Through training, reference testing services, technical support, and hosting of scientists, WHO Global Salm-Surv has been working to improve the laboratory capacity and thereby the data quality of WHO member states. The EQAS is one of the tools the program uses to assess the impact of its capacity-building efforts and to pinpoint areas for improvement.
The results from the first seven iterations of the WHO Global Salm-Surv EQAS, which to our knowledge is the largest external quality assurance program for the serotyping of Salmonella species, indicate that the majority of participating laboratories worldwide are capable of correctly serotyping Salmonella species. The number of participating laboratories increased consistently from 2000 to 2007, demonstrating an increased interest in quality assurance and an increased global capacity for Salmonella serotyping. However, fluctuations in the performance of some laboratories were observed. Some laboratories still do not meet the WHO Global Salm-Surv goal of performing serotyping on all eight strains with no more than one incorrect result. Efforts at building laboratory capacity for serotyping should focus on those laboratories in the future and should be directed at common difficulties.
Since test strains differ from year to year, an improvement in the performance of participating laboratories can be evaluated only on the basis of the internal quality controls. The results of quality control have remained fairly consistent during the seven iterations despite a large increase in the number of participants. The selection of a common serovar (Salmonella serovar Enteritidis) may have biased the results: this serovar is frequently encountered, and many laboratories are proficient at its identification. Additionally, this is a monophasic serovar. We have shown that most incorrect results appear to be caused by errors in the identification of phase two flagellar antigens. These factors might have contributed to the consistently high performance observed for the Salmonella serovar Enteritidis strain. Selection of a diphasic serovar may reduce this bias in the future.
Our data suggest that several factors contributed to the observed errors. Unpublished data from a needs assessment in the EQAS 2007 iteration, where 82 laboratories (56%) completed the survey, showed that nearly 1 out of 3 (30%) laboratories have limited access to high-quality antisera. Additionally, less-common serovars were included in some iterations (e.g., Salmonella enterica serovar Vinohrady [I 28:m,t:−]), and the same needs assessment found that 26% of laboratories had difficulty serotyping rare and unusual Salmonella strains. Finally, there are important regional differences in laboratory capacity. The needs assessment found that institutions in Africa, Asia and the Middle East, and Southeast Asia were more likely to report difficulty obtaining antisera, especially for serotyping unusual Salmonella strains, than were institutions in other regions.
The decline in the proportion of serotypes correctly identified in 2003 and 2004 was likely due to the selection of the Salmonella
isolates (Table ). In 2003 and 2004, laboratories needed less-common antisera in order to fully serotype all of the EQAS isolates. After 2004, only more-common serovars were included. The subsequent overall improvement in performance suggests that many laboratories have access only to commonly available antisera. WHO Global Salm-Surv has demonstrated that the predominant Salmonella
serotypes differ between regions (3
). Therefore, a broad selection of antisera for Salmonella
surveillance is needed globally. WHO Global Salm-Surv continues to provide information to participants on where to purchase high-quality antisera and to support many laboratories with antisera for surveillance purposes.
Many of the incorrect serotyping results were due to incorrect identification of phase two flagellar antigens (Table ). For example, common incorrect results included misidentification of Salmonella serovar Typhimurium (I 4,5,12:i:1,2) as Salmonella enterica serovar Farsta (I 4,5,12:i:e,n,x), Lagos (I 4,5,12:i:1,5), or Tumodu (I 4,5,12:i:z6). All four serovars share the same O and phase one flagellar antigens but differ in their phase two flagellar antigens.
Colonial form variation (the variable expression of minor antigens by different single-colony picks from the same strain) may occur with the expression of the O:61
antigen by some serogroup C2
). Therefore, although the current Kauffmann-White scheme regards O:6,8 and O:8 serovar pairs, such as Salmonella
serovars Newport (I 6,8:e,h:1,2) and Bardo (I 8:e,h:1,2), as distinct serovars, we allowed for colonial form variations. We considered correct identifications for Salmonella
serovars Newport, Kottbus, Hadar, Manhattan, and Bovismorbificans on the basis of the serogroup alone and accepted as correct for those serovars, respectively, Salmonella
serovars Bardo, Ferruch, Istanbul, Yovokome, and Hindmarsh.
The results from the WHO Global Salm-Surv EQAS also demonstrate important regional differences in the serotyping results for Salmonella species. Particular efforts should focus on Central Asia and the Middle East, but also on Africa, Russia, and the Caribbean, where a large proportion of the laboratories do not correctly serotype many of the strains. Addressing the regional differences will involve additional training courses in selected regions.
WHO Global Salm-Surv is a platform that can assist WHO member states to strengthen their core public health capacities under the International Health Regulations (IHR, 2005) for disease surveillance and response, which will in turn strengthen international public health security. WHO Global Salm-Surv promotes intersectoral collaboration among human health, veterinary, and food-related disciplines in food safety and other issues that arise at the human-animal interface. As the program continues to expand, it increasingly addresses regional training and support needs.
This study showed that there is a continuing need to improve Salmonella serotyping and that this need appears to be greater in specific regions. Detection of the phase two flagellar antigen is one of the more profound barriers to obtaining a satisfactory serotyping result. Future training efforts should be aimed at enhancing the ability to characterize the phase two flagellar antigen and disseminating information on where to purchase high-quality antisera.