Fusarium dimerum, typically a soil fungus, was isolated in South Africa from an adult male suffering from a corneal ulcer following an injury to the eye. Several species of the genus Fusarium cause disease in humans and animals. Keratomycosis is mainly caused by Fusarium solani, F. oxysporum and F. verticillioides (synonym: F. moniliforme), while F. dimerum (synonym: F. episphaeria) infections are considered rare. Fusarium cases are not only difficult to diagnose taxonomically, but have a very poor prognosis. In the literature, more than 3500 culture confirmed cases of keratomycosis, caused by filamentous fungi, have been reported. More than 850 of these were due to Fusarium species, forming the second most prevalent group of organisms recorded to cause this disease. Aspergillus species, particularly A. flavus, A. fumigatus and A. niger, apart from yeasts, are more often isolated. Few other fungi seem to play an active role in keratomycosis, with Alternaria, Curvularia, Acremonium and Penicillium species being sporadically isolated. Seasonal variation in the geographical area in which the studies are conducted also seems to have an influence on dominant species of fungi causing the infections. F. dimerum has not been described to cause human infections in South Africa and has not been recorded from soil, plant or organic material in this country. The macro- and microscopic characteristics of the eye isolate (MRC 7418) were found to be indistinguishable from strains described in the literature. The four F. dimerum strains examined in this study were slow growing, ranging from 8-18 mm in diameter at 25 °C and 4-21 mm at 30 °C after 72 h on PDA. Young colonies appeared yeast-like, white, becoming orange to deep apricot and slimy after 7 to 14 days. Both the surface and the reverse of the colonies had the same colour, with whitish floccose aerial mycelium present in some isolates. One strain (MRC 7528) became slightly atypical after 14 days on PDA as brown rings were formed at both temperatures. This strain and MRC 7418, both isolated from human eyes, grew faster at 25°C as well as 30°C than the other isolates examined. The most characteristic light and scanning electron microscopic features of the F. dimerum isolates examined include loosely branched conidiophores with densely aggregated monophialides producing curved macroconidia with pedicellate basal cells. The monophialides are erect, formed laterally in abundance on the vegetative hyphae, are either long subcylindrical or short and inflated (subglobose) with prominent collarettes. Immature macroconidia protruding from the phialides are cylindrical at first, becoming curved or falcate. Mature macroconidia are mostly 1-2 septate and vary in size depending on the number of septa present. One-septate macroconidia range from 11-18.6 x 3.3-4.2 µm and two-septate macroconidia range from 15.3-23.7 x 3.3-4.9 µm in size. Chlamydospores are found in all cultures and are mainly intercalary, rarely terminal, smooth-walled, often in chains or groups of two or more. Microconidia are not observed in any of the strains examined. The authenticity of the first report of F. dimerum from an eye infection in a human in South Africa was therefore confirmed by comparing it with other human isolates obtained in the USA.
Full conference title:
The 15 th Congress of the International Society for Human and Animal Mycology
- ISHAM 15th (2003)