Ringworm (Dermatophytosis)

Ringworm (dermatophytosis) is a fungal infection of the skin, hair follicles, and nails caused by dermatophyte fungi - most commonly Microsporum canis and Trichophyton mentagrophytes in dogs. Despite the name, it involves no worms whatsoever. The "ring" refers to the circular lesion pattern sometimes seen.

Ringworm is a zoonotic infection - it spreads readily between dogs, cats, other animals, and humans. Children, elderly individuals, and immunocompromised people are at higher risk of acquiring it from infected pets.

Signs in dogs

Ringworm in dogs doesn't always present as the classic circular lesion seen in humans. Signs include:

• Patchy hair loss (alopecia): Circular or irregular patches of hair loss, often with a scaly border
• Scaling and crusting: Flaking skin within the lesion
• Broken hair shafts: Stubble-like hair within affected areas
• Inflamed or reddened skin: Varying degrees of inflammation
• Nail changes: Brittle, deformed nails if nail beds are affected (onychomycosis)
• Minimal or no itching: Unlike many skin conditions, ringworm in dogs is often not particularly itchy

Subclinical carriers - dogs who carry dermatophytes without showing obvious signs - are common, particularly in certain breeds (Yorkshire Terriers, Persian cats for comparison). These animals shed spores and can infect humans or other pets.

Daily monitoring during treatment

Ringworm treatment takes weeks. Log daily:

• Lesion changes: Are existing lesions improving (hair regrowth within the lesion, reduced scaling)?
• New lesions: Note any new patches of hair loss
• Location and size: Measure or photograph lesions so you can assess progress
• Human household members: Note any circular, itchy lesions on family members - report these to a GP promptly

Diagnosis

Diagnosis is confirmed by:

• Wood's lamp: UV light causes some Microsporum canis strains to fluoresce apple-green. Not all ringworm fluoresces, so a negative result doesn't rule it out.
• Microscopy: Hair and skin samples examined under the microscope for fungal spores and hyphae
• Fungal culture: The most sensitive method; takes 1–4 weeks to grow. Definitive identification of the species.
• PCR testing: Rapid molecular testing available at some laboratories

Treatment

Ringworm treatment must be applied consistently for several weeks and typically involves a combination of topical and systemic therapy:

Topical:

• Antifungal shampoos (miconazole/chlorhexidine combination most commonly used in UK)
• Twice-weekly whole-body bathing for the duration of treatment
• Topical antifungal creams to specific lesions

Systemic (oral):

• Itraconazole or terbinafine - given daily or on pulse protocols; systemic treatment is typically necessary for widespread or persistent infection
• Treatment continues until the dog is culture-negative (two consecutive negative cultures spaced 2–4 weeks apart) - typically 8–16 weeks

Environmental decontamination: This is critical and often overlooked. Dermatophyte spores are extremely hardy and can persist in the environment for months to years. Thorough decontamination involves:

• Vacuuming thoroughly (dispose of bags)
• Mopping hard floors with a dilute bleach solution (1:10 household bleach in water; not safe for all surfaces)
• Hot-washing all bedding and fabric the dog has contact with
• Discarding items that cannot be adequately decontaminated

Without environmental decontamination, reinfection from the environment is likely even after the dog is successfully treated.

When to contact your vet

Contact your vet soon if:

• Circular, scaling areas of hair loss are present
• A human household member has developed a circular, itchy skin lesion
• A new pet has been introduced and skin changes develop in pets or people

Frequently Asked Questions

How did my dog get ringworm? Direct contact with an infected animal (dog, cat, rabbit, or other species), contact with contaminated soil, or contact with contaminated objects or bedding. Ringworm spores are very environmentally resilient.

Is ringworm dangerous to my family? Ringworm is uncomfortable but not dangerous in healthy adults. Children and immunocompromised individuals can have more significant infections. In humans, ringworm causes itchy circular lesions on skin (tinea corporis), scalp (tinea capitis in children), or between toes (athlete's foot). If family members develop lesions, see a GP.

My dog has completed treatment but lesions haven't fully resolved. Is that normal? Treatment takes time. Hair regrowth within lesions takes weeks after the fungus is cleared. A culture-negative result (on fungal culture) is the only definitive way to confirm the infection has cleared - surface lesion appearance alone can be misleading.

Can ringworm recur after treatment? Yes, if environmental contamination remains or if the dog is re-exposed. Completing the full treatment course and thorough environmental decontamination reduce the risk of recurrence.

Should I isolate my dog from other pets during treatment? Yes. Ringworm spreads readily between animals. Keep the infected dog separated from other pets until they have negative fungal cultures. If other pets have been exposed, have them assessed too.