Neurotropic Fungus – A Mini Review
Ajantha GS, Raghavendra D. Kulkarni
MBBS, MD (Microbiology), Associate Professor,
Department of Microbiology, SDM College of Medical
Sciences & Hospital, Sattur, Dharwad -580 009, India
MBBS, MD (Microbiology), Professor and Head,
Department of Microbiology, SDM College of Medical
Sciences & Hospital, Sattur, Dharwad -580 009, India
Ajantha GS, Department of Microbiology, SDM College of Medical Sciences & Hospital, Sattur, Dharwad -580 009 (Karnataka State, India) Email: firstname.lastname@example.org, email@example.com
Cladophialophora bantiana is the most common and dangerous neurotropic fungus which causes brain abscess. The infection which is caused due to this fungus is risky, not only to the patient due to the poor prognosis, but also to the clinician because of the difficulty in its management as well as to the laboratory personnel who handle it. Only few clinicians are aware of this fungus. Complete awareness among the clinicians and other health care personnel is essential for the successful management of these infections. On this basis, we have reviewed the microbiological, clinical and the therapeutic aspects of the C. bantiana infections.
cite this article :
Ajantha GS, Raghavendra D. Kulkarni. CLADOPHIALOPHORA
NEUROTROPIC FUNGUS – A MINI REVIEW. Journal of Clinical and Diagnostic Research [serial online] 2011 November [cited: 2015 Apr 1 ]; 5:1301-1306. Available from http://www.jcdr.net/back_issues.asp?issn=0973-709x&year=2011&month=November&volume=5&issue=6&page=1301-1306&id=1593
INTRODUCTION CNS infections which are caused by fungi except Cryptococcus neoformans are always a surprise, as the incidence is very less, especially in immunocompetent individuals (1). Apart from Cryptococcus neoformans, a few fungal species like Cladophialophora bantiana, Exophiala dermatitidis, Rhinocladiella mackenzie and Ochroconis gallopava are the recognized neurotropes. C. bantiana, a phaeoid fungus, is the commonest cause of cerebral phaeohyphomycosis (2),(3),(4). The purpose of this review was to provide adequate and clinico-microbiologically meaningful information to the health care personnel who were involved in the management of neurological infections. We searched the English language manuscripts which were published on ‘any date’, through PubMed, Google Scholar, ScienceDirect, MDConsult, Ovid and Medknow. The search terms which were used were ‘Brain abscess’, ‘Cerebral Phaeohyphomycosis’, ‘Phaeohyphomycosis’, ‘Cladophialophora’, ‘Cladophialophora bantiana’, ‘Cladophialophora bantianum’, ‘Xylohypha bantiana’, ‘Xylohypha emmonsii’, and ‘Cladosporium trichoides’.
Dematiaceous Fungi Dematiaceous, melanized or phaeoid fungi are a heterogeneous collection of generally unrelated fungi which share the common phenotypic feature of being darkly coloured. Their dark pigmentation results from the presence of dihydroxynaphthalene melanin in the cell walls. There is an argument that the term, ‘dematiaceous’ is epistemologically incorrect. Therefore, the term ‘phaeoid’ (which is derived from the Greek word “phaeo,” meaning “dark”) came to be used as a replacement. Both the terms, ‘dematiaceous fungi’ or ‘phaeoid fungi’ have no real taxonomic significance (5), (6). Nowa- days, the term “melanized” is more frequently used. Though thedebate about using different terms to describe the same character continues, all these terms are in common use. Dematiaceous fungi, though they are mainly saprophytic, are important agents of human and animal diseases. At present, over 150 species and 70 genera of dematiaceous fungi are known to cause various diseases and the list of potential pathogens is expanding rapidly (7). This group includes hyphomycetes, coelomycetes and ascomycetes. The clinical conditions which are associated with dematiaceous fungi are broadly classified into chromoblastomycosis, eumycotic mycetoma, and phaeohyphomycosis (Table/Fig 1)(6), (8),(9),(10).
The second nomenclature committee of the International Society for Human and Animal Mycology (ISHAM) recommends that the term “phaeohyphomycosis” should be a generic term for any mycosis which involves a dematiaceous fungus (5). Clinical laboratories have to maintain high vigil and diligence in the task of identifying the dark moulds and in assessing their clinical significance.
Taxonomy of C. bantiana C. bantiana is a melanized hyphomycetous (hyphomycetous - conidia not enclosed within any special structure) fungus which belongs to the order, Chaetothyriales (4). This fungus has been christened several times by taxonomists; hence, different names like Cladosporium bantianum, Cladosporium trichoides, Xylohypha bantiana and Xylohypha emmonsii have been found to be used for the same organism in the literature. (2),(3),(4),(5), (11).
The genus, Cladophialophora represents the anamorph members of ascomycetes in the order, Chaetothyriales, in the family, Herpotrichiellaceae. (9) These members are encountered in human infections which range from mild cutaneous lesions to fatal CNS infections. (12) C. bantiana was earlier under the genus, Cladosporium and it was later transferred to the genus, Xylohypha. (13) C. carrioniiand C. devriesii also were included in the genus, Cladosporium. Large subunit rRNA sequencing studies and nutritional physiology tests have led to the taxonomic reclassification of many fungal pathogens into the genus, Cladophialophora. Other species which are included under the genus, Cladophialophora are C. emmonsii, C. modesta, C. arxii, C. saturnica, and C. boppii. (6), (14), (15).
The type species of the genus, C. carrionii, causes chromoblastomycosis. C. bantiana and C. modesta cause brain infections, C. devriesii and C. arxii cause disseminated disease and C. boppii, C. emmonsii and C. saturnica cause cutaneous infections. (12), (16), (17).
Microbiology of C.bantiana Phaeoid fungi need intensive efforts for their identification upto the species level. Morphological, physiological and biochemical characters are used in their identification. Molecular studies now provide precision in the identification when the routine procedures are inconclusive. Among the vast array of these dematiaceous fungi, C. bantiana remains a comparatively easier species for identification,mainly because of its typical morphology and the source of the clinical specimen. It can grow on routine fungal culture media. The more commonly used media are potato dextrose agar, oatmeal agar and malt agar. Sabouraud’s dextrose agar is not generally preferred for black mould identification because the colonies may fail to develop their characteristic colour on this medium and the conidiation or sporulation may also be delayed or poor (6).
There are few important growth requirements and colony characteristics that may help towards the identification of this fungus. (6), (14), (18), (19) Its growth rate is moderate and it takes around 7-8 days of incubation to see visible growth. This fungus grows at both room temperature and at 370 C. The colonies mature within 15 days and they show a velvety texture and olivaceousgray to brown discolouration with a black reverse (Table/Fig 2). They do not produce any diffusible pigment.
C. bantiana has few important microscopic features which help in its identification. (6), (14), (18), (19) The fungus shows hyaline-to-brown, septate hyphae. Smooth walled, single-celled, pale olivaceous, ellipsoidal to spindle-shaped conidia of approximately 2.5-5 x 6-11 μm in size are seen. They are arranged in long, strongly coherent (non-fragile) chains and rarely show branching (Table/Fig 3) and (Table/Fig 4). The chains of the conidia arise directly from the hyphae. Chlamydoconidia are seen occasionally.
The Cladophialophora species are prone to identification problems because of the high degree of phenotypic similarity between its different species. Cladophialophora and Cladosporium can be differentiated, as the former do not display the dark scars of attachment. The type species of the genus Cladophialophora, Cladophialophora carrioni, shows densely-branched conidial chains. The chains arise from branched or unbranched denticles in C. carrioni.
There are some conventional biochemical tests and physiological tests which are commonly used in the identification of C. bantiana. (6), (20), (21) This fungus grows at 42-430 C, it can grow on media which contain cycloheximide, it is urease positive and the Fontana- Masson staining stains it black (Table/Fig 3). The Fontana- Masson stain is specific for melanin. This species is non-proteolytic on casein agar and it is unable to liquefy 12% gelatin. However, molecular methods like sequencing are the most dependable tools which can be used for its confirmation.
C. bantiana has rarely been isolated from nonhuman sources and only a few reports of its isolation from nature are on record. (20) The precise ecological niche of the fungus is unknown, but it is believed to be a soil fungus. (2), (12) The occupational association of the C. bantiana infection with farming is suggestive of the presence of this fungus in this environment.(22), (23).
Its primary pulmonary colonization may be important, following which it is transferred through the haematogenous route to the CNS. (2), (5), (16), (21), (24) This fungus has been shown to be present in the lungs of cerebral phaeohyphomycosis patients. (25) Animal experiments have shown that the organism is carried via blood to the CNS.(20) The presence of fungal elements in the arterial walls has been considered as the evidence of its spread to the CNS through the blood vessels. The multiple brain abscesses which are seen frequently also suggest its dissemination through the blood stream. (4) The rarely occurring cutaneous or subcutaneous infections result from its traumatic inoculation(26).
The laboratory personnel who handle this organism should not ignore the fact that pulmonary colonization is the initial eventand that the fungus is capable of infecting immunocompetent individuals. This species has been included in the list of fungi which should be kept under biosafety level-2 containment and therefore the use of a biosafety cabinet is imperative. (9), (18).
The fungi which are commonly encountered in non‑traumatic brain infections are C. bantiana, C. modesta, Exophiala dermatitidis, Fonsecaea monophora, Ochroconis gallopava and Rhinocladiella mackenzie. Amongst them, C. bantiana is the most frequentlyisolated fungus from cerebral phaeohyphomycosis. (2) This fungus has the ability to cause CNS infections in immunocompetent young individuals with no underlying risk factors for the fungal invasion. (2), (4) The infection with C. bantiana is commonly reported in transplant recipients, intravenous drug abusers or in patients on steroids. The condition may be mistaken for tubercular abscess. (27) Though extracranial infection is extremely rare, the fungus has also been reported to cause cutaneous and subcutaneous phaeohyphomycosis. (16), (24), (29), (26).
The virulence attributes of C. bantiana have not been adequately understood. Melanin production and thermotolerance (above 400C) have been considered to be the important virulence factors which are responsible for its pathogenicity. (12) Animal pathogenicity studies have confirmed its affinity to the glial tissue. (20) The evolutionary advantage, if any, of its growth in the living brain tissue remains unclear. (16) Neurotropism is related to the introns in the 18S rDNA subunit of the fungi. (30) Melanin may play a role in the CNS localization, probably, as it scavenges the free radicals which are produced by the phagocytic cells.(4) Melanin, as a virulence factor, has also been suggested in the CNS metastasis of malignant melanoma. Although the tumour spreads to many parts of the body, it has a predilection for the brain.(4), (31) However, melanin may not be the only factor for the CNS infection, as many other melanized fungi are not the frequent invaders of the CNS.
The infection provokes a granulomatous reaction which contains large numbers of giant cells. The fungal elements inside the phagocytes retain their viability. The suppression of innate cellular immunity by using cortisone is demonstrated to promote the C. bantiana infection. The first line of defense probably includes the complement system and PMNs, but T cell-mediated immunity also plays a role. (16), (22).
CerebralPhaeohyphomycosis C. bantiana infections are increasing in recent years.(2) The organism has a general preference for warmer climates with high humidity. Cases from arid climatic zones are rare.(16) Most infections occur in the 2nd or 3rd decades of life. However, it is noteworthy that the youngest patient who was reported was a six day old neonate(32).
The brain infection may present as a solitary, well-defined abscess, multiple abscesses or poorly demarcated cerebritis with extensive necrotic lesions.(2), (5), (7), (23), (25) Brain abscess is the commonest manifestation.(2) The abscesses are usually hemispheric and they affect the white matter and show a predilection for the frontal lobes. However, choroid plexus, thalamic, diencephalic, and cerebellar involvement can also occur.(16), (23) The extent and delineation of the lesions may vary according to the immune status and to immune response of the subject.(6) Rare presentations such as meningitis and meningoencephalitis have also been reported. (2), (32) Whether the character of the lesion is an important determinant of the clinical outcome or not, is not clear. (2), (23) Diffuse lesions or multiple lesions may complicate the surgical management(33).
CNS infections which are caused by C. bantiana have been reported worldwide. (16), (21), (22), (24), (25), (28), (33) The first cultureproven case was reported in 1952. 34 The first culture-proven Indian case was reported from Mumbai in 1966. (35) Many reports from various parts of India are available since then(23), (27), (36),(37),(38),(39),(40),(41).
Recently, we isolated this fungus from a case of brain abscess. During the literature search, we could not come across any report of the isolation of this pathogen from north Karnataka. Therefore,to the best of our knowledge, this is the first culture-proven case of cerebral phaeohyphomycosis which was caused by C. bantiana, from north Karnataka. The patient was treated by a near-total excision of the lesion and amphotericin B for two weeks, followed by fluconazole for six months. He was advised 5‑flucytocin in addition to amphotericin-B; however, he could not afford any antifungal other than fluconazole. The patient was doing well at follow up after three months. The case was discussed, where the radiological aspects were highlighted(42).
Management Cerebral phaeohyphomycosis is one of the most difficult conditions to treat. The mortality rate could reach up to as high as 70%, even in the treated cases. (2) When left untreated, the CNS infections which are caused by C. bantiana can be fatal within one to six months. (18) Complete excision of the brain abscess is better than simple aspiration or partial excision, and this is recommended. (2), (4), (5) Total excision of the well-defined abscess helps in reducing the pressure effects, reduction of the fungal load and in enhancing the response to the antifungal therapy. However, recurrence is a noted problem(23).
Antifungal therapy is essential in spite of surgical excision. The probability of residual fungal elements cannot be ruled out even after a thorough excision. (37) In addition, there are no standard guidelines for antifungal therapy which are available so far. The recommendations are based on anecdotal personal experiences. The rarity of this infection does not permit in gathering evidence which is based on authentic clinical trials. Monotherapy is not preferred as it often results in treatment failure. (4) Although amphotericin B has an antifungal activity against C. bantiana, the fungus may have innate or acquired resistance to it. (5) In an Indian case series, mixed results were noted with the drug. (23) Flucytosine has an antifungal activity against C. bantiana, and it also has excellent penetration into the CSF. (2) Because of the increasing risk of resistance, it has to be used in combination with other antifungal agents. (43) Ketoconazole, miconazole and fluconazole are not recommended. (18) In spite of having negligible activity, fluconazole has been used in some of the cases because of its low cost. (10) Identical problems were encountered in our case and because of the financial inability of the patient, we had to put him on fluconazole, hoping for at least a little support to the immune apparatus, to combat the residual fungal elements after thorough excision. Voriconazole has a fungicidal effect on C. bantiana. (44) It has good penetration into the CNS, but its MIC for C. bantiana is high. The clinical significance of the high MIC is not clear. Both success and failure have been reported. (10), (28), (33), (45), (46) Hence, it might not be the first choice of treatment in such infections(14).
Caspofungin has poor antifungal activity against this fungus. Posaconazole and itraconazole have the best in vitro activity against C. bantiana, followed by isavuconazole. (14) Concerns over the adverse effects and the lack of intravenous formulations are the problems with itraconazole. (7) Isavuconazole needs further evaluation. Experimental studies in murine models have shown best responses with posaconazole, where it prolonged the survival and reduced the level of the brain’s fungal burden significantly as compared to itraconazole. (47) At present, posaconazole appears to be the promising drug against C. bantiana as it also has good penetration into the CNS. (14) However, it must be noted that the action of posaconazole against C. bantiana is static and not cidal(47).
The number of isolates which have been tested against various antifungal drugs to date is low and therefore, the susceptibility profiles have a limited predictive value. The clinical correlation data are limited. (10) Complete surgical excision in combination with multi-antifungal therapy, seems to be the best treatment option at present. Initial combination therapy with liposomal amphotericin B and high dose azole (itraconazole, voriconazole or posaconazole), with or without 5-FC has been suggested in the literature. (2), (4), (14), (23), (27) Again, there is no standard guideline on the duration of the therapy. It can be as short as 3 months, which can be extendable up to or more than a year. The therapy should be continued until a complete radiographical resolution occurs. (4) The importance of a continuous long term follow up cannot be over emphasized.
Difficulties in the Developing World Most of the antifungals which are available for human therapy have two important drawbacks; the first being toxicity and the second, the cost. In the developing world, the cost becomes the primary issue. In addition to affordability, the uncertainty of the outcome is another deterrent which influences the decision making to initiate the therapy, as ultimately it is the patient who is going to select one of the different therapeutic options which are offered to him/her. The investigations and hospitalization charges add to the expenses. The approximate cost of only 6-8 weeks of the most economical therapy exceeds at least three hundred thousand Indian National Rupees. Because of this limitation, many cases in India are being treated with non-lipid based amphotericin B, with or without combination, which is within the reach of the Indian population. In many cases, even fluconazole has been used in spite of its proved inefficacy.
The development of a more effective as well as economical antifungal therapy, along with an accurate non-invasive diagnostic technique only will end this uncertainty about the outcome of the treatment and will ensure a better prognosis for this fatal fungal infection. Also, the pace of the research on this topic has to be stepped up further. It should be emphasized on the clinical fraternity that advanced imaging techniques including MR spectroscopy are far from being perfect in the establishment of the aetiological diagnosis. The culture and identification of the causative fungal agent is indispensible for its management. The identification of the fungus upto the species level is crucial in selecting the antifungal agent, as the choice of the antifungal agent varies with the species. When the fungus is isolated from clinical samples, a compatible histopathology is also a must.
Brain abscesses are diagnosed early and more frequently because of the advancements in radiology. C. bantiana is one of the most important fungal agents which cause nontraumatic brain abscess. Culturing and histopathological studies together give the confirmatory evidence of the aetiological diagnosis. The laboratory physicians need to exercise a meticulous approach while handling the specimens. Awareness, as well as a high index of suspension among the clinicians is the key in diagnosing this potentially fatal neurotropic fungal infection. Though the therapy is difficult, newer azoles like posaconazole have shown some promising results.
Conflict of Interest:None declared.
C. bantiana is the commonest cause of non-traumatic CNS infections.
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