What Is Ringworm in Pets?
Despite the misleading name, ringworm has nothing to do with worms. It is a fungal infection caused by a group of organisms called dermatophytes. In dogs and cats, the most common culprit is Microsporum canis, though Trichophyton mentagrophytes and Microsporum gypseum can also be responsible. These fungi colonise the outer layers of the skin, hair shafts and claws, feeding on keratin.
Ringworm is classified as a zoonotic disease, meaning it can spread from animals to people. This makes prompt identification and treatment particularly important, especially in households with children, elderly individuals or anyone who is immunocompromised.
How Does Ringworm Spread?
The infection spreads through direct contact with an infected animal, person or contaminated surface. Fungal spores are shed into the environment and can survive on bedding, grooming tools, furniture and carpets for many months. Cats are notorious carriers and may show minimal signs whilst remaining highly infectious — a phenomenon known as asymptomatic carriage.
Kittens, elderly cats and animals with compromised immune systems are most susceptible to developing visible lesions. In multi-pet households, a single infected animal can rapidly spread the infection to others if not isolated promptly.
Recognising the Signs
The classic presentation is a circular, scaly, hairless patch — though this textbook ring is not always present. In dogs and cats, you may notice:
- Patchy hair loss, often on the face, ears, paws and tail
- Scaly, crusty or inflamed skin within the bald patches
- Broken hair shafts giving a moth-eaten appearance
- Thickened or distorted claws (onychomycosis) in some cases
- Mild to moderate itching, though many cats show little discomfort
It is important not to assume every circular lesion is ringworm, as bacterial pyoderma, alopecia areata and other conditions can look similar.
Diagnosis: Why the Wood's Lamp Is Not Enough
Many pet owners and even some clinics rely on the Wood's lamp — an ultraviolet light that causes certain strains of Microsporum canis to fluoresce a characteristic apple-green colour. However, this test is only around 50 per cent sensitive. Not all strains fluoresce, false positives can occur from topical products, and the lamp must warm up properly and be used in a darkened room to have any value.
The gold standard for diagnosis is fungal culture, typically performed using Dermatophyte Test Medium (DTM). A sample of hairs or skin scale from the lesion border is inoculated onto the medium. Dermatophytes cause the medium to turn red as they metabolise protein, usually within 7 to 14 days. A microscopic examination of the colony then confirms the species.
PCR testing is increasingly available and offers faster results, but culture remains the most widely used confirmatory test in veterinary practice.
Treatment: Systemic and Topical Approaches
Effective treatment of ringworm in pets generally requires a combination of systemic antifungal medication and topical therapy.
Systemic Antifungals
Itraconazole is currently considered the first-choice systemic antifungal for cats with ringworm and is widely used in dogs as well. It is typically given orally on a pulse dosing schedule — one week on, one week off — for a minimum of six weeks. Terbinafine is another option, particularly useful in cases where itraconazole is not tolerated. Griseofulvin was historically popular but is used less frequently now due to its side-effect profile and the availability of better alternatives.
Treatment should always be guided by your vet, and fungal cultures should be repeated to confirm clearance before stopping medication — do not stop treatment simply because lesions appear to have resolved.
Topical Therapy
Topical treatment reduces the environmental shedding of spores and supports faster resolution. Twice-weekly whole-body shampooing with a miconazole and chlorhexidine shampoo is the most widely recommended approach. The shampoo should be left in contact with the coat for at least ten minutes before rinsing. In long-haired cats and dogs, clipping the coat can aid penetration of topical products, though this should be done carefully to avoid spreading spores further.
Lime sulphur dips are highly effective but are malodorous, staining and can be irritating, so they are less commonly used where modern alternatives are available.
Home Decontamination Protocol
Environmental decontamination is a critical and often overlooked step. Without it, reinfection from spore-contaminated surfaces is almost inevitable.
- Vacuum all carpets, upholstery and soft furnishings daily during treatment, disposing of the vacuum bag or emptying the canister outside after each use
- Wash all pet bedding, soft toys and fabric items at 60°C or above
- Disinfect hard surfaces with a 1:10 dilution of household bleach (sodium hypochlorite) or an antifungal surface disinfectant — most standard household cleaners do not kill dermatophyte spores
- Discard or thoroughly disinfect all grooming tools, collars and brushes
- Isolate the infected animal to a room that is easier to clean and decontaminate
- Repeat cleaning throughout the treatment period, not just once at the start
Who Is Most at Risk?
Whilst ringworm can affect anyone in contact with an infected animal, certain groups face a significantly higher risk of severe or difficult-to-treat infection:
- Children, who often have closer physical contact with pets and less robust immune defences
- Immunocompromised individuals, including those undergoing chemotherapy, organ transplant recipients or people living with HIV
- Elderly individuals with thinner skin and reduced immune function
If anyone in the household develops a circular, scaly skin rash during a pet's treatment, they should consult a doctor promptly and mention the animal diagnosis. Human ringworm is generally straightforward to treat with topical antifungals, though systemic treatment may be required for widespread or stubborn infections.
Prognosis and Prevention
With consistent treatment and proper environmental control, most pets recover fully from ringworm within two to four months. The temptation to stop treatment early when lesions appear to have healed is one of the most common causes of relapse.
Preventing future infections involves keeping new animals quarantined and checked before introducing them to the household, maintaining good general hygiene and ensuring pets are in optimal health with a balanced diet and appropriate veterinary care. Immunocompromised pets may need ongoing monitoring as they are more prone to recurrence.
If you suspect ringworm in your pet, seek veterinary advice promptly. Early diagnosis and treatment, combined with thorough home decontamination, are the most effective tools for protecting both your pet and your family.