Ringworm in Dogs and Cats: Symptoms and Treatment [Smentek, DVM]

Medically reviewed by: Kinga Wojtczak, Doctor of Veterinary Medicine, specialist in canine and feline diseases

Did you know that you can catch ringworm from your dog or cat even if the animal shows no signs of disease? Ringworm in dogs is especially dangerous for children and older people.

When we assess a puppy or kitten we are buying, there are many health parameters we cannot determine without medical knowledge. However, we should always assess the appearance of the largest organ—the skin. It is on the skin that we can see with the naked eye any abnormalities, defects and areas of hair loss, and spot parasites. Any deviation from normal in the appearance of the coat is one of the deciding factors when acquiring an animal.

Fungal infections make up a large group of diseases that show on the skin, and this is the topic I will try to explain in this article. You will learn what ringworm is, what its causes and symptoms are, and how this highly contagious disease is treated.

Ringworm in dogs and cats

Ringworm (dermatophytosis) is a highly contagious disease of animals and humans caused by microscopic fungi. It affects the skin and its appendages. An important feature of this group of diseases is their very high contagiousness, which in practice means the disease spreads easily between susceptible individuals, including between animals and humans. In addition, fungal infections are considered one of the main, and most serious, causes of infectious skin inflammation in animals. Because ringworm mimics other diseases so well, it often makes diagnosis and proper treatment seriously difficult.

Fungal infections (mycoses) are a very large group of diseases that is hard to define clearly, but in general we can divide them into:

• cutaneous mycoses (fungal skin infections),
• mycoses of the internal organs (systemic mycoses).

Mycoses of the internal organs, while very serious diseases, are fortunately not very common in our climate, because they mainly affect animals living in warmer climate zones. Cutaneous mycoses, on the other hand, especially dermatophytoses, can be a real problem for many animals, and because they are so common, this article will be mainly about them.

What is dermatophytosis?

Dermatophytosis is a disease of the skin and its appendages—the hair, claws and epidermis—caused by fungi with a multicellular thallus, that is, keratin-utilizing dermatophytes. The term dermatophyte, translated simply, means "skin plant"; it is a complex multicellular organism.

Depending on the environment they live in, we divide them into:

• geophilic dermatophytes (they live and multiply in soil, using organic matter, and do not pose a major threat to mammals),
• zoophilic dermatophytes, which occur in animals and are always pathogenic,
• anthropophilic dermatophytes, which live on humans.

Anthropophilic dermatophytes and zoophilic dermatophytes do not reproduce in the animal's surroundings, but they can be present there as resting spores, including on the animal's coat. We should also mention infections with single-celled fungi, namely yeasts of the genus Malassezia or Candida, which, although they cause skin disease (Malassezia dermatitis or otitis externa), are not dermatophytoses.

Causative agents of dermatophytosis

The main causes of dermatophytosis in dogs and cats are the following fungal species:

• Microsporum canis,
• Microsporum gypseum,
• Microsporum persicolor,
• Trichophyton mentagrophytes,
• Trichophyton verrucosum.

The most important fungus in this group is unquestionably Microsporum canis, which is estimated to cause over 90% of all dermatophytosis cases in cats and dogs. In the vast majority of cases it occurs in cats, which are its natural host—the reservoir of these infections. We also often see subclinical infection and long-term carriage, especially in young animals up to one year of age or those living in crowded settings, such as shelters, and therefore exposed to stress.

Dogs can also contract ringworm through direct contact with a sick cat. Asymptomatic carriage is most often seen in long-haired cats and in cats over two years of age. Remember that in a carrier, although the fungi multiply and are thus shed into the environment, there are no clinical signs, so the cat looks like a normal, healthy animal while posing a threat to the animals and people around it.

The other fungal species cause disease in animals less often, as they account for only about 10% of cases:

• Microsporum gypseum, a geophilic fungus that lives in soil, is a threat mainly to dogs that dig burrows or holes or bury food.
• Trichophyton mentagrophytes, in turn, is a zoophilic dermatophyte found in adult rodents.
• Trichophyton verrucosum lives mainly on cattle and, for obvious reasons, is mainly a concern in rural areas and for the animals that live there.

Microsporum canis spores are shown below.

How can my dog or cat get ringworm?

Dermatophytes, as I mentioned, are complex organisms that form mycelia, and thanks to their ability to produce keratinase and proteolytic enzymes, they can use the main protein of the epidermis, keratin. Fragmentation of the hyphae produces arthrospores—spores that are the infectious form of these fungi. They are extremely resistant in a dry environment, where they can survive for as long as a year, but they are quickly destroyed by high temperatures of around 100 °C.

Because they can survive for a long time in the immediate surroundings, many animals can carry spores on their own coats.

In shed skin scales, which of course cannot be seen with the naked eye, they can remain infectious for many months, just as they can in
shed hairs, which always contaminate pets' beds, carpets, furniture, walls, floors, curtains and drapes.

Brushes, combs, shared bowls, plush toys and other toys and chews also play an important role in spreading dermatophyte infection. Fungal spores present in dust easily get even into rooms and places where sick animals are not kept. You could say they are practically everywhere. And of course we cannot forget direct contact between animals, which can infect each other with ringworm through play, skin abrasions and also fights.

Spores can also be carried by any ectoparasites living on our animals, namely:

• fleas,
• sucking lice,
• chewing lice,
• flying insects, such as flies.

So we can see that fungi and their spores are all around us, and because they spread easily and survive for a long time in the environment, they pose a widespread threat.

Why doesn't every animal get sick?

There are three possible outcomes when an animal comes into contact with fungal spores.

1. First, no infection, only contamination of the skin, when the animal becomes a passive carrier of spores without getting sick itself.
2. Second, the animal may become an active carrier of spores, which is when we are dealing with fungal colonization.
   It is most often asymptomatic or presents with dandruff as the only clinical sign.
3. And finally, colonization by the spores may produce clinical signs—skin lesions—and we then speak of fungal disease, with typical signs that you, as the owner, can observe yourself.

So what determines which of these three situations we will be dealing with? Factors predisposing to infection and the state of the animal's immune system are very important.

Factors that favor dermatophyte infection

Not every animal that comes into contact with fungal spores shows clinical signs of ringworm. There are many factors that favor the development of clinical ringworm, and they are worth knowing about from a pet owner's point of view.

Insects

First of all, and this seems obvious, you need to take good care of your pet. Do not let it have fleas, chewing lice or sucking lice, and do not let it be bitten by black flies or other flies. On the one hand, they can carry spores; on the other, they can cause itching that makes the animal scratch, which in turn leads to abrasions of the epidermis and mechanical microtrauma to the skin.

Injuries to the epidermis create better conditions for dermatophyte spores to penetrate.

Housing and hygiene conditions

Environmental factors that lower the animal's immunity, including the local immunity of the skin, are also extremely important. All fungal infections are strongly favored by high temperature and high humidity. So let's make sure our pets live in proper housing and hygiene conditions, for example by avoiding overcrowding, which inevitably goes hand in hand with stress (especially in cats). It is stress and the release of steroid hormones that lower the body's immunity, making it more susceptible to ringworm.

Swimming

I would also not recommend swimming in natural bodies of water—ponds, reservoirs, ditches or backwaters—because it often ends in inflammatory skin disease, itching and, as a result, secondary skin diseases, including ringworm.

Diet

Proper, balanced nutrition is also very important, because nutrient deficiencies in the food, especially of protein, essential unsaturated fatty acids, or zinc, copper, sulfur and vitamins A and D, lead to a decline in the skin's immunity. With a well-functioning immune system, fungi really have a very hard time and cannot multiply freely, while any disease that causes, for example, cachexia (wasting) strongly predisposes the animal to clinical ringworm.

Medications

The medications used (e.g., steroid hormones, chemotherapy drugs) also matter, as while treating one disease they may favor another. So let's not artificially induce immunosuppression (lowered immunity) in our animals unless we absolutely have to.

Age

Importantly, ringworm is more likely in young animals, whose immune system is not yet working very efficiently, and in older dogs and cats, because of the natural decline in immunity.

Breed predisposition

Ringworm can occur in any dog or cat regardless of sex or breed, but we see it more often in:

• Persian cats,
• Exotic Shorthair cats,
• long-haired cat breeds,
• Pekingese,
• dogs that come into contact with wild animals.

How the disease develops

Spores that adhere to the stratum corneum (the outermost layer of the epidermis) germinate into mycelium, which leads to the appearance of clinical signs. The branching hyphae grow into the hair follicles, where they break up into spores. Importantly, because dermatophytes are sensitive to elevated temperature, they do not penetrate the deeper layers of the epidermis. The growing hair therefore breaks off just above its sheath, while the hair bulb remains healthy.

Inflammation may develop in the superficial layers of the skin, but this largely depends on the fungal species and the host's immunity. This matters because only this inflammation, even if minimal, can trigger cell-mediated immunity, which is protective. So if there is no inflammation, no cell-mediated immunity develops and there will be no recovery. In that case, reinfection will most likely occur.

Symptoms of ringworm in dogs and cats

Ringworm often causes considerable diagnostic difficulties because of its highly varied clinical signs and the fact that most skin diseases resemble ringworm lesions in appearance. Ringworm most often affects young animals under one year of age and typically presents as patchy, oval areas of hair loss with scaling skin. The lesions resemble a ring with a slightly reddened center and with papules, crusts and a red rim at the periphery.

The hairs break off a few millimeters above the skin surface, and the skin may be covered with a gray coating (resembling cigarette ash) or bran-like dandruff. The skin lesions may resemble what the veterinary literature describes as a frosted stubble field. After a few weeks, the lesions tend to regrow hair from the center. Ringworm is usually not a pruritic (itchy) disease and does not cause bothersome scratching. Moderate pruritus may sometimes occur, but this depends on any inflammation caused by the fungus. Importantly, we do not see systemic signs such as fever, decreased appetite or weight loss.

The sick animal feels well and stays as active and eager to play as usual.

In cats, ringworm lesions are found mainly:

• on the head,
• around the ears,
• on the bridge of the nose,
• on the limbs,
• sometimes on the sides of the body,
• on the tail,
• on the abdomen.

In dogs, on the other hand, the distribution of lesions is more random and not limited to the head; they can appear anywhere. Skin affected by the fungus may look as if it had been dusted with flour. In older animals, we may see only mild dandruff or thinning of the coat. Secondary bacterial infections sometimes occur as well, showing up as oozing lesions and crusts, but such cases are not common.

Ringworm in dogs and cats: pictures

Below are sample photos showing ringworm in animals.

Diagnosing ringworm

Keep in mind that ringworm in animals is a great mimic of other skin diseases, and a diagnosis should not be made, nor treatment started, on the basis of signs alone.

The diagnosis should therefore always be based on:

• history,
• physical examination,
• diagnostic tests.

History

From the history we can learn important information about a possible source of infection. The veterinarian should ask where the animal came from, that is, find out whether it has recently stayed in a shelter with other dogs and cats, whether it got lost and was out of our control, or whether it comes from a fully controlled source such as a good breeder. We should establish whether the animal took part in competitions or shows, whether it was transported in a crate, and whether it had direct contact with other animals.

This matters because it lets us establish whether direct infection was possible. We should determine whether there were any potential stressors, such as mating or heat (estrus), which can lower the body's overall immunity and facilitate infection. We also cannot overlook the patient's treatment history, since both medications used and past illnesses can affect the body's immune status.

Physical examination

Only then do we move on to the physical examination, which consists of a thorough inspection of the skin, an assessment of the hair and a description of any lesions and their location.

Diagnostic tests

In ringworm, diagnostic tests provide the final confirmation of a suspected diagnosis; they are, so to speak, the finishing touch. Examination under the ultraviolet light emitted by a Wood's lamp is a simple, painless test. It should always be performed first. Greenish fluorescence of the affected areas of skin may indicate infection with M. canis.

A negative result, that is, no fluorescence, does not rule out ringworm, because other fungal species do not fluoresce this way. Nonspecific results caused by tetracycline ointments, talc or dust are also possible, which is why we should next perform a microscopic examination. We examine the collected hairs and skin scrapings under the microscope, looking for fungal spores and assessing any abnormal changes.

It is therefore good practice to perform a culture on mycological media, which is still the gold standard in diagnosing dermatophytosis. Material collected from the lesions is inoculated onto special Sabouraud medium, then incubated for 30 days and evaluated. In a clinic setting, DTM (Dermatophyte Test Medium) proves very useful; in the presence of a fungal infection, it changes color from orange to red.

Treating ringworm in dogs and cats

Antifungal treatment may be topical or systemic, and the decision on which form to use depends on the nature of the lesions. Most often we need combination therapy—topical plus systemic treatment—because applying antifungal products to the skin alone very often proves ineffective. Topical treatment alone can be tried in cases of small, single, localized lesions in short-haired animals that are housed alone.

Topical treatment should be preceded by clipping the affected areas and their surroundings. This procedure will reveal the exact extent of the lesions, remove infected hairs and improve the penetration of the medications used. Remember that we should shave a large area around the lesions, because dermatophytes, invisible to the naked eye, can be found up to several centimeters from the edge of a lesion. The clipped hair must be disposed of without fail, preferably by burning it.

Animals with large, extensive, widespread lesions should be shaved completely. Don't worry—your pet's hair will grow back.

Antifungal medications

Only on skin prepared this way do we use antifungal medicated baths or ointments containing:

• clotrimazole,
• miconazole,
• terbinafine,
• enilconazole.

Treating ringworm is a long process and sometimes even requires medication for many months, as your veterinarian will tell you in advance.

The oral antifungal drugs currently used are mainly:

• itraconazole (Orungal),
• terbinafine (Lamisil).

We give them until a therapeutic effect is achieved, that is, until two negative fungal test results are obtained. Griseofulvin and, to a lesser extent, ketoconazole used to be commonly prescribed, but because of their high toxicity and side effects these drugs are not recommended and, fortunately, are being used less and less.

Decontaminating the environment

All measures that decontaminate (clean) the environment in which the sick animals live are very important in the treatment and prevention of ringworm in animals. They consist of vacuuming floors and walls and then disinfecting them. All objects and beds that the sick animals have been in contact with should also be cleaned. These measures aim to significantly reduce the number of fungal spores in the surroundings.

Remember, too, that healthy animals in contact with sick ones should also be treated, although lower doses of medication may be used in this case.

Ringworm vaccine

Vaccination deserves a mention as well, because there are products on the market for immunizing animals, but they work in cattle and fur-farmed animals. However, there is no scientific evidence that they also work in cats and dogs—so these vaccinations are not recommended, as they may do more harm than good.

Ringworm in people: catching it from cats and dogs

A fungal infection in animals must certainly not be downplayed, as it poses a threat to humans—especially children. These infections are zoonoses (diseases transmissible from animals to humans). The fungus Microsporum canis causes ringworm of the glabrous (hairless) skin as well as of the scalp. Infections in humans, unlike in animals, are severe, often with a lot of pus, and can lead to permanent hair loss.

I remember a woman from my own practice who had lesions on her hands and was seeing a dermatologist, and she could not believe she could have ringworm, since her long-haired cat had no noticeable lesions (apart from dandruff). An example of ringworm lesions in a human caused by dermatophytes is shown below.

Summary

Although fungal skin infections are common and very well researched, they still pose enormous difficulties in diagnosis and treatment, which may stem from their highly varied signs and the need for long-term therapy. Any skin condition the owner notices should be a reason to bring the dog or cat to a veterinary clinic without delay.

It is a good idea to keep a close eye on your four-legged companions, because some skin conditions can pose a serious threat to everyone who touches them, especially children. Remember that in this particular case, the success of treatment depends on very close cooperation between the veterinarian and the pet owner; without the owner's full commitment to the treatment, treatment cannot succeed.

It really is not hard to catch ringworm, especially from a pet that may not show full clinical signs. The treatment that follows can be really long and burdensome, so let's not downplay even minor skin changes, because the sooner we diagnose ringworm in a cat or dog, the sooner we will cure it. Best of all, though, is to follow the rules of prevention and not get ringworm at all, which is what I wish for both of us.

I hope that after reading this article you now know how to recognize ringworm in your furry friend and why this disease can be dangerous. Do you have questions about treating or preventing ringworm? Or perhaps your dog or cat has ringworm right now? Ask your questions now in the comments below the article, and I will answer as soon as possible.

FAQ