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Morphology of  Fusarium 

Fusarium microconidia and conidiophores in LPCB as shown above picture. Colonies are usually fast-growing, pale, or bright-colored (depending on the species) with or without a cottony aerial mycelium. The color of the thallus varies from whitish to yellow, pink, red, or purple shades. Species of Fusarium typically produce both macro-and microconidia from slender phialides. Macroconidia are hyaline, two to several-celled, fusiform to sickle-shaped, mostly with an elongated apical cell and pedicellate basal cell. Microconidia are one or two-celled, hyaline, smaller than macroconidia, pyriform, fusiform to ovoid, straight or curved. Chlamydospores may be present or absent.

Taxonomic Classification 

(described by Link ex Gray in 1821)

• Kingdom: Fungi
• Phylum: Ascomycota
• Order: Hypocreales
• Family: Hypocreaceae
• Genus: Fusarium
• Species: F. solani
• F. oxysporum
• F. verticillioides
• F. proliferatum
• F. sacchari
• F. anthophilum
• F. chlamydosporum and
• F. dimerum

Species of this genus, F. solani, and F. oxysporum are known to include the agents that cause human infections worldwide. F. solani can adhere to and damage the corneal membrane. Some Fusarium species, such as F. dimerum, are associated with keratomycosis, particularly in poor hygiene conditions.

Risk Group: They come in risk group 2 organisms.

Pathogenicity

Fusarium species cause a variety of infections including superficial, locally invasive, and disseminated infections. Infections due to Fusarium species are collectively referred to as fusariosis. Risk factors for severe fusariosis include prolonged neutropenia and T-cell immunodeficiency, especially in hematopoietic stem cell transplant recipients with severe graft-versus-host disease. Trauma is another major predisposing factor for the development of cutaneous infections due to Fusarium strains. Keratitis endophthalmitis,  otitis media, onychomycosis, cutaneous infections particularly of burn wounds, mycetoma, sinusitis, pulmonary infections, endocarditis, peritonitis, central venous catheter infections, septic arthritis, disseminated infections, perinephric abscess, and fungemia due to Fusarium spp. have been reported. Outbreaks of nosocomial fusariosis have also been reported due to being the presence of organisms in hospital water distribution systems. They also exist in the soil of potted plants in hospitals that constitute a hazardous mycotic reservoir for nosocomial fusariosis. Fusarium species produce mycotoxins which are very common worldwide. They exist in many plants and in various compositions. The major Fusarium mycotoxins are FB1, trichothecenes e.g. DON, nivalenol (NIV), T-2, and ZEN. Ingestion of grains contaminated with these toxins may give rise to allergic symptoms or be carcinogenic in long-term consumption.

Clinical Manifestation of Fusariosis in Immunocompromised Hosts

• Endophthalmitis
• Sinusitis
• Pneumonia
• Skin Involvement
• Fungemia
• Disseminated Infection

Laboratory Diagnosis 

Specimen: It depends on the nature of the infection site e.g. in case fungemia, blood, in the diagnosis of keratitis corneal scrapings (most frequent) or tissue biopsy and skin lesions (either cellulitis or metastatic lesions), and also blood for cultures for mold.

KOH mount: Presence of fungal elements

Fungal culture: To obtain growth of fungi.

LPCB Preparation: Observation of fungal structures from culture.

Serological test: β-d-Glucan Testing Is Important for Diagnosis of Invasive Fungal Infections but cannot distinguish Fusarium from other fungal infections (Candida, Aspergillus, Trichosporon, and others) which are also detected by the assay.

Histopathological Examination:  It is also a helpful tool for confirmatory diagnosis of fusariosis. In tissue, the hyphae are similar to those of Aspergillus species, with hyaline and septate filaments that typically dichotomize in acute and right angles. Although, adventitious sporulation may be present in the tissue, and the finding of hyphae and yeast-like structures together is highly suggestive of fusariosis in the high-risk population. In the absence of fungal growth, distinguishing fusariosis from other hyalohyphomycoses may be difficult and requires the use of another technique in situ hybridization in paraffin-embedded tissue specimens.

Molecular Test:  PCR-based method, using sequencing identification as a gold standard but why this, it verifies as identification of Fusarium species is often difficult due to the variability between isolates and because not all features required are always well developed (e.g. the absence of macroconidia in some isolates after subculture). It is possible to identify the genus Fusarium by several methods. On culturing, hyaline, banana-shaped, and multicellular macroconidia are very common; however, to identify them at the species level is not easy. Therefore, molecular methods are needed. Some of the most commonly used molecular methods are genus-specific PCR, 28 s rRNA gene sequencing, sequence-based PCR, multiplex tandem PCR, and automated repetitive sequence-based PCR.

Treatment

Useful anti-fungal drugs are-

• Itraconazole
• Voriconazole
• Amphotericin B and
• Posaconazole.

Keynotes

1. Fusarium differs from comparatively matching some fungi like Acremonium, Lecythophora, and Phialemonium by having macroconidia. It differs from Cylindrocarpon by having macroconidia with foot cells and pointed distal ends.
2. Fusarium oxsporum: In contrast to Fusarium solani the phialides are short and mostly non-septate.

References

1. https://www.intechopen.com/books/fusarium
2. https://cmr.asm.org/content/20/4/695
3. https://mycology.adelaide.edu.au/descriptions/hyphomycetes/fusarium/
4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC358338/
5. https://drfungus.org/knowledge-base/fusa
6. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3889722/
7. Clinical Microbiology Procedure Handbook, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
8. A Textbook of Medical Mycology. Editor: Jagdish Chander.  Publication Mehata, India.