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What is Ringworm? - Infections Caused by Trichophyton & Microsporum Fungi

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  • 0:05 Mistaken Identity
  • 2:03 Tricophyton and Microsporum
  • 3:35 Ringworm
  • 4:14 Diagnosis and Treatment
  • 5:16 Epidemiology
  • 6:34 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Ringworm is a common pathogen that infects the skin, hair, and nails of people and animals. This disease is not caused by a worm, however. In this lesson we will examine the fungus that causes ringworm and its impact on human health.

Mistaken Identity

Before the advent of DNA sequencing, it must have been tough to be a fungus. The poor fungi just never got the credit they deserved. They were originally classified as plants, which does make some sense. After all, a mushroom does appear to have more in common with an oak tree than an alligator. Many single-celled yeasts were considered bacteria or protozoa. Historically, the classification of different fungal groups and species was constantly changing as technology improved.

Comparing DNA sequences of all these different organisms has managed to finally give the fungi a rightful place of their own in the tree of life, properly grouping the mushrooms, molds, and yeasts in one big happy family. You would think this would clear everything up, but commonly used names, regardless of accuracy, can be hard to replace.

Let's take a look at an example. Have you ever had, or known someone else who's had, ringworm? The classic ring-shaped rashes associated with ringworm are not caused by a worm at all. The organism that causes the disease is a fungus. People thought the disease was caused by a worm for so long that the name has stuck. Often, ringworm is described with the word tinea followed by the body part of the rash. Tinea pedis is ringworm on the feet, also known as athlete's foot. Again, tinea means worm, not fungus. Another term used to describe ringworm is dermatophytosis. 'Dermato' means skin, 'phyt' means plant, and 'osis' refers to a disease. So this name literally translates to 'disease of the skin caused by plants'.

What I hope to have gotten across by this example is that ringworm is caused by a fungus, despite the many names that suggest otherwise. Let's take a closer look at this fungal pathogen.

Trichophyton and Microsporum

There are about 40 species of fungus that can cause ringworm in humans, but the most common belong to three genera. Trichophyton is the most common cause of ringworm in humans. Microsporum is the most common cause of ringworm in pets, but it can be transmitted to humans. Epidermophyton is the least common of the three main causes. All three cause similar disease, are acquired by similar routes of exposure, and require similar treatments, so from this point forwards, I will simply refer to them all by the common name ringworm.

The ringworm fungus thrives by digesting the protein keratin. This protein is a major structural component of the hair, skin, and nails, limiting ringworm infection to the body surfaces. The fungus does not penetrate into the body tissues, cause any internal systemic disease, or become life threatening.

Ringworm must be acquired by direct contact with the fungal organism. Contact can be with a person or pet that has an active infection, or the fungus can be picked up from contaminated objects. A fomite is any inanimate object that carries and transmits a disease-causing organism. In the case of ringworm, combs, clothing, hats, bedding, towels, and floors can harbor the fungus. In fact, any fomite that remains damp can support ringworm for weeks, infecting anyone that comes in contact.

Ringworm

Ringworm can occur on any body surface. Common areas of infection include the scalp, nail beds, feet (commonly called athlete's foot), and the groin (commonly called jock itch). Symptoms include the classic ring-shaped blister-like lesions, itchiness, thin discharge, and inflammation. Infected hair can change color or fall out. Infected finger and toenails often thicken, discolor, and may crumble. Rarely, irritated lesions can get infected, leading to abscesses, but nothing more severe occurs from ringworm infection.

Diagnosis and Treatment

Diagnosis of ringworm is usually based on symptoms. Most frequently, simply observing the lesions is enough to warrant treatment for the parasite. More in-depth diagnostic methods are available. Skin scrapes can be observed microscopically or cultured on dermatophyte test media, looking for the characteristic fluffy, white fungal growth. Some species of ringworm produce wastes that will glow when exposed to UV light. Shining a black light, called a Wood's lamp, on infected areas can positively identify the organism.

Treatments are considered very safe and effective, so these advanced methods of diagnosis are rarely needed. Over-the-counter powders and ointments dry the area while lowering the pH, making the skin inhospitable for ringworm growth. Common topical antifungals like Tinactin and miconazole are usually all that is required. Rarely, severe, widespread infections can be treated with oral griseofulvin.

Epidemiology

It has been estimated that upwards of 90% of men will have had ringworm at least once by the time they reach middle age. For some unknown reason, women don't seem to get the fungus as often, but they are still susceptible.

Ringworm can cause disease in any person, regardless of health, but there are a few groups at higher risk of ringworm infection. Athletes engaged in contact sports like wrestling; people that work with animals; anyone that frequently uses communal baths, like in gyms and fitness centers; and people that frequent hair and nail salons are more likely to come into contact with organisms and objects that harbor ringworm.

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