Community-acquired MRSA a growing problem in eye care

Infection caused by methicillin-resistant Staphylococcus aureus, or MRSA, is a growing concern in medicine, and eye care has not been spared.

For eye care practitioners, the potential for MRSA infection after refractive or cataract surgery represents a rare but potentially serious complication.

“Epidemiologic studies have shown that the most reported MRSA infection is conjunctivitis and the least reported is endophthalmitis,” Francis S. Mah, MD, Ocualr Surgery News Cornea/ External Disease Board Member, said.

Francis S. Mah

“It’s definitely a growing problem,” he continued. “However, in terms of the absolute numbers, it’s not overwhelmingly the most dominant etiology of bacterial infection that clinicians might imagine. We need to be cognizant of the rise of MRSA, but we need to also realize that other pathogens are probably more likely the cause of infections.”

A troubling aspect of MRSA is that the potential for infection affects empiric or prophylactic antibiotic choices.

“If you get a clue that a patient is at high risk, then you should start thinking about MRSA,” Dr. Mah said. “Otherwise, you should treat empirically, or prophylactically, the most likely pathogens.”

Ophthalmic MRSA

While numerous studies have implicated methicillin resistance as a significant public health threat in various medical disciplines, the best estimates of MRSA prevalence in ophthalmic infections suggest that it is implicated in a proportionally low percentage of cases, Dr. Mah said.

For instance, epidemiologic evidence suggests that MRSA is responsible for about 3% of all occurrences of endophthalmitis. However, among S. aureus, it is responsible for about 35% of all endophthalmitis, and this should be considered with the background that endophthalmitis is a relatively rare complication.

High-risk groups

In the ophthalmic setting, the greatest concern with MRSA surrounds postsurgical complications after cataract removal or LASIK. However, according to Dr. Mah, only about 2.5% of the general population carries MRSA, as reported in the literature, and so the risk of infection is relatively low. Where extra caution might be necessary, he said, is in the context of certain high-risk patients.

The percentage of MRSA carriers among health care workers is around 15% to 20%, and given the higher penetration of refractive procedures in this patient population, a prospective ocular culture might be an option. Patients who have concomitant risk factors such as diabetes or immune deficiency or who take steroids, which can inhibit immune function, are also known to be at higher risk.

Other high-risk groups have been identified in studies: patients who participate in athletics or who frequent gyms; patients in nursing homes; patients with a history of dialysis, previous colonization with MRSA or immune deficiency; and spouses of patients with relevant health care environmental exposure.

MRSA on the rise

Many of the epidemiologic studies to date in the ophthalmic literature have noted increasing prevalence of MRSA in local settings. Taken on the aggregate, these studies would suggest a problem that is not just regional.

As well, evidence is beginning to emerge from studies with a national scope that MRSA, specifically community-acquired MRSA, is a potential threat in the ophthalmic setting. In a poster presented at the 2008 meeting of the American Academy of Ophthalmology that reviewed swabs prospectively collected from refractive surgery candidates at 10 U.S. institutions, 321 of 399 patients had a positive lid culture, and 229 had a positive conjunctiva culture for staphylococcal species. Within the positive cultures, S. epidermidis was the predominant strain followed by S. aureus.

Of significance, according to Eric D. Donnenfeld, MD, an OSN Cornea/External Disease Board Member, is that the study was conducted in patients who were not health care workers. In addition, susceptibility testing revealed high levels of resistance to oxacillin.

A report based on data submitted by more than 200 laboratories across the United States to The Surveillance Network also showed an increase in the proportion of methicillin resistance among S. aureus ocular isolates from 29.5% in 2000 to 41.6% in 2005.

“The prudent course would be to consider the possibility of methicillin or multidrug resistance with any S. aureus ocular infection, even in the absence of recognized risk factors,” the study authors said.

The report concluded that “based on the rate of increase in the [The Surveillance Network] database, MRSA cultures from serious ocular infections could be more common than methicillin-susceptible S. aureus within 2 to 3 years.”

The report also found that MRSA isolates had a high level of multidrug resistance, defined as resistance to three or more antimicrobial agents. Only trimethoprim maintained adequate activity against MRSA (95%), while decreased susceptibility was noted for ciprofloxacin (31.6%), gatifloxacin (29%), levofloxacin (26.5%), moxifloxacin (27.4%) and tobramycin (44.9%).

“Those fluoroquinolones – gatifloxacin, moxifloxacin, levofloxacin – track the same, so if it’s resistant to one, it’s resistant to the others,” said Penny A. Asbell, MD, MBA, the lead author of the report. And if they are resistant to those three, “they are also going to be resistant to azithromycin, penicillin, perhaps reduced sensitivity to tobramycin, and the only one that we found might be effective is trimethoprim.”

Penny A. Asbell

A second report, also written by Dr. Asbell, OSN Contact Lens Section Editor, found high levels of methicillin resistance among S. aureus isolates. Her report was from a separate nationwide surveillance program, Ocular TRUST (Tracking Resistance in the United States Today), which collected ocular isolates from seven eyes hospitals and 28 community hospitals in 19 states.

“More than 50% of the isolates that were S. aureus were methicillin resistant,” Dr. Asbell said in reference to the surveillance data. “It is what we would expect: an increase in all infections across the board.”

Community-acquired MRSA

Within the growing incidence of MRSA, community-acquired strains are rapidly becoming the predominant threat. According to a widely cited review of all MRSA infections encountered in the Parkland Health and Hospital System in Dallas, looking specifically at the etiology and epidemiology of ocular infections, Preston H. Blomquist, MD, noted a flatline of nosocomial infections but a rise in the number and proportion of community-acquired strains of MRSA.

“In the hospital, 70% were community-acquired. For the ophthalmic setting, which accounted for 1.3% of our infections, 76% were community-acquired,” Dr. Blomquist said.

Identifying potential community-acquired MRSA may be a difficult proposition, however. In the Parkland study, patients with nosocomial ophthalmic MRSA tended to be younger than patients with community-acquired (average 24 years vs. 35.6 years, respectively). That finding was in contrast to the general hospital setting where patients with nosocomial MRSA were typically older (40.9 years) than patients with community-acquired MRSA (37.4 years).

Genetics affect expression of disease

More recently, Dr. Blomquist said, there has been a leveling of community-acquired infections in the Parkland system. However, it is difficult to attribute that to less prevalence, and instead, the drop in community-acquired cases may be due to use of empiric therapy that covers MRSA with less use of primary cultures.

“The good news is that for community-acquired MRSA, you don’t have to go to the big gun because many of the older antibiotics still work,” Dr. Blomquist said.

As in earlier studies, the ocular MRSA isolates in the Parkland setting were sensitive to trimethoprim-sulfamethoxazole.

Community-acquired MRSA is also more easily spread from patient to patient, according to James P. McCulley, MD, FACS, and because it produces increased levels of Panton-Valentine leukocidin, it more readily kills the body’s natural defense mechanisms.

“The genetic differences between the two manifest themselves in some very different expressions of disease and behavior of the bacteria,” Dr. McCulley said.

Despite the differences between the two pathogens, and despite the impact on antibiotic choice of determining the infectious etiology, genetic testing is impractical in the clinical setting. Instead, Dr. McCulley said, patient-specific factors may be telling: a young, otherwise healthy person, especially in the context of potential exposure to other possible carriers such as family members, fellow sports team members or a prison environment. The nature of the disease may also be a clue because community-acquired MRSA most often causes skin and soft tissue infection, so a presentation around the lids may be a sign.

“If it’s community, one has to be much more concerned about a more rapidly progressive, much more severe infection,” he said.

Antibiotic choices

More recently, vancomycin has demonstrated the greatest bactericidal effect against MRSA, but reports have surfaced of MRSA in the nosocomial setting with resistance to that antibiotic.

“The Hospital Infection Control Practice Advisory Committee did come out strongly advising against the routine use of vancomycin for prophylaxis,” Dr. Blomquist said.

It has been suggested recently that intracameral prophylaxis may have a role in the ophthalmic surgery setting to help offset risk of postsurgical infection.

Critics have countered that intracameral prophylaxis, while effective during the surgical setting, does not stay in the eye long enough to offer protection over the duration of the postsurgical period. As well, cost and the risk of dilution errors may outweigh the benefits of that strategy.

For more information:

• Francis S. Mah, MD, can be reached at University of Pittsburgh Medical Center, Eye and Ear Institute; (412) 647-2200; fax: (412) 647-5119; e-mail: mahfs@upmc.edu.
• Eric D. Donnenfeld, MD, can be reached at OCLI; (516) 766-2519; fax: (516) 766-3714; e-mail: eddoph@aol.com.
• Penny A. Asbell, MD, MBA, can be reached at Mount Sinai School of Medicine; (212) 241-7977; fax: (212) 289-5945; e-mail: penny.asbell@mssm.edu.
• Preston H. Blomquist, MD, can be reached at University of Texas Southwestern Medical Center; (214) 648-3770; fax: (214) 645-9482; e-mail: preston.blomquist@utsouthwestern.edu.
• James P. McCulley, MD, FACS, can be reached at UT Southwestern Medical Center at Dallas; (214) 648-3407; fax: (214) 648-9061; e-mail: james.mcculley@utsouthwestern.edu.

References:

• Asbell PA, Colby KA, Deng S, et al. Ocular TRUST: nationwide antibiotic susceptibility patterns in ocular isolates. Am J Ophthalmol. 2008;145(6):951-958.
• Asbell PA, Sahm DF, Shaw M, Draghi DC, Brown NP. Increasing prevalence of methicillin resistance in serious ocular infections caused by Staphylococcus aureus in the United States: 2000-2005. J Cataract Refract Surg. 2008;34(5):814-818.
• Blomquist PH. Methicillin-resistant Staphylococcus aureus infections of the eye and orbit (an American Ophthalmological Society thesis). Trans Am Ophthalmol Soc. 2006;104:322-345.
• Chung JL, Seo KY, Yong DE, et al. Antibiotic susceptibility of conjunctival bacterial isolates from refractive surgery patients [published online ahead of print April 23, 2009]. Ophthalmology. doi:10.1016/j.ophtha.2008.12.064.
• Solomon R, Donnenfeld ED, Perry HD, et al. Methicillin-resistant Staphylococcus aureus infectious keratitis following refractive surgery. Am J Ophthalmol. 2007;143(4):629-634.
• Witherspoon SR, Blomquist PH. Methicillin-resistant Staphylococcus. Ophthalmology. 2007;114(7):1420-1421.