Environmental factors influencing human viral pathogens and their potential indicator organisms in the blue mussel, Mytilus edulis: the first Scandinavian report

Abstract

This study was carried out in order to investigate human enteric virus contaminants in mussels from three sites on the west coast of Sweden, representing a gradient of anthropogenic influence. Mussels were sampled monthly during the period from February 2000 to July 2001 and analyzed for adeno-, entero-, Norwalk-like, and hepatitis A viruses as well as the potential viral indicator organisms somatic coliphages, F-specific RNA bacteriophages, bacteriophages infecting Bacteroides fragilis, and Escherichia coli. The influence of environmental factors such as water temperature, salinity, and land runoff on the occurrence of these microbes was also included in this study. Enteric viruses were found in 50 to 60% of the mussel samples, and there were no pronounced differences between the samples from the three sites. E. coli counts exceeded the limit for category A for shellfish sanitary safety in 40% of the samples from the sites situated in fjords. However, at the site in the outer archipelago, this limit was exceeded only once, in March 2001, when extremely high levels of atypical indole-negative strains of E. coli were registered at all three sites. The environmental factors influenced the occurrence of viruses and phages differently, and therefore, it was hard to find a coexistence between them. This study shows that, for risk assessment, separate modeling should be done for every specific area, with special emphasis on environmental factors such as temperature and land runoff. The present standard for human fecal contamination, E. coli, seems to be an acceptable indicator of only local sanitary contamination; it is not a reliable indicator of viral contaminants in mussels. To protect consumers and get verification of "clean" mussels, it seems necessary to analyze for viruses as well. The use of a molecular index of the human contamination of Swedish shellfish underscores the need for reference laboratories with high-technology facilities.

FIG. 1.

Map showing the coastal area of Skagerrak. The sampling sites, referred to as A, B, and C, are situated along the southwestern coast of Sweden.

FIG. 2.

Graphs from sites A, B, and C showing the occurrence of Ad, enterovirus, and NLV in relation to the presence of E. coli (indicated by columns). The water flow (cubic meters per second) in the river Göta Älv indicating regional land runoff is illustrated with an area diagram, and water temperature (Celsius) and salinity (PSU) are shown with line diagrams (right y axis). The left y axis that indicates the number of E. coli cells (MPN × 100 g⁻¹) is truncated in all three panels to better visualize the differences and fluctuations in the columns. The x axis indicates the time period from February 2000 to July 2001, with each month designated by the first letter of the name of the month. The correct number of E. coli microbes for the following sampling months are as follows: A, March 2001, MPN = 1.6 × 10⁶ × 100 g⁻¹; B, February and March 2001, MPN = 17,500 and 1.6 × 10⁶ × 100 g⁻¹, respectively; C, March 2001, MPN = 1.6 × 10⁶ × 100 g⁻¹.

FIG. 3.

The potential indicator organisms somatic coliphages, F-RNA phages, and phages infecting B. fragilis (PFU × 100 g⁻¹) measured in mussel tissue from sites A, B, and C during the period from February 2000 to July 2001, with each month designated by the first letter of the name of the month.

FIG. 4.

Graphs showing the human viruses detected in mussels from sites A, B, and C. For each site, the relationship between the percentages of virus-free samples and positive findings of enterovirus, Ad, and NLV, respectively, is presented for the investigational period from February 2000 to July 2001.