Nomenclature In Fungal Diseases
Ross Bond BVMS PhD DVD MRCVS DipECVD
Department of Veterinary Clinical Sciences
Royal Veterinary College, Hawkshead Lane
North Mymms, Hatfield, Herts AL9 7TA
Fungi are eukaryotic, unicellular or filamentous, multi cellular hyphal, mostly non-flagellate, nutritionally absorptive organisms with cell walls containing chitin and beta-glucans that reproduce sexually or asexually. The fungal kingdom is composed on 4 phyla, 103 orders, 484 families, 4979 genera, and 56,360 species, although it has been suggested that 1-1.5 million species might exist (Hawksworth et al 1995). Although the fungal kingdom contains important pathogens of animals, they are also relevant as plant pathogens, saprobes and biotechnological tools.
The diversity of fungi creates complex issues relating to taxonomy of the organisms and to the nomenclature of diseases caused by their invasion of animals. The vocabulary of mycology is also complex and contains many terms frequently unfamiliar to clinicians better versed in animal rather than fungal physiology. The application of molecular biology tools to fungal phylogeny and taxonomy has resulted in changing classifications which may confuse even professional mycologists, let alone veterinary clinicians faced with occasional cases caused by fungi. The situation is further complicated by variable pathological responses amongst host species leading to diverse clinical signs. Thus, there is an ongoing requirement for standard nomenclature to be adopted and propagated wherever possible, to promote accuracy and prevent confusion.
The International Society for Human and Animal Mycology created a sub-committee on mycoses nomenclature that reported in 1980 and in 1992 (Odds et al 1992). The purpose of this paper is to alert readers to the existence of these recommendations and to discuss how they might be best applied to selected fungal skin diseases of veterinary species. Terms written in bold text are approved and defined by the sub-committee.
A mycosis is an infectious disease caused by a fungus. The term ‘fungal disease’ is preferable to ‘mycotic disease’ as the former is less ambiguous; consider the example of ‘mycotic dermatitis’ being used as a synonym of dermatophilosis, a disease caused by the bacterium, Dermatophilus congolensis. A systemic mycosis is a fungal infection involving one or more deep solid organs. A disseminated mycosis is a mycosis spread haematogenously to involve two or more non-contiguous solid organs (and is therefore a subtype of systemic mycoses).
The general principle in fungal disease nomenclature promoted by ISHAM is to provide a description of the specific pathology and the causative organism (if known) in the format ‘pathology A caused by fungus X’, or ‘fungus X pathology A’. Adding the suffix ‘-mycosis’, ‘-osis’,’ -iasis’, or ‘-asis’ to the name of a fungus should be avoided unless this gives an unambiguous identity to a specific clinico-pathological process and also the casual agent. Certain names are, however, retained for traditional reasons; eg dermatophytosis, cryptococcosis, candidiasis.
The mycoses most familiar to veterinary dermatologists are those caused by dermatophytes and Malassezia spp. Dermatophytosis is an infection due to a fungus of the genera Microsporum, Trichophyton, or Epidermophyton. It must be differentiated from the term dermatomycosis, a more general term denoting an invasive fungal infection of the skin. Considering the recommendations referred to previously, it should be obvious that the terms ‘microsporosis’ and ‘trichophyton’ are to be avoided; for example, ‘dermatophytosis caused byMicrosporum canis’ is preferable. The term kerion denotes a severe form of dermatophytosis with deep, suppurant, inflammatory lesions.
Malassezia spp. have gained recognition as important commensal organisms and
cutaneous pathogens of dogs in recent years. The term ‘Malassezia dermatitis’
fits well with the ‘fungus X pathology A’ pattern. Although ‘dermatitis’
can be regarded as a vague term, the pathology in dogs seldom varies from a
hyperplastic, spongiotic, superficial perivascular to interstitial dermatitis
pattern. Malassezia does not conform to the suggested nomenclature. Pityrosporum
(hence ‘pityrosporosis’) was used as an alternative name but since
Malassezia was published before Pityrosporum, the former nomenclature priority.
There is greater potential for confusion resulting from the terminology of subcutaneous mycoses. Frequently nodular in clinical presentation, these constitute a heterogeneous group of diseases caused by a wide variety of fungi that usually invade the subcutaneous tissues after traumatic implantation. Mycetoma is a (i) localized deforming tumor with (ii) sinus tracts and (iii) ‘grains’ or ‘granules’ of the etiological agent. Two distinct groups are recognized: actinomycetic mycetomas associated with bacterial filaments on 1 um diameter or less, and eumycotic mycetomas (eumycetoma) associated with septate, hyphae of at least 2-4 um diameter. The diagnosis is dependent upon demonstration of the sinuses and granules. Periodic acid-schiff stain may assist in differentiating bacterial filaments from fungal hyphae when granules are observed in histological specimens. When the granule is composed of a pure population of non-filamentous bacteria, a diagnosis of bacterial granuloma (‘botryomycosis’) should be made (Chandler et al 1980).
‘Dermatophyte pseudomycetoma’ is a confusing term used to describe a form of dermatophytosis, this is, amongst the veterinary species, primarily reported in Persian cats with M. canis as the etiological agent. In human medicine, the term ‘pseudomycetoma’ has been adopted by some authors due to both clinical and histopathological differences from classical mycetomas. Clinically dermatophyte-associated lesions have tended to occur on the scalp, lacked sinus tracts and often occurred in association with widespread superficial dermatophytosis; eumycotic mycetomas are normally found on the hands and feet and are associated with traumatic implantation of the fungus (Chen et al 1993, Rinaldi et al 1983). Histologically, subtle differences in the ‘granule’ have been described in human cases; dermatophyte ‘pseudogranules’ were characterized by sequential development of small clusters of filaments to large mycelial aggregates, abundant Splendore-Hoeppli reaction, less compact mycelium, absence of a ‘cement’ and evidence of dermal penetration following follicular rupture (Ajello et al 1980). The author is unaware of detailed reviews of case series from veterinary species when granule morphology was subjected to detailed histological comparison. ‘Fungal granuloma caused by M. canis’ or ‘granulomatous dermatitis and panniculitis caused by M. canis’ fit more closely with the suggested nomenclature than ‘pseudomycetoma’. The dematiaceous fungi are a large, heterogenous group of moulds that have a brown or black melanotic pigment in their cells walls. Dematiaceous fungi are associated with mycetoma, phaeohyphomycosis and in humans, chromoblastomycosis (where the invading fungal cells contain septae in two perpendicular planes). Phaeohyphomycosis is a general term used describe infection by dematiaceous fungi but without reference to a specific pathology. Histologically, the pigmented, septae hyphae are scattered or form small aggregates but do not form granules (cf mycetoma with dark grains) (Chandler et al 1980). Some pathologists do not make this distinction when reporting on biopsy specimens from dogs and cats.
Veterinary clinicians and dermatopathologists should endeavor to adopt a standard nomenclature to ensure accurate communication and to facilitate the accumulation of experience of and expertise in these difficult diseases.
REFERENCES:
Ajello L, Kaplan W, Chandler F. Dermatophyte mycetomas: fact or fiction? In:
Fifth International Conference on Mycoses; 1980; Washington: Pan American Health
Organization; 1980: 135-140.
Chandler F, Kaplan W, Ajello L. A colour atlas and textbook of the histopathology of mycotic diseases. London: Wolfe Medical Publications Ltd; 1980.
Chen AW, Kuo JW, Chen JS, Sun CC, Huang SF. Dermatophyte pseudomycetoma: a case report. Br J Dermatol 1993; 129: 729-32
Hawksworth D, Kirk P, Sutton B, Pegler D. Dictionary of the fungi, Eighth edn. Cambridge: CAB International; 1995
Odds F, Arai T, Disalvo A, et al. Nomenclature of the fungal diseases: a report and recommendations from a sub-committee of the International Society for Human and Animal Mycology (ISHAM). Journal of Veterinary and Medical Mycology 1992; 30: 1-10
Rinaldi MG, Lamazor EA, Roeser EH, Wegner CJ. Mycetoma or pseudomycetoma? A
distinctive mycosis caused by dermatophytes. Mycopathologia 1983; 81: 41-8.
8909 Iverleigh Court 
Potomac, Maryland
20854
301-983-8387 800-755-4738
Fax 301-365-0191
E-mail dermapet@aol.com