Staphylococcus pseudintermedius is a bacterium commonly found on the skin and in the nasal and oral cavities of healthy dogs, where it is a normal part of the microbial flora. This bacterium can also be found on cats, although it is less prevalent. In most healthy animals, its presence is benign and coexists with the host as a commensal organism.
How a Common Bacterium Causes Infection
Staphylococcus pseudintermedius is an opportunistic pathogen, causing disease when a pet’s natural defenses are lowered. This transition requires a factor that disrupts the skin’s barrier or compromises the immune system. Triggers include underlying allergic skin disease, like atopic dermatitis, which causes inflammation and weakens the skin’s integrity.
Physical damage to the skin from wounds, scratches, or parasitic infestations like fleas or mites can also provide an entry point for the bacteria. Hormonal imbalances, such as hypothyroidism or Cushing’s disease, can alter skin health and immune function. When these defenses are breached, the bacteria can proliferate, leading to an active infection.
Recognizing an Infection in Pets
The most frequent sign of a Staphylococcus pseudintermedius overgrowth is a skin infection, technically called pyoderma. Signs include redness, small, pus-filled bumps (pustules), or circular crusts on the skin. Affected areas often experience hair loss, and the skin might feel greasy or have an unpleasant odor. These symptoms are common on the abdomen, in skin folds, and in areas where the pet frequently scratches or licks.
Beyond skin issues, this bacterium is a primary cause of ear infections (otitis externa) in dogs. Symptoms of an ear infection include head shaking, scratching at the ears, redness inside the ear canal, and a dark, waxy discharge. Less frequently, S. pseudintermedius can cause urinary tract infections, characterized by increased urination frequency or straining. It can also infect surgical sites, leading to post-operative wound complications.
Veterinary Diagnosis and Treatment
A veterinarian typically begins the diagnostic process by examining the pet’s clinical signs and history. To confirm a bacterial infection, a procedure called skin cytology is often performed. This involves pressing a microscope slide against the affected skin to collect a sample of cells and microorganisms. Under the microscope, the veterinarian can identify the characteristic cocci (round) bacteria and inflammatory cells, confirming pyoderma.
For recurrent or severe infections, a bacterial culture and sensitivity test is recommended. A sterile swab is used to collect a sample from the infected site, which is then sent to a laboratory to grow the bacteria and test its susceptibility to various antibiotics. This test definitively identifies S. pseudintermedius and guides the selection of an effective medication. Treatment often involves a multi-modal approach, combining topical therapies like medicated shampoos or ointments with systemic oral antibiotics. It is important for pet owners to complete the full course of treatment prescribed by the veterinarian to ensure the infection is fully resolved.
Antibiotic Resistance and MRSP
A significant challenge in treating these infections is the emergence of antibiotic-resistant strains, particularly Methicillin-resistant Staphylococcus pseudintermedius (MRSP). MRSP is a strain of the bacterium that has developed resistance to a class of commonly used antibiotics known as beta-lactams, which includes many penicillins and cephalosporins. This resistance means that standard antibiotic choices are often ineffective against MRSP infections.
The development of resistance is frequently linked to the incomplete or inappropriate use of antibiotics. When an antibiotic course is stopped early, the more susceptible bacteria are killed off, but the more resilient ones can survive and multiply, passing on their resistance traits.
Prevention and Contagion Risks
Preventing Staphylococcus pseudintermedius infections primarily involves managing the underlying conditions that trigger them. For dogs with allergies, this includes working with a veterinarian to control allergic reactions through diet, medication, or other therapies. Maintaining good skin health through regular grooming and using medicated shampoos as recommended by a vet can help manage bacterial populations on the skin. Promptly cleaning and caring for any wounds also reduces the opportunity for infection to take hold.
The bacterium can be transmitted between pets in the same household, particularly if one animal has an active infection. While the risk of transmission to healthy humans is very low, it is not zero. Immunocompromised individuals, the elderly, or those with open wounds should exercise caution. Simple hygiene practices, such as thorough hand washing with soap and water after handling an infected pet or applying topical medications, can significantly minimize any risk of transmission.