What is Sporothrix schenckii?
Sporothrix schenckii is a dimorphic fungus that is widely distributed in soil and in and on living and dead decaying environments.
- It is a pathogenic fungus that causes an infection known as sporotrichosis, in both animals and humans.
- Sporotrichosis is a chronic mycotic infection that affects the cutaneous and subcutaneous tissues.
- The fungal infection is associated with the formation of small, ulcerative, and suppurative lesions on and in the skin tissues.
- It can also affect the lymphatic system by forming nodular lesions which can be ulcerative and suppurative.
- The fungus gains entry into the host through skin abrasions and sometimes through inhalation into the lungs.
- Evidence of secondary spread to the muscles and bones is rare, with occasional infection of the central nervous system, lungs, and the genitourinary tract have been reported.
- Sporothrix schenckii was first isolated in 1896 by Benjamin Schenck, a medical student at the Johns Hopkins Hospital in Baltimore, MD, from a 36-year-old male patient presenting lesions on the right hand and arm.
Habitat of Sporothrix schenckii
- This fungus is commonly found in soil.
- It can also be found on living plants such as barberry shrubs and roses or in plant debris such as sphagnum moss and pine bark mulch.
- Sporothrix schenckii grows as a mold in the environment and at temperatures below 35°C.
- It transforms into a yeast form in vivo and at temperatures above 35°C.
- Sporotrichosis, an infection caused by Sporotrix schenckii is an occupational disease to florists, gardeners, and foresty workers.
- It is commonly prevalent in Tropical and subtropical regions.
Morphology of Sporothrix schenckii
- Sporothrix schenckii is a dimorphic fungus.
- In the basic mycological culture at 25°C, it has a filamentous form that is composed of hyaline, septate hyphae 1 to 2 μm wide.
- The fungal growth in the colonies is characterized by branched septate hyphae which produce small distinct asexual spores known as conidia of 3–5 μm, which are brown in color.
- The conidia are produced by conidiophores, which arise at right angles from the septate hyphae.
- The conidiophore, are tapered at the ends.
- The formed conidia are clustered on tiny denticles at the apex of the conidiophore forming a flower-like appearance.
- The conidia are ovoid or elongated, 3-6 x 2-3 µm, hyaline, single-celled, and smooth-walled.
- Large singly occurring conidia may also be formed as the fungal culture ages.
- The large conidia are dark, thick-walled obovate to angular.
- In rich media budding yeast cells which are fusiform of about 1–3 × 3–10 μm are formed which are spherical, or oval-shaped.
Figure: Culture of Sporothrix schenckii (Sabouraud media, 28 °C) Filamentous state with thin hyphae and denticle microconidia like “daisy flowers” (Erythrosine, 40×). Image Source: MDPI.
Cultural Characteristics of Sporothrix schenckii
- In routine mycological agar media such as malt extract agar and potato dextrose agar, at room temperature 25°C, they grow slowly, forming blackish/greyish and shiny colonies that mature to moist, glabrous, wrinkled, and fuzzy colonies, colored in black.
- In a rich medium such as brain heart infusion media (BHI) at 35-37°C, Sporotrix schenckii is thermally dimorphic whose growth is characterized by multiple budding yeast cells.
- In the BHI medium, they produce colonies that are glabrous, white-greyish to yellowish color, yeast cells.
Transmission of Sporothrix schenckii
- The fungus inoculation into the host is via traumatic implantation of the fungus from contaminated soil, plants, and organic matter with the fungus.
- It is common in florists, gardeners, miners, and forest workers.
- Zoonotic transmission from cats has also been reported in a few cases in Brazil.
- It has a 1-12 week incubation period.
Pathogenesis of Sporothrix schenckii
Virulence Factors of Sporothrix schenckii
- Sporothrix schenckii has the ability to grow in temperatures of 35-37°C.
- At 35°C, it can cause lymphatic sporotrichosis and at 37°C, it can cause disseminated and extracutaneous lesions of sporotrichosis.
- Sporothrix schenckii has the ability to synthesize melanin, an insoluble compound that has been linked to the virulence of many fungal groups.
- The melanin is found on the dematiaceous conidia of the fungus
- Conidial melanization enhances the resistance of Sporotrix schenckii to macrophage phagocytosis initiating the first stage of infection by the conidia spores, the infective particle of the fungus.
- The melanin pigmentation plays a major role in causing cutaneous sporotrichosis by enhancing the invasiveness of the fungus into the host.
Figure: Structures of some intermediates in the pentaketide pathway of melanin biosynthesis in S. schenckii. Image Source: Wikipedia.
- Sporotrix schenckii has adhesins such as integrins and adhesin lectin-like molecules that are able to recognize the glycoproteins on the extracellular matrix of the cutaneous tissues i.e fibronectin, laminin and type II collagen.
- The fibronectin adhesins are located on the surface of yeast cells and they attribute to the fungal adherence factors to the host.
- Laminin receptors are located on the fungus hyphae and yeasts which have the ability to bind to the extracellular matrix.
- The presence of these adhesins favors the fungus adherence to the host tissues and enhance disease dissemination.
Clinical Manifestations of Sporotrichosis
- It is the most common manifestation of Sporotrix schenckii
- Lesions occur on the hands and arms, lower extremities, trunk, and face.
- Primary lesions occur within the first few weeks of fungal inoculation.
- The lesions are small nodules that progress to ulceration
- The lesions are a bit painful and nonpruritic.
- New nodules spread on the lymphatics and become ulcerated.
- Lymphangitis may develop, making the lymph nodes swollen and painful.
- Sporothrix schenckii is a common causative agent of suppurative and granulomatous inflammatory reaction in the dermis and subcutaneous tissue.
- It is characterized by microabscess and fibrosis, hyperkeratosis, parakeratosis, and pseudoepitheliomatous hyperplasia.
Fixed Cutaneous Infection
- This is the development of a single lesion normally on the face.
- The lesion may be ulcerated or verrucous
- The lesion only disappears on antifungal therapy.
- It is a rare condition of sporotrichosis, commonly affecting alcoholics
- It is characterized by septic arthritis which occurs due to traumatic inoculation which spreads to the joints.
- Bursitis and tenosynovitis can occur presented with carpal tunnel syndrome.
- It is a subacute to chronic infections
- It is common in persons with Chronic Obstructive Pulmonary Disease (COPD).
- It occurs after the inhalation of conidia.
- It is characterized by fever, fatigue, weight loss, coughing, sputum productions, and hemoptysis
- Infected individuals often have chronic cavitary pulmonary histoplasmosis, tuberculosis, or atypical mycobacterial infection.
Diagnosis of Sporotrichosis
Specimen: Tissue biopsy, pus from lesions, sputum, urine, blood, and cerebrospinal and synovial fluids.
- 10% potassium hydroxide (KOH) wet mount to observe for budding yeast cells
- Gram staining stains the yeast cells gran positive.
Figure: Sporothrix schenckii conidiophores and conidia at 25°C. Image Source: The University of Adelaide.
Figure: Sporothrix schenckii PAS stained tissue section showing budding yeast-like cells. Image Source: The University of Adelaide.
- Sabouraud agar with chloramphenicol and on media with cycloheximide, such as mycobiotic agar, growth occurs in 5-7 days at 25°C, forming filamentous hyaline colonies which become dark at the center as the culture ages. Dematiaceous conidia are also observed.
- Potato Dextrose Agar and Cornmeal agar are used to show conidiogenesis.
- Brain Heart Infusion Agar, Chocolate agar and blood agar are used to show dimorphism at 35 to 37°C. Growth of colonies develops within 5-7 days to form yeast cell colonies which are creamy and yellow, to tan color.
Figure: Sporothrix schenckii culture at 25C and budding yeast cells in BHI at 37°C. Image Source: The University of Adelaide.
- Detection of PCR amplicons
- Oligonucleotide primers for differentiation of Sporortrix schenckii from other fungal species.
- Tube agglutination test to detect for agglutinin antibodies (high titers)
- Latex Agglutination – Antigen-coated latex particle to detect for sporotrichin antigen in the sera of infected patients
Sporotrichin Skin Test
- This test is meant to detect delayed hypersensitivity (cellular immune response).
- It can detect and confirm the present and previous infection by Sporothrix schenckii.
- The sporotrichin skin test has been used to confirm the diagnosis of bulbar conjunctival sporotrichosis.
Figure: Recognition of Sporothrix species using classical and molecular approaches. Image Source: https://doi.org/10.1093/mmy/myx103
Treatment of Sporotrichosis
- Oral administration of a saturated solution of potassium iodide
- Itraconazole is currently the first-choice treatment
- Systemic infection can be treated with amphotericin B
Prevention and Control of Sporotrichosis
- Putting on protective clothing such as gloves and long sleeves during high-risk activities, for example, handling sphagnum moss, wires, rose bushes, hay bales, conifer (pine) seedlings, or other materials that may facilitate the exposure to the fungus
- Cats with sporotrichosis should be correctly treated and kept isolated in a proper place to prevent zoonotic transmission.
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