross et al., 1992).

Sterile tissue culture for M. canis can be performed. In those countries where systemic fungi occur, tissue culture should not be performed until they have been ruled out by histopathological examination, as culture of some organisms may be dangerous (Gross et al., 1992). 

Clinical management and prognosis 

Solitary nodules should be excised if possible and may result in a cure. If a cure is to be obtained, a combination of surgical excision and medical treatment has the best chance of success. Itraconazole 10mg/kg every 24 hours by mouth is the medical treatment of choice and is licensed for dermatophytosis in the UK. Treatment may be required for many months (Medleau and Rakich, 1994). Terbinafine (not licensed) 30-40mg/kg should be given every 12 hours by mouth (Hnilica and Paterson, 2017). 

Some cases do not respond and therefore the prognosis is guarded with relapse common even in those cases that do respond initially. The difficulties of treatment are well illustrated in the case of an eight-year-old female neutered Persian cat that failed to respond to treatment of more than four years, comprising repeated surgical resection, long courses of griseofulvin, itraconazole and finally eight months of terbinafine (Bond, 2001).

David Grant, MBE, BVetMed, CertSAD, FRCVS, graduated from the RVC in 1968 and received his FRCVS in 1978. David was hospital director at RSPCA Harmsworth for 25 years and now writes and lectures internationally, mainly in dermatology.

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