How Do Dogs Get Ringworm?
Post Date:
December 10, 2024
(Date Last Modified: November 13, 2025)
Ringworm is a contagious skin infection in dogs caused by certain fungi that digest keratin in hair and skin. It often presents as circular patches of hair loss but can vary widely in appearance and severity.
What Is Ringworm?
Dermatophytosis, commonly called ringworm, is a superficial fungal infection involving the stratum corneum, hair shafts, and occasionally nails rather than a parasitic worm; the fungi produce infectious spores that adhere to hair and skin. Spores from dermatophytes can remain viable in the environment for long periods, commonly surviving about 12–24 months under favorable conditions[1].
Causative Organisms
Several dermatophyte species infect dogs; the main genera are Microsporum and Trichophyton, each with different ecology and host tendencies. Common species include:
- Microsporum canis
- Microsporum gypseum (also called Nannizzia gypsea in some taxonomies)
- Trichophyton spp., including T. mentagrophytes complex
These fungi form arthroconidia (spores) during their lifecycle; the spores are robust, adhere to hairs, and are the primary infectious stage that contaminates animals and the environment[2].
How Ringworm Spreads
Dogs most commonly acquire infection by direct contact with an infected animal’s hair or skin. Indirect transmission via contaminated fomites such as bedding, brushes, collars, and kennel surfaces is also frequent because spores are shed with hair and can persist on objects and in dust for many months[1].
Airborne dispersal of dry hairs and spores can contribute to spread within crowded or poorly ventilated environments, increasing exposure risk for other animals and people[3].
Risk Factors for Dogs
Certain situations raise the likelihood that a dog will contract ringworm. Young animals, particularly puppies, are more susceptible to infection because of developing immune responses, and immunosuppressed dogs have a higher risk of clinical disease[4].
Skin damage from trauma or coexisting dermatologic conditions, crowded or unsanitary housing such as shelters, and warm, humid environments that favor fungal persistence all increase transmission and clinical expression of disease[2].
Clinical Signs in Dogs
Typical presentations include circular or irregular patches of alopecia (hair loss), scaling, and crusting; lesions may be single or multiple and range from small focal spots to generalized involvement. Pruritus is variable and can be absent, mild, or intense depending on secondary infection and individual response[2].
Localized disease often appears on the face, ears, and limbs, while generalized disease may involve most of the body and is more common in young or immunocompromised animals; subclinical carriers can shed infectious spores without obvious skin lesions[3].
Diagnosis and Testing
Diagnosis combines clinical assessment with tests: Wood’s lamp screening, fungal culture on dermatophyte test medium (DTM), PCR for fungal DNA, and cytology or microscopic examination of plucked hairs and skin scrapings.
| Test | Typical turnaround | Relative sensitivity/notes | When useful |
|---|---|---|---|
| Wood’s lamp | Immediate | Positive in about 50% of M. canis infections, not reliable for other species | Rapid screening for fluorescence |
| Fungal culture (DTM) | 7–14 days, read up to 21 days | Gold standard for species identification when positive | Confirmatory testing and outbreak control |
| PCR | 1–3 days | High sensitivity and can detect species quickly | Rapid confirmation and species typing |
| Microscopy (hair/root exam) | Immediate | Can show fungal elements but less specific than culture or PCR | Initial screening and sample triage |
Wood’s lamp screening yields fluorescence in a subset of infections—roughly one-half of Microsporum canis cases—so a negative lamp exam does not rule out disease[3]. Fungal culture on DTM commonly requires 7–14 days of incubation and should be observed up to 21 days before calling a sample negative[2]. PCR assays can shorten diagnostic time with results often available in 1–3 days depending on the laboratory workflow[2].
Treatment Options for Dogs
Management combines topical and systemic antifungals along with environmental decontamination. Topical therapy options include medicated dips such as lime sulfur or shampoos containing miconazole and chlorhexidine applied at intervals recommended by a veterinarian[5].
Systemic therapy is indicated for generalized disease, severe localized disease, or to shorten the infectious period; itraconazole is commonly used at approximately 5 mg/kg/day, which equals 0.5 mL/kg/day if a 10 mg/mL oral suspension is used for dosing calculations[2].
Typical systemic courses range from at least 4–6 weeks and are often continued until 2 consecutive negative fungal cultures or until a period specified by the clinician beyond clinical cure; some protocols extend therapy for several weeks after clinical resolution to reduce relapse risk[2]. Secondary bacterial infections should be identified and treated with appropriate antimicrobials when present[5].
Environmental and Household Control
Because spores can survive for many months, environmental decontamination is an essential part of control. The CDC and veterinary authorities recommend a 1:10 dilution of household bleach (one part bleach to nine parts water) for hard, nonporous surfaces with a contact time of about 10 minutes for effective sporicidal activity[1].
Wash bedding and removable fabrics in hot water and dry on high heat; laundering in hot cycles and drying on high temperature for the full drying cycle helps inactivate spores, and vacuuming followed by wiping hard floors reduces environmental hair and debris[1]. In multi-dog households or shelters, isolate infected animals until they meet clinical and laboratory criteria for noninfectious status, commonly defined as two negative fungal cultures collected at least one week apart or as advised by the attending veterinarian[4].
Human Risk and Zoonotic Precautions
Ringworm is zoonotic and can spread from infected pets to people; children and immunocompromised persons are at higher risk for symptomatic infection and may develop more extensive lesions[1].
Precautions to reduce transmission include wearing gloves when handling infected animals or contaminated bedding, washing hands thoroughly with soap and water after contact, and laundering clothing and linens separately; these measures reduce household exposure and environmental contamination[1]. Seek medical advice if a suspicious skin lesion appears, particularly if it spreads or does not respond to basic topical care within about 2 weeks[1].
Follow-up, Prognosis, and Monitoring
Most uncomplicated dermatophytosis cases in otherwise healthy dogs respond to appropriate topical and/or systemic therapy and show substantial clinical improvement within 4–8 weeks, although complete mycological cure can take longer[6].
When systemic azoles such as itraconazole are used, baseline biochemical screening including alanine aminotransferase (ALT) and alkaline phosphatase (ALP) is commonly recommended, with repeat testing every 2–4 weeks during treatment to detect hepatotoxicity early[7].
Clients are frequently advised that stopping therapy solely on apparent clinical resolution risks relapse; many clinicians continue treatment until mycologic confirmation of cure, which often means two consecutive negative fungal cultures collected at least 1 week apart[6].
Sample Collection and Laboratory Tips
Obtaining representative diagnostic samples improves test sensitivity: collect hairs and scales from the margin of active lesions, from multiple lesion sites if present, and include at least 10–20 hairs per sample when possible for culture or microscopy[6].
For fungal culture, laboratories commonly incubate dermatophyte test medium for 7–14 days and often hold plates up to 21 days before calling them negative because some isolates grow slowly; informing the lab that dermatophytosis is suspected improves handling and identification priority[6].
When using Wood’s lamp as a screening step, remember that only a subset of Microsporum canis isolates fluoresce and fluorescence can be obscured by topical products; therefore positive fluorescence suggests but does not definitively confirm infection, and negative fluorescence does not rule it out[6].
Shelter and Outbreak Management
In group housing such as animal shelters or breeding facilities, dermatophyte prevalence among dogs during an outbreak can vary widely and has been reported in the literature at rates commonly in the low- to mid-double-digit percentages; targeted screening and segregation are essential to limit spread[6].
Practical outbreak control measures include isolating suspect or confirmed cases, prioritizing rapid diagnostic testing, and instituting environmental cleaning protocols with effective sporicidal agents; programs that combine topical treatment of affected animals, environmental decontamination, and monitoring typically achieve control within several weeks to a few months depending on facility size and compliance[6].
In high-risk settings, some facilities implement triage screening and temporary intake suspension or cohorting, since even brief delays in identifying and isolating infected animals can substantially increase the number of exposed animals and the workload for decontamination teams[6].
Practical Prevention Advice for Owners
Reduce exposure risk by minimizing contact between infected and uninfected animals and by laundering pet bedding and soft toys separately in hot water; washing machines and dryers on high-temperature settings and drying for a complete cycle help inactivate spores[6].
Grooming tools that contact hair should be cleaned and disinfected or replaced; hard surfaces treated with an appropriate sporicide and allowed the product’s recommended contact time will reduce environmental contamination and the chance of recurrent infection in the household[6].
When to Involve Specialists or Public Health
If a household has multiple infected pets, infections in immunocompromised people, or a large-scale outbreak in a kennel or shelter, consult a veterinary infectious disease specialist or the local public health authority because coordinated animal and human health measures may be required to manage risk and reporting[6].
Consider referral for cases that fail to improve after an appropriate course of therapy, that have unusual clinical features, or when drug toxicity or complex management decisions (for example, when treating valuable breeding animals) necessitate specialist input[7].
Key Practical Takeaways for Clinicians
Use a combination of screening and confirmatory tests to guide treatment decisions; collect adequate samples, expect culture turnaround times commonly measured in 1–3 weeks, and use PCR where rapid species identification is important for outbreak response[6].
When prescribing systemic antifungal medication, inform owners about the need for monitoring and the typical duration of therapy, and emphasize strict environmental hygiene to shorten infectious periods and reduce reinfection risk[7].
