Small Animal Dermatology, 3rd edition

CHAPTER 7 ANTIBIOTIC STEWARDSHIP AND EMERGING RESISTANT BACTERIAL INFECTIONS 145

 Superantigens (toxic shock syndrome toxins, TSST-1) cause severe, fatal inflamma-

tion by provoking nonspecific activation of T cells and massive release of cytokines.

 Alpha toxins (cytolysin) form pores in the keratinocytes.

HISTORICAL BACKGROUND TRACKING

METHICILLIN RESISTANCE

 Staphylococcus aureuscommonly colonizes the nasal passages in 29–38% of the

human population.

 Approximately 0.84% of the United States population (2 million people) have nasal

colonization with MRSA with no clinical signs.

 Individuals in close contact (spouse, parent/child, caregiver) with a human patient

diagnosed with MRSA infection are at a 7.5-fold greater risk of carriage than those

individuals with a casual relationship (roommates, friends, etc.).

 The incidence of methicillin resistance has increased rapidly in human hospital

strains ofS. aureussince the early 1960s.

 Two main categories – hospital acquired (HA-MRSA) and community acquired (CA-

MRSA):

HA-MRSA is the primary pathogen causing nosocomial infections in people;

risk factors include immunosuppressive disease/medications, surgery, hospital-

izations, etc.

CA-MRSA has significantly increased in recent years with at least 50% of those

individuals known to be colonized with MRSA carrying the CA-MRSA strains

Community-acquired strains differ genetically from HA-MRSA and may express

a toxin called Panton-Valentine leukocidin

Risk factors for the community form are crowded living conditions, military

facilities, prisons, sports equipment/locker rooms, geriatric/nursing homes, day-

care facilities, etc.

CA-MRSA is usually more sensitive to antibiotics than HA-MRSA, but life-

threatening necrotizing fasciitis, necrotizing pneumonia, and sepsis can develop

There has been a recent shift in the traditional strain locations, with HA-MRSA

being found in the community setting and CA-MRSA in hospital environments;

a new term, HCA-MRSA (healthcare associated-MRSA), has evolved.

 A 2007 report by the American Medical Association (AMA) estimated that MRSA

infections occurred in 95 000 Americans in 2005, with 18 650 resulting in death; the

highest rate of MRSA-related deaths in human patients (58%) was found in the hos-

pital environment.

 Methicillin resistance has been recognized in domestic animals since the early 1970s

and became clinically importance in the 1990s.

 There has been a precipitous increase in reports of methicillin resistance in recent

years.

 In 2005, one national laboratory reported only 19% ofS. aureusisolates were methi-

cillin resistant: in 2007, the percentage had increased to 42%.