What are mold infections?
A mold infection, or mycosis, is the growth of mold or fungi in the body. Molds are fungi that grow in a filamentous form. Generally, an infection implies active growth and not merely presence in a particular body site.
Mold infections are caused by fungi. Fungi (the plural of fungus) are eukaryotic and nonphotosynthetic, and they (usually) contain the chemical compound chitin in their cell walls. All of these features distinguish fungi from other classes of infectious agents such as bacteria, viruses, and parasites.
Fungi are divided into yeasts and molds. Molds grow by branching and longitudinal extension (adding cells to the end of filament), while yeasts grow by budding or by binary cell division. Molds are composed of long, thin hyphae that aggregate to form a mycelium. The mycelium (plural mycelia) is the mass formed when hyphae grow extensively around and on top other hyphae.
Although there are thousands of species of molds, most do not cause disease in healthy people. However, almost all fungi have the potential to colonize humans, especially people with severely compromised immune systems. Molds are acquired from an environmental source and not through person-to-person contact.
Healthy people generally have the ability to combat the fungi they encounter. Accordingly, the most important risk factor for developing mycoses is the health of the host. Deficiencies in the immune system, such as human immunodeficiency virus (HIV) infection, acquired immunodeficiency syndrome (AIDS), and neutropenia, and deficiencies caused by immunosuppressive therapy and even old age, substantially increase the risk of mycoses. Other risk factors include poor lung function from other conditions such as chronic obstructive pulmonary disease, bronchiectasis, tuberculosis, sarcoidosis, and asthma.
Molds thrive in soil and moist environments, so people exposed long-term to soil, dust, and dirt are at greater risk of developing mold infections. Also, environments with poor ventilation can allow mold growth and spore formation, resulting in a higher risk of mold infections.
The symptoms of mold infection depend on the nature of the fungus and the body site affected. Mold can cause disease in humans in three ways: by ingestion or inhalation of toxins, by infection (mycosis), or by triggering allergic responses. For example, Stachybotrys (also called black mold) in buildings causes an allergic response triggered by environmental exposure.
The most common sites of infection are the respiratory tract (especially the lungs) and the skin and nails. Fungi that are invasive, especially in immunocompromised persons, can infect the internal organs, including the kidneys. It also can infect the central nervous system, the urogenital tract, and the lymphatic system.
Infections of the skin and nails, while serious to the person affected, almost never proceed to more serious invasive or systemic infections. These infections are usually caused by fungi capable of degrading keratin. Skin infections are often called ringworm, while nail infections are called ringworm of the nail. Infection of the toenails or fingernails is more formally called onychomycosis, a common affliction of persons with poor circulation, especially the elderly. The most common fungi causing these infections are dermatophytes, principally of the genera Epidermophyton, Microsporum, and Trichophyton.
The most serious fungal infections are often transmitted through the respiratory tract, that is, through inhalation of airborne spores. Symptoms of respiratory mold infections are often nonspecific and can include fever, cough, headache, rash, muscle aches, night sweats, and hemoptysis (coughing up blood).
The diagnosis of fungal infections generally involves an examination, or clinical observation, to check for particular symptoms. Also, diagnosis may include a laboratory fungal culture from affected body sites, serological tests for antibodies to a specific fungi, and radiologic imaging. Definitive diagnosis usually requires laboratory culture of the fungus and identification based on morphological characteristics. Histologic examination of biopsy material is often used to suggest the existence of a mold infection.
The more recent use of molecular identification tests (tests for specific genes of a fungus) has led to more rapid identification and to avoiding the need for identification based on sporulation. An example of a molecular test is the polymerase chain reaction (PCR), which can rapidly identify an organism both from culture and from affected clinical material (tissue or fluids).
Historically, mold infections, especially invasive infections, have been difficult to treat. Molds are not susceptible to antibiotics. Antifungal drugs have not usually matched antibiotics for convenience, efficacy, or safety. The most effective antifungal for serious or systemic fungal infections for many years was amphotericin B. However, while effective, amphotericin B has many deleterious side effects and must be administered intravenously. After being injected, many people experience high fever, hypotension, vomiting, headache, and nausea; these side effects subside within several hours.
Newer antifungals or new formulations of older antifungals have been approved for human use. Liposomal formulations of amphotericin B have significantly less toxicity, but they are still effective against many invasive fungi. Liposomal formulations of amphotericin B are not effective against dermatophyte fungi.
New drugs in the azole class of antifungals have been developed and may be of use both in systemic and in other fungal infections. These triazoles include itraconazole, voriconazole, ravuconazole, and posaconazole. An entirely new class of antifungals, the echinocandins, has recently been developed. These compounds act by inhibiting a specific step in the synthesis of fungal cell-wall components. They are effective at preventing fungal growth but have minimal toxicity to humans. Caspofungin was the first of the echinocandins to receive approval from the U.S. Food and Drug Administration. Other echinocandins available are micafungin and anidulafungin.
Most mold infections are very difficult, if not impossible, to prevent. Fungi are present in all environments. Generally, mold infections begin with airborne spores, which makes it impossible to avoid infection.
For persons who are immunocompromised, some measures that may be helpful include the avoidance of dusty environments and activities where dust exposure is likely (such as construction zones), the wearing of respirators when in or near dusty environments, and the avoidance of activities that disturb dirt or soil (such as gardening and yard work). In health care settings, air quality measures, such as high-efficiency particulate air (HEPA) filtration, should be followed.
Midgley G., Yvonne M. Clayton, and Roderick J. Hay. Diagnosis in Color: Medical Mycology. Chicago: Mosby-Wolfe, 1997. A medical mycology textbook with many color images. Includes detailed descriptions of common mycoses and the organisms that cause them.
Patterson, Thomas F. “Fungal Infections.” Infectious Disease Clinics of North America 20 (2006): 485-734. This special journal issue covers fungal infections and includes many useful articles on specific fungal diseases, emerging fungi, diagnosis, and therapy.
Richardson, Malcolm D., and Elizabeth M. Johnson. Pocket Guide to Fungal Infection. 2d ed. Malden, Mass.: Blackwell, 2006. A handy guide, with much visual information for both the nonexpert and thespecialist. Includes clinical presentation, diagnosis, and treatment for the major fungal diseases of humans.
Zumla, Alimudin, Wing-Wai Yew, and David S. C. Hui, eds. Emerging Respiratory Infections in the Twenty-first Century. Philadelphia: Saunders/Elsevier, 2010. A comprehensive work that includes discussion of the relationship between respiratory infection and molds and yeasts.