Once indoor relative humidity passes 65% and the room holds 24 to 30 C, ringworm stops being a skin case and becomes a household problem. Spores of Microsporum canis survive 12 to 18 months in carpet, on brush bristles, and in the 2 mm gap behind baseboards, and damp air shortens the time they need to germinate. Three numbers set the pace: hold rooms at 40 to 60% RH, disinfect high-touch surfaces once a week, and treat for 4 to 6 weeks even when the skin looks clean on day 14.
Why 65% is the switch, not 90%
Fungal spores do not need standing water, only enough water activity to restart metabolism. In a closed apartment at 26 C, air above 65% RH pushes the microclimate inside fabric, carpet, and wood over that line. Below 40% the air is too dry for arthroconidia to germinate, but dryness does not kill them: a 6-month-old spore in a vacuum bag is still infective. Put a hygrometer in the room the cat sleeps in, log it twice a day for 7 days, and treat any reading above 65% as an event rather than weather.
Reading the first 3 lesions
Lesions usually appear 7 to 21 days after exposure. Look for 1 to 3 circular patches 1 to 4 cm across with a raised scaly rim and a clearing center, most often on the head, ear pinnae, muzzle, and front legs, with broken hairs and 2 to 5 mm stubs at the edge. In longhaired cats the classic bald circle may never appear; the coat just looks dull and clumped, so part the fur in 6 places along the spine and check for 1 cm crusts. A cat can carry spores with zero lesions, and those carriers are the ones that re-infect a treated housemate 4 weeks later.
Wood's lamp: plan for a 30% miss
A Wood's lamp is fast and cheap, and it misses about 30% of infections. Only a share of M. canis strains make the porphyrin that glows apple green under 365 nm, so a strain can be fully infective and stay dark. Even a glowing strain will not fluoresce if the cat had a topical antifungal in the previous 3 to 7 days, if the lamp ran less than 5 minutes of warm-up, or if it was held closer than 10 cm and washed out the contrast. A positive glow is useful evidence; a dark result proves nothing. Never close a 4 to 6 week course because the lamp is negative at week 3.
Culture timing and the 21-day rule
Ask for a dermatophyte culture on DTM agar: colonies start to grow in 7 to 14 days and turn the medium from yellow to alkaline red as the fungus digests it. PCR returns an answer in 1 to 3 days and is worth the extra cost when 2 or more cats or a child share the flat. Sample by brushing a sterile toothbrush 10 to 20 strokes across the lesion edge and the surrounding 2 cm of coat, then press the bristles into the agar. A culture still negative at day 21 is a real negative, not a slow one. Document 2 negative cultures 2 weeks apart before you call a cat clear.
Treat for 4 to 6 weeks, not 10 days
Topical therapy does most of the decontamination work: a lime sulfur dip at 1:16, or a 2% miconazole and chlorhexidine shampoo, twice weekly for the whole course. Whole-body clipping helps only in longhaired cats and only if the clipper is disinfected between cats. Oral itraconazole at 5 to 10 mg/kg once daily, or terbinafine at 30 to 40 mg/kg, is the usual protocol for multifocal lesions or a multi-cat household; cats with raised liver values, FIV, or 2 or more concurrent drugs need bloodwork at week 0 and week 4. Most cats reach 2 negative cultures between week 4 and week 6. Stopping at day 10 is how a flat relapses in week 5.
The weekly disinfection routine
Once a week for the whole course, twice a week for the first 2 weeks. Mechanical removal beats spraying: vacuum with a HEPA machine for 5 to 10 minutes per room, then wipe. Dilute household bleach to 1:10, roughly 0.5% sodium hypochlorite, leave 10 minutes of wet contact time, then rinse and dry, because a damp surface undoes the work. Wash bedding at 60 C for 30 minutes. Throw out what cannot be washed: a 20 cm scratch post core, cardboard scratchers, cheap rugs. Change AC and furnace filters every 30 to 90 days in the wet months, and run a dehumidifier 8 to 12 hours a day set to 50%, emptying the tank before it sits full overnight.
| Indoor RH | Spore pressure | Dehumidifier hours per day | Disinfections per week | Recheck interval |
|---|---|---|---|---|
| under 40% | low, germination stalled | 0 | 1 | 14 days |
| 40 to 55% | baseline for a dry flat | 4 | 1 | 14 days |
| 56 to 65% | rising, watch carrier cats | 6 to 8 | 1 | 7 days |
| 66 to 75% | germination likely in fabric | 10 to 12 | 2 | 7 days |
| over 75% | treat as an outbreak zone | 12 to 16 | 2 to 3 | 3 to 5 days |
Paws, floors, and standing water
Wet floors carry spores into the paw pads, and the interdigital skin stays damp for hours after a cat walks across a mopped room. Dry between the toes with a paper towel once a day during the rainy weeks, and keep the litter mat dry; a mat holding 100 ml of water is a culture plate. Standing water after flooding is a separate risk, and a cat that walks through it needs the same paw drying plus a check for cuts. The wet-season paw routine is in monsoon paw skin care, and flood-water exposure is covered in monsoon flood water safety. If the cat licks or chews a plant on a wet balcony, check it against the toxic plant list before assuming the drooling is ringworm related.
When to call the vet today
Book a visit if 3 or more lesions are expanding, if a lesion sits within 2 cm of the eye or on a nail bed, if the cat is on prednisolone or cyclosporine, or if any person in the house has a new ring-shaped rash, since human and cat cases need to be treated on the same calendar. A kitten under 6 months, a senior over 12 years, or a cat with FeLV should not wait out a 2-week home trial.
- Buy a hygrometer today and log 2 readings a day for 7 days; act above 65%.
- Book a DTM culture, not just a lamp exam, and write the day 21 date on the calendar.
- Set 2 weekly alarms: topical treatment and surface disinfection, for the full 4 to 6 weeks.
- Photograph every lesion at week 0, week 2, and week 4 at the same distance.
- Stop only after 2 negative cultures 2 weeks apart, then do 1 final wipe-down.
Terms mentioned in this article
Frequently asked
The lamp showed nothing at week 3, so can I stop treatment?
No. A Wood's lamp misses roughly 30% of infections, and topical drugs from the previous 3 to 7 days can suppress fluorescence even in a strain that would normally glow. Finish the planned 4 to 6 weeks and confirm with 2 DTM cultures taken 2 weeks apart. Stopping early on a dark lamp is the single most common reason a flat relapses about 5 weeks later.
How often should I disinfect, and does bleach actually work?
Once a week for the whole course, twice a week for the first 2 weeks after diagnosis in a damp flat. Bleach works only with contact time: dilute to 1:10, about 0.5% sodium hypochlorite, keep the surface wet for 10 minutes, then rinse and dry. Vacuum with a HEPA machine first, because spores sit in fabric and dust and a wet wipe alone leaves them behind.
My cat has no lesions but the other cat keeps getting ringworm. What now?
Assume a carrier. Brush-culture the asymptomatic cat on the coat and around the ears, and expect 1 to 3 weeks for a result. Carriers are common in multi-cat homes, and treating only the cat with visible patches leaves the household cycle running. Meanwhile keep humidity under 60%, disinfect weekly, and do not share brushes or bedding between the 2 cats.