CONDITION

Dermatophytosis (Ringworm)

Dermatophytosis is a fungal infection of the skin, hair, and sometimes claws. The fungi involved, most often a species called Microsporum canis in dogs and cats, live on and digest keratin, the structural protein that makes up the outer layers of skin and the shaft of each hair. This can lead to patchy hair loss, scaling, and sometimes reddened or crusted skin, though the appearance varies widely between individuals. Many owners arrive here after noticing circular areas of thinning fur, broken hairs, or flaky skin, often on the face, ears, or paws. The name "ringworm" refers to the round shape these patches can take, though not every case looks this way, and the infection is caused by a fungus, not a worm. Young animals, those in multi-pet households, and those with other health pressures tend to show signs more readily, though healthy adults can carry the organism without visible change. This page explores what dermatophytosis may look like in practice, how the infection establishes itself and spreads, the ways it can be identified in the clinic, and the range of approaches used to manage it in the home and environment.

Why this matters now

Dermatophytosis can affect dogs and cats of any age, though young animals and those with compromised immune systems appear more susceptible. In cats it is believed to be the most common fungal infection worldwide. The condition is frequently seen in animals from shelters, catteries, or multi-pet environments where close contact facilitates transmission. Some animals can carry the fungus without showing obvious signs, potentially spreading it to others.

Without intervention, ringworm lesions may gradually expand and new areas can become affected. In otherwise healthy animals, the condition can be self-limiting, though the time this takes allows continued environmental contamination and potential transmission to other animals and humans. The pattern of spread and severity can vary considerably between individuals based on immune response and the specific fungal species involved.

Signals & patterns

Early signals

Circular patches of hair loss

Classic ring-shaped areas where hair has fallen out, often with a slightly raised, scaly border and clearer centre.

Scaly or crusty skin

Affected areas typically show scaling, crusting, or a grey, powdery appearance where the fungus is active.

Broken or brittle hairs

Hair shafts in affected areas become weakened and break off, sometimes appearing stubbly rather than completely bald.

Mild itchiness

Some animals may show increased scratching or grooming of affected areas, though ringworm is often less itchy than other skin conditions.

Later signals

Spreading or coalescing lesions

Multiple patches may develop and can merge together, creating larger irregular areas of hair loss.

Claw involvement

In some cases, the fungus can affect the claws, causing brittleness, distortion, or discolouration of the nails.

Secondary bacterial infection

Damaged skin may become infected with bacteria, leading to more pronounced inflammation, discharge, or crusting.

Click to read about the biological mechanisms

How this is usually investigated

Diagnosing dermatophytosis requires specific testing because the clinical appearance can resemble other skin conditions. Confirmation of fungal infection guides appropriate management and helps assess zoonotic risk.

Wood's lamp examination

Purpose: Uses ultraviolet light to check for the apple-green glow of hairs infected with Microsporum canis, which is seen in most untreated M. canis infections.
Considerations: A useful screening tool when positive, but many ringworm infections do not fluoresce, so a negative result does not rule out the condition.

Fungal culture

Purpose: Hair and scale samples are placed on special growth medium to identify the specific fungus causing infection.
Considerations: Results take 1 to 3 weeks. A positive culture identifies the fungal species, though it can also come from spores carried on the coat without active infection, and no single test is accurate in every case. Important for monitoring treatment success.

Microscopic examination (trichogram)

Purpose: Direct examination of plucked hairs under the microscope can reveal fungal spores around or within the hair shaft.
Considerations: Provides rapid results when positive but requires skill to interpret and may miss infections where fungal elements are sparse.

PCR testing

Purpose: Molecular testing can detect fungal DNA with high sensitivity, identifying the specific species involved.
Considerations: Faster than culture with good accuracy. May detect DNA from non-viable organisms, so context is important.

Skin biopsy

Purpose: Tissue sampling with histopathology can confirm fungal infection and assess the depth of involvement.
Considerations: Rarely needed for straightforward cases but may help when the presentation is unusual or other conditions are suspected.

Options & trade-offs

Management of dermatophytosis typically combines treating the infected animal with reducing environmental contamination. The approach balances speed of resolution against practicality and potential side effects.

Topical antifungal treatment

Medicated shampoos, dips, or localised applications help reduce fungal load on the skin and limit environmental shedding.

Trade-offs: Used together with systemic treatment, which is the combination the World Association for Veterinary Dermatology recommends. Labour-intensive with whole-body applications. Spot treatment of single lesions is advised only alongside other treatment, not as the sole therapy.

Systemic antifungal medication

Oral antifungal drugs reach the infection through the bloodstream and are typically continued until cultures confirm cure.

Trade-offs: Generally effective but treatment duration is often prolonged (weeks to months). Some medications require monitoring for potential side effects. Stopping too early risks relapse.

Environmental decontamination

Thorough cleaning and disinfection of the environment helps eliminate spores and reduce reinfection risk.

Trade-offs: Reduces the risk of spread to other animals and people, and stops spores picked up from the home from giving false positive culture results. Can be challenging in complex environments. Spores can persist for many months, but thorough physical cleaning and washing remove them.

Isolation of affected animals

Keeping infected animals separate from others reduces transmission during the treatment period.

Trade-offs: Important in multi-pet households. Can be stressful for social animals. Duration depends on treatment response.

Clipping of hair coat

Removing hair from around affected areas can reduce fungal burden and improve topical treatment penetration.

Trade-offs: Helps limit contamination of the environment when combined with topical treatment. Clipping can spread spores, and feline guidance advises against clipping a cat's whole body for this reason. Clipped hair carries spores and is a source of contamination.

Common misconceptions

Misconception:

"Ringworm is caused by a worm"

Reality:

Despite its name, ringworm is a fungal infection, not a parasitic worm. The name derives from the ring-shaped appearance of skin lesions in humans.

Misconception:

"Only visibly affected animals can spread ringworm"

Reality:

Some animals, particularly cats, can carry and shed fungal spores without showing obvious lesions. These asymptomatic carriers can be a source of infection for other animals and humans.

Misconception:

"Once the lesions look better, treatment can stop"

Reality:

Treatment length is usually guided by fungal culture or PCR results rather than appearance. The infection can persist below the surface even when hair regrowth begins, and stopping treatment prematurely often leads to relapse.

Related conditions

Malassezia Dermatitis

Malassezia dermatitis can occur alongside dermatophytosis when the disruption to the skin's normal barrier and microbiome creates an environment in which both fungal organisms (the dermatophyte and the yeast) overgrow at the same time. In some cases, what appears to be a persistent ringworm infection may involve secondary Malassezia colonisation that prolongs signs even as the dermatophyte is treated.

Demodectic Mange

Demodectic mange can resemble dermatophytosis in its presentation of patchy hair loss, scaling, and sometimes crusting, particularly in young animals or those whose immune system is under pressure. Both conditions may be considered when investigating hair loss in a puppy or kitten, and in some cases they can occur together if the immune challenge that allows mites to proliferate also reduces resistance to fungal colonisation.

Pyoderma

Pyoderma can develop as a secondary complication when dermatophytosis damages the skin's protective barrier, allowing surface bacteria to multiply and establish infection in the inflamed or broken tissue. Conversely, some cases initially assumed to be bacterial skin infection may turn out to involve dermatophyte colonisation, particularly when the response to antibacterial treatment is incomplete.

Otitis Externa in Cats

Dermatophytosis can rarely extend into the ear canal in cats, contributing to inflammation and discharge that resemble other causes of otitis externa. When ear signs appear alongside skin or claw changes typical of ringworm, the fungal infection may be playing a role in both locations.

Documenting the location and progression of any lesions with photographs can help track response to treatment. Being aware of the zoonotic potential is relevant for all household members, particularly children or immunocompromised individuals who may be more susceptible. Understanding that resolution typically takes weeks rather than days helps set realistic expectations for the treatment period.

Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS