Staphylococcus pseudintermedius catheter-related bloodstream infection after exposure to domestic dogs and a cat

Abstract

We report a case of a catheter-related bloodstream infection due to oxacillin-susceptible Staphylococcus pseudintermedius in a patient receiving haemodialysis who possibly acquired the organism from her pets. Because of persistent bacteremia and the organism’s ability to form biofilm, catheter removal and antimicrobial therapy were indicated to attain source control. Both clinical and microbiological cure were confirmed. Catheter care education should include information about pet exposure and the possibility of zoonotic infections.

Background

Staphylococcus pseudintermedius is a rare opportunistic pathogen in humans but the leading cause of skin and wound infections in dogs and cats. S. pseudintermedius is found on canine and feline skin, and this opportunistic pathogen is easily transmitted from dogs and cats to humans and vice versa. This staphylococcal organism is coagulase positive, as are Staphylococcus aureus and Staphylococcus intermedius. Microbiological identification may be challenging, and treatment is difficult because of prevalent antimicrobial resistance patterns. S. pseudintermedius has virulence factors similar to those of S. aureus and forms multilayered biofilm in most cases of infection.¹

To our knowledge, this is the second reported case of a catheter-related bloodstream infection (CRBSI) caused by S. pseudintermedius that may have been acquired from a pet.² Because the biofilm-formation mechanism of this organism is a virulence factor, and because our patient had persistent bacteremia, the catheter was removed and antimicrobial therapy was given; these actions led to a cure. Although S. pseudintermedius infections are rare in humans, patients with catheters would benefit from risk assessments that involve their pets and education on preventing such infections. Moreover, patients are often their best advocates for care when provided with pertinent information about how to control the spread of organisms from pets.

Case presentation

A 36-year-old woman came to our emergency department and reported fever, nausea and vomiting for 1 week. On evaluation, she was afebrile but hypotensive. She had end-stage renal disease secondary to a congenital renal anomaly, had received three failed renal transplants and had been receiving haemodialysis via a femoral haemodialysis catheter for 5 years without complications. Skin excoriations without signs of purulence were present at the site of her femoral line. She had no murmur or cutaneous manifestations of endocarditis. She had a cat and four dogs that frequently sat on her lap.

Investigations

Laboratory studies determined the following values: haemoglobin, 116 g/L (reference range, 121–151 g/L); white blood cell count, 6.8×10⁹/L (reference range, 3.4–10.8×10⁹/L); platelet count, 179×10⁹/L (reference range, 150−450×10⁹/L); sodium, 131 mmol/L (reference range, 134–144 mmol/L); potassium, 3.0 mmol/L (reference range, 3.5–5.2 mmol/L); serum urea nitrogen, 33 mg/dL (reference range, 8–27 mg/dL); creatinine, 5.68 mg/dL (reference range, 0.76–1.27 mg/dL); lactate, 1.0 mmol/L (reference range, 0.5–1.0 mmol/L); and procalcitonin, 4.19 ng/mL (reference range, <0.15 ng/mL). The patient was admitted to the intensive care unit, and fluid bolus therapy and broad-spectrum intravenous antibiotics (vancomycin and piperacillin–tazobactam) were initiated immediately. Two sets of aerobic and anaerobic peripheral blood cultures yielded Gram-positive cocci in clusters that were identified as S. pseudintermedius. The susceptibility profile showed that S. pseudintermedius was sensitive to oxacillin, cefazolin, ceftriaxone, vancomycin, trimethoprim–sulfamethoxazole and tetracycline. Although the patient was given antibiotics, repeated blood cultures remained positive for S. pseudintermedius for several days and until the catheter was removed. Cultures from the haemodialysis catheter grew >15 colony-forming units of the same organisms. Transesophageal echocardiography did not show evidence of endocarditis.

Treatment

The division of infectious diseases was consulted. Given the findings of S. pseudintermedius in the removed catheter, vancomycin was discontinued and cefazolin was administered. After 48 hours of treatment with cefazolin, bacteremia clearance was documented. A new tunnelled catheter was placed 48 hours after the first documented negative blood culture result. Before the patient was discharged, cefazolin was discontinued, and vancomycin was administered owing to logistic issues with administering antimicrobial therapy in the haemodialysis unit.

Outcome and follow-up

The patient’s blood pressure improved over 3 days, and her symptoms resolved. After haemodialysis catheter removal and 2 weeks of antistaphylococcal therapy, the infection was microbiologically and clinically cured. Surveillance blood cultures 2 weeks after antimicrobial therapy completion were negative. The patient has remained well and stable.

Discussion

S. pseudintermedius is a common skin and mucosal commensal organism in dogs, with a carriage rate as high as 80% among healthy dogs.³ It is frequently isolated from canine skin and ear infections. In 2006, Van Hoovels et al ⁴ reported the first human infection with this pathogen. Since then, the veterinary literature has reported the pathogenic mechanisms, clinical importance and antimicrobial resistance of S. pseudintermedius in the canine population.

Clinical laboratories have become more aware of the microbiological characteristics of S. pseudintermedius, which can be confused with S. aureus, because both are coagulase positive, or can be misidentified as S. intermedius by routinely used biochemical identification systems. In 2010, Chuang et al ² reported the first case of an S. pseudintermedius CRBSI in a haemophilic child who had contact with a dog. Antibiotic lock therapy failed, and catheter removal was needed for bacteremia clearance, similar to the treatment administered to our patient. Investigations by Pompilio et al ⁵ into the pathogenicity of S. pseudintermedius demonstrated that this organism can form dense biofilms and microcolonies over a polystyrene surface.^5–7 The complex structure of this biofilm may function as a potential reservoir for bacterial growth or barrier to antibiotic penetration, or both, thereby possibly resulting in antimicrobial resistance. Hence, effective infection treatment and cure may require removal of the infected foreign material.

Few S. pseudintermedius infections have been reported in humans. Somayaji et al ⁶ analysed 27 isolates of S. pseudintermedius from 24 patients. Eighteen patients (75.0%) had a skin and soft tissue infection; two (8.3%), invasive infection associated with prosthetic joint and bloodstream infection; and four (16.7%), colonisation (three, skin; one, lung). Multidrug resistance was commonly identified, and methicillin resistance was identified in three cases on six total isolates (22.2%). Of these, five patients with reported outcomes received antimicrobial therapy guided by antibiotic sensitivity studies and had resolution of infection.⁷ The majority of patients (22 (92.1%)) had confirmed contact with dogs at the time of infection. Similarly, our patient had confirmed contact with multiple dogs and a cat. S. pseudintermedius was likely transmitted from her dogs, cat, or both via translocation from a skin wound to the catheter site or direct contamination of her intravenous line, resulting in the CRBSI and bacteremia.

If a catheter-associated infection has been diagnosed, catheter removal is often necessary for source control. Infectious Diseases Society of America guidelines strongly recommend catheter removal in cases of complicated infections associated with severe sepsis; haemodynamic instability; endocarditis; evidence of metastatic infection, erythema, or exudate due to suppurative thrombophlebitis; or persistent bacteremia after 72 hours of antimicrobial therapy to which the organism is susceptible.^8–10 In addition, long-term (≥14 days) indwelling catheters should be removed in the case of CRBSI due to S. aureus, Pseudomonas aeruginosa, fungi or mycobacteria.¹¹ According to existing guidelines, catheter removal was indicated in our patient on the basis of her haemodynamic instability and persistence of bacteremia.

Pet owners derive psychological and physiological benefits from their pets.¹² However, all patients should know that pets are repositories of various zoonotic pathogens. Weingart et al ¹³ reported that cancer patients with central venous catheters may express ‘confidence in their knowledge of line care’ but are unaware of the critical aspects of line care and recognition of infection. Information about pet-associated diseases and safe ownership practices is an important aspect of infection prevention that needs to be included in the education of patients with central lines. Patients should be encouraged to communicate with veterinarians and physicians and to learn healthy animal-contact practices.

In summary, we have reported the second case of a CRBSI due to S. pseudintermedius, a rare zoonotic pathogen that can be acquired from dogs and cats. It can be microbiologically misidentified as S. aureus or S. intermedius. In cases of CRBSI, catheter removal may be necessary for source control. Greater awareness of this pathogen by veterinarians and healthcare providers is needed to educate patients with central line catheters and to prevent transmission of this zoonotic disease from pets.

Learning points.

• Staphylococcus pseudintermedius is a rare zoonotic, opportunistic pathogen associated with dogs and cats.

• Source control, not antibiotics alone, is the key to treatment of persistent and complicated infections because of this pathogen’s predisposition for biofilm formation.

• If catheter-associated infection has been diagnosed, catheter removal may be necessary for source control.

• To prevent catheter-related bloodstream infections, all patients with central venous catheters should receive environmental risk assessments and infection prevention education, including information about safe pet ownership.