Ringworm presents a curious set of paradoxes. While often not a major concern, the strain most problematic for shelters and rescues, Microsporum canis, is highly contagious to both humans and animals, making its spread feel significant.
Another paradox lies in the diagnosis and treatment of this seemingly simple skin infection, akin to athlete's foot in humans. There are nuances to consider, including three main types of ringworm encountered in animal care settings, each requiring a different treatment approach. Fortunately, our understanding of ringworm has advanced considerably, though myths and misinformation still persist.
Understanding Ringworm
Ringworm, also known as feline dermatophytosis, is a common fungal skin infection affecting cats worldwide and can also impact humans. Despite its name, it is not caused by a worm but by microscopic parasitic fungi called dermatophytes, which thrive on the skin, particularly in hair follicles. Ringworm is a resilient fungus that can persist in soil.
Cats and cattle are most frequently affected, though dogs and other animals can contract it. Humans can get ringworm from infected pets or their environment. Cats contract ringworm through exposure to infected soil, direct contact with infected animals or people, or indirectly via their living environment. Spores can spread between cats through shared bedding, grooming tools, carriers, furniture, and carpeting.
Kittens and cats under one year old, whose immune systems are still developing, are most susceptible. Cats with suppressed immune systems, such as FIV+ cats, and longhaired cats are also more prone to infection.
Symptoms of Ringworm in Cats
The most common symptoms of ringworm in cats include:
- Circular, red, scaly patches on the skin.
- Bald patches or hair loss.
- Itching or scratching.
- Crusty or scaly skin.
Ringworm lesions and hair loss are frequently localized to the face, ears, and limbs but can appear anywhere on the body. Patches may not always be circular, sometimes presenting as patchy baldness, with or without redness and inflammation. Mild itching is common, and cats may scratch their ears as an early sign. Hair-loss patches can fluctuate during the infection cycle.
Ringworm is highly contagious to other pets and humans. However, ringworm in cats typically self-resolves within two to four months, meaning it often goes away on its own without intervention. For feral cats, ringworm is usually not treated and resolves naturally.
Diagnosis of Ringworm
Accurate diagnosis is paramount. While fungal cultures can be performed in-house, interpreting the results requires expertise. Dermatophyte test media on culture plates turn red if ringworm is growing, but this can also occur with certain contaminants. Careful observation of colony growth under a microscope is necessary.
Types of Ringworm
In small animal medicine, three main types of ringworm are encountered:
- Microsporum canis (M. canis): The most common type in cats and the one of greatest concern due to its high contagiousness.
- Microsporum gypseum (M. gypseum): Less concerning than M. canis.
- Trichophyton: Similar in appearance to M. gypseum and not as concerning.
Treating all ringworm types uniformly can be problematic. A cat with an M. gypseum lesion might require minimal treatment and no isolation, whereas a full treatment protocol for M. canis could be unnecessarily intensive.
The Role of the Wood's Lamp (Black Light)
A Wood's lamp, a type of ultraviolet lamp, is a quick screening tool for ringworm. Under the lamp in a dark room, the ringworm fungus fluoresces an apple green color. Typically, the entire hair shaft will glow, especially at the base, indicating infection within the hair follicle.
Important Considerations for Wood's Lamp Examination:
- Fluorescence Mechanism: The glowing effect is due to a metabolite produced by the ringworm fungus that coats the hair as it grows.
- Distinguishing True Infection: Look for hair that glows all the way to the base. Glowing only at the tips may indicate medication or resolving ringworm where new hair is growing out.
- Wipe Test: A glow from ringworm cannot be wiped off with a damp cloth, unlike some other substances.
- Other Glowing Substances: Sebum wax, scabs, and crusts can also fluoresce under a Wood's lamp.
- Lamp Specifications: Use a corded Wood's lamp with a wavelength of 360-365 nanometers for optimal results.
- Limitations: Only about 60% of ringworm fungi fluoresce under a Wood's lamp. Therefore, a negative result does not rule out ringworm. M. gypseum and Trichophyton do not typically glow under a Wood's lamp.
Other Diagnostic Methods
Fungal Culture: Considered the most accurate diagnostic method. A veterinarian takes a skin scraping, often with a toothbrush, and cultures it in a lab. This process can take 10-21 days. A positive culture shows cottony fungal growth and a color change in the culture media (often to red).
PCR Test: Polymerase chain reaction tests are highly sensitive and can detect even a single spore. However, they do not provide quantitative results, only positive or negative. This means they can detect dead ringworm spores, making them unsuitable for monitoring treatment effectiveness.
Trichogram: This involves the direct examination of plucked hair under a microscope. If hair glows under a Wood's lamp, examining it under a microscope can provide a diagnosis that is close to a rapid test result.
Species Identification is Crucial: It is essential to identify the specific species of ringworm. For instance, ringworm in dogs is most often M. gypseum, not the highly contagious M. canis. Misidentification can lead to unnecessary isolation and intensive treatment protocols.
Treatment of Ringworm
Treatment protocols vary depending on the type of ringworm and the severity of the infection. It is important to obtain an accurate diagnosis before commencing treatment.
Clipping the Hair
Clipping the hair around affected areas is important to reduce spore spread and allow treatments to reach the skin more effectively, especially for longhaired cats. This should be done with extreme care to avoid further skin trauma or spreading the infection.
Topical Treatments
- Lime-Sulfur Dips: Often prescribed, these dips are applied as baths twice a week for 2-4 weeks. They have a strong odor and can temporarily discolor the coat to a yellowish hue.
- Enilconazole (Clinafarm EC): Another topical treatment option.
- Antifungal Liquids: Containing agents like itraconazole, these are sponged onto the coat every other week for six weeks.
- Topical Antifungal Creams: Containing miconazole or thiabendazole, these are applied once or twice daily for 2-4 weeks and are effective for both pets and humans.
Oral Medications
Oral antifungal medications, such as ketoconazole and itraconazole, may be prescribed for severe lesions or infections involving the nail beds.
Monitoring Treatment
Cats undergoing treatment should be re-evaluated by a veterinarian every 2-4 weeks with new cultures. A cat is considered cleared of infection only after two consecutive negative cultures taken two weeks apart.
Ensuring the cat is on a high-quality diet, particularly good quality wet food, is also recommended, as ringworm can tax the animal's system.
Preventing the Spread of Ringworm
Catching ringworm on intake is the most effective way to prevent outbreaks. A thorough intake examination, especially for cats, looking for lesions and hair loss, is recommended. Wood's lamp exams should be performed on cats upon intake.
Environmental Disinfection
Thorough and daily cleaning is critical:
- Vacuuming: Vacuum all surfaces, furniture, drapes, and carpets daily to remove lightweight spores. Empty vacuum bags or canisters frequently and disinfect canisters with a 1:10 bleach-water solution.
- Bleach Solution: Use a 1:10 bleach-water solution to disinfect all hard surfaces, allowing it to sit for about 10 minutes. Bleach should also be used in laundry for pet bedding and towels.
- HVAC Systems: Consider having heating and cooling ducts professionally vacuumed.
- Grooming Tools and Carriers: Thoroughly bleach grooming tools and cat carriers, especially when shared or used at adoption events.
Isolation and Quarantine
- New Cats: Isolate new cats for 14-21 days before introducing them to other animals. Any contagious diseases typically manifest within this period.
- Infected Cats: Consider restricting an infected cat to a single room to simplify cleaning.
- Multi-Cat Households: If a ringworm-infected cat enters a multi-cat household, all cats should be checked by a veterinarian. Given the high likelihood of spore transmission, other cats may contract ringworm.
- Reintroduction: Cats should only be co-mingled again after the infected cat tests negative on two consecutive cultures, two weeks apart.
For rescuers, extreme caution is advised when using shared grooming tools or carriers at adoption events.