Microsporum: General Characteristics, Pathogenesis, Clinical Findings, Laboratory Diagnosis, Epidemiology, Prevention and Control

Microsporum: General Characteristics, Pathogenesis, Clinical Findings, Laboratory Diagnosis, Epidemiology, Prevention and Control

 Introduction of Microsporum

Microsporum is a genera  dermatophytes that causes dermatophytosis, or cutaneous infections  of the hair and skin.  They are ascomycetous molds that produce macroconidial spores that are unique to this group of dermatophytes, distinguishing them from Trichophyton and Epidermophyton. Microsporum spp. are keratolytic, meaning they contain keratinase, an enzyme that digests skin and hair particles, causing cutaneous diseases. Since it cannot develop in temperatures above 37°C, it is restricted to nonviable skin tissues as a dermatophyte. It is a zoonotic fungus since it is extremely transmissible from animals to humans.

Risk Group Classification

Risk Group 2 pathogens.

Clasification of Microsporum

(Gruby 1843)

Kingdom: Fungi

Division: Ascomycota

Class: Eurotiomycetes

Order: Onygenales

Family:  Arthrodermataceae

Genus:  Microsporum

Other species: Microsporum fulvum, Microsporum amazonicum, Microsporum boullardii, Microsporum cookei, Microsporum distortum, Microsporum duboisii, Microsporum ferrugineum, Microsporum fulvum.

Habitat of Microsporum

Microsporum live in the soil and are found all over the world. They grow at a room temperature of 25-27 degrees Celsius, on keratin surfaces with low temperatures. They are keratotic, so they tend to live in keratin-rich environments including hair and skin.

Susceptibility to Disinfectants

Microsporum are susceptible to phenolic compounds, formaldehyde, glutaraldehyde, iodophors and sodium hypochloride (1%).

Physical Incativation

The infectious substance can be inactivated by UV , gamma and microwave (aerosol)  radiation; moist heat (121°C for at least 20 minutes; and dry heat (165-170°C for 2 hours).

Morphology

Asexual reproduction occurs in Microsporum, which produces macroconidia and microconidia. Macroconidia are asexual spores of large size. They are hyaline, multiseptate, with a range of shapes ranging from spindle-like to obovate. The macroconidia have a thin or thick echinulate to the verrucose cell wall and are 7–20 µm by 30–160 µm in height. These fungi can be distinguished from other dermatophytes by their distinct form, which is dense and rough. Microconidia are smaller than macroconidia and are therefore asexual Microsporum spores. Microconidia are single-celled hyaline organisms.   Microconidia have smooth cell wall and are pyriform to clavate in shape.

 Microconidia are 2.5–3.5 µlong and 4–7 µwide.

Culture Characteristics

At 25°C, they develop quickly on Sabouraud dextrose agar (SDA). Microsporum spp. can produce a perfect shape, which is a fungus that can complete meiosis.  In rice grain medium with pigmented peripheries, they grow well. The basic media for the growth and identification of Microsporum species is Trichophyton agar.

Common species of Microsporum 

Microsporum audouinii

They are anthropophilic fungi that cause non-inflammatory scalp and skin infections in children. It is a natural cause of tinea capitis, entering hair shafts and inducing an ectothrix infection. The ectothrix fluoresces a bright yellow under Wood’s ultra-violet light.  They form spread flat colonies that vary in colour from whitish-grey to tanned white on simple media. They have a furry look about them. They produce macroconidia and very rarely microconidia. The macroconidia are long and smooth, with a spindle shape that is irregular. Microconidia are pyriform to clavate in shape if they form. They also produce pectinate (comb-like) and racquet hyphae (segmented series of hyphae with swollen tips).

Microsporum canis

This is a zoophilic dermatophyte that is spread worldwide and is transmitted from dogs and cats to humans. Ringworms are commonly caused by it, particularly in children.  It causes an ectothrix infection, which can be detected using Wood’s ultra-violet light, which fluoresces as a bright greenish-yellow light. They grow well in rice grain agar, producing white aerial mycelium as well as a yellow pigment. They form white to cream-colored spread flat colonies with a cottony surface in basic media. The colonies also have a brownish-yellow edged pigmented surface. They create spindle-shaped macroconidia with 5-15 cells, a verrucose thick wall, and a terminal end.They may also produce microconidia that are pyriform to clavate and produce fewer microconidia. Microsporum canis var. distortum, which causes infection in cats and dogs, and Microsporum canis var. equinum, which causes ringworms in horses, are two other dysgonic strains of Microsporum canis.

Microsporum ferrugineum

It’s an anthropophilic fungus that causes human tinea capitis epidemics in children. It infects people in the same way that Microsporum audouinii does.  It infects the hair shafts, causing an ectothrix infection that is visible as greenish-yellow fluorescence in the Wood’s ultra-violet light. Asia, Russia, Africa, and Eastern Europe are all familiar with it. Colonies form slowly in basic mycological media (SDA, PDA) and have a waxy glabrous, convoluted thallus with a creamy to buff-colored surface.  Surface pigmentation ranges from yellow to creamy to dark red. The colonies are pleomorphic and downy. They also have irregular branching hyphae that look like bamboo hyphae and carry chlamydospores. The bamboo-branched hyphae is a specific and distinct characteristic of Microsporum ferrugineum as hown above image.

Mode of Infection

Direct or indirect contact with desquamated epithelium-contaminated skin or scalp lesions of infected individuals, animals, or fomites. In people with weakened cell-mediated immunity, infection can spread via broken skin.

Pathogenesis of Microsporum

Microsporum spp. virulence  is attributed to their ability to colonize keratin-containing materials like skin, hair, and nails, indicating that it is a fungal keratinase. Keratinase is an enzyme produced by fungi that degrades and breaks down keratin materials. Because the body’s defense mechanisms prevent the fungi from invading the circulatory system, the immune response elicited by the host is a localized inflammatory response. When fungi colonize the mucous membrane, they release the elastase enzyme, which activates the immune system. Microsporum, like other dermatophytes, causes cutaneous infections on hair, skin, and nails. As a result, the fungi are known to cause a variety of cutaneous infections (dermatophytosis), such as: Tinea capitis and Tinea corporis.

Incubation Period

The incubation period is from several days to a few weeks, depending on the species and the host.

Tinea capitis

This is a fungus that infects the hairs on the scalp and is also known as ringworms or herpes tonsurans. Epidermophyton and Trichophyton spp. are also responsible. The fungi infiltrate the hair shafts by penetrating the outer root sheath of the hair follicles. It may result in either an inflammatory or non-inflammatory infection. Inflammatory tinea capitis is marked by painful pus filled nodules and scarring alopecia, in which the affected area loses hair and leaves a scar. Eyelashes and brows may also be affected. It is most common in children aged 3 to 14, but it can affect people of any age, particularly those who are immunocompromised, such as diabetics or cancer patients. Tinea capitis is a Microsporum spp. specific ectothrix infection in which the fungi attack the outer sheath root. Subacute and chronic dermatitis with or without follicular inflammation and destruction characterize it. It’s possible that suppurative folliculitis is present. Hyperkeratosis is a type of hyperkeratosis. Parakeratosis is a type of keratosis that affect (a mode of keratinization characterized by the retention of nuclei in the stratum corneum).

Tinea corporis

It is a pink-reddish round-shaped patch with plaques and a raised scaly border that extends on the periphery and clears at the center, also known as ringworms. Small pus-filled papules have appeared on the periphery. It causes itching, scaling, redness, and rashes on the arms and legs, and it can be dry and flaky.

Spongiosis

An inflammatory infiltrate in the perivascular space, black dot tinea capitis, which infects the hair, is one of the clinical symptoms. Kerion is characterized by inflammation that leads to alopecia scarring (condition that causes hair to fall out in small patches, which can be unnoticeable). Favus is a bog inflammatory condition characterized by deep-seated oozing nodules, abscesses, crusting, or scutula. The distinctive lesion of favus, a yellow saucer-shaped crust containing a mass of hyphae and spores).

Laboratory Diagnosis of Microsporum

Specimen: It depends on the site of infection. e.g. in skin infection , skin scrapings, nail infection-nail scrapings whereas in hair infection- hair scraping, pus, tissue biopsies.

Differential diagnosis: It is necessary to clear from othre resembling conditions like bacterial abscess, psoriasis, eczema, syphilis, lupus erythematosus.

Direct Microscopy

Potassium hydrooxide (KOH) Wet Mount: May show fungal elements

Culture Characteristics

Potato Dextrose Agar (PDA): M. audouinii produce pinkish-brown or salmon-colored fluffy colonies while M.  canis produces bright yellow colonies.

Trichophyton Agar

M. audouinii produces flat, white, suede-like to downy, with yellow-brown reverse colonies with a furry texture whereas M.canis forms flat, white, suede-like to downy, with a yellow to pale yellow-brown reverse colonies.

Rice Grain Agar

M. canis produces white aerial mycelium with the production of yellow pigment

Histological Identification

The periodic acid-Schiff (PAS) stain is a special stain that will help in identifying dermatophytic fungi.

LPCB Tease mount 

It is used for observation of fungal structures like hyphae and spores or conidia. Conidia is of  two types i.e. Microconidia :- small unicellular

Macroconidia:- Multicellular, septate

Special hyphae:- hyphae such as spiral hyphae, racquet hyphae and favic chandeliers

Biochemical test

Urease test:- Microsporum species are urease negative

Other methods of diagnosis

Hair perforation test: – fungi pierce hair producing wedge – shaped perforations.

Positive: – Trichophyton mentagrophytes and Microsporum canis.

Molecular methods: PCR assay

Woods Lamps Examination: – Positive for various Microsporum species and Trichophyton schoenleinii. Fluorescence is due to the presence of pteridine pigment in cell wall. Fluorescence seen under wood’s lamp-

Microorganism    Fluorescence Color

  1. Microsporum audouinii : Bright – green
  2. M. canis : Bright – green
  3. M. ferrugineum:  Blue – green
  4. M. distortum: Blue – green
  5. M.gypseum: Dull – yellow
  6. Trichophyton schoenleinii: Dull – green
  7. Malassezia furfur: Golden – yellow

Treatment

  • Antifungals in cream, lotion, or gel, such as imidazole, ciclopirox, naftifine, or terbinafine, for mild-moderate lesions caused by tinea corporis and tinea capitis.
  • Oral itraconazole can be used to treat severe infections for an extended period of time.
  • Antifungal shampoos are used for both treatment and prevention of fungal infection spread. Tinea capitis and tinea corporis can both be prevented with antifungal creams.

Prevention and Control

  • Handwashing after handling and exposure to animals, soil, and plants should be done in a sanitary manner.
  • Touching lesions on the skin and scalp of infected individuals should be avoided.
  • On the affected areas, loose-fitting clothing is recommended.
  • When participating in activities that require close human contact with skin, it is important to maintain good hygiene.

Epidemiology

This pathogen is found all over the world, and infections are fairly common. They are the most common pathogens in northern Europe and North America. Southern Europe and the Arabic countries have a higher prevalence of zoophilic dermatophytes.

Keynotes

  • Absence of microconidia and macroconidia, having bamboo hyphae(  irregular branching  hyphae with prominent cross walls) and presence of chlamydospores are the diagnostic features of  Microsporum ferrugineum.
  • Microsporum persicolor properties-

Colony characteristics

White to pinkish in colour

Reverse pigment: orange

Microconidia: thin walled and cigar shaped

-Abundant, spherical to pyriform

4-7 celled micrconidia may also be found but rarely.

  • Pectinate body is found in M. audouinii.
  • Nodular organ is found in M. canis.

Further Reading

  1. https://www.sciencedirect.com/topics/medicine-and-dentistry/microsporum
  2. https://www.sciencedirect.com/topics/medicine-and-dentistry/microsporum-audouinii
  3. https://cmr.asm.org/content/cmr/8/2/240.full.pdf
  4. https://www.cdc.gov/fungal/diseases/ringworm/definition.html
  5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5283510/
  6. https://www.msdsonline.com/resources/sds-resources/free-safety-data-sheet-index/epidermophyton-floccosum/
  7. https://pedsinreview.aappublications.org/content/33/4/e22
  8. https://en.wikipedia.org/wiki/Microsporum
  9. https://mycology.adelaide.edu.au/mycoses/cutaneous/
  10. https://www.esccap.org/uploads/docs/1afu1a79_0765_ESCCAP_Guideline_GL2_v6_1p.pdf
  11. http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1413-86702014000200181
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