Hot Topic

Superficial Fungal Infections: Dermatophytes

Part 1


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Published: July 2012

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Dermatophytes infect the skin, hair, and nails in humans. Some are transferred from contact with soil, some from animals to humans, and some only infect humans. Dr. Roberts shares his lifetime of experience to assist you in differentiating the organisms under consideration in your patient’s differential diagnosis. Part 1 provides an overview of the dermatophytes and their ecology and microscopic and clinical features. Part 2 provides additional clinical presentations with a focus on individual species.

Presenter: Glenn D. Roberts, PhD

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Welcome to Mayo Medical Laboratories Hot Topics. These presentations provide short discussion of current topics and may be helpful to you in your practice. Our speaker for this program is Dr. Glenn Roberts, a Professor of Laboratory Medicine and Pathology and Microbiology at Mayo Clinic, as well as a consultant in the Division of Clinical Microbiology. Dr. Roberts discusses the most commonly seen dermatophytes in clinical specimens in North America. This is Part 1 of a 2-part presentation. Thank you, Dr. Roberts. Thank you for that introduction, Sharon.

I have nothing to disclose.

Today we're going to talk about the subject of superficial fungal infections and these include the dermatophytes that can infect a normal host, as well as the immunocompromised host. But the normal host and often they involve the hair, skin, or nails. With the immunocompromised host, since the immune system is diminished, these dermatophytes can attack potentially any site and so you can expect even a systemic disease with these dermatophytes.

In terms of the microbiology, there are 3 genera of dermatophytes that we deal with in the clinical laboratory. And the first genus is the genus Trichophyton. And it’s characterized by macroconidia that are elongated, and pencil to cigar-shaped. And it’s smooth-walled, multiseptate, and thin-walled. The problem is that most of the time you don’t see those macroconidia when you are looking at these, this particular genus. You see microconidia, and the arrangement and sometimes the shape of these microconidia is what we use for identifying dermatophytes on the traditional basis. So in general, the microconidia, the things that are predominate in most of the cultures, and sometimes they are arranged in clusters, like grapelike clusters, other times they are lined up along the sides of the hyphae, and then there are times when we just have problems getting them to sporulate at all and we can’t tell what they are.

This is a slide showing you a dermatophyte and it belongs to the genus Trichophyton and you can see the small spores, these are the microconidia, the smaller spores. And the large ones you see in there that are elongated and kind of maybe a little bit pencil-shaped are the macroconidia that characterizes the genus. But finding those really doesn’t tell us what the organism is, it’s those microconidia that we deal with.

The next slide shows that other genus, the second genus that we are going to talk about is the genus Epidermophyton or Epidermophyton, depending on how you want to pronounce it, this genus produces, has, or contains 1 organism, Epidermophyton floccosum and it produces these large, multiseptate, smooth-walled, club-shaped macroconidia and no microconidia at all. And that’s what you see to characterize this genus. The other genus that we deal with in the clinical laboratory is the genus Microsporum. And it is characterized by having large, multiseptate, rough-walled, and spindle-shaped macroconidia. And we see these macroconidia most of the time in the cultures and that’s what helped us to identify the genus. However, though, we still see some microconidia in select cultures and so it’s a little bit confusing sometimes, but we’ll see and talk about these and some of these and talk about them as we go along here.

This is a genus Epidermophyton and it has these club-shaped macroconidia that are smooth-walled. There are no little spikes sticking out of them at all. And they occur singly or in clusters of 2, and no more than 3. And you can see on here there is a cluster of 3 on this one here at the top and so this is what we use to characterize the Epidermophyton.

And then the genus Microsporum is characterized by these rough-walled macroconidia in contrast to what we just saw a second ago. The shape of these macroconidia varies depending upon the organism. This one has happens to be a bit spindle-shaped, you can see here.

The next slide shows you the common dermatophytes that we see in the clinical laboratory, kind of based on the order of predominance in some ways depending on where you are, though. But by far and large, the most common dermatophyte that we see in North America probably in the world is Trichophyton rubrum. Trichophyton mentagrophytes; Trichophyton tonsurans; Trichophyton verrucosum that you find infecting cattle and we see it in humans; Microsporum canis, the one that infects cats and dogs and we do see it affecting humans; Microsporum audouinii; and Epidermophyton floccosum.

The next slide talks about, a little bit about how these dermatophytes work. They are keratinophilic. That’s what they invade, the keratinized tissue and sometimes you can, you can almost guess what organism or group it might belong to, which genus by looking at what it involves. The genus Trichophyton involves the hair, skin and nails, so it is all of the keratinized tissue. Microsporum infects the hair and skin, and not the nails. And Epidermophyton infects the skin and nails, and not the hair. So this is just something that you use as a little bit of a guide. The thing about dermatophytes is they infect normal hosts, all of the time. Probably one of the most common groups of fungi that we see in the clinical laboratory, dermatophytes, don’t see them involving immunocompromised patients very often, but certainly immunocompetent patients, yes.

The next slide talks about how you can group these dermatophytes in another way if you wish. This slide shows you how they are grouped in the environment. Dermatophytes, 3 major groups: geophilic, the ones found in the soil; zoophilic, the ones that infect living animals including humans; and then anthropophilic, the ones that involve only humans, infect humans and that’s all, and they don’t infect any other animal source at all. So, 3 genres. We don’t use this characterization very often in the clinical lab. It doesn’t have a lot of relevance as to what we do.

The next slide shows you some examples, though, of these 3 groups. The geophilic group, Microsporum gypseum, this is one that is found in the soil and people like florists and other people who handle soil are the ones who acquire infection by this organism. The zoophilic group contains the organism that infects dogs and cats, Microsporum canis, and this is one that is highly infectious and not only infects the animals but human sources as humans as well. And there have been a number of outbreaks where someone will bring some kittens into a nursing home or some other place where there are some folks all gathered together or live together, and you will find that pretty soon that whole population if they have handled that cat or dog will come down with Microsporum canis infection. Trichophyton mentagrophytes is a zoophilic organism that infects animals, but it can infect humans as well. And then Trichophyton verrucosum is one I mentioned and it infects cattle, and people that have cattle herds, dairy herds, often times will get the infection from those infected cows. So these are examples of these 2 groups.

And then the anthropophilic group that only infects humans. Really, the 3 here that infect humans exclusively would be Trichophyton rubrum, Trichophyton tonsurans, and Epidermophyton floccosum. Trichophyton mentagrophytes can infect animals as well as humans. And so there is some overlap with these groups and they don’t fit nicely into compartments that you would like to put them in. So this is kind of how we can group these organisms to make a little more sense out of them.

And the next slide shows you kind of what goes on in a clinical laboratory. One of the first things that a dermatologist will generally ask for, and they do it often times in their own office, is what are you going to see in a direct microscopic examination of a piece of skin or nail or maybe a piece of hair. You may find just hyphae that are septate and have no pigment in, so they are hyaline, and then you may find some of the hyphae that are breaking down into arthroconidia, those rectangular spores. And that may be what you will see in a direct microscopic examination. It doesn’t tell you for sure that it is a dermatophyte and it certainly doesn’t tell you which one it is, but it suggests that there is a dermatophyte here if it is from one of the samples that we talked about, the hair, the skin, or the nails. There are other things that can infect those sites, but for by and large for the most part the dermatophytes are the ones that we see doing that.

The next slide shows you a KOH prep, potassium hydroxide prep, just showing you hyphae in there that has septations. And it looks a little brown, it just happens to be the way this slide is, but you can see the septate hyphae in there and that’s maybe all that you see. And that’s good enough to suggest a dermatophyte.

The next slide shows you the same thing except it shows you the hyphae are kind of intertwined amongst the epithelial cells that you see there and it’s hard to see if it’s septate or not, but they are narrow hyphae.

The next slide shows you another classification for things in this group and that is a classification based on which anatomic site these organisms infect. And this helps the clinician because we often know which dermatophytes involve the hair, the skin, or the nails, not necessarily the hair, skin and nails, but may involve just the feet, it may involve the hands, it may involve the scalp, facial area, and we have names for those clinical conditions. So what we now, we characterize these clinical presentations is by calling them ringworm of whatever it happens to be and tinea pedis means ringworm of the foot. tinea refers to ringworm. So tinea pedis is Latin for infection of the feet. Primarily involving the soles, the toe webs, and areas between the fourth and the fifth toes, and that seems to be a characteristic that you see.

The next slide shows you a foot with infection between the fourth and fifth toes. And you may not see anything, but just some simple scaling of the skin or you may see an area that is really infected and you see a lot of erythema and a lot of exfoliation of the skin.

And then in the next slide you see an extreme case of tinea pedis where this person obviously had been in contact with some water for a long period of time and this dermatophyte invaded the feet and the nails. And you can see everything is infected and it would take a long time to treat a patient with that sort of thing. These dermatophytes for the most part are nuisances more than they are anything severe, except in a case like this.

The next slide shows you another clinical condition called tinea cruris, ringworm of the groin or the perianal area. And we know of at least 1 dermatophyte that causes infection in that area.

And the next slide shows you what tinea cruris looks like. This is where the area in the groin, the inguinal area or the perianal area gets all red and inflamed and it has a very active border that you can see on the left-hand side of the slides you see where the redness stops, that’s the active border. And on the right hand side, you can see the same thing. If you were going to make a KOH preparation from a patient who had a dermatophyte infection, whether it is tinea cruris or something else, where you would like to go to get that sample would be at the advancing edge of this infection. So you would be right there where the redness stops, that’s where the dermatophyte would start to invade the normal tissue. And that’s where you are going to find it. Also on the right-hand side of the slide, you see there is a satellite lesion sitting over there. It’s a typical ringworm lesion. So in reality if you get right down to it, this is a case of what we call tinea corporis, because it means that it involves more than just the groin and it probably involves some other sites, and if you were to see that patient you more than likely would see some sites on the smooth skin of the trunk. So tinea cruris does look like what you see there along the groin area.

The next slide talks about tinea corporis. Tinea corporis is an infection of the smooth skin of the trunk and the extremities. And that happens frequently in some patients and it is exclusive of the face, the foot, the groin, the hands, or the scalp area. So it’s the smooth skin of the trunk and extremities for sure, that’s how it presents. So these clinical presentations help the clinician because we can sort of link which dermatophytes cause these particular entities and we will show you that as we go along here.

The next slide shows you a case of tinea corporis. This lady has lesions that you see on the smooth skin of the trunk and they are typical ringworm lesions. The one down at the lower right-hand side is actually a very typical one. What happens with these lesions is that as they begin to heal, the center of the lesion shows the first area of healing. And then you see, after a while, a little evidence of infection in the center, but you see an advancing border and you see an area within there that is still erythematous and that is what gives it kind of a ringworm appearance they talk about.

The next slide is tinea capitis. This is infection of the scalp, the eyebrows, and the eyelashes, and it primarily occurs in young children before the age of puberty. It has been in the past, it used to be a real common thing but I don’t know how common it is these days. Tinea capitis though, generally, for the most part refers to the infection of the scalp.

The next slide shows you a child’s scalp and, if you look, you see that some of the hair has gone off of there. That is called alopecia and that just means baldness. And that may be all the child may experience is the loss of hair. In some cases the lesions may become much more apparent and go down much deeper in the tissue and look much more severe. And we have seen cases like that before. And sometimes what happens with the ringworm, all the ringworm infections for that matter, is they make it secondarily infected with something like staph and you get foliculitus where the hair follicles are and they will get terribly infected with something that you have to treat with antibiotics.

The next slide is a term that is used as kind of a general term. It’s called onychomycosis. And it kind of means the infection of the nails and the surrounding tissue, the paronychia of the nails. In these cases, things other than dermatophytes can cause onychomycosis. Things like Aspergillus, Fusarium, and other things that we see in the clinical laboratory besides dermatophytes can actually be involved in causing onychomycosis. So this is where we branched out just a little bit more.

The next slide shows you a case of onychomycosis where all these nails are involved. And to treat these nails with an antibiotic, antifungal antibiotic, would take an awful long time because what has to happen is the nail bed has to be sterilized down below the nail, you see going towards the hand, has to be sterilized and as it grows, a new nail grows out sterile, the old nail has to be clipped off. A lot of times what the dermatologist may do, is just to avulse these nails and take them right off and then start treating with an antifungal agent and sterilize the nail bed so that when the new nails grow back out they’re sterile.

The next slide shows you another ringworm, tinea pedis. Tinea pedis involves Trichophyton rubrum, Trichophyton mentagrophytes, and Epidermophyton floccosum. Notice- you see the same 3 again causing tinea cruris. And you are going to see that pretty much the same dermatophytes cause the same clinical entities most of the time. So, the common ones that we see are Epidermophyton floccosum, Trichophyton rubrum, and Trichophyton mentagrophytes. They cause tinea cruris.

Tinea corporis, same 2 with the addition of Microsporum canis. Tinea capitis, there are a couple of other. Actually, there are 3 new ones we haven’t talked about. Trichophyton tonsurans, Microsporum audouini, and Trichophyton violaceum are some that are seen infrequently but, nevertheless, are seen and they cause tinea capitis along with Microsporum canis. So, you never know what you are going to get when you try to culture one of these things. It depends on where the patient’s been, the contact they’ve had, and so one. So, you kind of have to be ready to identify the ones that we know are commonly associated with these clinical entities.

The next slide shows you a schematic of how to identify these dermatophytes, at least one. And I did it for all of the dermatophytes that we deal with on a routine basis. Not every one of them out there, because there are a huge number of dermatophytes ,most of which we don’t see. So, Epidermophyton floccosum. If you look at a culture, generally it’s a culture that you are going to have on Mycosel agar or cornmeal agar. Cornmeal agar, uses in the clinical laboratory where we find the white cornmeal agar is very good for sporulation of these dermatophytes. Mycosel agar is nothing more than Sabouraud’s Agar with chloramphenicol and cycloheximide in it. And it allows these dermatophytes to grow in primary recovery, and so it’s a good primary recovery medium for dermatophytes, then if you want a subculture to cornmeal agar, then you begin to see the sporulation. But what you’re going to see with the Epidermophyton floccosum is these large smooth-walled, club-shaped, macroconidia that we saw a while ago that occur singly or in clusters of no more than 2 or 3. And a colony is described in all of the literature as khaki colored. It’s kind of a yellow-green color. And in the center of the colonies, most of the time you are going to see a white area. And that is called an area pleomorphism. That means that what has happens to this culture is, that as it grows the center part of it becomes sterile. And if you try to subculture that center part for a laboratory work or something, you can end up with nothing more than just white sterile hyphae. So, if you take it from an area where you see sporulation, or from an area where the youngest part of the culture is, or in between there in the center, you are going to see the culture is sporulated. And it will produce those large club-shaped, smooth-walled, macroconidia and that would be Epidermophyton floccosum.

So, the next slide just talks about what you might see. There are other things that you might see with this organism. One would be chlamydoconidia. These are kind of over-wintering spores and Epidermophyton floccosum is kind of notorious for producing these nonspecific chlamydoconidia in hyphae, sometimes on the end of a hyphal strand, sometimes in the middle. These macroconidia are blunt, although you see they are rounded on the ends, pretty club-shaped and they don’t have any rough wall at all and they are multicelled, and we mentioned they occur in clusters of 2 or 3, and sometimes singly. Another thing that I want to mention, is that for those of you who do teaching, is that if you decide to keep your cultures in the refrigerator to keep them for subculture later, with the Epidermophyton floccosum it will die. It is not thermo tolerant. It is killed by the cold. It is just a bit of trivia, but sometimes, when you are in a teaching environment you kind of need to know things like that.

The next slide shows you what you saw a while ago, just upside down from what you saw it looks like a different slide but it is the same one. Epidermophyton floccosum with those multicelled, smooth-walled, club-shaped, macroconidia occurring in cluster of 2 or 3 and sometimes even singly, so this is what it would look like. You have to remember another thing and that is when we take photographs of these things, we look for the picture perfect ones and it takes a long time to find them because they don’t all look this good.

The next one is that we have seen in the clinical laboratory and you can see it doesn’t look like it’s exactly as clear as the others, but it has the same shape and you see the very same arrangement of the conidia.

The next slide talks about the culture morphology, and I mentioned before the khaki color, pale olive-green, maybe that would do it. Some of them are white in the center. A lot of times they have these folds in them as they are growing up and they look kind of velvety, and I mentioned this white sterile hyphae that will grow up in the center of it after about 2 or 3 weeks of incubation. This one is one you can identify by just looking at the microscopic morphology after you have subcultured it from a primary recovery medium.

The next slide shows you what it looks like. And this is the khaki color that people talk about. I don’t know if I would call it khaki color or not, but it has this kind of greenish yellow looking colony, and you will notice there is a little bit of white on all of these colonies here some on the top. It is the heaped up area in the center is what I was talking about that becomes pleomorphic.

This ends part 1 of the superficial fungi and the fungal infections that we will talk about some of the other groups in part 2.