Staphylococcus pseudintermedius (SPI) is a bacterium commonly found on dogs and is the most frequently isolated bacterial pathogen in canine skin and soft tissue infections. Previously known as Staphylococcus intermedius, SPI is a normal part of the canine microbiome. However, it can become an opportunistic threat, causing stubborn infections and various diseases. Understanding this dual nature is important for managing and preventing recurrent issues in dogs.
Understanding the Commensal Organism
Staphylococcus pseudintermedius is naturally present on the skin and mucous membranes of a large percentage of healthy dogs. SPI can be isolated from the skin, mouth, nose, or gastrointestinal tract of approximately 50% to 90% of dogs without causing disease. It forms part of the dog’s resident microbiota, often found in high concentrations in the nasal, perianal, and oral regions.
The shift from a harmless inhabitant to a pathogen occurs when the dog’s defenses are compromised, making SPI an opportunistic organism. Underlying factors disrupt the skin barrier or suppress the immune system, creating a favorable environment for bacterial overgrowth. Triggers include allergies, hormonal imbalances like hypothyroidism or Cushing’s syndrome, or physical trauma. This allows the bacterial population to multiply rapidly and invade the superficial layers of the skin, initiating an infection.
Recognizing Signs of Active Infection
When SPI shifts to an aggressive phase, it typically causes pyoderma, a bacterial skin infection and one of the most common manifestations of active SPI infection. Visible signs include papules (small, red bumps) and pustules (small, pus-filled blisters).
The infection often leads to hair loss, crustiness, and excessive itching, which can result in the dog chewing or licking the affected area. Other common presentations include otitis externa (inflammation of the external ear canal) and deeper infections such as folliculitis. A veterinarian may use cytology (microscopic examination of skin samples) to confirm the presence of Staphylococcus cocci. For definitive identification and to determine the most effective treatment, especially in recurrent cases, a bacterial culture and sensitivity test is necessary.
Treatment Strategies and Managing Antibiotic Resistance
Treatment for SPI infections involves a combination of systemic and topical therapies to reduce the bacterial load. Systemic treatment uses oral antibiotics selected based on the infection’s severity and location. Topical treatments, such as medicated shampoos, sprays, or wipes containing biocidal agents like chlorhexidine, are also important and can resolve superficial infections.
A significant challenge is the emergence of Methicillin-Resistant Staphylococcus pseudintermedius (MRSP). MRSP strains have acquired a specific gene that makes them resistant to an entire class of antibiotics, including common ones like cephalexin and amoxicillin. The prevalence of MRSP is increasing, and many strains are multidrug-resistant, complicating treatment choices.
Since MRSP infections look identical to susceptible infections, culture and sensitivity testing is essential to guide antibiotic selection. This testing ensures the prescribed oral medication will be effective against the specific bacterial strain. Owner compliance is paramount; the full course of antibiotics must be administered exactly as prescribed, even if symptoms improve quickly. This ensures the bacteria are fully eradicated and prevents the development of further resistance. Topical therapy with 2-4% chlorhexidine is highly recommended, as resistance to these biocides has not been observed, making them a reliable option even for MRSP cases.
Preventing Recurrence and Household Transmission
The most effective way to prevent SPI recurrence is to identify and manage the underlying cause that compromised the dog’s defenses. This requires a long-term strategy focused on controlling conditions like canine atopic dermatitis or treating endocrine diseases. Regular, preventative bathing with appropriate shampoos helps maintain the skin barrier and reduces the bacterial population on the skin surface.
To minimize the spread of the organism, particularly MRSP, simple household hygiene measures should be implemented. Owners should practice frequent hand washing after handling the dog, especially after touching infected areas. Cleaning and disinfecting items that contact the dog, such as bedding, towels, and toys, can reduce the environmental bacterial load.
Transmission of SPI from dogs to humans is possible, but the risk of infection in a healthy person is low. SPI is primarily adapted to canines, and human infections are rare, usually occurring only in individuals who are immunocompromised or have broken skin. While transmission potential exists, it most often results in colonization rather than an active human infection.