Unmasking MRSA
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Danville, Pa. — When a patient presents with a spontaneous abscess with surrounding cellulitis, physicians should suspect methicillin-resistant
Staphylococcus aureus (MRSA), according to Dirk M. Elston, M.D.
Dr. Elston practices in the department of dermatology, Geisinger Medical Center, here.
"A spontaneous abscess in a young person, especially one involved in sports, strongly suggests MRSA," Dr. Elston tells Dermatology Times. MRSA incidence is much higher in people in close physical contact with one another, especially members of sports teams who
have abrasions and share common equipment, for instance, wrestlers who share mats.
Two forms
MRSA exists in two forms: healthcare-associated (HA) and community-acquired (CA).
"CA-MRSA is not just HA-MRSA that went home from the hospital with someone," Dr. Elston says. The two forms are genetically
different organisms. HA-MRSA has a longer history, tends to be resistant to multiple antibiotics, and has a much larger resistance
gene cassette than CA-MRSA. The type IV gene cassette of CA-MRSA is small and only codes for CA-MRSA. New strains of multi-drug
resistant CA-MRSA have been identified in Asia and may spread to U.S. shores, Dr. Elston adds.
CA-MRSA is more virulent than methicillin-stable S. aureus (MSSA) and colonies are more likely to progress to clinical infection, Dr. Elston explains. CA-MRSA has caused severe or
fatal pneumonitis, especially in children, and those patients may also experience bone or joint infections. CA-MRSA may also
cause very serious skin and soft tissue infections including necrotizing fasciitis. Fortunately, most skin infections present
as abscess and respond to drainage.
CA-MRSA initially was reported in Los Angeles among homosexuals, prison inmates and members of sports teams, Dr. Elston says.
"Since then, it's popped up in widely disseminated foci all over the United States, Europe and Asia."
Dr. Elston says risk factors for HA-MRSA include:
- previous hospitalization
- a stay in the intensive care unit
- surgery (particularly abdominal)
- total parenteral nutrition or enteral feedings
- mechanical ventilation
- previous antibiotic therapy
- endotracheal, tracheostomy, nasogastric tubes
- earlier MRSA infection leading to long-term colonization
- residence in a nursing home
- dialysis
Risk factors for CA-MRSA include:
- young age
- sports participation
- exposure to a prison
- parenteral substance abuse
- race (black, Native American)
Treatment of MRSA
Dr. Elston says MRSA abscesses may respond to drainage alone, as do other abscesses.
"Any collection of purulent material must be drained. With MSRA, as with other staphylococcal abscesses, drainage is the single
most important aspect of treatment, and antibiotic therapy is secondary," he says, and notes that this was demonstrated by
Fridkin and colleagues (N Engl J Med. 2005;352:1436-1444) and discussed in an editorial by Chambers (N Engl J Med. 2005;352:1485-1487).
Outcomes data reported by Fridkin show that patients who had incision and drainage did well, even when they received an antibiotic
to which the organism was resistant.
A treatment algorithm for the management of uncomplicated skin and skin structure infections (including MRSA infections) in
the primary care setting has been published (Scher RK, et al. Cutis. 2005;75[1 Suppl]:3-23) and a revised algorithm is in press (J Drugs Dermatol.), Dr. Elston says. Both algorithms recommend drainage of any abscess. When an antibiotic is necessary, choices for CA-MRSA
include trimethoprim/sulfamethoxazole, a tetracycline, vancomycin, linezolid, lincosamides and fluoroquinolones.
"Vancomycin by infusion can be given by home nurses, so it does not necessarily require a hospital admission," Dr. Elston
says. He further notes that some data suggest that linezolid can be more effective than vancomycin, but linezolid should be
reserved for serious or life-threatening infections and for patients who cannot be placed in a long-term facility because
of the organism they carry.
According to Dr. Elston, a sulfa drug (e.g., trimethoprim sulfa) or tetracycline (e.g., doxycycline) is a good choice for
most patients with CA-MRSA.
"In areas where the HIV prevalence is high, sulfa may not be the best choice because so much sulfa has been used for pneumonia
pneumocystis carinii prophylaxis," he says. "In those populations, sulfa resistance among the staphylococci may be present."
He suggests that local surveillance data should be reviewed. Lincosamides (e.g., clindamycin) can be used, but inducible lincosamide
resistance has recently been recognized.
The presence of inducible lincosamide resistance in Staphylococcusspp. can be detected with the "D test," Dr. Elston says.
"In the D test, they put an erythromycin and clindamycin disc next to each other. Instead of seeing a circle zone of inhibition
around the lincosamide, it's flattened on the side toward the erythromycin, like the letter D. A D-positive test indicates
inducible lincosamide resistance. In these cases, clindamycin may be a poor choice."
Dr. Elston adds that fluoroquinolones should be reserved for resistant infections, because as a class, they are losing their
efficacy due to overuse.
When to refer
According to Dr. Elston, the primary indication for referral is the presence of osteomyelitis, necrotizing fasciitis or other
deep purulent infection that cannot be drained in the physician's office. He further elaborates that other indications for
referral might be the presence of synergistic infection or complex drug allergies when the antibiotic choice is in question.
Antibiotic prophylaxis
The current recommendations for antibiotic prophylaxis (e.g., surgery through infected skin) have addressed MRSA, according
to Dr. Elston.
No antibiotic prophylaxis is recommended for cutting through intact, nondiseased skin, he says. For grossly infected skin
in a high-risk individual (for example, a patient with a recent prosthetic joint or artificial heart valve), the current recommendation
is still dicloxacillin or cephalosporin or clindamycin. For a patient with a spontaneous abscess that needs to be drained
and whose risk is high for endocarditis, Dr. Elston says he would choose clindamycin because neither dicloxacillin nor cephalosporin
is an MRSA drug. Studies are needed to address the best antibiotic in this setting, he adds.
Although MRSA is becoming a fairly prevalent organism, the majority of skin infections in the community are still not MRSA,
Dr. Elston says.
"You should think of MRSA when you see someone at high risk (sports team, prison) or anyone with spontaneous abscess or
cellulitis."
Disclosure: Dr. Elston is a consultant for and serves on the Speakers Bureau of Abbott Laboratories.
For more information: Fridkin SK, Hageman JC, Morrison M, et al. Active Bacterial Core Surveillance Program of the Emerging Infections Program
Network. Methicillin-resistant Staphylococcus aureus disease in three communities. N Engl J Med. 2005;352:1436-1444. Erratum in: N Engl J Med. 2005;352:2362.
