The threat of CA-MRSA is no longer emerging; it’s here

September 2006

Two months ago, I discussed a presentation that I had given at the May meeting of the Southeastern Society for Emerging Biological Threats. In that presentation, I reviewed the threat posed by hospital-acquired staphylococcal infection during the late 1950s and early 1960s.

These epidemic penicillin-resistant staphylococci were mostly of bacteriophage type 52/52A/80/81, and for the most part seemed to disappear during the late 1960s and early 1970s. The reasons for the virtual disappearance of this epidemic threat were never clarified, whether it was due to infection control practices, the introduction of methicillin and its congeners, or simply a long-term secular trend. I also alluded to some limited evidence that the current CA-MRSA of the USA 300 type was genetically related to the epidemic 52/52A/80/81 staphylococci of 50 years ago.

There were a number of other presentations at that meeting that were devoted to the challenges posed by CA-MRSA. I will discuss only two of these: a presentation by Scott Weese, MD, of the Ontario Veterinary College, on CA-MRSA in animals, and one by Elizabeth Bancroft, MD, from the Acute Communicable Disease Control section of the Los Angeles County Department of Health Services.

Transmission of MRSA

In his presentation about CA-MRSA in animals, Dr. Weese outlined several concerns: the transmission of MRSA from infected animals to humans (and vice versa); whether colonized animals acted as reservoirs of MRSA in the community and whether these reservoirs were of human or animal origin; and the nature and extent of disease that occurred in animals. Each major animal species presented different issues: Among horses, the major issues were nasal or facial contact, fecal contamination and international movements, particularly among race horses; among household pets the significant issues were the degree, duration and intensity of contact; among pet birds the issues were fecal contact and aerosolization of fecal matter. Finally, there are issues in processing animals raised for human consumption as food.

In horses, MRSA appears to be endemic in certain horse populations worldwide. The organism seems to transmit regularly between horses and humans. The USA 500 strain appears to dominate globally. Infection control measures, entirely similar to those we employ in a human health care setting, can eradicate MRSA from horse farms.

Among household pets, there have been rapid increases in reported cases of MRSA infection in recent years. However, colonization has been generally uncommon in household pet populations. Where studied, colonization rates have varied from zero to 2% in dogs, and there was no colonization at all in cats. However, there have been sporadic episodes in veterinary clinics in which colonization rates have been as high as 9%. In general, pet isolates have been indistinguishable from common human clones.

It is clear that there is both intra- and inter-species transmission of MRSA in veterinary clinics. Does this occur in households as well? Is there a household cycle that can be broken? How common are such household “outbreaks”? Answers to these questions await further study. It is clear, however, that colonization of veterinary personnel with MRSA from infected animals does occur and appears to be a risk factor for infection of veterinary personnel.

Finally, important questions arise in considering the threat, if any, posed by therapy animals. For example, should therapy animals and their contacts be screened in some way before visitation? Have infection control protocols been established to deal with MRSA colonization or infection of therapy dogs? There seems to be no end of legitimate questions that could be asked about the implications in this area.

Pediatric MRSA

Through the Los Angeles County Department of Health Services, Dr. Bancroft carried out an MRSA surveillance study of pediatric MRSA hospital admissions during a six-month period in 2003. There were 140 such admissions to Los Angeles hospitals during this period, 92% of which were skin and soft tissue infections.

Frequency was highest in those younger than 2 years, but cases occurred at all ages. Twenty-three percent of the cases were initially misdiagnosed as spider bites. Most of the strains (96%) were of the USA 300 type. Significant risk factors that emerged were: exposure within the previous month to someone who had been incarcerated; the presence of traditional risk factors for nosocomial MRSA; and household contact with a skin/soft tissue infection within the previous month. Participation in contact sports also seemed to be a risk factor.

Specific recommendations for outbreak control in athletic teams, correctional institutions, and day care or developmentally disabled settings were developed and are available at the following Web site: http://lapublichealth.org/acd/MRSA.htm.

Skin and soft tissue infections

Viewed against this background, the findings of Moran, et al and the EMERGEncy ID Net Study Group, (N Engl J Med; 2006;355;7) should not be a huge surprise. This study was carried out in 11 university-affiliated emergency departments throughout the United States during the month of August 2004, and focused specifically on patients presenting with skin and soft tissue infections.

There were 422 such patients, and S. aureus was recovered from 320 (76%) of them. Spider bites again was a significant early misdiagnosis. Of the 320 isolates, MRSA accounted for 59% overall. In individual EDs, the MRSA rate ranged from a high of 74% in Kansas City to a low of 15% in New York City. However, in Philadelphia, just a short distance from New York, the rate was 55%. Among the MRSA isolates, 97% were USA 300 type, and most were of a single strain, USA 300-0114. Similarly, 98% of the strains possessed the Panton-Valentine leukocidin toxin gene.

Antimicrobial susceptibility patterns were also quite similar; 95% were susceptible to clindamycin (not tested for inducible clindamycin resistance), 6% were susceptible to erythromycin, 100% to rifampin and sulfa-trimethoprim, and 92% to tetracycline. In 57% of patients treated with antibiotics, with or without incision and drainage, treatment was not concordant with the results of susceptibility testing. Interestingly, among patients with adequate follow-up information, there was no significant difference in outcome whether the antibiotics given were active against MRSA or not. This confirms the long-standing belief that incision and drainage are not only necessary but also sufficient for treatment of most skin and soft tissue infections. Antibiotic therapy should probably be reserved only for these with underlying disease or those with significant systemic signs.

The major message, however, is quite clear: it is high time for emergency department physicians to re-examine their own treatment protocols, and if they haven’t done so already, consider whether to add MRSA coverage to patients with skin and soft tissue infection who are considered candidates for empiric antibiotic therapy.