Current Vet

In this episode of Current Vet, Dr. Lottie talks about dermatophytosis, why this fungus targets the superficial skin layer, what diagnostic tests are available and how we can treat the infection.

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Timeline:
00:00 Intro
00:36 Case
02:08 Aetiology & Pathogenesis
04:58 Clinical Signs
06:39 Diagnosis
10:23 Treatment
14:03 Zoonotic Potential
14:54 Prevention
15:46 Key Points
16:55 Outro

References

Current Vet is an educational podcast intended for veterinary students, veterinary professionals, and individuals with an interest in veterinary medicine.

All content provided in this podcast and its associated materials is for educational and informational purposes only. It is not intended as, and must not be considered a substitute for, professional veterinary advice, diagnosis, or treatment.

Any clinical cases discussed in this podcast are fictional, and are designed to reflect typical or likely clinical scenarios for educational purposes. They do not represent specific real-life cases, clients, or animals.

While every effort is made to ensure accuracy and alignment with current evidence at the time of publication, veterinary medicine is a rapidly evolving field, and recommendations may change over time.

Creators and Guests

Host
Dr. Lottie Wilkinson
Creator and host of the Current Vet podcast

What is Current Vet?

The podcast that makes veterinary medicine simple.

In each episode, Dr. Lottie breaks down clinical conditions, cases, and concepts across species, focusing on pathophysiology, decision-making, diagnostics, and what actually matters in practice. It’s the kind of context that makes your knowledge finally click.

Every month, we’ll also have honest conversations with guests about the incredible variety of veterinary medicine, what you can do with a vet degree and how to think bigger about your career.

Whether you’re cramming for exams or looking for a soundtrack for your dog walk, Current Vet will make vet med simple

Intro

Hello, hello, and welcome back to Current Vets. For anyone who's new here, this is the podcast that makes vet new medicine simple. I'm your host, Dr. Lottie, and today we're covering a condition that you're very likely to see in first opinion practice on your farm placements or a mixed practice. It is dermatophytosis, also known as ringworm.

It's common, it spreads easily, and it's zoonotic, so it's definitely a top condition to be aware of, even if it's not causing severe illness. So let's get straight into our case.

Case

You're called out to a dairy unit in late winter. The farmer says there have been a few calves with crusty bald patches on their faces.

It's spreading through the group with more calves recently showing signs, and one of the farmhands has a rash on his arm. You walk into the calf shed and notice that they are housed in fairly close contact and that the air is a bit stale. It's a little bit musty. You examine a four-month-old calf who is bright and alert, eating normally, but you notice that they have several circular areas of alopecia around the eyes and muzzle, a little bit of gray/white crusting around these lesions, and some scaling at the edges.

You check on a few more calves and see the exact same pattern, so at this point, ringworm is going to be high on your list. But you still wanna think about a couple of other differentials because a few conditions can mimic this. Your main differentials here are going to be obviously dermatophytosis, or ringworm.

We also have lice, which is especially high risk in housed young stock. It can cause rubbing and patchy hair loss. We have mange mites and then a couple of lower down differentials might include dermatophylosis, nutritional dermatoses, or photosensitization can cause similar lesions, but they probably wouldn't fit well with this clinical picture, so they're very low on our list. Of course, the fact that there is a human case also pushes our suspicion of ringworm much higher,

Aetiology & Pathogenesis

what is dermatophytosis?

So it is a fungal infection of keratinized tissues. So the fungus lives in the outer layer of the skin, known as the stratum corneum, and the hair shafts, as well as hoof horn and nails can also become infected, but this is less common. Dermatophytes are filamentous fungi, so they grow as those branching hyphae instead of as yeast, and they produce enzymes which break down keratin to use as a nutri-nutrient source, and that is why they target these keratinized tissues.

So in cattle, the classic organism is Trichophyton verrucosum. In other species, they typically get other key fungal infections. So in cats and dogs, it's often Microsporum canis. Horses often get Trichophyton equinum, and sheep and goats get various Trichophyton species infections.

These dermatophytes produce infective spores, known as arthroconidia, which are shed into bedding and into dust. They can stick to things in the environment, things like fences or gates, crush rails, maybe calf jackets and halters or grooming tools for horses, and they can persist in the environment for a long time, especially if they are in organic material and cool, shady environments.

the way they cause disease is fairly straightforward. These spores land on the skin or the hair, often helped to spread by moisture or overcrowding, or minor skin trauma. These spores then germinate and grow as hyphae in the outer skin and the hair shaft.
The fungus produces those keratinase enzymes that break down keratin. The hair shafts become brittle and break, and that is causing that alopecia and stubbly broken hair appearance. The skin is gonna respond with inflammation secondary to this infection, producing exudate and cell debris, which causes that scaling and crusting.

As the fungus spreads outwards in a circle, it then creates that ring appearance because the center of the ring where the infection originated can actually heal over as the fungus expands outwards most healthy animals eventually develop a cell-mediated immune response that clears the infection, which is why ringworm is described as self-limiting.
But infected animals can keep shedding infective material while the lesions are resolving. So in a housed group, that means infection is spread very quickly through the group, and even apparently recovered animals can still be sources of that infection.

Clinical Signs

What clinical signs are we going to see? So the typical signs are that circular alopecia, gray white crusts around the lesion, scaling, and typically it's not very pruritic, so minimal pruritus and itching, but this can vary depending on the case. The common sites of infection are gonna be around the eyes, around the muzzle and the face, the neck, and then sometimes they spread further in heavier outbreaks

Dermatophytosis typically affects young animals as they have a naive immune system , or it's also common in immunocompromised animals. Infection often appears when animals are housed, especially in winter and multiple animals are typically affected within a short period in this time.

In horses, lesions are often seen under tacking areas, so the girth, the saddle, and the withers, and they're also linked to shared grooming kit, tacks, rugs, and clippers, so the distribution of lesions can be a bit different

In sheep and goats, the circular alopecia and that scale crusting around the face and neck is the same, but the lesions may not be as visible in sheep because of their wool. So parting that wool and getting a really good look at the actual skin layer is really important. So the pattern across species is essentially the same.

It's a superficial fungal infection that causes hair breakage, scaling, crusting, and that characteristic circular area of alopecia

Diagnosis

Let's go back to our calves and talk through what the next steps are gonna be for diagnosis
In practice, clinical diagnosis may be sufficient , but a confirmation is most useful when lesions appear atypical. Animals are needing to be moved or go to market. You need to document clearance in a yard or for a herd plan or if there is a significant zoonotic concern.
So first up, we've got that clinical diagnosis. So in a classic housed calf outbreak, clinical diagnosis is typically sufficient based on those characteristic lesions, the younger age group, the fact that they're housed with close contact, poor ventilation, overcrowding perhaps, and if multiple animals are affected as with our case.

But you should still keep ectoparasites on your radar, especially if there is quite marked pruritus

If we do want to go down the confirmation route, the first step we could do is microscopy, and this is with a potassium hydroxide prep. This uses hairs and scale from the active edge of a lesion and potassium hydroxide preparation works because it breaks down keratin and clears debris, making the fungus much easier to see under the microscope.

So you prep that slide, you add the potassium hydroxide. If you see hyphae or spores associated with the hair shafts, that is supportive of dermatophytosis. This test is really quick, it is cheap, but its sensitivity depends heavily on the quality of the sample, basically whether you actually get a sample that contains the fungus and the interpretation of the findings,

Next up, we could do a fungal culture. Culture is still the kind of gold standard confirmatory test because you can grow that specific fungus and confirm the species. It uses hair plucks and scale as the sample from those lesion margins again, and the lab grows the organism in dermatophyte test media, which is specialized media for the fungus, and then can identify it once that culture has grown.

So the pros of this are obviously that it's gonna confirm infection, and it can identify species, but it is very, very slow. Often takes, up to three weeks, and it may be contaminated with other pathogens, which is gonna complicate the interpretation of results. So in practice, it's probably not that useful.

It's definitely not gonna be guiding your treatment decisions if you're waiting three weeks for confirmation, so typically not used in practice but is available as a test and something you should be aware of

Finally, we also have PCR. PCR is gonna detect the dermatophyte DNA. It is rapid, it's sensitive, it's really useful for outbreak decisions if you need confirmation. But it is more expensive, it's not available everywhere, and it can also amplify contaminating pathogens. So again, we need to interpret it within the clinical context

A quick little sampling rule that is important before you go into treatment, take samples from the active lesion edge because that is where the fungi are most abundant and they're actually growing from, and avoid aggressively cleaning the site before you sample. You wanna sample the organism. You do not want a sterile sample.

So don't scrub it, and make sure you're sampling the active lesion edge

Treatment

Okay, now treatment has two main goals. First is to reduce the fungal burden and shedding in infected animals, and then second is to stop ongoing transmission by controlling the environment and fomites

our first treatment option is topical antifungals. But before we put these on, we should be cleaning the animal, maybe with some shampoos, to remove any organic debris, and also clipping the hair around lesions to really make sure that the topical treatments can be as in close contact and as effective as possible.

Topical therapy is really practical because the fungus lives superficially, so it targets the fungus best as opposed to systemic therapies. The options are of course gonna vary a little bit by country and depending on licensing, but commonly used options include washes and sprays, and other azole-based topical products and shampoos such as a ketoconazole
A quick little summary of how azoles work. So basically, fungi rely on ergosterol for their cell membrane. Azoles are gonna block ergosterol production. So this means that the membrane fails, it's dysfunctional, and then fungal growth is inhibited and the fungi die. Another alternative to these antifungals is lime sulfur dips.

Lime sulfur actually has antifungal, sporicidal, and ker- keratinolytic activity. So it acts to make a really hostile environment for the fungi, and they die off. But we need to warn owners and farmers that it can cause permanent hair discoloration in animals, especially with the lighter or white coats.

So that is something that they need to watch out for, and if it's something that's gonna bother them, for example, if they are a show dog or a highly valuable animal, that is maybe something that they would not want

Okay, next we have systemic antifungals. So the options here include itraconazole, grijosea fulvin, terbinafine, but especially in large animals, systemic therapy is going to be limited by the cost of the treatments, they have long withdrawal times,

And also the fact that many cases actually resolve over time if infection control approaches are really strict

So what is gonna be important here is our environmental management approaches. So let's quickly talk about those Assume anything that touches infected animals can spread spores. So we're thinking calf jackets, horse halters, grooming brushes, gates, rails, and fences, any tacks or halters, and rugs. The goal here is gonna be to reduce sharing of equipment, and if sharing is unavoidable, making sure that anything being shared is cleaned and disinfected between each animal.

Disinfection works best after physically cleaning the object because organic debris is gonna protect those fungal spores from disinfection if it's still there. In housing, especially focus on any rubbing points, so corners of pens or railings or gates, as this is where spores are most likely to accumulate and spread from.

Improving ventilation and reducing humidity whenever possible, and making sure that we're avoiding overcrowding because this will increase contact and environmental contamination, so reducing that stocking density whenever possible is also really useful

Zoonotic Potential

I just wanna touch on the zoonotic risk of ringworm. So ringworm is zoonotic, and people typically get infected through handling, through direct touching or handling of lesions, contaminated clothing, or contaminated equipment. In humans, it usually causes, like, that itchy, circular expanding skin lesion, and what we should be doing to avoid infection is making sure we're really hot on our PPE, so wearing gloves, wearing that protective clothing when handling potentially infected animals, washing hands really thoroughly, keeping work clothing separate from household laundry when possible, and if someone develops suspicious lesions, they must speak to their GP for appropriate treatments

Prevention

Prevention's mainly gonna focus on those environmental management tips, as we said before. So disinfection, physical cleaning of objects, focusing on any of those rub points, highly contaminated areas, the ventilation, the overcrowding and stocking density. But there is also actually vaccines for ringworms in some regions, especially for cattle, but the availability and licensing are country dependent.

So where they are used, they're typically part of a wider control plan, and they should not be considered a replacement for good hygiene and good management. Basically, even if vaccines are available in your region, check local guidance and make sure we're still prioritizing that environmental control

Key Points

Okay, here are the top things that you need to know about dermatophytosis. Ringworm is a contagious zoonotic fungal infection of the hair and superficial skin layer. It's caused by dermatophyte fungi, which are filamentous and spore-forming. Lesions are usually areas of circular alopecia , with that scale and gray-white crusting, most often found around the eyes, the muzzle, the face, and the neck.

Diagnosis is typically based on clinical presentation, especially in outbreak scenarios, but confirmation can be done with microscopy, with culture, or PCR. Treatment relies on topical antifungal therapy, but it must be used alongside environmental control, otherwise reinfection and spread can continue.

Don't forget that PPE and hand hygiene are essential for people, and any suspicious lesions in humans must be seen by a doctor.

Outro

And that is dermatophytosis or ringworm. If you found this helpful, follow and like the podcast, Current Vet, wherever you're listening from. Share it with a friend or colleague who would find it useful and of course, let us know if there's any topics that you want to hear next.

You can find us on Instagram and TikTok at VeterinaryVista or leave a comment below. We'll see you next time.