Wednesday, September 20, 2006
The threat of CA-MRSA is no longer emerging; it's here
Doctors may need to consider adding MRSA coverage to patients with skin and soft tissue infection who are candidates for empiric antibiotic therapy.
by Theodore C. Eickhoff, MD IDN Chief Medical Editor
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: Infectious Disease News