The impact of calicivirus mixed infection in an oyster-associated outbreak during a food festival

Background: Despite calicivirus food-borne outbreaks posing major public health concern worldwide, little information is at present available about the impact of caliciviruses mixed infection in an oyster-associated outbreak in China. Objectives: To investigate the clinical and epidemiologic characteristics of an oyster-associated calicivirus outbreak initiated by a food festival in Shanghai, China, in April 2014. Study design: Molecular epidemiological studies based on nucleotide sequencing and phylogenetic analysis of calicivirus strains from patients. Results: A total of 65 of the 78 (83%) cases from this outbreak were associated with raw oyster consumption. Forty-six calicivirus strains were identified from 25 stool specimens with norovirus (NoV) GII.4 Sydney_2012, GII.13, GI.2, GI.5 and sapovirus (SaV) GI.2 being predominant genotypes and with a prevalence of triple-, double- and single-infection being 20%, 48% and 28%, respectively. Meanwhile, 13 putative NoV recombinants were indicated by the phylogenetic inconsistency between capsid and polymerase genotype, mainly including GII.Pe/GII.4 Sydney_2012. Molecular epidemiological investigation showed possible multiple route transmission in the field. The clinical and epidemiologic characteristics of the mixed point-source calicivirus outbreak also conformed to Kaplan's criteria. Conclusions: This is the first reported oyster-associated calicivirus outbreak with a high prevalence of mixed infection during a food festival described in China. Our investigation underscores the importance of early surveillance and comprehensive etiologic identification of mixed point-source outbreaks and the need for reliable standards of monitoring oysters to prevent and control calicivirus food-borne outbreaks in China. (C) 2015 Elsevier B.V. All rights reserved.