Bacterial vs. Fungal Skin Infection in Dogs: Key Differences

By Emiel Maddens  ·  Reviewed in consultation with licensed veterinary professionals  ·  Updated April 2026  ·  11 min read

Photo by Karsten Winegeart on Unsplash

Skin infections are among the most common reasons dogs visit the veterinarian, and they fall into two broad categories: bacterial and fungal. While both cause visible skin damage and discomfort, their underlying biology, treatment requirements, and long-term management strategies differ in important ways. Treating a fungal infection with antibiotics, or a bacterial infection with antifungals alone, wastes time and money while your dog continues to suffer.

This guide compares the two infection types in detail, covering how they look, what causes them, how veterinarians diagnose them, and which treatments work best for each. We also address the common scenario where both types coexist, requiring a combined approach for successful resolution.

What Are Bacterial and Fungal Skin Infections?

Bacterial and fungal organisms are normal residents of canine skin. Problems arise when these populations grow out of control, usually because an underlying condition has disrupted the skin's natural defenses.

Bacterial Skin Infections (Pyoderma)

Pyoderma literally means pus in the skin. The most common culprit is Staphylococcus pseudintermedius, a bacterium that normally lives on canine skin without causing problems. When the skin barrier is compromised by allergies, parasites, or hormonal imbalances, bacteria multiply and invade, producing pustules, papules, and crusts. Pyoderma is classified as surface, superficial, or deep based on the layers of skin involved.

Fungal Skin Infections

Two main types of fungal infection affect dogs. Malassezia dermatitis is caused by the yeast Malassezia pachydermatis, which naturally inhabits the skin and ears but can overgrow in favorable conditions. Dermatophytosis (ringworm) is caused by filamentous fungi that invade hair and the outer skin layer. These are distinct organisms requiring different antifungal approaches.

Why They Commonly Coexist

Allergic dermatitis is the most frequent underlying trigger for both infection types. The inflamed, damaged skin creates conditions favorable for both bacterial and yeast overgrowth simultaneously. Studies show that 30 to 50 percent of dogs with atopic dermatitis have concurrent bacterial and yeast infections.

Surface Colonization vs. Tissue Invasion

Surface and superficial bacterial infections affect the epidermis and hair follicles, while deep pyoderma extends into the dermis and subcutaneous tissue. Malassezia typically stays superficial, while dermatophytes invade hair shafts. The depth of infection influences treatment intensity and duration.

Cytology can quickly reveal whether bacteria, yeast, or both are responsible for a dog's skin infection.

Photo by Mikhail Nilov on Pexels

What Causes Bacterial and Fungal Skin Infections in Dogs?

Understanding the predisposing factors is essential because treating the infection without addressing the underlying cause leads to frustrating recurrence.

1. Allergic Dermatitis

Environmental and food allergies are the number one predisposing factor for both bacterial and fungal skin infections. The chronic inflammation and skin barrier dysfunction create an ideal environment for microbial overgrowth.

2. Hormonal Disorders

Hypothyroidism and Cushing's disease alter skin immunity and structure, predisposing dogs to recurrent infections. Hypothyroid dogs often develop a dull, dry coat with chronic, low-grade pyoderma.

3. Skin Fold Anatomy

Breeds with prominent skin folds (Bulldogs, Shar-Peis, Pugs) trap moisture and warmth, creating microenvironments that favor bacterial and yeast proliferation.

4. Immunosuppression

Dogs on immunosuppressive medications, young puppies with immature immune systems, and dogs with underlying immune deficiencies are more susceptible to both infection types.

5. Moisture and Humidity

Excessive bathing, swimming, or living in humid climates can macerate the skin and promote microbial overgrowth, particularly Malassezia yeast and surface bacteria.

6. Prior Antibiotic Use

Repeated or prolonged antibiotic courses can disrupt the normal skin microbiome, sometimes allowing fungal organisms to proliferate in the absence of bacterial competition.

Which Breeds Are Most Affected?

Certain breeds face a lifelong battle with skin infections due to their anatomy, coat characteristics, and genetic predispositions to allergic or immune-mediated disease.

• English Bulldog: English Bulldogs are extremely prone to both bacterial and yeast skin infections due to their extensive skin folds, allergic predisposition, and compromised skin barrier.
• Cocker Spaniel: Cocker Spaniels have a well-documented predisposition to Malassezia dermatitis, often presenting with chronic greasy, odorous skin and recurrent ear infections.
• West Highland White Terrier: Westies are notorious for developing severe Malassezia dermatitis alongside atopic dermatitis, often requiring lifelong management of both conditions.
• Basset Hound: Basset Hounds' loose, folded skin and pendulous ears create environments where both bacteria and yeast thrive, leading to chronic skin and ear infections.
• Shar-Pei: Shar-Peis combine deep skin folds with a unique immune profile that predisposes them to severe bacterial pyoderma and recurrent skin infections.

Signs and Symptoms

Recognizing the visual and sensory differences between bacterial and fungal infections helps guide initial treatment decisions while awaiting diagnostic confirmation.

Pustules and Papules

Bacterial infections characteristically produce small, pus-filled bumps (pustules) and raised red bumps (papules) that may form rings or collarettes when they rupture. Fungal infections rarely produce true pustules.

Scaling and Flaking

Fungal infections tend to produce more scaling, flaking, and dandruff-like debris on the skin surface. Bacterial infections can cause scaling, but it is usually secondary to other more prominent signs.

Greasiness and Odor

Malassezia yeast infections produce a characteristic greasy, waxy skin texture and a distinctive musty or yeasty odor often compared to corn chips. Bacterial infections have a different, sometimes sour smell.

Circular Hair Loss

Dermatophyte (ringworm) infections produce well-defined circular patches of hair loss with broken hair shafts. Bacterial infections cause more diffuse or patchy hair loss related to folliculitis.

Epidermal Collarettes

These circular, peeling lesions with a ring of loose skin at the edges are a hallmark of superficial bacterial pyoderma. They represent ruptured pustules and are highly suggestive of bacterial rather than fungal infection.

Ear Involvement

Malassezia is the leading cause of otitis externa (ear infections) in dogs, producing dark, waxy discharge. Bacterial ear infections produce more purulent, yellowish discharge and tend to be more painful.

Interdigital Involvement

Both infection types frequently affect the spaces between toes. Malassezia causes reddish-brown staining and a greasy texture, while bacterial infections produce swelling, draining tracts, and pain.

Diagnosis

Diagnostic testing is straightforward for most skin infections and provides the information needed to select targeted, effective therapy.

Impression Cytology

Pressing a microscope slide directly against the skin surface, staining it, and examining under a microscope can reveal bacteria (cocci or rods), yeast (budding organisms), or both within minutes. This inexpensive test guides initial treatment decisions.

Bacterial Culture and Sensitivity

For deep or recurrent pyoderma, culture identifies the specific bacterium and determines which antibiotics will be effective. This is especially important with the rise of methicillin-resistant Staphylococcus infections (MRSP).

Fungal Culture

Dermatophyte test medium (DTM) culture identifies ringworm species. For Malassezia, cytology is usually sufficient, as culture is not routinely needed given the organism's characteristic appearance.

Skin Biopsy

In chronic or unusual cases, histopathology can reveal the depth and type of infection, identify granulomatous responses, and rule out other conditions that mimic infectious skin disease.

Wood Lamp and PCR

Wood lamp screening and PCR testing provide additional tools for diagnosing dermatophytosis. PCR offers rapid, highly sensitive detection of fungal DNA.

Treatment

Effective treatment requires identifying the correct infection type, choosing the appropriate antimicrobial category, and addressing the underlying condition that allowed the infection to develop.

Systemic Antibiotics

Oral antibiotics such as cephalexin, clavamox, or clindamycin are the standard treatment for superficial and deep bacterial pyoderma. Treatment typically lasts 3 to 4 weeks for superficial infections and 6 to 8 weeks or longer for deep infections.

Systemic Antifungals

Oral ketoconazole, itraconazole, or fluconazole are used for widespread Malassezia dermatitis. Terbinafine or itraconazole is preferred for dermatophytosis. Duration varies from 4 to 12 weeks depending on severity.

Topical Antimicrobial Therapy

Chlorhexidine shampoos (2-4%) are effective against both bacteria and yeast. Miconazole-chlorhexidine combination shampoos address both infection types simultaneously and are a cornerstone of maintenance therapy.

Medicated Wipes and Sprays

For localized infections or maintenance between baths, medicated wipes containing chlorhexidine, ketoconazole, or acetic acid can target specific problem areas like skin folds, paws, and ears.

Treating the Underlying Cause

Addressing allergies with immunotherapy, managing hormonal disorders with appropriate medication, and maintaining skin fold hygiene are all essential for preventing recurrence of both infection types.

Prevention

Prevention strategies focus on maintaining skin health and controlling the underlying conditions that make dogs vulnerable to microbial overgrowth.

Regular Medicated Bathing

Routine bathing with chlorhexidine or medicated shampoos every 1 to 2 weeks can keep bacterial and yeast populations in check, especially in dogs with recurrent infections.

Skin Fold Maintenance

Cleaning and drying skin folds daily with antiseptic wipes prevents the moisture accumulation that drives both bacterial and yeast infections in brachycephalic breeds.

Allergy Management

Keeping allergies well-controlled with appropriate medications, immunotherapy, or dietary management reduces the skin inflammation that predisposes dogs to secondary infections.

Omega-3 Fatty Acid Supplementation

Fish oil supplementation supports skin barrier function and has mild anti-inflammatory properties, helping reduce the frequency of secondary skin infections in allergic dogs.

Related Symptoms

Dogs with this condition often show these symptoms. Our guides explain each one:

• Why Your Dog's Skin Smells Bad, A musty or yeasty odor often points to Malassezia overgrowth, while a sour smell may indicate bacterial infection.
• Dog Ear Infections: Causes and Treatment, Ear infections are frequently caused by the same bacteria and yeast that affect the skin.
• Why Dogs Lick Their Paws Excessively, Paw licking often signals interdigital infections involving bacteria, yeast, or both.

FAQs: Bacterial vs. Fungal Skin Infections in Dogs

Q: q

a

Q: q

a

Q: q

a

Q: q

a

Sources

Hillier, A. et al. Guidelines for the diagnosis and antimicrobial therapy of canine superficial bacterial folliculitis. Veterinary Dermatology, 2014.

Bond, R. et al. Biology, diagnosis, and treatment of Malassezia dermatitis in dogs and cats. Veterinary Dermatology, 2020.

Moriello, K.A. et al. Diagnosis and treatment of dermatophytosis in dogs and cats. Veterinary Dermatology, 2017.

Morris, D.O. et al. Recommendations for approaches to meticillin-resistant staphylococcal infections. Veterinary Dermatology, 2017.

Related Reading

• Dog Skin Infections: Types and Treatment, A comprehensive overview of all skin infection types in dogs and their management.
• Yeast Infections on Dog Skin, Focused guide on Malassezia dermatitis, its triggers, and effective antifungal strategies.
• Pyoderma in Dogs: Diagnosis and Treatment, Detailed information on bacterial skin infection classification and antibiotic therapy.
• Chlorhexidine for Dogs: Uses and Benefits, How chlorhexidine products help manage both bacterial and fungal skin infections.

Emiel Maddens

Founder of Vetified. Develops topical antifungal and antimicrobial formulations for companion animals. Vetified products are listed on DailyMed and manufactured through FDA-registered facilities in the United States.