HA-MRSA also appear to have an advantage over MSSA in colonizing patients after
transmission (74). During an epidemic of HA-MRSA colonizations and infections in a surgical
ICU, 23 patients were exposed to six patients admitted to the ICU with HA-MRSA
colonization. PFGE of isolates showed that all secondary cases had HA-MRSA PFGE patterns
identical to the PFGE patterns of the strain recovered from the patients to whom they were
exposed. None of the PFGE patterns of the isolates of MSSA cultured from patients and HCWs
were the same. The authors concluded that HA-MRSA may have spread more easily between
patients due to selection through antibiotic pressure.
Airborne transmission of HA-MRSA may occur, but the importance of this route of
transmission has not been established. The CDC has not recommended airborne precautions
for patients with HA-MRSA colonization or infection (75). Theoretically, HA-MRSA could be
transferred by the airborne route after aerosolization from contaminated environmental
surfaces or by aerosolization from nasal carriers. One study has shown that HA-MRSA can be
aerosolized from environmental surfaces, i.e., changing bed sheets (76). Molecular typing
showed that environmental isolates and patient isolates were identical. However, the authors
did not investigate other possible routes of transmission of HA-MRSA to the patients.
Several studies have been published on the dissemination ofS. aureusfrom the upper
respiratory tracts of HCWs. To the author’s knowledge, no such studies have been published
on dissemination of HA-MRSA from HCWs. One study has epidemiologically implicated a
HCW with chronic sinusitis and nasal colonization withS. aureusin the spread ofS. aureusto
patients. The relationship was confirmed by molecular typing (67). There appears to be a
strong relationship between shedding of S. aureusby HCWs and having a viral upper
respiratory tract infection (77,78). In one study, nasal carriers ofS. aureuswho volunteered
were experimentally infected with rhinovirus (78). Investigators were able to quantify the
S. aureuscolony-forming units (CFU) released into the air under varying conditions, including
type of clothes worn and whether or not a mask was worn. They documented that theS. aureus
released into the air was from the experimentally infected volunteers by molecular typing.
Studies on airborne dissemination of HA-MRSA using these techniques are needed.
Risk factors for acquisition of HA-MRSA.Risk factors for acquisition of HA-MRSA in ICUs
vary depending on the type of ICU. Risk for HA-MRSA colonization/infection identified in
recent well-designed studies making use of multivariable analysis is shown in Table 2.
Neonatal ICUs.The epidemiology of HA-MRSA colonization and infection has been less well
studied in NICUs than in adult ICUs. Few, if any, reports on outbreaks of HA-MRSA in NICUs
published in the 1990s and up to the present have included data on the risk of acquisition of
HA-MRSA during outbreaks or analytic epidemiologic studies to identify risk factors for
acquisition. One study provided time-and-intensity-of-care-adjusted incidence density for
infections. In the intensive care section of the unit this incidence density was 0.73 infections/
1000 patient-care hours (47). In the intermediate-care area the incidence density was
0.62 infections/1000 patient-care hours. There are no data on the rate of acquisition of HA-
MRSA colonization.
There are few data on the source of HA-MRSA in NICUs. In one recent study, patients
would have to be presumed to be the source of HA-MRSA, as personnel or the environment
could not be implicated (49). In another study based on molecular typing, environmental
cultures were all negative and a HCW was thought to have transferred the HA-MRSA
outbreak strain from an adult hospital (51). However, the HCW was not epidemiologically
implicated as the source. In all of the latter studies, transmission between patients by the hands
of HCWs is suggested (47,49,51). In a prospective surveillance study in an NICU risk factors
for colonization with HA-MRSA included delivery by cesarean section and receipt of systemic
antibacterial therapy immediately before delivery. Absence of smoking by the mother
appeared to be protective (21). No case-control studies to identify risk factors for colonization
or infection with HA-MRSA in NICUs have been published to the author’s knowledge. Using a
different approach, one study implicated overcrowding and understaffing as risk factors for
acquisition of HA-MRSA colonization or infection (47).
106 Mayhall