Researchers at UCLA have developed a mathematical model that mimics a particularly nasty and ongoing outbreak at the Los Angeles County Jail of the flesh-eating bacteria known as methicillin-resistant Staphylococcus aureus, or MRSA.
Reporting in the September issue of Nature Reviews Microbiology (currently online), Sally Blower, a professor of biomathematics at the Semel Institute for Neuroscience and Human Behavior at UCLA, and colleagues constructed a simple model of the outbreak in order to assess its severity, predict the consequences of a catastrophic outbreak and suggest effective interventions to stop or control it.
Blower was intrigued by the outbreak at the jail of a form of MRSA called community-acquired methicillin-resistant S. aureus (CA-MRSA), a "super-bug" that is difficult to eradicate and that spreads easily under crowded conditions with less than optimal hygiene. In infected individuals, the bug can cause symptoms ranging from minor skin infections to severe skin ulcers, and in some cases life-threatening diseases.
Incarceration has been identified as a major risk factor for CA-MRSA infection. While large outbreaks have been reported in jails around the country, the researchers chose the Los Angeles County Jail for two reasons: It is the nation's largest jail, housing some 165,000 inmates a year and approximately 20,000 at any given time, and it has a high rate of CA-MRSA infection. An outbreak was first reported in 2002, and it continues to this day.
To date, nearly 8,500 cases have been reported at the jail, and, said Blower, "inmates, once they are released, are spreading the pathogen throughout the community as well."
With cooperation from jail officials, the researchers compiled information that determined booking rates (inflow), duration of jail stay (outflow), the rate of CA-MRSA transmission within the jail and the prisoners' status with regard to CA-MRSA infection — not infected; asymptomatic but infectious (colonized bacteria living on the skin); or infected and infectious (ulcers appearing on the skin).
The researchers used the data to establish the parameters of the disease and then built a mathematical model that established the extent of the outbreak and suggested the best way to control the pathogen.
The research showed that the jail outbreak is extremely large but not catastrophic and that it would have become catastrophic if inmates had been incarcerated for more than two to two-and-a-half months. If catastrophic, thousands of infected inmates would have been released each month. The model also revealed that the outbreak was sustained because of a continuous inflow of colonized and infected individuals who had picked up the bug in the community and brought it into the jail, not because of transmission within the jail.
"That's the value of such modeling," Blower said. "One of the things it can do is help to pinpoint where the best point is for intervention, which in this case is at the point of inflow. This model also shows that it is very likely that jails are 'hot-spots' for contributing to the spread of CA-MRSA in the community."
More complex models can be developed using the simple transmission model as a platform so that additional quantitative insight can be gained into the outbreak dynamics of such nasty pathogens.
Funding for the study was provided by the National Institutes of Health. Other authors included Emily Kajita, Justin T. Okano, Erin N. Bodine and Scott P. Layne, all of UCLA.
The Semel Institute for Neuroscience and Human Behavior at UCLA is an interdisciplinary research and education institute devoted to the understanding of complex human behavior, including the genetic, biological, behavioral and sociocultural underpinnings of normal behavior, and the causes and consequences of neuropsychiatric disorders. In addition to conducting fundamental research, the institute seeks to develop effective treatments for neurological and psychiatric disorders, improve access to mental health services and shape national health policy regarding neuropsychiatric disorders.