The report presents the seventh round of the external quality assessment (EQA-7) scheme for typing of Salmonella enterica spp. organised for laboratories in the Food- and Waterborne Diseases and Zoonoses network (FWD-Net).
The objectives are to assess the quality and comparability of the typing data produced by the national public health reference laboratories in the FWD-Net.
Since 2012, it has covered the molecular typing methods used for EU-wide surveillance, Pulsed Field Gel Electrophoresis (PFGE) for all serovars and Multiple Locus Variable number of tandem repeat Analysis (MLVA) for Salmonella Typhimurium.
Twenty seven laboratories completed the EQA exercise and submitted results. The majority (93%) of laboratories did the PFGE part and more than half (56%) participated in the MLVA part.
Variable PFGE gel quality
PFGE is the gold standard for high discriminatory typing of Salmonella and the only generic molecular method for typing of all Salmonella serovars.
ECDC said the majority (74%) of laboratories produced a PFGE gel of sufficient quality to allow inter-laboratory comparison. However, the overall quality varied considerably.
“Compared to the previous EQA, fewer gel quality parameters generated a score of 1 [Poor], reflecting an improvement in the parameters ‘Restriction’ and ‘DNA degradation’. In both the previous and the current EQA, almost all non-comparable gels obtained a score of 1 [Poor] in the parameter ‘Bands’, stressing particular difficulties with this parameter,” said the agency.
“Measures need to be taken in the laboratories to improve the quality of ‘Bands’ to ensure inter-laboratory comparison of PFGE profiles. Most of the low ‘Band’ scores were due to thick or fuzzy bands. In a few cases, the entire lane was distorted as well. The easiest, and often best, way to improve the sharpness of the bands is to use wider wells.”
Gel analysis was performed by 18 participants in the PFGE part and all laboratories produced gel analyses in accordance with guidelines; an improvement compared to previous EQAs.
The molecular surveillance system TESSy-MSS relies on FWD-Net laboratories producing comparable typing results.
MLVA typing results
Multiple Locus Variable number of tandem repeat Analysis is a newly introduced typing method for S. Typhimurium, increasingly used due to fast turn-around-time and low-cost, providing high discrimination within one of the most prevalent serovars.
Quality of the MLVA typing of S. Typhimurium was high; twelve laboratories (80%) reported correct allelic profiles for all test strains and improvement during the EQAs was demonstrated (60% of the participants reported correct MLVA types for all test strains in EQA-4).
The majority of errors (six of eight) were caused by misreading raw data and errors upon submission of results, which are easily correctable, said the agency.
A laboratory standard operating procedure (SOP) for MLVA of S. Enteritidis was published this year.
In 2014, Salmonellosis was the second most commonly reported zoonotic disease in the European Union, with a notification rate of 23.4 cases per 100,000 population.
Although there was a decrease in the annual total number of Salmonella outbreaks in the EU between 2008 and 2014, it was still the second most common cause in the EU in 2014.
The EQA, arranged by the Unit of Foodborne Infections at Statens Serum Institut (SSI), Denmark, is the final one under the contract with ECDC and was between October 2015 and February 2016.