Biomathematics and Statistics Scotland, Rowett Research Institute
The aim of this study was to assess the monitoring programme conducted by the Food Standards Agency Scotland (FSAS) for determining the prevalence of toxins responsible for diarrhetic shellfish poisoning (DSP), paralytic shellfish poisoning (PSP) and amnesic shellfish poisoning (ASP), in shellfish harvested from classified inshore production areas in Scotland. The toxicity patterns observed at designated sites throughout the year were established using data collected over a three-year period from April 2004 to November 2006. The current study is a follow-up on a previous risk assessment (report S01026), which was concerned with assessment of the inshore monitoring programme during April 2001 – March 2004.
The current (as implemented in April 2006) FSAS monitoring programme was assessed for the risk of a toxic event at a particular site being undetected. Alternative schemes that offered a more targeted allocation of resources or an improved level of public health protection were also considered.
Mussels are currently used as indicator species, and as it was the only species with data from all across Scotland, findings from this report tend to focus on mussels.
Analysis of monitoring data
The data analysis was concerned with toxin concentrations of ASP, DSP and PSP detected in mussels, pacific oysters, king scallops, queen scallops, and cockles (1788, 518, 31, 95, 26, and 30 samples, respectively). The numbers of samples collected during April 2004 – November 2006 were similar to those during the earlier period of April 2001 – March 2004, except for king scallops and cockles, which were only 20 to 30% of the numbers collected during April 2001 – March 2004.
Initial analysis of the monitoring data from 2004/6 revealed the following
- Prevalence of toxins during April 2004 – November 2006 was markedly different from the prevalence observed during April 2001 – March 2004. Not only was the average prevalence lower during 2004-2006, the pattern of prevalence of toxin during the 12 months of the year was different for these two data sets also.
To get a better understanding of variation in toxin patterns from year to year, it was decided to analyse the entire data set (from April 2001 to November 2006) in a combined, coherent, manner. All findings mentioned hereafter relate to the 2001/6 period, and are based on the model assumptions outlined in the report.
All samples were assigned to Pods (these are designated sample locations, introduced by FSAS in November 2006). In some instances the sample site was not covered by an existing pod, in which case a new pod was introduced. This resulted in 84 pods. To allow for modelling of toxin patterns, pods with limited data that had similar toxin patterns and were in close proximity of each other, were grouped. This resulted in 25 sites.
Analysis of the monitoring data from 2001/2006 revealed the following:
- Toxin levels of DSP, PSP and ASP varied significantly over time (between months and between years), across sites (with some sites showing a tendency for higher toxin levels) and across shellfish species.
- DSP was most often detected in queen scallops (10%) and mussels (5%). For the remaining species, fewer than 4% tested positive.
- Prevalence of DSP was high in 2001, 10% in mussels (20% in queen scallops), but gradually declined to 4% in mussels in 2006 (3% in queen scallops). This decline with time was seen in all shellfish species.
- DSP in mussels was present throughout the year but peaked at 13%, for an average site, during June – August. For certain sites prevalence went up to 31%.
- Nearly all sites tested positive for DSP in mussels at some time during 2001/6.
- PSP levels exceeding the regulatory limit of 80 μg/100g only occurred in mussels, except for 1 cockle and 2 clam samples in 2006 that exceeded 80 μg/100g.
- From 2001 to 2005 there was a decline in the proportion of mussel samples exceeding the regulation limit; 2.6% in 2001 down to 0.5% in 2005. In 2006 levels went up again (1.7%).
- A peak occurred in early summer (May and June, up to 4.6%) and a lesser peak in September (1.2%), although for some sites in Shetland this went up to 20% in June. Prevalence was low during the winter months.
- There were several sites for which PSP in mussels tested negative during the entire period of investigation.
- PSP was nearly always absent in pacific oysters, with only 2 samples (out of 852) testing between 0 and 40μg/100g (test results exceeding 40 μg/100g result in more frequent sampling).
- King scallops is the only shellfish species for which ASP levels exceeded regulatory limits (> 20μg/g), although low levels were detected in all shellfish species.
- There were only four sites at which king scallops were sampled and the ASP prevalence was similar for these sites.
- Prevalence of ASP exceeding regulatory limits in king scallop gonads was high during 2001/3 (up to 50%, for an average site and average month) then dropped down to 3% in 2005 and increased to 17% in 2006.
- In mussels, although approximately 40% of samples tested positive for ASP, only 3 out of 3791 samples exceeded field closure limits.
- Prevalence of positive ASP levels in mussels was high in 2001/4 at around 50%, but dropped to 20% in 2005 and 8% in 2006.
- ASP was detected in mussels at all 25 sites and prevalence across sites was similar.
- ASP in king scallop gonads and mussels tended to be present throughout the year although prevalence tended to be lower in late winter and early spring.
- For a given toxin, toxicity patterns over time (e.g. decline in DSP over the 2001/6 period) were similar for all shellfish species, although the actual level varied from species to species.
- Toxin prevalence varies between sites, even for sites that are close geographically.
Risk assessment of present and alternative monitoring schemes
The monitoring programme in place in 2006 consists of
- PSP: weekly all year round for all species
- DSP: weekly from April to November, fortnightly in December, monthly from January to March for all species
- ASP: weekly from July to November, fortnightly from April to June, monthly from December to March for all species. The only exception is king scallops, which are tested weekly all year round.
The monitoring data from April 2001-November 2006 provided sufficient information on levels of DSP in mussels, PSP in mussels and ASP in mussels and king scallops for each site during each month to enable a risk assessment to be carried out. The risk assessment was concerned with the monitoring programme failing to detect a toxic event, i.e. that a site is not tested while toxin levels exceed field closure (for example, a monthly sampling scheme would fail to detect that a site might become toxic only one week after a negative test result. This is referred to as the risk of non-detection. It was assumed that the test result is valid for one week.
- As PSP was tested on a weekly basis, the risk of non-detection in mussels was zero. As several sites always tested negative, alternative schemes were developed that considered reduced sampling effort at those sites.
- The maximum risk of non-detection was 3.7% for DSP in mussels during December (currently fortnightly sampling). Monthly testing during January-March seems acceptable as the maximum risk of non-detection was only 1.4%. Overall, the current scheme appears to be largely appropriate.
- ASP in king scallop gonads was tested weekly throughout the year and hence the risk of non-detection was zero. As high levels of ASP were prevalent throughout the year the current scheme appears to be appropriate.
The risk assessment enabled the following recommendations to be made.
- For the monitoring of PSP in mussels, sampling effort could be made more efficient by either reducing the sampling frequency or using simple screening methods for sites that have always tested negative for PSP.
- For a given toxin, toxicity patterns were similar for all shellfish species, supporting the use of mussels as indicator species.
- It is important to note that these findings are based on only six years’ of data and therefore there is a considerable amount of uncertainty in the estimates. There is no guarantee that sites, species, or months that were clear during this six-year period will remain clear in the future as toxin patterns may change. Therefore,
- - Some level of shellfish monitoring should be continued at all sites in order to reduce the risk of toxic events being overlooked.
- - Sampling schemes should be flexible so that adjustments in sampling frequency can be easily and quickly made when necessary.
Project Code: S14036