Staph aureus Bacteremia

1) Risk Factors for Staph aureus bacteremia (SAB)

• Prosthetic devices
• Intravascular catheters
• Immunocompromised state
• Malignancy
• Nasal colonization

** A substantial portion of patients present with SAB with no identifiable source.   This is actually bad, as it is predictive of subsequent complications.

Factors which suggest that blood cultures are truly positive:

• Growth within 72h
• Suspicion for systemic illness
• Degree of positivity
• Pathogen consistent with infection

2) Complications of Staph aureus bacteremia
Basically, realize that Staph aureus is an extremely virulent, "sticky" bug that likes to "metastasize" any and everywhere.
Despite effective antibiotics, mortality is still high (~20-40%)

1. Infective Endocarditis - can cause infection even of normal valves, let alone prosthetic or damaged valves.  Incidence of endocarditis with SAB is extremely high - estimated anywhere from 10-30% --> any pt with SAB automatically warrants a transthoracic echo at the least.  Indications for transesophageal echo are more controversial, but many advocate jumping straight to a TEE, given the extremely high pretest probability.   Staph aureus often causes invasive complications in the heart, such as perivalvular abscesses, valvular destruction, conduction abnormalities, etc.
2. Prosthetic Device Infection - if any hardware in the body, be extremely concerned that it is going to get seeded!! (i.e. prosthetic joints, ICD or pacemakers)
3. Osteomyelitis, especially Vertebral
4. Septic arthritis - up to 10% incidence, most common = knee.  Having a baseline abnormal joint, as in rheumatoid arthritis, is a big risk factor.
5. Splenic abscess - usually with endocarditis, can get splenic infarcts as well.  Sometimes requires splenectomy.
6. Meningitis - uncommon, usually with overwhelming disease.  Very bad.
7. Septic emboli to the lungs - typically with right-sided endocarditis
8. Renal infarcts and bacteruria - if you see Staph aureus in the urine, and no foley, esp with hematuria --> suspect Staph aureus bacteremia and endocarditis!!
9. Abscesses anywhere else - as in our patient, abscesses can develop virtually anywhere in the body.

3) Treatment for Staph aureus bacteremia

• Empiric treatment typically is Vancomycin, to cover both MRSA and MSSA.
• If susceptibilities reveal MSSA, drug of choice is Nafcillin.  In general, beta-lactams such as Nafcillin are the most efficacious cidal agents that we have.  Keep in mind that Vancomycin is inferior to Nafcillin for MSSA - this is because Vancomycin is slowly cidal, compared to Nafcillin.
• THERE IS NO ROLE FOR PO ANTIBIOTICS IN STAPH AUREUS BACTEREMIA.  With rare rare exceptions, all antibiotics must be given IV!!
• If pt develops complications as above, treatment will also include source control as indicated (drainage of abscesses, septic joints, removal of hardware, etc).
• As a general rule, central lines that were present during SAB must come out. 
• For uncomplicated SAB with a suspected removable source (i.e. infected catheter), duration is typically at least 14 days.  For most other complicated infections, duration can range from 4-8 weeks or sometimes more.

(Christopher Woo MD, 8/10/10)

(Chanu Rhee MD, 9/30/10)