Staphylococcus aureus bacteremia, Europe.

To the Editor: In their article, Collignon et al. (1) present a table comparing absolute numbers and incidence rates of Staphylococcus aureus bacteremia (SAB) in Australia to those of 5 other countries, and state that "some data are available from other countries for comparison" and "only 2 countries, Denmark and England, appeared to have comprehensive collection systems."

We would like to add data from the European Antimicrobial Resistance Surveillance System (EARSS) to their table. EARSS is a multinational surveillance system that links national networks by collecting comparable and validated data on the prevalence of antimicrobial resistance of 5 microorganisms, including S. aureus (2). A total of 30 countries participate in EARSS. To ensure representativeness of the data it publishes, EARSS has set criteria, which can be found elsewhere (3).

In 2003, through a questionnaire, we collected background information on all hospitals served by laboratories participating in EARSS, including the estimated hospitals' catchment populations. Proportion of the country population covered by EARSS was then calculated by dividing the sum of catchment populations by the total population of the country (4). Catchment populations of hospitals providing single specialty or supraregional type of care were not counted to avoid overlap with other hospitals within the same country. Only the countries that provided denominator data for at least 60% of the isolates were included to ensure that the sample of hospitals was still representative of the country as a whole.

The number of SAB and the incidence of SAB per 100,000 inhabitants in 2003 were calculated from EARSS data, adjusted for population coverage, and are presented in the Table. These are crude estimates of the true number of SAB and should, thus, be interpreted with caution. For example, we assumed that the isolates for which hospital background information was not available did not differ from isolates for which we had this information, and that hospitals that participated in EARSS in 2003 were a representative sample of the countries' hospitals. Additionally, the incidence of SAB was positively correlated with the blood culturing rate (Spearman r = 0.74, p = 0.002), which means that the incidence of SAB is likely to have been underestimated in countries that reported few blood cultures. Although some countries did not report their blood culturing rate, the incidences of SAB in these countries were among the highest and are unlikely to be underestimated. For example, the EARSS data for Denmark and Ireland nicely fitted those presented in Table 4 of the article by Collignon et al. (1). Finally, reporting to the EARSS system greatly improved over the years, which is why this study was performed on the last available year, 2003. Nevertheless, one cannot exclude underreporting of SAB by EARSS participating hospitals since EARSS is a voluntary reporting system. For example, England reported 18,403 SAB cases or an incidence of 37 SAB per 100,000 inhabitants from April 2002 to March 2003 through its mandatory surveillance scheme (5), whereas an estimate for the United Kingdom from the EARSS database would only give 7,800 SAB cases for 2003. However, it is impossible to determine whether this discrepancy was due to poor voluntary reporting of SAB cases, a lower blood culturing rate in EARSS participating hospitals, or a poorly representative sample of the country's hospitals. Data from the United Kingdom were excluded from the present study on the basis of the latter possibility; denominator information for <60% of the isolates was available.

In conclusion, EARSS is the first comprehensive surveillance system on antimicrobial resistance in Europe. Within certain limitations, EARSS can also provide valuable information on blood-culturing practices and the incidence of SAB in Europe. The system is continuously being improved, and additional information on the representativeness of EARSS data is being collected. This will allow us to improve the quality and accuracy of the reported incidence rates. In the future, the system should allow reporting of similar data for an even larger number of European countries and for additional microorganisms, such as Escherichia coli.

Edine W. Tiemersma, * Dominique L. Monnet, ([dagger]) Nienke Bruinsma, * Robert Skov, ([dagger]) Jos C.M. Monen, * Hajo Grundmann, * and European Antimicrobial Resistance Surveillance System Participants

* National Institute for Public Health and the Environment, Bilthoven, the Netherlands; and ([dagger]) Statens Serum Institut, Copenhagen, Denmark

References

(1.) Collignon P, Nimmo GR, Gottlieb T, Gosbell IB, Australian Group on Antimicrobial Resistance. Staphylococcus aureus bacteremia, Australia. Emerg Infect Dis. 2005;11:554-61.

(2.) Tiemersma EW, Bronzwaer SL, Lyytikainen O, Degener JE, Schrijnemakers P, Bruinsma N, et al. Methicillin-resistant Staphylococcus aureus in Europe, 1999-2002. Emerg Infect Dis. 2004;10: 1627-34.

(3.) European Antimicrobial Resistance Surveillance System. EARSS manual 2004 [cited 2005 July 25]. Available from http://www.earss.rivm.nl

(4.) Central Intelligence Agency. The world fact book 2003 [cited 2005 July 26]. Available from http://www.cia.gov/cia/download 2003.htm

(5.) Health Protection Agency. The second year of the Department of Health's mandatory MRSA bacteraemia surveillance scheme in acute trusts in England: April 2002-March 2003. CDR Weekly. 2003;13:1-9. [cited 2005 July 26]. Available from http://www.h pa.org.uk/cdr/archives/2003/cdr2503.pdf

Address for correspondence: Edine W. Tiemersma, National Institute for Public Health and the Environment, Center for Infectious Diseases Epidemiology, PO Box 1, 3720 BA Bilthoven, the Netherlands; fax: [phone]; email: [e-mail]

Table. Absolute numbers, rates of Staphylococcus aureus bacteremia
(SAB), and percentage of methicillin-resistant Staphylococcus aureus
(MRSA), European Antimicrobial Resistance Surveillance System (EARSS),
2003 *

                                      % population      blood culture
                    Population         covered by        sets/1,000
Country           ([dagger]) (4)    EARSS ([dagger])     inhabitants

Austria              8,188,207            42.7               NA
Bulgaria             7,537,929           100                  2
Croatia              4,422,248            81.3                7
Czech Republic      10,249,216            92.3               11
Denmark              5,384,384            46.2               NA
Estonia              1,408,556           100                 <1
Finland              5,190,785            94.3               27
Hungary             10,045,407           100                  1
Iceland                280,798           100                 28
Ireland              3,924,140            89.2               NA
Israel               6,116,533            39.7               42
Malta                  400,420           100                  4
Poland              38,622,660            24.3                3
Romania             22,271,839            59                 <1
Slovenia             1,935,677           100                 17
Spain               40,217,413            24.3               21
Sweden               8,878,085           100                 28

                    No. SAB      No. SAB for      SAB/100,000
                  reported to      country        inhabitants
Country              EARSS       ([section])      ([section])

Austria                871          2,038              25
Bulgaria               157            149       2 ([paragraph])
Croatia                360            443              10
Czech Republic       1,387          1,503              15
Denmark                671          1,451              27
Estonia                 98             98       7 ([paragraph])
Finland                727            771              15
Hungary                859            859       9 ([paragraph])
Iceland                 64             64              23
Ireland              1,108          1,243              32
Israel                 368            926              15
Malta                  122            122              31
Poland                 166            684        2 ([paragraph])
Romania                 85            144       <1 ([paragraph])
Slovenia               299            296              15
Spain                1,391          5,731              14
Sweden               1,855          1,760              20

Country           % MRSA

Austria             15
Bulgaria            31
Croatia             37
Czech Republic       7
Denmark             <1
Estonia              5
Finland             <1
Hungary             14
Iceland              0
Ireland             42
Israel              43
Malta               43
Poland              19
Romania             46
Slovenia            13
Spain               25
Sweden              <1

* Only countries that provided hospital background information for at
least 60% of the isolates were included; NA, not available.

([dagger]) Source: (4).

([double dagger]) Population coverage rate as calculated from EARSS
hospitals that provided background information was adjusted for
nonresponding hospitals as follows: population coverage as calculated
divided by proportion of isolates with hospital background information.

([section]) The total number of SAB per country was calculated as
follows: number of S. aureus isolates in EARSS divided by adjusted
proportion of population covered.

([paragraph]) These rates are grossly underestimated because of the
very low blood culturing rate.