Section of Clinical Pathology, Faculty of Veterinary Science, University of Pretoria
Onderstepoort, Republic of South Africa
The reader should be warned at the outset that this paper is not a treatise on the systematics and/or diagnosis of fungal disease. The author is not a mycologist nor a microbiologist. Over a period of 25 years of cytological evaluation of Fine Needle Aspirates (FNA), impression smears and fluid concentrates (body cavity effusions, nasal flushes, trans-tracheal aspirates and bronchial lavage), the author has occasionally "stumbled" upon evidence of systemic fungal disease without that possibility actually featuring on the list of differentials (except as a remote possibility). Upon reflection, many of these cases presented with some tell-tale indication or diagnostic "trigger", which, if recognized, would have directed the search at a much earlier stage.
It is important, in this context, to recognize that it is accepted that fungal culture and lactophenol cotton blue (or PAS) staining will be a much better diagnostic method than "messing around" with rapid-Romanowski type stains. But, more often than not, the thought of "is this fungal?" has not entered the cytologist's mind and other "triggers" are required to create the "Hey, I wonder if..." situation.
Predisposition and diagnostic "triggers"
Probably the first trigger to be alert to is the occurrence of chronic disease, especially in breeds with a high prevalence of immunodeficiency diseases. Breeds that immediately come to mind are:
Gray collies (cyclic haematopoesis)
Irish setters (Beta-2-integrin adhesion deficiency)Doberman (Poorly-defined neutrophil function defect)
Miniature dachshund (Pneumocystis pneumonia)
Chronic disease, especially respiratory (nasal and even pneumonic), is often a trigger in its own right. Recurrent bouts of cystitis and epistaxis should always be considered as potential cases.
Pyrexia of unknown origin must always be considered as potential cases of systemic mycosis.
Young poor-doers (runts) should raise the index of suspicion.
Patients with diseases that are known to be immunosuppressive such as demodicosis, distemper virus, diabetes mellitus and canine ehrlichiosis, dermatophytosis (especially if difficult to cure) to name but a few. Obviously, FIV and even FeLV must always open the possibility of systemic mycotic infection.
A non-breed-related cystitis-spondylosis association with fungal disease is also observed from time-to-time.
Iatrogenic factors are also worth considering:
Prolonged antibiotic therapy
Regardless of these fairly obvious associations, most of the cases will still be diagnosed as total fortuitous "finds". There are, however, some further cytological triggers.
Cytological features (other than the organism)
Marked inflammation without any obvious cause (especially bacteria). This may occasionally be made more difficult to recognize if the patient has already been on antibiotics, as is so often the case.
Inflammation with a preponderance of macrophage/monocyte cell series present.
"Floccular" inflammatory cell aggregates that tend not to "smear" out easily.
Cytological features of the organism
Fungal spores and yeasts can resemble erythrocytes "that just stain badly"
Often they exhibit a negative staining (non-staining) "halo" or shell.
Sometimes, particularly with yeasts, there may be a vague bluish-staining "core" surrounded by poorly staining material.
The "budding" of yeasts is a classical feature but it may take the cytologist quite a few fields before the pattern strikes home.
Unusually vacuolar macrophages with "something" in the vacuoles.
Fungal mycelia may stain poorly (even negatively) with the rapid stains and consequently the septate nature may remain difficult to see. Any regularly, parallel-sided light-coloured "hair-like" structures should always be given a second look. These are often best visualized by examining the thicker portions of the slide where the negative staining is more obvious.
Conidia (chains and oval) and conidiophores ("lollipops") as well as septate and elongate macro and microconidia are often found in cytological material derived from external lesions (including the nose) and usually represent airborne contamination. The sheer number found may be the first "trigger" that there is something amiss.
There is absolutely no doubt that, when the index of suspicion is very high, it is to be advised to send some of the material for culture and lactophenol staining. There is, however, no harm in reprocessing some of the sample material (squash preparations if the material is grainy or floccular) and staining for longer and or with supravital stains like New Methylene Blue.
Most systemic fungal diseases are known or potential zoonoses and the diagnostician is cautioned to work carefully with the material.