MicrobiologyBytes

Subscribe to podcasts (free):
[iTunes] Enhanced podcasts
[RSS] mp3 podcasts (audio only)
Download this podcast (free):
Enhanced version
mp3 version (audio only)

Fungal infections or “mycoses” are classified by the degree of tissue involvement and mode of entry into the host:

• Superficial infections are localised to the skin, the hair, and the nails.
• Subcutaneous infections are confined to the dermis, subcutaneous tissue or adjacent structures.
• Systemic infections occur deep within the body and involve the internal organs.
• Opportunistic infections only affect the immunocompromised.

Human fungal infections in are uncommon in normal healthy people, and are usually confined to conditions such as candidiasis (thrush) and dermatophyte skin infections such as athlete’s foot. However, in immunocompromised hosts, a variety of normally mild or nonpathogenic fungi can cause potentially fatal infections. In addition, the relative ease with which people can now visit “exotic” countries provides the means for unusual tropical fungal infections to be imported.

In superficial mycoses, infection is localised to the skin, the hair, and the nails. An example is “ringworm” or “tinea”, an infection of the skin by a dermatophyte. Ringworm refers to the characteristic central clearing that often occurs in dermatophyte infections of the skin.
Candida albicans is a yeast which causes candidiasis or “thrush” in humans. As a superficial infection, candidiasis typically infects the mouth or vagina. C. albicans is part of the normal flora of the vagina and gastrointestinal tract and is termed a “commensal” organism. However, during times of ill health or impaired immunity the balance can alter and the organism multiplies to cause disease. Antibiotic treatment can also alter the normal bacterial flora allowing C. albicans to flourish.

Subcutaneous mycoses penetrate a little deeper into the body than superficial infections, but are still confined to the dermis, subcutaneous tissue or adjacent tissues. These infections may occur following wounding of the skin and the introduction of foreign matter such as soil or vegetation. These mycoses are rarer and mostly occur in tropical regions. They tend to be slow in onset and chronic in duration. An example is sporotrichosis caused by Sporothrix schenckii. This fungus is dimorphic, being a mould that can convert to a yeast form when grown at 37°C on rich laboratory media or during infection. Sporotrichosis was once common in Europe but cases are now rare. The disease is most prevalent the American tropics, South Africa and Australia. Infection usually follows an insect bite, thorn prick or scratch from a fish spine. Certain occupations appear to have increased risk of infection. These include florists, farm workers and others who handle hay and moss.

Systemic mycoses are invasive infections of the internal organs with the organism gaining entry by the lungs, gastrointestinal tract or during medical treatment through non-sterile intravenous lines. They may be caused by fungi which are primarily pathogenic or by opportunistic fungi that are only marginally pathogenic but can infect immunocompromised hosts. Systemic fungal infections in previously healthy people arise most frequently from the respiratory tract. Examples include histoplasmosis, blastomycosis, coccidiomycosis and paracoccidiodomycosis. These fungi occur throughout the world but infections are not common in industrialized countries.
Histoplasmosis is caused by Histoplasma capsulatum. This organism is also dimorphic (being a mould that can convert to a yeast form). H. capsulatum is endemic in many parts of the world including North and South America. It is found in the soil and growth is enhanced by the presence of bird and bat excrement. Environments containing such material are often implicated as sources of human infection. The lungs are the main site of infection but dissemination to the liver, heart and central nervous system can occur. Pulmonary infection can resemble the symptoms seen in tuberculosis, complicating diagnosis.

Opportunistic fungi affect only patients usually have some underlying immune or metabolic defect, or have undergone surgery. Such diseases include aspergillosis, systemic candidosis and cryptococcosis. Exceptionally, other fungi that are normally not normally pathogenic, such as Trichosporon, Fusarium or Penicillium, may cause systemic infections.
Aspergillosis is the name given to a number of different diseases caused by the mould Aspergillus. This organism produces large numbers of spores and occurs world-wide. In the United Kingdom, A. fumigatus is the most common species causing disease. The organism can infect the lungs, inner ear, sinuses, and sometimes the eye of previously healthy people. In immunosuppressed hosts, Aspergillus can become disseminated throughout the body. In severely immunocompromised patients (e.g. those receiving chemotherapy) Candida albicans, which is a part of the normal human flora, can proliferate and become disseminated throughout the body.

Cryptococcosis is a systemic infection caused by the yeast Cryptococcus neoformans. The commonest type of this disease is a subacute or chronic form of meningitis resulting from the inhalation of the organism. Pulmonary infection involving the lungs can also occur. This disease affects both healthy and immunosuppressed individuals and occurs world-wide. C. neoformans can be isolated in large numbers from pigeon droppings in the environment, although the birds themselves do not appear to harbour the organism.

Pneumocystis is an infection of the lung caused by Pneumocystis carinii. This organism is a common cause of a potentially fatal pneumonia in AIDS patients. A complex intracellular parasite, with a life cycle of trophozoite and cyst, it was formerly considered to be a protozoan. However, comparison of DNA and RNA sequences have established that it is one of the group of red yeast fungi. P. carinii is a commensal organism of many wild and domestic animals and evidence suggests that human infection is commonly derived from dogs.

Constant exposure to fungal spores in the atmosphere can induce respiratory allergies. Elevated antibodies to a range of common spore forming fungi have been demonstrated in occupational diseases such as Humidifier fever, Malt workers’ lung and Wheat threshers’ disease.
Certain fungi, such as mushrooms, can produce poisonous toxins that may prove fatal if ingested (e.g. Amanita phalloides, the “death cap”). Others such as Psilocybe (SY-low-sy-bee) (magic mushrooms) affect the central nervous system inducing hallucinogenic responses.
Many moulds produce secondary metabolites (mycotoxins) which are highly toxic to humans. Ergotism is caused by eating bread prepared from rye infected with the fungus Claviceps purpurea. Historically, several large scale outbreaks of madness in local populations have been attributed to ergotism.

To sum up, like other micro-organisms, fungi are all around us. Most of them are very helpful in the natural environment and only a few are villains, causing disease in humans and animals. Don’t loose any sleep!

This entry was posted on Tuesday, January 9th, 2007 at 9:49 am and is filed under Biology, Environment, Fungi, Health, HIV/AIDS, Medicine, Microbiology, Mycology, Podcast, Science. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.