Abstract
Methicillin-resistant Staphylococcus aureus (MRSA) is the super bug that has recently been causing so much concern for hospital health care workers. Rosemary Zvonar, an antimicrobial pharmacy specialist at the Ottawa Hospital, spoke to conference delegates about the new threat of community-acquired MRSA.
Key points
CA-MRSA and HA-MRSA are genetically unique strains
At-risk populations for CA-MRSA include children, especially those in daycare centres, soldiers, prisoners, homeless persons, intravenous drug users, men who have sex with men, native populations, and athletic teams
Incidence in Canada is low, but continues to rise
It is important to recognize the possibility of a CA-MRSA infection
Basic infection control practices are important in preventing spread
Methicillin was the first anti-staphylococcus drug developed after penicillin (in 1959); it was then quickly replaced with cloxacillin due to the harmful side effect profile of methicillin. Resistance developed quickly in the hospital setting (1961), and was first identified in Canada in 1981. Methicillin resistance in S. aureus arises from the presence of a gene (Mec A) that allows the bacteria to withstand all beta-lactam antibiotics; in many cases, it also confers co-resistance to other antimicrobial agents (macrolides, clindamycin). MRSA had always been a hospital-acquired infection; its presence in the community, making it an “old foe with new fangs,” is now a huge cause for concern, says Zvonar.
Community-acquired MRSA (CA-MRSA) and hospital-acquired MRSA (HA-MRSA) are 2 unique strains that are genetically distinct from one another. The good news is that CA-MRSA does not appear to be co-resistant to as many non-beta-lactam antibiotics as HA-MRSA. The problem, however, is that MRSA is less likely to be suspected in a community setting, and those who are colonized are more likely to develop severe infections. The most common types of infections caused by CA-MRSA are skin and soft tissue infections and severe pneumonia.
The difficulty comes from recognizing a CA-MRSA infection in an individual. Zvonar says that suspicions should arise when the individual infected belongs to a high-risk population — children, soldiers, prisoners, the homeless, intravenous drug users, athletic teams, native populations, and those who live in crowded quarters. Lack of response to antibiotics, or description of the infection as a “spider bite” should also raise red flags. If cultures are available and over 10%–15% of S. aureus isolates are MRSA, then CA-MRSA is likely. Zvonar was quick to remind her audience that it is important to note that lack of response to antibiotics may not be due to the presence of MRSA, but may in fact be due to an insufficient dose.
Treatment of CA-MRSA differs depending on the site and severity of the infection. The infection is often not treated with antibiotics at all; however, if the infection is severe enough, or if the infected individual is young or immunocompromised, first-line oral therapy includes clindamycin, cotrimoxazole, and doxycycline (not to be used in children under the age of 8, or in pregnancy). IV medications for those with serious infections, which present as extensive cellulitis or multiple abcesses, include vancomycin, cotrimoxazole, clindamycin, and linezolid.
Zvonar finished by saying that although the current incidence of CA-MRSA in Canada is low, the number of cases continues to rise; it is therefore extremely important to prevent the spread with basic infection control practices. It is also critical to be aware of CA-MRSA as a real possibility in certain situations. Education and vigilance are extremely important in controlling the spread of CA-MRSA.