Dermatophytosis (Ringworm) in Guinea Pigs: Symptoms, Diagnosis, and Treatment
TL;DR. Dermatophytosis is a highly contagious, zoonotic fungal skin infection in guinea pigs that causes patchy hair loss and crusting, requiring veterinary-prescribed antifungals and rigorous environmental decontamination to cure.

Dermatophytosis in guinea pigs typically begins with hair loss and crusting around the eyes, nose, and ears.
What is it?
Dermatophytosis, commonly known as ringworm, is a superficial fungal infection of the skin, hair follicles, and occasionally the nails. Despite its common name, ringworm is not caused by a worm or any other parasite. Instead, it is caused by microscopic fungi known as dermatophytes. In guinea pigs, the vast majority of cases are caused by the fungal species Trichophyton mentagrophytes. This organism is highly contagious and poses a significant zoonotic risk, meaning it can easily spread from infected animals to humans and other household pets.
The infection begins when fungal spores (arthrospores) come into contact with compromised skin or hair shafts. The fungus produces specialized enzymes that digest keratin, a structural protein found in the outer layer of the skin, hair, and nails. As the fungus feeds on this keratin, it invades the hair follicles and surrounding tissue, causing the hair shafts to weaken, break, and fall out. This process leads to the classic circular patches of hair loss and scaling associated with the disease.
For guinea pig owners, dermatophytosis is a condition that demands prompt attention. Because the fungal spores are incredibly resilient and easily shed into the environment, an untreated infection can quickly spread through an entire colony of guinea pigs and infect human family members. Managing this disease requires a coordinated approach of targeted veterinary treatment and aggressive environmental sanitation.
Causes & risk factors
The primary cause of dermatophytosis in guinea pigs is direct or indirect exposure to Trichophyton mentagrophytes spores. Direct transmission occurs when an uninfected animal comes into physical contact with an infected guinea pig or an asymptomatic carrier—an animal that harbors the fungus on its skin and hair but shows no clinical signs of illness. Indirect transmission occurs when a guinea pig comes into contact with contaminated objects, such as bedding, cages, grooming brushes, toys, or human clothing.
Several environmental and host factors increase a guinea pig's susceptibility to developing an active infection:
- Stress: High stress levels suppress the immune system, making it harder for the body to fight off fungal colonization. Stressors include overcrowding, sudden environmental changes, or the introduction of new cage mates.
- Poor Husbandry: High humidity, poor cage ventilation, and infrequent cleaning create an ideal breeding ground for fungal spores.
- Age and Health Status: Young, growing guinea pigs, pregnant females, and senior animals with underlying health conditions are particularly vulnerable due to immature or compromised immune systems.
- Skin Microtrauma: Tiny abrasions on the skin from rough bedding, external parasites (like mites or lice), or fighting among cage mates provide an entry point for fungal spores.
While any guinea pig can contract ringworm under the right conditions, certain companion animal breeds in other species show suspected genetic predispositions to dermatophytosis. These include Persian cats, Yorkshire Terriers, and Jack Russell Terriers. In guinea pigs, however, environmental conditions and exposure play a far more significant role in infection rates than breed genetics.
Signs to watch for
Clinical signs of dermatophytosis in guinea pigs typically appear first on the face, particularly around the nose, eyes, and ears. From there, the infection can spread to the forelimbs, back, and rest of the body.
Cardinal Signs
- Alopecia (Hair Loss): This is the most prominent sign, often presenting as circular, patchy, or irregular areas of complete or partial hair loss.
- Scaling and Crusting: The skin within the hairless patches often becomes dry, flaky, and covered in gray-white scales or thick, crusty scabs.
Common Signs
- Stubbled or Broken Hairs: Hairs at the margins of the hairless patches are weakened by the fungus and easily break off close to the skin surface.
- Seborrhea: The skin may appear unusually greasy or excessively flaky.
- Erythema: The affected skin often appears red and inflamed.
- Papules: Small, raised red bumps may develop on the skin surface.
- Asymptomatic Carrier State: Some guinea pigs carry the fungal spores on their fur without displaying any visible skin lesions, acting as a hidden source of infection for other pets and humans.
Occasional Signs
- Pruritus (Itching): Itching is highly variable. As noted in a leading veterinary dermatology reference:
"Pruritus, if present, is usually minimal to mild but occasionally may be intense."
- Facial Folliculitis and Furunculosis: Deep inflammation of the hair follicles on the face, which can lead to localized swelling and pustules.
- Paronychia and Onychodystrophy: Fungal invasion of the nail bed, leading to inflammation around the claws (paronychia) and misshapen, brittle, or crumbling nails (onychodystrophy).
- Kerions: Raised, boggy, highly inflammatory nodular lesions that can ooze pus, representing an intense immune response to the fungus.
Rare Signs
- Peripheral Lymphadenomegaly: Swollen lymph nodes near the site of infection.
- Draining Tracts: Deep, chronic channels that break through the skin surface and drain fluid.
- Miliary Dermatitis: Small, crusty bumps scattered across the body (more commonly observed in feline dermatophytosis but occasionally seen in other species).
- Dermal Nodules: Firm lumps within the deeper layers of the skin.

Classic lesions of dermatophytosis feature dry, flaky scales, redness, and hairs that break off easily.
How vets diagnose it
Diagnosing dermatophytosis accurately is essential because other common guinea pig skin conditions, such as sarcoptic mange (Trixacarus caviae mites) or bacterial dermatitis, can cause similar symptoms of hair loss and crusting. Your vet will begin with a thorough physical examination and history, followed by specific diagnostic tests.
Because much of the standardized dermatological literature on ringworm focuses on dogs and cats, veterinarians frequently extrapolate diagnostic and treatment protocols from these species to manage guinea pigs safely.
Wood's Lamp Examination
Your vet may perform a Wood's lamp examination, which involves shining a specialized ultraviolet light over the skin lesions in a dark room. However, this test has significant limitations. The ultraviolet light only causes fluorescence when it interacts with specific chemical metabolites produced by certain strains of Microsporum canis (a common ringworm fungus in cats and dogs). As detailed in a prominent veterinary dermatology textbook:
"This unique combination causes tryptophan metabolites produced by some strains of M. canis to fluoresce a bright apple green color. Unfortunately, not all Microsporum strains produce this cell product, making the Wood's lamp useless for other species."
Because guinea pig ringworm is almost exclusively caused by Trichophyton mentagrophytes, which does not fluoresce, a negative Wood's lamp test does not rule out dermatophytosis.
Direct Microscopic Examination (Trichogram)
Your vet may pluck several hairs from the active edge of a skin lesion, place them on a glass slide with mineral oil or a clearing agent, and examine them under a microscope. This allows the vet to look for fungal hyphae or spores (ectothrix spores) coating the hair shafts, as well as characteristic