59
Risk Factors for Nosocomial Bloodstream Infections Due to Acinetobacter
baumannii: A Case-Control Study of Adult Burn Patients
Hilmar Wisplinghoff, Walter Perbix, and Harald Seifert
From the Institute of Medical Microbiology and Hygiene, University of
Cologne, and the Department of Plastic Surgery, Städtisches
Krankenhaus Köln-Merheim, Cologne, Germany
Risk factors for Acinetobacter baumannii bloodstream infection (BSI) were studied in patients
with severe thermal injury in a burn intensive care unit where A. baumannii was endemic. Of 367
patients hospitalized for severe thermal injury during the study period, 29 patients with nosocomial
A. baumannii BSI were identified (attack rate, 7.9%). Cases were compared with 58 matched controls
without A. baumannii BSI. The overall mortality rate was 31% among cases and 14% among
controls; only two deaths (7%) were considered directly related to A. baumannii BSI. Molecular
typing of A. baumannii blood isolates by means of randomly amplified polymorphic DNA analysis
and pulsed-field gel electrophoresis revealed the presence of three different strain types. Multivariate
analysis showed that female gender (P Å .027), total body surface area burn of ú50% (P Å .016), prior
nosocomial colonization with A. baumannii at a distant site (P Å .0002), and use of hydrotherapy
(P Å .037) were independently associated with the acquisition of A. baumannii BSI in burn patients.
These data underscore the need for effective infection control measures for this emerging nosocomial
problem.
During the past 20 years, Acinetobacter baumannii has
emerged as a significant nosocomial pathogen mainly affecting
patients with impaired host defenses in the intensive care unit
(ICU) setting [1]. Clinical illnesses associated with A. baumannii include pneumonia, meningitis, endocarditis, peritonitis,
skin and soft-tissue infections, urinary tract infections, and
bloodstream infections (BSIs) [2]. The clinical course of
A. baumannii BSI may range from benign transient bacteremia
to fulminant septic shock, with a crude mortality rate as high
as 52% [3 – 5]. Outbreaks of infections have been linked to
contaminated respiratory therapy equipment [6], intravascular
access devices [3], bedding materials [7, 8], and transmission
via hands of hospital personnel [9]. Multidrug resistance is
common among these organisms and may complicate the treatment of serious infections [10, 11]. As with other nosocomial
pathogens, various risk factors have been found to be associated
with A. baumannii colonization and infection, such as extended
hospital stay, severity of illness markers, organ failure, mechanical ventilation, presence of intravascular access devices, prior
antibiotic therapy, and male gender [12 – 16].
Nosocomial infection remains the leading cause of morbidity
and death in patients with burns [17]. These patients constitute
Received 5 May 1998; revised 6 August 1998.
This work was presented in part at the 37th Interscience Conference on
Antimicrobial Agents and Chemotherapy held on 28 September – 1 October
1997 in Toronto (abstract J-202).
Grant support: This work was supported by the Medical Center of the
University of Cologne (Köln Fortune no. V99 / 1996).
Reprints or correspondence: Dr. Harald Seifert, Institute of Medical Microbiology and Hygiene, University of Cologne, Goldenfelsstrasse 19 – 21, 50935
Cologne, Germany ([email protected]).
Clinical Infectious Diseases 1999;28:59–66
q 1999 by the Infectious Diseases Society of America. All rights reserved.
1058–4838/99/2801–0010$03.00
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a homogeneous patient population who usually do not have
other serious underlying diseases. However, they present with
a unique combination of factors that are of potential significance for the development of nosocomial infection, such as
large burn wound areas, mechanical ventilation, multiple surgical procedures, parenteral nutrition, and decreased fluid, electrolyte, and serum protein levels.
Previous studies that have tried to identify risk factors for
the acquisition of nosocomial A. baumannii infections did not
include a significant number of burn patients [12 – 16]. The
goal of this study was to analyze risk factors specific for
A. baumannii bacteremia in burn patients. In addition, we present the largest series of BSI due to A. baumannii in burn
patients reported to date. Molecular typing methods were used
to analyze strain relatedness among A. baumannii isolated from
blood and other clinical specimens.
Materials and Methods
Background
Krankenhaus Köln-Merheim is an 800-bed university-affiliated tertiary care teaching hospital in Cologne, Germany. It
has four separate ICUs, including one of the major burn centers
in Germany with 10 beds in the burn intensive care unit (BICU)
and 10 beds in a separate plastic surgery ward for convalescing
patients that is adjacent to the BICU. Annually, about 120 –
150 thermally injured adults and children from all parts of
Germany are admitted. All patients in the BICU are cared for
in single cubicles. All microbiological testing for the hospital
was performed at the Institute for Medical Microbiology and
Hygiene, University of Cologne.
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Wisplinghoff, Perbix, and Seifert
Study Design
To identify risk factors associated with the acquisition of
A. baumannii BSI, a retrospective case-control study was conducted. A case was defined as any patient admitted to the BICU
during the study period (1 January 1990 through 31 December
1992) for whom one or more blood cultures were positive for
A. baumannii and who met the Centers for Disease Control
and Prevention’s definition for nosocomial BSI [18]. Second
episodes were excluded. Controls were randomly selected from
burn patients without A. baumannii bacteremia and were required to have been hospitalized in the same unit and during
the same period as the cases. A control was included only if
his or her BICU stay was at least as long as that of the corresponding case before the onset of A. baumannii BSI. Controls
were matched to cases on a 2:1 ratio for age ({10 years) and
length of stay before A. baumannii bacteremia.
Data Collection and Definitions of Clinical Parameters
Data recorded at the time of admission included age, sex,
severity of underlying burn injury as determined by the percent
of total body surface area (TBSA) burn [19], and the abbreviated burn severity index (ABSI) [20]. The ABSI is a prognostic
score system that correlates the probability of survival with the
presence of adverse prognostic factors such as age (1 point for
every 10 years), percent of TBSA burn (1 point for every 10%),
female gender (1 point), inhalation injury (1 point), and fullthickness burn (1 point). The probability of survival with a
score of ú12 points is õ10%. The cause of the burn injury
(exposure to flames, electricity, acid, hot surfaces, or explosion), accompanying traumata (fractures, severe tissue damage,
and head injury), and the presence of other significant comorbidities were also recorded.
In addition, the following data were extracted from the laboratory and medical records for the patients: length of BICU
stay before A. baumannii bacteremia; number of days in the
BICU before the first culture was positive for A. baumannii;
site of infection or colonization with A. baumannii; presence
of prior bloodstream infections, burn wound infections, and/or
urinary tract or respiratory tract infections due to agents other
than A. baumannii; duration of mechanical ventilation; presence of any of the five organ system failures defined below;
presence of peripheral, central venous, or arterial intravascular
catheters, urinary catheters, and nasogastric tubes; number and
type of prior surgical procedures, in particular surgical wound
excision and grafting; use of hydrotherapy; use of prior antimicrobial therapy; and outcome. Controls were followed up until
the date of bacteremia in the corresponding case.
Colonization and infection were defined according to previously published criteria for the study of nosocomial infections
[18]. Sepsis, severe sepsis, and septic shock were defined according to criteria reported by the American College of Chest
Physicians/Society of Critical Care Medicine Consensus Con-
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CID 1999;28 (January)
ference Committee [21]. The presence of organ system failure
was assessed by using the criteria described by Fagon et al.
[22] with the following minor modifications: respiratory failure,
FiO2 of ú0.4 for ú3 days during mechanical ventilation; cardiovascular failure, any need for epinephrine or norepinephrine
or a need for dopamine (ú5 mg/[kgrmin]) to maintain a systolic blood pressure of ú90 mm Hg; renal failure, decreased
urine output (õ500 mL/d), acute increase in serum creatinine
level of ú2.0 mg/dL, or requirement of acute dialysis or ultrafiltration; hematopoetic system failure, platelet count of õ100
1 109/L; and liver failure, serum bilirubin level of
ú5 mg/dL.
Microbiology
For surveillance, tracheal aspirate and burn wound specimens for cultures were obtained from burn patients at the time
of admission and twice weekly thereafter. Quantitative cultures
of burn wound biopsy specimens were not performed for the
diagnosis of burn wound infection. A. baumannii isolates were
identified by using the simplified identification scheme of
Bouvet and Grimont [23]: growth at 377C, 417C, and 447C;
production of acid from glucose; gelatin hydrolysis; and use
of 14 different carbon sources. All A. baumannii isolates recovered from blood as well as from other body site specimens
were prospectively collected during the study period and stored
in glycerol broth at 0707C until further use. MICs of selected
antibiotics were determined with a broth microdilution method
(MicroScan; Baxter Laboratories, West Sacramento, CA) in
accordance with guidelines established by the National Committee for Clinical Laboratory Standards [24]. Isolates of
A. baumannii were further characterized by means of molecular
typing methods such as randomly amplified polymorphic DNA
PCR analysis with use of two different primers (ERIC-2 and
M13) and Ready-To-Go RAPD Analysis Beads (Pharmacia
Biotech, Freiburg, Germany) [25] and analysis of genomic
DNA by pulsed-field gel electrophoresis with use of the restriction enzyme ApaI as described previously [26].
Statistical Analyses
Results were expressed as the mean {SD or as a proportion
of the total number of patients. Univariate analysis was
conducted by the x2 test for categorical variables and the MannWhitney U test for continuous variables. Factors were considered significant at a P value of £.05. All P values are twotailed. Risk factors independently associated with A. baumannii
BSI were identified by stepwise logistic regression analysis of
variables selected by univariate analysis, with a limit for entering and removing variables at 0.05. Only variables observed
in at least 10% of the patients were included. All statistical
analyses were done by using SPSS software (SPSS, Chicago).
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A. baumannii Bloodstream Infections in Burn Patients
61
Table 1. Characteristics of burn patients with (cases) and without (controls) Acinetobacter baumannii
bloodstream infection at the time of admission to the BICU.
Characteristic
No. (%) of females
Mean age { SD in y (range)
Mean ABSI { SD (range)
Mean percentage of TBSA burn {
SD (range)
No. (%) of patients with inhalation
injury
Mean length of stay in BICU { SD
in d (range)
No. (%) of patients who died
Cases
(n Å 29)
Controls
(n Å 58)
Total*
(n Å 367)
17 (59)
36 { 15 (9 – 75)
8.1 { 2.5 (4 – 13)
16 (28)
36 { 14 (6 – 74)
6.2 { 2.2 (2 – 12)
108 (29)
39 { 19 (1 – 96)
6.3 { 2.5 (1 – 16)
42 { 21 (14 – 95)
26 { 18 (2 – 80)
25 { 18 (2 – 99)
15 (52)
19 (33)
102 (28)
50 { 27 (5 – 106)
9 (31)
30 { 23 (6 – 86)
8 (14)
26 { 24 (3 – 156)
70 (19)
NOTE. ABSI Å abbreviated burn severity index; BICU Å burn intensive care unit; TBSA Å total body surface
area.
* Total no. of burn patients treated for §48 hours.
Results
Study Population and Patient Characteristics
Between 1 January 1990 and 31 December 1992, 389 patients with severe thermal injury were admitted to the BICU;
22 of these patients were treated for õ48 hours and were
excluded from the study. Of the remaining 367 patients, 35
had A. baumannii BSI (attack rate, 9.5 cases per 100 admissions). Six cases were excluded from further considerations
because their medical records were not available (four patients)
or because appropriate controls could not be identified (two
patients). The study cohort comprised the remaining 29 patients.
Patient characteristics at the time of admission are shown in
table 1. Seventeen cases (59%) and 16 controls (28%) were
female. The mean age {SD for the cases was 36 { 15 years
(range, 9 – 75 years), and the mean total length of stay in the
BICU { SD for the cases was 50 { 27 days (range, 5 – 106
days). The mean percent of TBSA burn { SD was 42 { 21
(range, 14 – 95) for the cases and 26 { 18 (range, 2 – 80) for
the controls; the mean ABSI { SD was 8.1 { 2.5 for the cases
and 6.2 { 2.2 for the controls. Fifteen cases (52%) and 19
controls (33%) had inhalation injury. Exposure to open fire led
to thermal injury in 54% of patients, 21% of burns were secondary to an explosion, 6% were caused by electricity, and 19%
were due to other causes such as contact with hot surfaces,
liquids, or gases. There was no significant difference between
cases and controls in terms of the type of burn injury.
The origin of A. baumannii bacteremia in 18 cases could
not be determined, mainly because A. baumannii was isolated
from multiple sites. Nine cases were considered catheterrelated, and in two cases, bacteremia was considered secondary
to a burn wound infection. A. baumannii bacteremia secondary
to a respiratory tract infection was probably underreported because of difficulties in performing chest roentgenography as
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well as chest physical examinations on these cases. Only two
cases had A. baumannii bacteremia without prior colonization
at a distant site.
Clinical sepsis at the time of bacteremia was evident in all
cases. Seven cases (24%) met the criteria for severe sepsis,
and three cases (10%) presented with septic shock. Bacteremia
was polymicrobial in 14 cases; Pseudomonas aeruginosa, enterococci, and coagulase-negative staphylococci were isolated
most frequently as copathogens.
The overall in-hospital mortality rate among all patients admitted for severe burns during the study period was 19%. Nine
cases and eight controls died during their hospital stay; therefore, the crude mortality rate among cases was more than twice
that among controls (31% vs. 14%, respectively). In only two
cases, however, was death considered related to A. baumannii
bacteremia.
Microbiological Data
During the study period, A. baumannii was highly endemic
in the BICU, with maximum rates of colonization as high as
48 cases per 100 admissions [27]. The most common microorganisms isolated from burn patients during the study period
were, in decreasing order of frequency, P. aeruginosa (21.5%),
A. baumannii (19.5%), and Staphylococcus aureus (11.9%);
11.2% of blood culture isolates were A. baumannii. All
A. baumannii isolates were multidrug-resistant with three different resistance patterns: strain A was susceptible to amikacin
and imipenem only; strain B, amoxicillin/clavulanate, piperacillin, and imipenem; and strain C, tobramycin and imipenem.
Forty-nine patients (27 cases [93%] and 22 controls [38%])
were colonized with A. baumannii; A. baumannii was not isolated from any of these patients at the time of admission. The
mean duration of hospitalization { SD from the time of admission to the first culture positive for A. baumannii was 10 { 10
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Figure 1. Fingerprint patterns for blood culture isolates of Acinetobacter baumannii recovered from burn patients following PCR amplification with the primer ERIC-2. Three different A. baumannii strain
types were found: A (lanes 2 – 7), B (lanes 8 and 9), and C (lanes
10 – 18). Lanes 1 and 19, 100-bp ladder.
days (range, 1 – 41 days) for both groups. For cases, the mean
time { SD between colonization and the onset of bacteremia
with A. baumannii was 13 { 10 days (range, 1 – 78 days). The
primary site of colonization was most often the respiratory tract
(57% of cases) followed by contaminated burn wounds (37%).
Randomly amplified polymorphic DNA PCR typing of blood
culture isolates revealed that three different A. baumannii strain
types infected 13, 12, and 4 cases, respectively (figure 1). These
strains corresponded to the strains isolated from a distant site
in a given patient as well as to the A. baumannii strains found
to be endemic in the BICU. Typing results obtained by PCR
fingerprinting were confirmed by pulsed-field gel electrophoresis (data not shown).
Case-Control Study
Clinical characteristics of cases and controls and risk factors
for A. baumannii BSI according to univariate analysis are
shown in table 2. Cases had more severe burn injuries than did
controls as determined by a higher mean ABSI (8.1 vs. 6.2,
respectively; OR, 5.8), and more cases had a TBSA burn of
ú50% than did controls (34% vs. 9%, respectively; OR, 4.8).
The severity of illness was greater in cases than in controls as
illustrated also by a higher frequency of organ failure (69%
vs. 43%, respectively; OR, 2.9) and a greater mean total number
of organs affected (1.6 vs. 0.8, respectively; OR, 3.5).
Cases received support with mechanical ventilation more
frequently (97% vs. 64%, respectively; OR, 18.4) and for
longer periods than did controls. The mean duration of mechanical ventilation was 22 days for cases and 15 days for controls.
Cases were more likely than controls to have received hydrotherapy (86% vs. 55%, respectively; OR, 5.1) at least once
during their hospital stay as part of their burn wound care. The
mean number of courses of hydrotherapy was five for cases
and three for controls.
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CID 1999;28 (January)
Prior antimicrobial therapy was administered more often for
cases than for controls. Cases were significantly more likely
than controls to have received therapy with broad-spectrum
penicillins, cephalosporins, and aminoglycosides. Twentyseven cases and 22 controls were colonized with A. baumannii
prior to bacteremia (93% vs. 38%, respectively; OR, 22.1).
Prior BSIs due to organisms other than A. baumannii were
more frequent in cases than in controls (41% vs. 12%, respectively; OR, 5.1).
All cases and 50 controls (86%) had at least one intravascular
catheter in place (OR, 4.5). Cases had a mean { SD of 2.2 {
0.4 intravascular access devices per patient, whereas controls
had a mean { SD of 1.5 { 0.9. Intravascular catheters were
in place for a mean duration of 23 days for both cases and
controls. Urinary catheters (93% vs. 74%, respectively; OR,
4.7) and nasogastric tubes (97% vs. 67%, respectively; OR,
13.6) were also more frequent in cases than in controls.
Multivariate logistic regression analysis identified four independent risk factors associated with the acquisition of A. baumannii BSI in burn patients: female gender (P Å .027), prior
nosocomial colonization with A. baumannii (P Å .0002), TBSA
burn of ú50% (P Å .016), and use of hydrotherapy (P Å .037)
(table 3).
Discussion
Nosocomial infections in thermally injured patients remain
a common complication, contributing substantially to burnassociated morbidity and mortality [17, 28, 29]. P. aeruginosa
is considered the major cause of life-threatening infections in
these patients; the overall mortality rate among patients with
P. aeruginosa bacteremia has been reported as high as 77%
[30, 31]. In recent years, A. baumannii, another opportunistic
gram-negative nosocomial pathogen, has also emerged as an
important organism in major burn units around the world [7,
32 – 37]. Multidrug resistance in these organisms, including
resistance to imipenem, is a cause of concern [5, 11, 36, 37].
A. baumannii was highly endemic in our hospital during the
entire study period. The overall incidence of colonization and
infection with A. baumannii (48 cases per 100 admissions) was
higher than in most other previous investigations performed in
various types of ICUs for which reported attack rates ranged
from 3% to 14% [6, 12, 13, 15]. This difference may be explained by the fact that large burn wounds contaminated with
A. baumannii provide an excellent reservoir for patient-to-patient spread of this organism. Only two previous studies reported comparable colonization rates of 61% and 51%, respectively [7, 32]. Both studies were from other burn centers, and
attack rates were recorded during hospital outbreaks of limited
duration.
The present study included 29 cases of A. baumannii BSI
in burn patients, which to our knowledge is the largest series
of cases in this patient population that has been reported to
date. During the study period, A. baumannii was the second
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A. baumannii Bloodstream Infections in Burn Patients
63
Table 2. Results of univariate analysis of potential risk factors for acquisition of Acinetobacter baumannii bacteremia in burn patients.
Finding
Female gender
TBSA burn of ú50%
Mean ABSI { SD
Mechanical ventilation
Organ failure
Respiratory
Cardiovascular
Hematologic
Renal
Mean total number of organs affected { SD
Prior colonization with A. baumannii
Prior surgical procedures
Use of hydrotherapy
Prior BSI not caused by A. baumannii
Prior RTI not caused by A. baumannii
Prior use of antibiotics
Broad-spectrum penicillins
Cephalosporins
Aminoglycosides
Central venous catheter
Arterial catheter
Urinary catheter
Nasogastric tube
Cases
(n Å 29)
Controls
(n Å 58)
P value
17 (59)
10 (34)
8.1 { 2.5
28 (97)
20 (69)
11 (38)
16 (55)
13 (45)
5 (17)
1.6 { 1.2
27 (93)
27 (93)
25 (86)
12 (41)
12 (41)
16 (28)
5 (9)
6.2 { 2.2
37 (64)
25 (43)
12 (21)
17 (29)
11 (19)
5 (9)
0.8 { 1.1
22 (38)
49 (84)
32 (55)
7 (12)
15 (26)
.0049
.0073
.0002
.0004
.0229
.0856
.0191
.0110
.7805
.0110
õ.0001
.2541
.0041
.0018
.1403
3.7
4.8
5.8
18.4
2.9
.0002
.0002
.0005
.0358
.0003
.0175
.0022
11.8
11.8
5.4
4.5
14.7
4.7
13.6
27
27
21
29
29
27
28
(93)
(93)
(72)
(100)
(100)
(93)
(97)
31
31
17
50
38
43
39
(53)
(53)
(29)
(86)
(66)
(74)
(67)
OR (95% CI)
3.0
3.5
3.2
22.1
5.1
5.1
(1.5 – 9.5)
(1.4 – 16.0)
(2.2 – 15.5)
(2.3 – 145)
(1.1 – 7.5)
NS
(1.2 – 7.5)
(1.3 – 9.3)
NS
(1.1 { 11.4)
(4.8 – 102)
NS
(1.6 – 16.5)
(1.7 – 15.2)
NS
(2.6 – 54.1)
(2.6 – 54.1)
(2.0 – 14.4)
(1.7 – 23.8)
(1.9 – 116)
(1.0 – 22.2)
(1.7 – 108)
NOTE. ABSI Å abbreviated burn severity index; NS Å not significant; RTI Å respiratory tract infection; TBSA
Å total body surface area. Unless otherwise indicated, all values represent no. (%) of patients.
most common pathogen isolated; 11.2% of blood cultures were
positive for this organism. The incidence of bacteremia due to
A. baumannii was 7.9 cases per 100 admissions. Lyytikäinen
and co-workers [37] isolated A. baumannii from blood specimens from eight patients with thermal injury, but these investigators did not provide data on infection rates. Sherertz and
Sullivan [7] found acinetobacter BSI in seven (6.8%) of 103
patients admitted to the hospital during a 21-month epidemic
period. A. baumannii BSI was rare in other studies of burn
patients [32, 36]. In our study, the mortality rate was 31%
among burn patients with A. baumannii BSI (cases), 14%
among burn patients without A. baumannii BSI (controls), and
19% among the total burn population; however, these findings
may rather reflect more severe burn injuries in cases than the
intrinsic virulence of A. baumannii, since death directly related
to A. baumannii bacteremia was observed only in two (7%) of
29 cases.
Sherertz and Sullivan [7] also observed a lower mortality
rate among patients with acinetobacter bacteremia (14%) than
among patients with BSI due to other organisms (44%). These
investigators even speculated that Acinetobacter might be
functioning in a protective role by preventing colonization
and infection with more virulent organisms. These findings
contrast the attributable mortality rate associated with A. baumannii infection that ranged from 19% to 34% in other recent
series [4, 5, 13]. In our experience, A. baumannii bacteremia
Table 3. Results of multivariate logistic regression analysis of factors associated with the acquisition
of Acinetobacter baumannii bacteremia in burn patients.
Factor
Coefficient
SE of
coefficient
P value
OR (95% CI)
1.6351
1.8136
3.2668
1.7135
0.7390
0.9290
0.8794
0.8215
.027
.016
.0002
.037
5.13 (1.20 – 21.83)
6.13 (1.21 – 37.80)
26.23 (4.68 – 147)
5.5 (1.11 – 27.76)
Female gender
TBSA burn of ú50%
Prior colonization with A. baumannii
Use of hydrotherapy
NOTE. TBSA Å total body surface area.
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Wisplinghoff, Perbix, and Seifert
in burn patients often results from noninvasive burn wound
infection and sometimes may be transient after burn wound
manipulation [28], thus running a more benign course with a
lower mortality rate. A. baumannii respiratory tract infection,
which is associated with a less favorable prognosis [4, 38],
is less frequently observed in this patient population [7].
Unfortunately, the source of A. baumannii bacteremia was
not identified in most of our patients, since invasive diagnostic techniques such as quantitative cultures of protected specimen brush or bronchoalveolar lavage samples and quantitative burn wound biopsy cultures that were proposed by other
investigators [38, 39] were not employed.
Investigations of hospital outbreaks have suggested that exposure to colonized patients and contaminated medical equipment is associated with A. baumannii colonization and infection
[6, 9, 14, 37]. Sherertz and Sullivan [7] identified contaminated
mattresses in a burn unit as the source of an outbreak of acinetobacter infection that could be controlled only after each patient’s mattress was discarded after the patient was discharged
from the unit. PCR fingerprinting as well as DNA macrorestriction analysis showed that three different A. baumannii strain
types, which corresponded to the strains known to be endemic
in the BICU, caused bacteremia in our patients. These strains
were all multidrug-resistant. Increased resistance among
A. baumannii was generally observed in more recent studies
[4, 11, 36, 37]. The concurrent transmission of two epidemic
A. baumannii strains was also observed in previous reports [14,
37]. The retrospective design of our study, however, did not
allow us to identify any point source of infection or to prove
patient-to-patient transmission.
Various studies have analyzed potential risk factors predisposing to the nosocomial acquisition of A. baumannii. Risk
factors identified previously included male sex [14], underlying malignancy [16], APACHE II score [13, 16], prior length
of stay in the unit [7, 13 – 15, 40], mechanical ventilation [12,
14 – 16, 40], prior use of antibiotics [12 – 14, 16] (in particular,
third-generation cephalosporins [12, 14, 15] and aminoglycosides [12]), enteral hyperalimentation [14, 15], presence of
a nasogastric tube [14], exposure to invasive procedures such
as prior surgery [15], tracheostomy [14, 15, 40], placement
of intravascular catheters [3, 14 – 16] and/or urinary catheters
[14 – 16], prior infection, and organ system failure [13]. Most
of these potential risk factors were identified by univariate
analysis and may simply reflect the severity of the underlying
disease and the current management of severe diseases in the
modern ICU setting that leads to the frequent impairment of
natural host defenses and provides multiple opportunities for
the invasion of bacterial pathogens. Severity of illness [13],
male sex [14], duration of ICU stay [13 – 15], mechanical
ventilation [16], prior infection [13], and use of third-generation cephalosporins [14, 15] were also confirmed by multivariate regression analysis as independent risk factors for acquisition of A. baumannii.
Previous investigators have not differentiated between colonization and infection with A. baumannii in their analysis of
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CID 1999;28 (January)
risk factors, however. Most researchers analyzed the acquisition of A. baumannii in the setting of a nosocomial outbreak
and only rarely included a sufficient number of patients with
A. baumannii BSI [9, 12, 13, 15, 16]. In addition, previously
reported data may be difficult to compare since they were
obtained for a mixed population of patients with multiple underlying diseases who were cared for in various types of ICUs.
In the present study, risk factors for A. baumannii BSI were
analyzed in a population of burn patients (who are rather homogeneous with regard to their underlying illnesses, invasive procedures, and type of care). Several of the factors found to be
significant by univariate analysis in previous studies were also
found to be significant for our patients, including mechanical
ventilation, prior use of antimicrobials, prior infection due to
organisms other than A. baumannii, presence of organ system
failure, and use of intravascular catheters, urinary catheters,
and nasogastric tubes. The severity of the underlying burn
injury is expressed by both the ABSI and the percent of TBSA
burn rather than by the APACHE II score. Both markers are
specific for thermally injured patients, and the percent of TBSA
burn was identified as a significant risk factor by logistic regression analysis. However, most of the other factors that mainly
reflect the severity of the underlying burn injury were not found
to be independently associated with A. baumannii BSI in our
patient population.
Hydrotherapy administered to patients with extensive fullthickness burns continues to be an integral part of wound care
in many burn centers in the United States and elsewhere [41].
However, hydrotherapy (which usually includes immersion of
the patient in tap water during administration of intravenous
narcotic analgesics) may facilitate the dissemination of nosocomial pathogens to other wounds in the same patient and may
serve as a reservoir for patient cross-contamination [41]. The
risk of infection associated with hydrotherapy was recently
assessed in an outbreak of wound infections with P. aeruginosa
in burn patients [31]. In contrast to the findings of Sherertz
and Sullivan [7], hydrotherapy was found to be independently
associated with the development of A. baumannii BSI in our
study. The mechanism by which hydrotherapy may favor
A. baumannii BSI has not been established. It seems possible
that meticulous cleaning and disinfection of hydrotherapy
equipment alone may be sufficient to eliminate this risk factor.
Of note, female gender was also found to be an independent
risk factor for nosocomial A. baumannii bacteremia in burn
patients in our study. There is currently no explanation for this
finding, which is in contrast to data reported by Mulin and
colleagues [14] who found that male sex was an independent
risk factor in their study. In our series, on the other hand, 14
(82%) of 17 female patients with A. baumannii BSI survived,
while only six (50%) of 12 male patients survived. This finding
is even more surprising since female gender is usually considered an adverse prognostic factor for survival in patients with
severe burn trauma [20].
Prior colonization with A. baumannii was the most significant independent risk factor for A. baumannii BSI in our study.
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A. baumannii Bloodstream Infections in Burn Patients
None of our patients were colonized at the time of admission.
The mean interval between colonization with A. baumannii at
a distant site and the onset of bacteremia was 13 days. This
finding is not surprising since colonization is a prerequisite for
infection, but it underscores the need for effective infection
control measures.
In conclusion, this study demonstrates that burn patients with
large burn wound areas are at high risk for the development
of nosocomial BSI due to A. baumannii in a unit where this
organism is endemic. Mortality rates among these patients are
high. Epidemiological typing revealed that three different
A. baumannii strain types were responsible for BSI. Prior colonization with A. baumannii was a prerequisite for the acquisition of BSI with this organism and was the most significant
independent risk factor. Other independent risk factors for burn
patients included female gender, percent of TBSA burn, and
hydrotherapy. Our data substantiate the need for intense surveillance of A. baumannii in high-risk areas and the institution
of effective infection control measures to prevent the spread
of this organism. In addition, the role of hydrotherapy in burn
care should be reconsidered.
Acknowledgments
The authors thank A. Schulze and D. Stefanik for excellent
technical assistance, R. Lefering for help in the statistical analysis,
and R. P. Wenzel for critical review of the manuscript.
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