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, those with compromised immune systems, and certain breeds appear more susceptible. Cats, particularly long-haired breeds, are more commonly affected than dogs. 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 healthy animals, the condition can be self-limiting over several months, though this extended period 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.
Nail bed 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
Fungal culture
Microscopic examination (trichogram)
PCR testing
Skin biopsy
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: Often used alongside systemic treatment for faster resolution. Labour-intensive with whole-body applications. May be sufficient alone for very localised infections.
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: Essential for preventing spread to other animals and humans. Can be challenging in complex environments. Fungal spores are resistant and require specific cleaning approaches.
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 affected areas or the entire coat can reduce fungal burden and improve topical treatment penetration.
Trade-offs: Accelerates resolution when combined with topical treatment. Must be done carefully to avoid spreading spores. Contaminated hair must be disposed of properly.
Common misconceptions
"Ringworm is caused by a worm"
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.
"Only visibly affected animals can spread ringworm"
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.
"Once the lesions look better, treatment can stop"
Fungal cultures rather than appearance should guide treatment duration. 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 often develops 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 occasionally 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.
Feline Stomatitis
Feline immunodeficiency virus can weaken a cat's immune response to dermatophyte infection, making clinical signs more persistent, widespread, or difficult to resolve. Cats with FIV may carry dermatophytes for longer periods and may require more prolonged or intensive management to clear the organism.
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: 31 July 2026 · Dr Alastair Greenway MRCVS