The short answer
Dermatophytosis, or ringworm, is a superficial fungal infection of the skin and hair. In cats it is caused mostly by Microsporum canis, and it can spread to people. The published sources describe diagnosis as a combination of physical and Wood's lamp examination, microscopic examination of hairs and fungal culture, with PCR as a further option. The treatment they describe pairs a systemic antifungal, most often itraconazole, with twice-weekly whole-body topical therapy and environmental cleaning, continued until follow-up testing is negative. The sources disagree on several practical points, including how reliable the Wood's lamp is, how many negative cultures are needed, and whether clipping helps.
Key takeaways
- The 2013 ABCD guidelines attribute over 90% of feline cases worldwide to M canis [1].
- In one shelter's records covering 5644 cats over 24 months, Moriello reported that 10.3% were culture positive but only 1.6% were both lesional and culture positive [2].
- The ABCD guidelines call itraconazole the systemic "drug of choice", with terbinafine as an alternative [1].
- Moriello writes that topical therapy is the only way to kill spores on the hair coat, because systemic drugs act at the hair follicle [2].
- ABCD and Moriello describe treating until at least two negative cultures [1][2]. The Merck Veterinary Manual says one negative PCR result supports "mycological cure", and that one negative culture also does unless the animal has systemic illness [3].
- Moriello and the Merck manual both treat mechanical cleaning and detergent washing as the most important decontamination step [2][3].
Doses on this pageEvery dose below is quoted from the cited source. Verify against the current label/formulary and the patient's full clinical picture.
Cause, transmission and risk factors
Dermatophytes are fungi that live on keratin in the outer skin, hair and sometimes claws. According to the ABCD guidelines, their infective spores (arthrospores) can survive in a dry environment for 12 months or more, though they are short-lived in humid conditions [1]. The guidelines list direct contact with sick or subclinically infected animals and indirect spread through brushes, collars, toys and dust as routes of transmission [1].
Moriello identifies direct cat-to-cat contact as the most important route. She notes that in a reconstructed feline epidermis model, M canis spores began adhering within 2 hours, with invasion documented histologically within 5 days [2]. The Merck manual adds that lesions need enough spores, skin microtrauma and moisture. It also says transmission from environmental spores is inefficient without microtrauma such as clipping with contaminated clippers [3].
The ABCD guidelines list young age (the first 2 years), immunosuppression, other disease, nutritional deficits and heat and humidity as predisposing factors, and describe conflicting survey data on FIV [1]. The Merck manual states that FIV or FeLV status does not predispose cats to infection [3]. On how common the disease is, the ABCD guidelines describe it as the most common fungal infection of cats [1]. The Merck manual puts true disease prevalence at under 4% of skin disorders and says it is "not the most common skin disease of cats" [3].
Clinical signs and case grouping
The ABCD guidelines describe the typical lesion as circular hair loss with broken hairs, scaling and sometimes a red margin with a healing center, most often on the head. Young cats show lesions on the bridge of the nose that extend to the temples and ears [1]. Itching is described there as variable and generally mild to moderate. Less common forms include miliary dermatitis on the back and nodular pseudomycetomas, which the guidelines say are seen more often in Persian cats [1].
Moriello lists the muzzle, lips, area around the eyes, ears and ear margins, digits, inner limbs, axillae and tail as the places lesions are most often overlooked [2]. The Merck manual states that severe infections reflect a failure of the host's cell-mediated immunity, not a more virulent strain [3].
Moriello groups cats into three categories for management [2]:
- Simple infection: otherwise healthy, unstressed cats with limited lesions.
- Complicated infection: widespread or inflammatory lesions, long or matted coats, concurrent illness such as upper respiratory infection, a history of prior treatment or surrender for "resistant dermatophytosis", or feral status.
- Lesion-free, culture positive: either fomite carriers mechanically carrying spores, or cats with very early lesions.
Diagnosis
The sources agree that several tests are used together, but they weigh them differently.
| Test | ABCD guidelines [1] | Moriello review [2] | Merck manual [3] |
|---|
| Wood's lamp | About 50% of M canis strains fluoresce | Difficult to find cat studies supporting the 50% figure; fluorescence usually present in lesional, infected cats | Review found 91-100% of untreated, infected animals had glowing hairs |
| Direct hair exam | Sensitivity assessed at 59%; 76% with calcofluor white | Mineral oil mount, no clearing agent needed | Confirms infection in more than 85% of cases |
| Fungal culture | Described as the "gold standard" | Remains the "gold standard" | "No single test is a gold standard" |
On Wood's lamp technique, the Merck manual describes holding the lamp within 2-4 cm of the skin, starting at the head and moving slowly, and looking for apple-green fluorescence of hair shafts [3]. Moriello reported that in her unpublished laboratory work, re-examining toothbrush samples under magnification found over 50% of previously Wood's lamp negative brushes to be positive [2].
Sampling advice differs. The ABCD guidelines describe brushing for 5 minutes with a sterile toothbrush when subclinical infection or carriage is suspected, and swabbing lesion margins with alcohol before collection [1]. The Merck manual says only lesions should be sampled and that plucked hair should not be wiped with alcohol because it may produce a negative result [3].
For culture handling, Moriello reports that 99% of cultures from M canis-infected cats in her laboratory were positive by day 14 and recommends holding negative plates for 21 days [2]. The ABCD guidelines note that suspect colonies on dermatophyte test medium must be confirmed microscopically [1]. Moriello describes counting colony forming units per plate (a "pathogen score") to track infection, and reports that fomite carrier cats typically had fewer than 10 cfu per plate [2]. The Merck manual notes PCR cannot distinguish viable from nonviable spores and advises submitting more than 20 hairs and crusts [3].
The shelter figures above come from one retrospective dataset, and Moriello cautions that culture surveys can overestimate disease because of fomite carriage [2]. In that dataset only a quarter of cats with skin lesions had dermatophytosis [2].
Treatment
All three sources describe the disease as self-limiting in healthy cats. The ABCD guidelines say isolated lesions resolve in 1-3 months [1], Moriello says resolution can take 70-100 days [2], and the Merck manual frames treatment as a way to shorten the course and limit spread [3]. Moriello summarizes the components as the "CCATS plan": confinement, cleaning, assessment, topical therapy and systemic therapy [2].
Systemic drugs
The ABCD guidelines describe itraconazole pulse therapy of "5 mg/kg/day for 1 week, every 2 weeks for 6 weeks" and note use is possible in kittens as young as 6 weeks [1]. The Merck manual gives "5 mg/kg, PO, every 24 hours on a week on/week off schedule". It says most infections resolve after 3 or 4 cycles and warns that compounded itraconazole has poor bioavailability [3]. The ABCD guidelines give terbinafine at 30-40 mg/kg once daily [1]. Moriello reports that in shelter cats, the prolonged hair persistence seen in healthy cats after terbinafine was not observed, with colony counts rising quickly after the drug stopped [2].
The Merck manual states ketoconazole should not be used in cats, fluconazole is the least effective option, griseofulvin is no longer recommended and lufenuron is ineffective [3]. The ABCD guidelines do not recommend vaccination because a safe and efficient vaccine is not available. They also describe a placebo-controlled double-blind study of 55 cats in which improvement after vaccination did not differ significantly from placebo [1].
Topical therapy
The ABCD guidelines describe whole-body 0.2% enilconazole twice weekly, or 2% miconazole with or without 2% chlorhexidine as a twice-weekly rinse or shampoo [1]. The Merck manual lists lime sulfur 1:16 or enilconazole 1:100 twice weekly as rinses with residual activity. It lists miconazole/chlorhexidine shampoo 2-3 times a week, which has no residual activity [3]. Moriello considers lime sulfur or enilconazole first choice [2]. Both the ABCD guidelines and Moriello advise against relying on spot treatment alone [1][2].
The sources differ on two handling questions. The first is collars. The ABCD guidelines state that side effects of enilconazole are very seldom reported provided grooming is prevented (Elizabethan collar) until the cat is dry [1]. Moriello reports a barrier collar is not necessary and that oral ulcers in her shelter work were associated with upper respiratory infection, not the topicals [2]. The second is clipping. The ABCD guidelines describe whole-coat clipping as useful in generalized disease and long-haired cats [1]. Moriello warns that clipping can cause microtrauma and therapy burns. She reserves it for matted coats, extensive lesions in long-haired cats or nonresponse, and uses combing as an alternative [2].
Monitoring and end of treatment
The ABCD guidelines describe treating until the dermatophyte cannot be cultured on at least two sequential brushings 1-3 weeks apart [1]. Moriello advocates weekly cultures as the fastest way to detect a negative result and notes that falling colony counts track resolution [2]. The Merck manual describes clinical resolution first, then testing. One negative PCR result supports mycological cure, and one negative culture also does unless the animal has systemic illness [3].
For cats that look resolved but keep culturing positive, Moriello lists three main causes: fomite contamination, unhealed subclinical lesions, or spores remaining on the distal hair shaft in hard-to-treat areas such as the ears [2].
Decontamination and confinement
Moriello states that cleaning mainly removes infective material and reduces false-positive cultures, because transmission occurs primarily through direct contact [2]. She and the Merck manual describe the same sequence: a "hard clean" with detergent, rinsing, then a disinfectant labeled effective against Trichophyton. Both describe thorough cleaning once or twice weekly and laundry washing of soft items [2][3]. The ABCD guidelines list 1:33 lime sulphur, 0.2% enilconazole and 1:10 to 1:100 household bleach as most effective against spores in catteries and shelters [1].
The Merck manual and Moriello both warn that long confinement of kittens risks undersocialization [2][3]. In catteries and shelters, the ABCD guidelines describe separating infected and uninfected cats and treating all animals in the group [1].
