Researchers at UCLA have developed a mathematical model that mimics a
particularly nasty and ongoing outbreak in the Los Angeles County Jail
(LACJ) of the flesh eating bacteria Staphylococcus Aureus.
Reporting
in the September issue of Nature Reviews Microbiology and 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 in the
jail, and suggest effective interventions to stop or control it.
Blower was intrigued by the outbreak in the LACJ of community-acquired
meticillin-resistant Staphylococcus aureus (CA-MRSA), a "super bug"
that's difficult to eradicate, and easy to catch through crowded
conditions and less than optimal hygiene. When someone is infected, the
bug can cause illnesses that range from minor skin infections, to
severe ulcers on the skin, to life-threatening diseases.
A major
risk factor for CA-MRSA has been identified as incarceration. While
large outbreaks have been reported in jails around the country, the
researchers choose the LACJ for two reasons-it is the nation's largest
jail, housing some 165,000 inmates per year and 20,000 inmates at any
given time, and it has a high rate of CA-MRSA-an outbreak was first
reported in 2002 and continues to this day. To date, nearly 8,500 cases
have been reported in the jail, and, said Blower, "Inmates, once they
are released, are spreading the pathogen throughout the community as
well."
With cooperation from the LACJ, the researchers compiled
information that determined booking rates or inflow, duration of stay
or outflow, the rate of transmission of the bug within the jail, and
the three "states" the prisoners were in while imprisoned: not
infected, asymptomatic but infectious (colonized bugs 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 LACJ outbreak is extremely large but not
catastrophic, but 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. Their model also revealed that the outbreak was sustained
because of a continuous inflow of colonized and infected individuals
who had picked up the bug from the community and brought it into the
jail, and not from within-jail transmission.
"And that's the
value of such modeling," said Blower, "because 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.
Source: University of California - Los Angeles. August 2007.
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