Pyoderma in dogs: bacterial skin infection

Overview

Three depths of infection

Surface pyoderma is confined to the outermost horny layer of the skin. Examples include skin fold dermatitis and acute moist dermatitis, or hot spots. Superficial pyoderma extends into the epidermis or follicle openings. Superficial folliculitis affects the upper part of a hair follicle.

Puppy impetigo can cause pustules between follicles on sparsely haired skin. Deep pyoderma extends into the dermis and sometimes subcutaneous tissue. Deep infection can remain localised or spread across a large area.

Pyoderma is common in dogs. Deep pyoderma is less common than infections closer to the surface. A single pustule or crust does not reliably reveal the depth of infection.

Causes

Staphylococcus pseudintermedius is the most common cause of bacterial skin infection in dogs. The same bacterium also occurs on healthy dogs' skin and mucous membranes. Infection can begin when the skin's defences weaken and its normal bacteria multiply excessively. In allergic skin disease, itching and skin damage can predispose to bacterial infection.

S. pseudintermedius has accounted for over 90% of pathogens isolated from superficial pyoderma. In deep pyoderma, it has accounted for around 60%. Deep infection can also involve Gram-negative bacteria such as Pseudomonas. Follicle rupture can carry hair fragments deep into the skin, where they maintain a foreign-body reaction.

A localised deep infection can also start with a bite, wound or penetrating foreign body. Some surface infections involve large numbers of both bacteria and Malassezia yeast.

Risk factors

Warm, moist skin folds are particularly prone to infection. In short-muzzled dogs, infection can develop in deep facial folds, for example. Repeated pressure at the elbow can damage hair follicles. Interdigital inflammation may involve paw conformation, local pressure or constant licking.

Demodex mite overgrowth can predispose to deep bacterial infection. Recurrent pyoderma may be associated with hypothyroidism or hyperadrenocorticism. When infections begin before three years old and recur, allergic skin disease is among the most likely underlying causes. Previous repeated systemic antibiotic treatments increase the risk of MRSP infection.

Opposing surfaces in a deep skin fold can rub together and maintain inflammation. Excess weight and weight-bearing on haired skin can predispose to interdigital inflammation. A plant fragment embedded in the skin may underlie a single inflamed interdigital nodule.

Typical symptoms

Different stages of superficial infection can be visible in the same dog at once. A red papule can become a small pustule at a hair follicle. After a pustule ruptures, a scaly ring may remain around it.

• Small red papules may appear on the skin.
• Some bumps are pale blisters containing pus.
• Crusts and scaly edges can develop.
• The coat may thin in several small patches.
• Deep infection can cause painful nodules and swelling.
• A deep focus of infection can discharge blood-stained pus.

Superficial folliculitis lesions often occur on the trunk and underside. In a short-haired dog, multiple small hairless areas can make the coat look patchy. Deep infection may be tender even if the dog does not scratch or lick it. Post-grooming deep folliculitis symptoms can begin within 24–48 hours of bathing and skin-damaging handling.

Deep skin infection can enlarge nearby lymph nodes. The affected area may smell unpleasant.

Complications and consequences

Deep skin infection can spread into subcutaneous tissues. In severe deep infection, bacteria can enter the bloodstream. A persistently inflamed area may scar. Scarred skin may remain permanently hairless.

Bacterial infection can recur if the predisposing skin disease remains untreated. Meticillin-resistant S. pseudintermedius is abbreviated to MRSP. MRSP infection can be harder to treat because fewer antibiotics are effective.

In one 2024 surveillance report from northern Europe, around 5% of S. pseudintermedius strains isolated from canine infection samples were MRSP. In the same surveillance, 81% of 298 tested S. pseudintermedius strains produced penicillinase. Penicillinase is a bacterial enzyme that makes certain antibiotics ineffective against the strain. The figures describe bacteria isolated from clinical infection samples; carriage screening samples were excluded.

Scarred skin can remain more fragile than normal. In the most severe cases, bacterial spread into the bloodstream can cause a serious systemic infection.

Diagnosis at the vet

Cytology establishes the nature of inflammation

The vet assesses lesion type and location to help determine infection depth. Redness or a pustule can also occur with other skin diseases, so bacterial infection is confirmed with cytology. A cell sample is taken from a lesion before antimicrobial medication is chosen. Bacteria found inside inflammatory cells under the microscope support a diagnosis of bacterial infection. The same sample may show Malassezia yeast contributing to inflammation.

In superficial infection, a sample can be taken from an intact pustule, beneath a crust or at the edge of a scaly ring. An adhesive tape sample can be taken from a dry or greasy lesion. A deep nodule may require a fine-needle sample or tissue biopsy. In a chronically licked limb lesion, surface culture matched bacteria found deeper in only eight of 22 dogs.

Bacterial culture and investigating the underlying cause

Culture identifies bacteria growing from the lesion being treated. Susceptibility testing helps establish which antibiotics may work against the bacteria found. Culture and susceptibility testing are particularly important in deep or recurrent pyoderma when a systemic antibiotic is needed. Staphylococci can grow in cultures even from healthy skin, so results are interpreted alongside cytology and lesions.

Skin scrapings can look for Demodex mites maintaining follicular inflammation. Hair or fungal samples can investigate whether dermatophytes are causing similar lesions. If skin samples and symptoms do not reveal the underlying cause, blood or urine tests may be needed.

Treatment

Topical treatment for surface and superficial infection

The vet prescribes treatment and any medication. Topical antimicrobial treatment is the first choice for surface and superficial pyoderma. An antiseptic such as chlorhexidine is generally chosen before a topical antibiotic. Shampoo may suit widespread lesions better than a wipe or cream intended for a small area. Removing discharge and crusts from the skin can support topical treatment.

Clinical resolution and normalised cytology in surface pyoderma can usually be established within 7–14 days. In topical-treatment studies of superficial folliculitis, lesions began improving within one to two weeks. In most of these dogs, lesions resolved within three to four weeks. The response of superficial infection to topical treatment is usually assessed at two to three weeks.

Long or matted hair can be shortened so treatment reaches the skin directly. Topical treatment was effective for superficial bacterial folliculitis in 12 of 13 published clinical studies.

Deep infection and monitoring the response

Deep pyoderma usually also needs a systemic antibiotic. Some national guidance recommends choosing the antibiotic for deep skin infection according to culture and susceptibility testing. In a serious infection, the vet may start treatment after sampling, before culture results are available. Treatment may also include topical skin care once the area is no longer too painful to handle. A dog with painful deep infection may need pain relief.

Antibiotics may also be considered for superficial infection if topical treatment is impractical or gives an inadequate response. If deep lesions stop improving, repeat cytology can assess whether bacterial infection persists. A new culture may be needed if intracellular bacteria are still seen. Red skin does not always mean persistent bacterial infection: inflammation can continue after the infection has been treated.

Prevention

Recurrent pyoderma is prevented by treating the disease maintaining it, such as allergy. Regular external parasite control reduces parasite-induced skin irritation that can predispose to another infection. If the reason for recurrence remains unclear, underlying causes may be reassessed every six to 12 months according to symptoms.

MRSP most commonly spreads between dogs through contact. It can also spread through brushes, bowls and other shared equipment. Avoiding unnecessary antibiotic treatment helps limit the spread of resistant bacteria.

If skin damage is related to repeated pressure, soft bedding may reduce the load on the skin. Weight loss in an overweight dog can reduce repeated mechanical stress.

Important for dog owners

Tell the vet about previous antibiotics and whether lesions return in the same places. If your dog has been diagnosed with MRSP, mention it when booking so the practice can prevent spread. Prevent licking as directed by the vet, for example with a suitable protective collar. Contact the practice if lesions do not decrease as expected, as the skin may need re-examination.

Do not start leftover antibiotics on your own: a new infection may need a different medicine. Treatment success is assessed through lesion improvement and, when needed, repeat cytology.

Wash your hands after handling infected skin, as resistant bacteria can travel on hands.

Keep chlorhexidine out of the eyes because it can damage the cornea. Apply the vet's chosen topical product so it reaches the affected skin through the coat.

Frequently asked questions

Can pyoderma spread to another dog?

Bacterial infection often starts from the dog's own skin bacteria when another factor weakens skin defences. However, MRSP can pass between animals through contact. Carrying the bacterium does not mean a dog has skin infection. If your dog is known to have MRSP, mention it when booking an appointment.

Does pyoderma always need an antibiotic?

Topical antiseptic treatment is often sufficient for superficial pyoderma. The vet considers a systemic antibiotic particularly for deep infection or when superficial infection does not improve with topical treatment. Susceptibility testing helps choose an appropriate antibiotic.

Why does the skin infection return after treatment?

Infection can recur if the underlying cause, such as allergic skin disease, continues. Residual skin changes after treatment may also be unrelated to a new bacterial infection. Repeat cytology helps distinguish persistent infection from other skin inflammation.

Can MRSP be recognised by a lump's appearance?

No: lesions caused by MRSP can look the same as those caused by a more antibiotic-susceptible staphylococcus. Meticillin resistance is established through bacterial culture and susceptibility testing. However, superficial MRSP pyoderma can still respond to topical antiseptic treatment.

When should topical treatment start to improve the skin?

Superficial folliculitis lesions have begun improving within one to two weeks of topical treatment. The vet usually assesses the response at two to three weeks. Seek earlier assessment if the skin worsens or your dog is painful.