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Molecular detection of Toxoplasma gondii in ready-to-eat salad mixes: multi-country survey using a validated and harmonised standard operating procedure, Europe, 2021 to 2022
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Rafael Calero-Bernal1,*
, Martha Betson2,*
, Iva Slana3
, Barbora Bartosova3
, Gianluca Marucci4
, Alessia Possenti4
, Gema Álvarez-García1
, Nadja Bier5
, Anne Mayer-Scholl5
, Rebecca P Berg6,7
, Umer Chaudhry2
, Nadia M López-Ureña1,8
, Weronika Piotrowska9
, Jacek Sroka9
, Gro S Johannessen10
, Rebecca Davidson11
, Filip Dámek12
, Radu Blaga12,13
, Sandra Thoumire12 , Barbora Zalewská3
, Helga C Waap14
, Pikka Jokelainen15
, Marco Lalle4,*
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View Affiliations Hide AffiliationsAffiliations: 1 Animal Health and Zoonoses (SALUVET) Research Group, Animal Health Department, Faculty of Veterinary Sciences, Complutense University of Madrid, Madrid, Spain 2 Department of Comparative Biomedical Sciences, School of Veterinary Medicine, University of Surrey, Guildford, the United Kingdom 3 Department of Microbiology and Antimicrobial Resistance, Veterinary Research Institute, Brno, Czechia 4 Department of Infectious Diseases, Istituto Superiore di Sanità, Rome, Italy 5 Department of Biological Safety, Unit Diagnostics, Pathogen Characterization, Parasites in Food, German Federal Institute for Risk Assessment (BfR), Berlin, Germany 6 Laboratory of Parasitology, Department of Bacteria, Parasites & Fungi, Statens Serum Institut, Copenhagen, Denmark 7 Lewyt College of Veterinary Medicine, Long Island University New York, New York, the United States 8 Microtech S.R.L. (Laboratory), Moca, Espaillat, Dominican Republic 9 Department of Parasitology and Invasive Diseases, Bee Diseases and Aquatic Animal Diseases, National Veterinary Research Institute, Pulawy, Poland 10 Section for Food Safety and Animal Health Research, Norwegian Veterinary Institute, Tromsø, Norway 11 Section for Food Safety and Animal Health Research, Norwegian Veterinary Institute, Ås, Norway 12 Anses, INRAE, Ecole Nationale Vétérinaire d’Alfort, Laboratoire de Santé Animale, BIPAR, Maisons-Alfort, France 13 University of Agricultural Sciences and Veterinary Medicine, Cluj-Napoca, Romania 14 Laboratory of Parasitology, The National Institute for Agricultural and Veterinary Research (INIAV), Oeiras, Portugal 15 Secretariat for Infectious Disease Preparedness and One Health, Statens Serum Institut, Copenhagen, Denmark* These authors contributed equally to this work and share first authorship.Correspondence:Marco Lallemarco.lalle iss.it
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View Citation Hide Citation
Citation style for this article: Calero-Bernal Rafael, Betson Martha, Slana Iva, Bartosova Barbora, Marucci Gianluca, Possenti Alessia, Álvarez-García Gema, Bier Nadja, Mayer-Scholl Anne, Berg Rebecca P, Chaudhry Umer, López-Ureña Nadia M, Piotrowska Weronika, Sroka Jacek, Johannessen Gro S, Davidson Rebecca, Dámek Filip, Blaga Radu, Thoumire Sandra, Zalewská Barbora, Waap Helga C, Jokelainen Pikka, Lalle Marco. Molecular detection of Toxoplasma gondii in ready-to-eat salad mixes: multi-country survey using a validated and harmonised standard operating procedure, Europe, 2021 to 2022. Euro Surveill. 2025;30(22):pii=2400594. https://doi.org/10.2807/1560-7917.ES.2025.30.22.2400594 Received: 06 Sept 2024; Accepted: 27 Jan 2025
Abstract
Most Toxoplasma gondii infections in humans are considered foodborne, but the relative importance of the various routes of infection is largely unknown. Consumption of green produce contaminated with T. gondii oocysts has been identified as a possible source.
We aimed to estimate the occurrence and prevalence of T. gondii oocysts in commercially available ready-to-eat (RTE) salad mixes in 10 European countries.
A real-time PCR-based method for oocyst detection was developed and optimised by two laboratories and validated in an interlaboratory test. This detection method and a harmonised sampling strategy were applied in a multi-country study. Multivariable logistic regression was used to investigate risk factors for oocyst contamination of RTE salad.
The real-time PCR method had a detection limit of 10 oocysts per 30 g of salad. We collected 3,329 RTE salad samples (baby leaf and cut leaf mixes) from October 2021 to September 2022. The prevalence of T. gondii oocyst contamination was 4.1% (95% confidence interval (CI): 3.4–4.8%; n = 3,293). In multivariable regression analysis, winter season, sampling and packaging of salad in Northern Europe and production of salad in Western Europe were associated with detection of T. gondii, with no statistically significant differences between salad types.
We estimated the prevalence of T. gondii oocysts in RTE leafy green salads using a validated and standardised procedure to assess the potential risk for human infection; highlighting the need to address this risk at each critical point of the salad production chain.

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