Microsporum gallinae

Microsporum gallinae is a fungus of the genus Microsporum that causes dermatophytosis, commonly known as ringworm.^[1] Chickens represent the host population of Microsporum gallinae but its opportunistic nature allows it to enter other populations of fowl, mice, squirrels, cats,^[2] dogs and monkeys.^[3] Human cases of M. gallinae are rare, and usually mild, non-life-threatening superficial infections.^[2]

Taxonomy and naming

Microsporum gallinae was first identified in 1881 by Megnin from chicken favus,^[4] and named Epidermophyton gallinae.^[5] It was later transferred from the Epidermophyton genus, and classified in the Trichophyton genus, as T. gallinae.^[4]

The identification of rough-walled macroconidia, a hallmark of the Microsporum genus, lead to the dermatophyte being classified as M. gallinae.^[1][4] There is still debate about the phylogenetic placement of this dermatophyte, but the accepted name is Microsporum gallinae.^[5] Analysis of its DNA sequences by PCR shows M. gallinae is most closely related to Microsporum vanbreuseghemii and Arthroderma grubyi, both are also zoophilic fungi.^[3][6]

Distribution

Microsporum gallinae is distributed throughout the world, with cases reported in: Europe, the Middle East, South America, North America, Africa and Japan.^[2] The fungus seems to be isolated more often in areas with high humidity and temperature.^[3]

Morphology

In culture, M. gallinae produces a white, satiny colony.^[7][8] The colony appears flat with radial folds and irregular edges as it grows in culture^[7] The colony reverse produces a strong pigmentation that changes from pink to a deep red and diffuses through the growth medium.^[5][7][8] Optimal growth of M. gallinae occurs at 26-28 °C,^[3] and no special nutritional requirements are necessary for its growth.^[9] Additionally, M. gallinae is urease positive.^[9]

During sporulation, septate hyphae,^[2][7] with macroconidia with tappered tips, and club shaped microconidia are present.^[7][8][10] The addition of thiamine or yeast extracts promotes sporulation of M. gallinae.^[7] Hair invasion observed in M. gallinae is large spored ectothrix type, meaning that the spores form outside of the hair shaft.^[5]

Pathogenicity

Microsporum gallinae is a cosmopolitan zoophilic fungi that very rarely affects humans.^[10] It obtains nutrients from keratin-rich skin, nails and hair, releasing enzymes during its digestion that elicit a host immune response as seen in ringworm.^[11][12] Microsporum gallinae infection is diagnosed by culturing the scrapings from skin lesions.^[9]

Chickens

Microsporum gallinae causes a superficial infection^[13] that forms white lesions on the wattles and combs of chickens.^[1][10] Lesions may spread to the head and neck.^[2] The feathers are normally not affected by the dermatophyte, although some feather loss can occur.^[1] Roosters and chicks tend to be more susceptible to the infection, with fighting cocks having the highest rates of M. gallinae dermatophytosis.^[2] Although, it tends to resolve with treatment, the M. gallinae infection may become chronic.^[1] The lesions can also become infected with bacteria, complicating the infection.^[1] In some cases, it can resolve naturally without treatment; however, the infection may persist for weeks prior to clearance.^[5]

Humans

Microsporum gallinae has been isolated from the scalp, and smooth skin in human populations.^[10]Microsporum gallinae infections are most commonly tinea capitis and tinea corporis.^[3] Very few human cases of M. gallinae infection have been reported, none of which were life-threatening.^[9] Of the reported cases, individuals ranged from 3–96 years old.^[3] They had cutaneous lesions on the glabrous skin or the scalp.^[3] These localized lesions are frequently accompanied by itching.^[2][9] The cutaneous manifestations are very similar to those of Microsporum canis therefore many cases of Microsporum gallinae could have been unreported.^[3]

In rare cases, immunocompromised individuals form severe dissemination on the skin, instead of small localized lesions^[3] as a result of handling infected animals.^[2][9] Only one case of extensive dermatophytosis was reported involving M. gallinae infection of a person with AIDS.^[2][3][9]

Treatment

The treatment of M. gallinae involves both topical and oral antifungals.^[1][3] Topical antifungals, such as: Terbinafine,^[3] Tolnaftate,^[1] and orally administered Griseofulvin^[1][9] have been used successfully to treat M. gallinae infections in humans and animals.

References