Staphylococcus pseudintermedius

Staphylococcus pseudintermedius (previously called S intermedius), a gram-positive bacterium, is a leading cause of opportunistic infections, particularly of the skin and soft tissue, in dogs and cats and is predominantly found on mucosal surfaces.¹ Infections typically occur after normal host defenses are compromised, particularly via damage to the skin barrier.

Organism

S pseudintermedius is a coagulase-positive Staphylococcus spp found in the mucosal and skin microbiotas of up to 85% of healthy dogs^2-5; however, comprehensive testing may find it in nearly all dogs. Although there are few studies on S pseudintermedius in cats, rates of 9% to 22% have been reported.^6,7

Antimicrobial Resistance

S pseudintermedius is potentially susceptible to most antimicrobials but easily develops resistance to many antimicrobials. Beta-lactamase–mediated resistance to penicillins (eg, amoxicillin) is common.^2,8 Methicillin resistance associated with resistance to penicillins, cephalosporins, and carbapenems is a significant concern, particularly in dogs.

Methicillin-resistant S pseudintermedius (MRSP) is resistant to many drug classes and can be found in healthy dogs and cats (prevalence of 0%-9% in dogs and <1% in cats).^9-16 Primary risk factors for infection with MRSP include recent antimicrobial use, previous MRSP infection, and previous hospitalization or visit to the clinic.^17-19 Prolonged colonization with MRSP can be observed in some dogs, as S pseudintermedius is a canine commensal, resulting in potential infection with the bacterium for months after successful clinical cure.²⁰

Disease

S pseudintermedius can cause opportunistic infections in almost any organ system, is the predominant cause of superficial folliculitis (eg, pyoderma), and is a common (possibly the most common) cause of wound and surgical site infections.^21-26 Disease can range from mild and self-limiting to rapidly fatal, depending on infection location (eg, skin infection vs sepsis) and patient factors (eg, immunocompetence).

Diagnosis

Culture and susceptibility testing of appropriate specimens is required for definitive diagnosis of staphylococcal infection. Adjunct testing, including cytology (eg, identification of cocci in neutrophils), can support culture results and clinical suspicion in lieu (or in advance) of culture. Methicillin resistance is detected via routine antimicrobial susceptibility testing. Oxacillin is most commonly used as the marker of methicillin resistance; cefoxitin can also be used. Resistance to oxacillin or cefoxitin indicates MRSP. Resistance to amoxicillin/clavulanic acid or cephalosporins indicates strong suspicion of methicillin resistance.

Treatment

Certain types of topical therapy can be effective for treatment of superficial infections and is a first-line option.²² Chlorhexidine bathing is an effective alternative to systemic antimicrobials in dogs with superficial folliculitis and has been recommended as a first-line treatment.²² Other topical agents (eg, benzoyl peroxide) have less supporting evidence but may be effective in some situations.^27,28 Topical biocides or antimicrobials (eg, mupirocin, fusidic acid) can be effective for treatment of other superficial (eg, wound) infections. In vitro studies have suggested additional treatments (eg, honey) may also be useful.²⁹

Drug choice should ideally be based on susceptibility testing when systemic antimicrobials are required. Penicillins (eg, amoxicillin) are highly effective against susceptible strains, but beta-lactamase–producing species are common in many regions. Empirical treatment with a potentiated penicillin (eg, amoxicillin/clavulanic acid), clindamycin, or first-generation cephalosporin (eg, cephalexin) is reasonable when MRSP is not considered highly likely (eg, no previous MRSP infection or recent antibiotic treatment). Doxycycline and potentiated sulfonamides are also effective antistaphylococcal drugs. Fluoroquinolones and third-generation cephalosporins have activity against staphylococci but are preferred for use against gram-negative bacteria, and use of these higher-tier antibiotics offers little to no benefit compared with other options, except in patients in which once-daily or single-injection administration is required. 

Treatment options for MRSP are usually limited and should be guided by susceptibility testing. MRSP is typically susceptible to amikacin and chloramphenicol, but resistance can occur. Chloramphenicol should be used with caution in cats. Nitrofurantoin can be useful for treatment of lower urinary tract disease. Fosfomycin can be used in dogs but is toxic in cats.³⁰ Rarely, higher-tier drugs (eg, linezolid) can be considered but should be reserved for cases in which there are no other options, treatment has a good chance of success, and an expert in infectious diseases or clinical pharmacology has been consulted. Because staphylococcal infections are almost invariably secondary, underlying causes (eg, allergic skin disease) should be addressed concurrently when possible.

Prevention

Because S pseudintermedius can cause opportunistic infection, the goal of preventive treatment for MRSP and methicillin-susceptible strains is to reduce the risk for secondary infection by controlling underlying risk factors (eg, allergic skin disease), using surgical asepsis, and providing good wound care.

Zoonotic Risks

S pseudintermedius is potentially zoonotic, but the risks are low. Human infections with methicillin-susceptible S pseudintermedius and MRSP have been reported^31-35; however, the relatively ubiquitous nature of S pseudintermedius in dogs, high human-to-dog exposure, and rarely reported infections of methicillin-susceptible S pseudintermedius and MRSP indicate that risks are limited. There is no evidence or suspicion that MRSP has greater risk for transmission to humans. Good hygiene (particularly hand hygiene) and avoiding contact with infected sites can help prevent zoonotic transmission.