Up to 53 million people are thought to carry MRSA. Scientists estimate that around 2 billion people, some 25-30 percent of the world's population, have a form of the Staphylococcus aureus bacteria.
Because cystic fibrosis patients are often treated with multiple antibiotics in hospital settings, they are often colonised with MRSA, potentially increasing the rate of life-threatening MRSA pneumonia in this group. The risk of cross-colonisation has led to increased use of isolation protocols among these patients. In a hospital setting, patients who have received fluoroquinolones are more likely to become colonised with MRSA, this is probably because many circulating strains of MRSA are fluoroquinolone-resistant, which means that MRSA is able to colonise patients whose normal skin flora have been cleared of non-resistant Staph. aureus by fluoroquinolones.
In the US there are increasing reports of outbreaks of MRSA colonisation and infection through skin contact in locker rooms and gymnasiums, even among healthy populations. MRSA also is becoming a problem in paediatrics.
MRSA causes as many as 20% of Staphylococcus aureus infections in populations that use intravenous drugs. These out-of-hospital strains of MRSA, now designated as community-acquired, methicillin-resistant staph. aureus, or CA-MRSA, are not only difficult to treat but are especially virulent. CA-MRSA apparently did not evolve de novo in the community, but represents a hybrid between MRSA which escaped from the hospital environment and the once easily treatable community organisms. Most of the hybrid strains also acquired a virulence factor which makes their infections invade more aggressively, resulting in deep tissue infections following minor scrapes and cuts, and many cases of fatal pneumonia as well.
As of early 2005, the number of deaths in the United Kingdom attributed to MRSA has been estimated by various sources to lie in the area of 3000 per year. The staphylococcus bacteria accounts for almost half of all UK hospital infections.
During the summer of 2005, researchers in The Netherlands discovered that three pig farmers or their families were infected by MRSA bacteria that were also found on their pigs. Researchers from Radboud University Nijmegen are now investigating how widespread the MRSA bacteria is in pigs, and whether it will become characterised among the zoonoses.
Recently, it has been observed that MRSA can replicate inside of Acanthamoeba, increasing MRSA numbers 1000-fold. Since Acanthamoeba can form cysts easily picked up by air currents, these organisms can spread MRSA via airborne routes. Hence, control of Acanthamoeba in the clinical environment may be advisable.