Ringworm

Laurie Anne Walden, DVM

Cat with ringworm. Scaly red lesions on the nose were caused by Microsporum canis infection. Photo by CDC/Dr Lucille K. Georg.

Ringworm (dermatophytosis) is a skin infection caused by fungi. It’s called ringworm because in humans it sometimes looks like a small circular skin rash. Ringworm isn’t caused by a worm, despite the name.
 
Ringworm is zoonotic: it spreads between animals and people. Many animal species carry ringworm. The infection is transmitted by contact with an infected host or with contaminated soil, bedding, or other objects in the environment.
 
Causes and Risk Factors
Many types of fungi cause ringworm in different host species (animals and people). The most common fungi that infect cats and dogs are Microsporum canis and Trichophyton mentagrophytes, whose natural hosts are animals, and M gypseum, which is found in soil.
 
Fungi tend to infect abraded skin, not healthy intact skin, so scratches and scrapes—for example, from scratching itchy skin—make ringworm more likely.
 
The pet animals most likely to have ringworm are young puppies and kittens, free-roaming animals, and animals living in warm or humid climates. Working and hunting dogs are more likely than indoor dogs to encounter fungal spores in the environment.[1]
 
Signs of Infection
In dogs and cats, ringworm doesn’t typically cause the ringlike skin lesion that it causes in humans. Ringworm in animals mimics other skin diseases and can’t be diagnosed just by its appearance. Some animals have no visible signs at all. These are some of the signs of ringworm in dogs and cats:

• Hair loss
• Scaly, crusty, or flaky patches of skin
• Pink, red, or darkened patches of skin
• Small red bumps on the skin
• Change in appearance of a toenail (from nail bed infection)

 
Ringworm isn’t usually itchy. However, skin conditions that an animal might have at the same time as ringworm could cause itchy skin, and these conditions can also make a fungal infection more likely.
 
Diagnosis
A combination of diagnostic tests might be needed to confirm a fungal infection and monitor the response to treatment. M canis infection causes hairs to glow fluorescent green under a Wood’s lamp, a type of ultraviolet light source, although the fluorescence can be difficult to see in some patients. Samples collected by plucking hairs or brushing affected skin can be submitted for fungal culture or polymerase chain reaction testing.
 
Treatment
In animals and people with normal immune function, ringworm can clear on its own without treatment, although this can take weeks or months. The goals of treatment are to clear the infection more quickly and prevent the infection from spreading to others.
 
In dogs and cats, treatment often involves both topical treatment and oral medication. Topical treatment (like antifungal shampoos) reduces the risk of disease transmission and environmental contamination. Oral medication treats the fungal infection at its source.
 
Decontaminating an infected animal’s surroundings might be part of the treatment plan. Environmental decontamination prevents false-positive culture results (caused by noninfective fungal spores carried on the fur) that might lengthen the course of treatment.[1]
 
Prevention
These steps help prevent ringworm from spreading to people and other animals:

• If your pet has ringworm, treat it according to your veterinarian’s recommendations and have all other contact animals checked for ringworm.
• Wash your hands with soap after handling animals in general.
• Wear gloves and long sleeves when handling animals with ringworm; wash your hands afterward.
• Do not handle animals with ringworm if you have a compromised immune system.
• To clean the environment of an animal with ringworm, vacuum floors, disinfect hard surfaces, and wash bedding. In areas where bleach is safe to use, 1/4 cup chlorine bleach in 1 gallon of water kills fungal spores.[2]

 
References
1. Moriello KA, Coyner K, Paterson S, Mignon B. Diagnosis and treatment of dermatophytosis in dogs and cats. Clinical Consensus Guidelines of the World Association for Veterinary Dermatology. Vet Dermatol. 2017;28(3):266-e68. doi:10.1111/vde.12440
2. Ringworm. Centers for Disease Control and Prevention. Accessed September 9, 2021. https://www.cdc.gov/healthypets/diseases/ringworm.html

Image source: CDC Public Health Image Library

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