Researchers have evaluated Belgium’s surveillance system for Salmonella infections and the potential role of whole genome sequencing.

Surveillance of salmonellosis in Belgium depends on voluntary referral of human Salmonella isolates to the National Reference Center (NRC). Isolates are accompanied by a form with epidemiological information that includes the age, gender, and postal code of the patient, associated clinical picture and recent travel history.

The work, funded by Sciensano (the Belgian Institute for Health) contributed to a more accurate estimate of the burden of salmonellosis in Belgium and shows the systems help with interpreting surveillance data and trends over time.

Researchers assessed coverage of the NRC surveillance system based on a survey among licensed Belgian medical laboratories in 2019 and a study from 2016 to 2020 using the sentinel network of laboratories surveillance system. The number of labs in this network ranged between 38 and 47 in these five years. Results were published in the journal PLOS One.

WGS potential
Coverage of the NRC surveillance system was estimated to be 83 percent and 85 percent, based on results of the survey and the study. These figures are higher than reported in other European countries such as France and Netherlands.

Molecular subtyping by multiple-locus variable number tandem repeat analysis (MLVA) is routine for the two most important serotypes which are Enteritidis and Typhimurium. Whole genome sequencing is used in cases involving multidrug-resistant, invasive or outbreak-related strains.

High coverage of the NRC surveillance system advocates for the implementation of WGS at this central level to help earlier detection of outbreaks, said researchers.

Changes in laboratory practice such as use of culture-independent diagnostic tests (CIDTs) may impact the current surveillance system that relies upon culture confirmation and referral of isolates. However, the survey found use of CIDTs to identify Salmonella was limited in January 2020 in Belgium. Only five of 113 laboratories used a CIDT such as multiplex PCR to diagnose Salmonella cases.

Survey and study findings
The survey was linked to the mandatory external quality assessment (EQA) in January 2020 for medical labs to assess the quality of laboratory analyses.

It revealed labs do not make a selection based on serotype when sending isolates to the NRC. No regional differences in lab practice were seen that explain the higher incidence of Salmonella Typhimurium in Flanders.

Scientists said they are confident that the observed difference in incidence in serotypes between the different regions reflects the reality and is not because of selective sending of isolates. Possible explanations for the higher incidence of Salmonella Typhimurium in Flanders could be differences in food consumption patterns and/or a higher environmental spread due to abundant pig farms.

The main factors for sending isolates to the NRC were epidemiological reasons, for confirmation and/or antibiotic resistance, and for further serotyping.

The capture-recapture study showed coverage of the NRC surveillance network remained stable in the past five years. Even in 2020, when there was a decrease in Salmonella cases, likely related to the impact of COVID-19, coverage of the NRC surveillance remained high. The NRC database showed 1,631 Salmonella infections in 2020 compared to 2,619 in 2019.

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