Scalp Infections (Malassezia)
Malassezia (formerly known as Pityrosporum) is a genus of related fungi, classified as yeasts, naturally found on the skin surfaces of many animals including humans. It can cause hypopigmentation on the trunk and other locations in humans if it becomes an opportunistic infection.
Some confusion exists about the naming and classification of Malassezia yeast species due to a series of changes in their nomenclature. Work on these yeasts has been complicated because they are extremely difficult to propagate in laboratory culture.
Malassezia were originally identified by the French scientist Louis-Charles Malassez in the late 19th century. Raymond Sabouraud identified a dandruff-causing organism in 1904 and called it "Pityrosporum malassez", honoring Malassez but at the species level, not the genus. When it was determined that the organisms were the same, the term "Malassezia" was judged to possess priority.
In the mid 20th century, it was reclassified into two species:
- Pityrosporum (Malassezia) ovale which is lipid dependent and found only on humans. P. ovale was later divided into two species, P. ovale and P. orbiculare, but current sources consider these terms to refer to a single species of fungus, with M. furfur the preferred name.
- Pityrosporum (Malassezia) pachydermatis, which is lipophilic but not lipid dependent and found on the skin of most animals.
In the mid 1990s, scientists at the Pasteur Institute in Paris, France discovered additional species.
Currently there are 10 recognized species:
- M. furfur
- M. pachydermatis
- M. globosa
- M. restricta
- M. slooffiae
- M. sympodialis
- M. nana
- M. yamatoensis
- M. dermatis
- M. obtusa
Role in human diseases
Recently, identification of Malassezia on skin has been aided by the application of molecular or DNA based techniques very similar to those used by forensic scientists to identify criminal suspects. These investigations show that in humans the species causing most skin disease, including the most common cause of dandruff and seborrhoeic dermatitis is M. globosa (though Malassezia restricta is also involved.) The skin rash of tinea versicolor (pityriasis versicolor) is also due to infection by this fungus.
As the fungus requires fat to grow, it is most common in areas with many sebaceous glands: on the scalp, face, and upper part of the body. When the fungus grows too rapidly, the natural renewal of cells is disturbed and dandruff appears with itching (a similar process may also occur with other fungi or bacteria).
A project in 2007 has sequenced the genome of dandruff-causing Malassezia globosa and found it to have 4,285 genes. M. globosa uses eight different types of lipase, along with three phospholipases, to break down the oils on the scalp. Any of these 11 proteins would be a suitable target for dandruff medications.
Another surprising finding is M. globosa's potential ability to reproduce sexually, though this has not been seen in the laboratory or elsewhere.
It is hypothesized that individuals with Trichotillomania suffer from a sort of autoimmune-disordered reaction to Malassezia and/or Candida yeast. Since Malassezia is especially present in the hair follicles and scalp, "hair pulling is like sneezing: the body is attempting to rid itself of an allergy-causing irritant."
The number of specimens of M. globosa on a human head can be up to ten million.
Treatment of symptomatic scalp infections
Symptomatic scalp infections are often treated with selenium disulfide or ketoconazole containing shampoos. Other treatments include ciclopirox olamine, coal tar, zinc pyrithione (ZPT), miconazole, or tea tree oil medicated shampoos. Hydrogen peroxide is also used to manage symptoms of itching.