Despite its name, ringworm is not a worm -- it is a fungal infection (dermatophytosis) caused primarily by Microsporum canis. It is one of the few cat diseases that readily spreads to humans, making it both a veterinary and public health concern. Kittens and immunosuppressed cats are most susceptible.
Identification
- Classic sign: Circular patches of hair loss with scaly, crusty skin
- Common locations: Face, ears, paws, but can appear anywhere
- Some cats are asymptomatic carriers (no visible lesions but spread the fungus)
- Kittens: Most commonly affected, often acquired in shelter/cattery environments
Diagnosis
- Wood's lamp: Some Microsporum canis strains fluoresce green under UV light (not all -- only 50%)
- Fungal culture: Gold standard -- definitive but takes 7-14 days for results
- PCR test: Faster than culture, highly accurate
- DTM (Dermatophyte Test Medium): Color-change culture medium used in some clinics
Treatment
- Oral antifungal: Itraconazole is the drug of choice (typically 4-8 weeks)
- Topical treatment: Lime sulfur dips or miconazole/chlorhexidine shampoo (2x weekly)
- Both oral AND topical together: Most effective approach
- Continue treatment until two negative fungal cultures (not just until lesions resolve)
Environmental Decontamination
This is crucial -- ringworm spores survive in the environment for 18+ months.
- Vacuum daily (dispose of bag or empty canister outside)
- Wash all bedding, blankets in hot water with bleach weekly
- Hard surfaces: clean with 1:10 bleach solution
- Confine infected cat to one easy-to-clean room during treatment
- HEPA air purifiers help reduce airborne spores
Human Transmission
- Ringworm readily transmits from cats to humans (zoonotic)
- Appears as red, circular, itchy rash on human skin
- Children, elderly, and immunocompromised people are at highest risk
- Wash hands after handling infected cat
- Human treatment: topical antifungal cream (OTC), oral antifungal if severe
Frequently Asked Questions
How long is my cat contagious with ringworm?
A cat remains contagious until treatment produces two consecutive negative fungal cultures (usually 6-10 weeks into treatment). Lesion resolution does NOT mean the cat is no longer contagious -- active fungal spores may still be present on the coat. This is why treatment must continue past visual improvement. During treatment, minimize the cat's access to carpeted areas and fabric furniture, and maintain rigorous environmental cleaning.
