David Brill
The emergence of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) as a nosocomial pathogen presents a range of new challenges for both prevention and management, according to a leading infectious disease specialist.
Professor Richard Wenzel, president of the International Society for Infectious Diseases, told the audience at the ICID that CA-MRSA appears to be more virulent than other nosocomial strains and has an unknown epidemiology.
He described the case that first “gave him respect for CA-MRSA:” a 22-year old male who deteriorated rapidly and died within 36 hours.
The patient had normal heart sounds on admission, yet developed a systolic murmur and symptoms of endocarditis after 16 hours. CT at presentation was also normal, yet MRI performed shortly before death showed around 100 brain abscesses which were later confirmed by biopsy.
It is unclear exactly what makes CA-MRSA so virulent, said Wenzel. The Panton-Valentine leucocidin (PVL) gene and alpha hemolysin have both been implicated in animal models, but their precise roles remain unclearly defined.
Screening also appears to be a complicated issue: a prospective cohort study of 51 CA-MRSA patients found that only 41 percent were nasal carriers, while a report of five patients who received the infection via heterosexual transmission revealed that only one was a nasal carrier.
Screening would therefore underestimate the prevalence of the USA-300 and USA-400 strains, Wenzel said, whereas in cases of nosocomial MRSA the majority of patients test positive for USA-100.
“This is important because people in the US are saying we just have to screen people so we know who’s out there. But it’s not that simple,” he said.
The antiobiogram for CA-MRSA also seems to differ from other types of infection, and the optimal treatment strategy remains unclear.
“The problem is what drugs do you use? I want to tell you this is getting complicated,” said Wenzel, who is chairman of the department of internal medicine at Virginia Commonwealth University, US.
“For life-threatening infections, antibiotics that inhibit protein synthesis and intravenous immunoglobulin may be useful,” he said, but noted that side effects are an issue with many of the available drug treatments.
“Linezolid we think is a relatively good drug but neuropathy, lactic acidosis and serotonin syndrome have all been described.
“Daptomycin looks good in comparative trials for skin and soft tissue. However, it’s not recommended for pneumonia because surfactant inhibits the antibacterial activity.”
He added that vancomycin can lead to renal failure when given with amino glycosides, while trimethoprim causes hyperkalemia in the majority of patients.
Definitive answers are still lacking on what treatments to use either for preventative prophylaxis of CA-MRSA or for treating ventilator-associated pneumonia, Wenzel said, noting that there are currently no clinical trials which can inform these decisions.
Improving our knowledge of resistant pathogens, drug interactions and the adverse events of antibiotics may help in future, he said.
“The plans for changing the prophylactic and empirical therapy for nosocomial infections need to begin right now,” he concluded.
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