Mycoses

3 Key excerpts on "Mycoses"

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  eBook - ePub

  Fungi

  Biology and Applications

  • Kevin Kavanagh(Author)
  • 2017(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  For the most part, transient exposure to fungi or fungal colonization occurs without the knowledge of the affected individual. This is primarily due to the inherent low virulence of most fungi, especially when confronted with the full arsenal of the human immune system. However, some species of fungi are long‐established members of the human microflora (i.e. commensals in the oral cavity and the gastrointestinal tract) and under certain conditions some fungi can cause disease and even death. As many as 200 fungal species have been associated with human infections (known as Mycoses); however, only a handful of these species are responsible for the majority of infections. Although, less well known than bacterial or viral pathogens, over a billion people are affected by fungal disease and approximately 1.5 million deaths are attributable to fungi annually. To put this in context, a similar number of people die from tuberculosis each year.

  Some of the most common microbial infections in humans are caused by microscopic fungi (e.g. thrush, dandruff, and “athlete’s foot”). These are superficial and relatively innocuous; however, fungi can also cause far more devastating diseases, such as invasive aspergillosis and systemic candidiasis, both of which have very high associated mortality rates. The incidence of these latter infections has been increasing in recent decades and this has fueled a heightened interest in Mycoses and the fungal species responsible for them among the clinical and scientific communities.

  The types of infections caused by fungi can be classified in a number of ways. One division is based on whether the infection occurs in an otherwise healthy host (i.e. primary Mycoses) or whether the host has an underlying medical condition causing impaired immune function (i.e. opportunistic Mycoses). These groups of diseases can be further subdivided depending on whether the infection is confined to the outer layers of the epithelia (i.e. superficial Mycoses) or whether the infecting organisms penetrate through this barrier into the bloodstream and disseminate throughout the body (i.e. systemic or disseminated Mycoses).

  One of the most exciting developments in the field of microbiology in recent years, made possible by the introduction of next‐generation sequencing, has been the ability to study the entire microbial populations present in specific parts of the body (i.e. the microbiome). These studies, which have largely been directed towards the bacterial component of the microbiome, have identified that fungi in general only represent approximately 0.1% of the total human microbiota. However, despite being present in comparatively small numbers, these studies have confirmed the diversity of fungi and have also shown that a significant proportion of the mycobiome (all of the fungal species) present in niches such as the oral cavity are noncultivatable and taxonomically unclassified. Studies are currently underway to investigate how the mycobiome evolves and varies in health and disease and to identify how fungi interact with other members of the microbiome.

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  eBook - ePub

  Fungi

  Biology and Applications

  • Kevin Kavanagh(Author)
  • 2011(Publication Date)
  • Wiley

    (Publisher)

  The ubiquity of fungi in the environment has already been alluded to in earlier chapters of this book. Fungal species have evolved and adapted to live in a wide variety of environments and ecological niches and, consequently, constitute a very diverse group of organisms. There are fewer than 100 000 species of fungi that have been identified to date; however, this is likely to be just the tip of the iceberg and it has been conservatively estimated that there are probably at least 1.5 million fungal species inhabiting our planet. Given this vast number of species and their prevalence in the environment, it is hardly surprising that humans unwittingly come into contact with many different types of microscopic fungus every day and that humans offer a potential source of nutrients for some of these fungal species. For the most part, transient exposure to fungi or fungal colonization occurs without the knowledge of the affected individual. This is primarily due to the inherent low virulence of most fungi, especially when confronted with the full arsenal of the human immune system. However, some species of fungi are long-established members of the human microflora (i.e. commensals in the oral cavity and the gastrointestinal tract) and under certain conditions some fungi can cause disease and even death. As many as 200 fungal species have been associated with human infections (known as Mycoses); however, only a handful of these species are responsible for the vast majority of infections. Fortunately for us, fungi are far more important pathogens of plants (see Chapter 12) and insects.

  Some of the most common microbial infections in humans are caused by microscopic fungi (e.g. thrush, dandruff and ‘athlete's foot’). These are superficial and relatively innocuous; however, fungi can also cause far more devastating diseases, such as invasive aspergillosis and systemic candidosis, both of which have very high associated mortality rates. The incidence of these latter infections has been increasing in recent decades, and this has fuelled a heightened interest in Mycoses and the fungal species responsible for them amongst the clinical and scientific communities.

  The types of infection caused by fungi can be classified in a number of ways. One division is based on whether the infection occurs in an otherwise healthy host (i.e. primary Mycoses) or whether the host has an underlying medical condition causing impaired immune function (i.e. opportunistic Mycoses). These groups of diseases can be further subdivided depending on whether the infection is confined to the outer layers of the epithelia (i.e. superficial Mycoses) or whether the infecting organisms penetrate through this barrier into the bloodstream and disseminate throughout the body (i.e. systemic or disseminated Mycoses).

  10.2 Superficial Mycoses

  The human body is covered by skin, hair and nails which, given their location in the body, are continuously exposed to the environment and, consequently, a wide variety of environmental microbes. For the most part, the keratinised epithelia which comprise the outer layers of the skin constitute an effective barrier that excludes microorganisms from gaining entry to deeper tissues. In addition, the skin also produces secretions, including sweat, sebum, transferrin and antimicrobial peptides known as defensins, that have the ability to kill many bacterial and fungal species. The skin is also equipped with intra-epithelial T and B cells, as well as a range of phagocytes. However, a small number of fungal species have evolved mechanisms of overcoming these defensive mechanisms and can actively colonize the skin surface, becoming established as members of the normal skin microbial flora. From time to time (mainly for reasons that are still unclear) these fungi can cause disease (Table 10.1 ). Two examples of such infections are pityriasis versicolor and tinea nigra. The former is caused by a yeast-like organism known as Malassezia furfur . This species thrives on the fatty acids found in sebum secreted by the skin and affects pigment-producing cells, resulting in a pink rash on pale skin and hypopigmentation in darker skin. Interestingly, this species and the related species Malassezia globosa have also been associated with dandruff, a common ailment characterized by increased shedding of skin cells from the scalp. Tinea nigra, a rare dermatomycosis characterized by a rash caused by the mould species Hortaea werneckii , results from the production of melanin by the fungus that causes the formation of brown macular patches on the palms and soles of the feet. As well as infecting the skin, fungi can also infect hair and nails. For example, fungi belonging to the genus Trichosporon cause a disease in hair known as white piedra (from the Spanish for stone), while Trichophyton rubrum

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  Evidence-Based Dermatology

  • Michael Bigby, Andrew Herxheimer, Luigi Naldi, Berthold Rzany, Robert Dellavalle, Yuping Ran, Masutaka Furue(Authors)
  • 2014(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  An important subset of systemic Mycoses is referred to as the opportunistic Mycoses, because there is always an underlying abnormality such as neutropenia or AIDS. Systemic candidosis and aspergillosis belong to this group. Skin involvement is rare and is usually the result of bloodstream spread. This chapter is largely concerned with the subcutaneous Mycoses. The systemic Mycoses are, by definition, severe internal infections, and a discussion of their management is beyond the scope of a dermatological work. In addition, there are no clinical studies directly relevant to the cutaneous manifestations of the systemic diseases, with the exception of a debate about the relevance of direct cutaneous invasion in their pathogenesis. Subcutaneous infections are rare and are generally confined to developing countries. There are few well-organized clinical studies in these infections, and randomized double-blind controlled trials are exceedingly rare. The evidence search for this chapter is based on the Cochrane Central Register of Controlled Trials (version 3, 2007) and my own collection of studies and personal contacts in the field. Most drugs used for these infections have been developed to treat other mycotic infections, and their application to deep Mycoses is based on individual cases or case clusters. Each of the subcutaneous Mycoses is dealt with separately. Mycetoma Definition Mycetoma is a subcutaneous infection caused by either fungi (eumycetoma) or actinomycetes (actinomycetoma) (Figure 45.1). The focus of infection is the subcutaneous tissue, including subcutaneous fat. The hallmark of the infection is that the microorganisms involved form into clusters of filaments called grains, which are surrounded by a dense neutrophil response, forming an abscess. These abscesses subsequently discharge onto the skin surface via draining sinuses, but may affect underlying bone, resulting in osteomyelitis

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