Biomathematics and Statistics Scotland, Rowett Research Institute
Holtrop, G ; Horgan, G
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 2001 to March 2004.
The current (as implemented in 2003) 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.
Analysis of monitoring data
The data analysis was concerned with toxin concentrations of ASP, DSP and PSP detected in mussels, Pacific oysters, scallops, queen scallops, and cockles (2132, 389, 144, 88 and 77 samples, respectively). Data from April 2001 to March 2004 were used.
Analysis of the monitoring data 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 and mussels, with 18% of the queen scallop samples and 12% of the mussel samples giving a positive test result. For the remaining species less than 5% of the samples tested positive. DSP was present throughout the year but peaked in August, when for certain sites the probability of mussel samples being positive was estimated to be 30-60%.
- PSP levels exceeded the regulatory levels in cockles and mussels only. Less than 2% of the cockles and mussels analysed during the 3-year period exceeded the regulatory limit for PSP. PSP toxins were not detected in shellfish from October-March, and peaked in June, with a few sites having an estimated probability of 20-34% for samples to exceed field closure levels during May and June.
- ASP was most often present in King Scallops, with over 90% of the samples tested on the whole scallop (the standard procedure for testing scallops for DSP and PSP) exceeding the closure limit. When tested on the gonad, 37% of King Scallop samples were found to exceed the closure limit. ASP levels in gonad tissues peaked in August-November, with the chance of a sample exceeding field closure estimated to be 57-77% during this period. Although ASP was detected in all other species, it resulted in field closure for only a small number of mussel and queen scallop samples (3 out of 2043 and 3 out of 81, respectively).
- The data suggest that levels of PSP can change rapidly at a site, increasing at some sites from zero to field closure levels within one week.
- It is important to note that these findings are based on only three years of data and therefore there is a considerable amount of uncertainty in the estimates. There is no guarantee that sites (or species, or months) that were clear during this three-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.
Risk assessment of present and alternative monitoring schemes
The present monitoring programme consists of monthly sampling throughout the year with certain sites being sampled fortnightly during April-September. The monitoring data from April 2001-March 2004 provided sufficient information on levels of DSP in mussels, PSP in mussels and ASP in 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 could become unknowingly toxic (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.
The maximum risk of non-detection using the present monitoring scheme was determined as being 46% for DSP in mussels, 26% for PSP in mussels, and 57% for ASP in King Scallop gonads.
Alternative monitoring schemes were devised, based on a combination of monthly, fortnightly and weekly sampling. These schemes are toxin, species, site and month specific, based on the principle that when toxin levels are low monthly sampling would be sufficient, while weekly sampling would be required when toxin levels are high. Fortnightly sampling would apply for intermediate toxin levels.
Furthermore, it was assumed that with weekly sampling the risk of non-detection is zero. Two schemes were considered, namely one where the risk of non-detection does not exceed 10% at any one time at any site, and a stricter scheme where the risk of non-detection does not exceed 5%.
The risk assessment enabled the following recommendations to be made.
- For the monitoring of PSP in mussels, sampling effort could be made more efficient, resulting in a reduced risk of non-detection while using fewer samples than is the case at present. This would require a more targeted allocation of samples to those sites and months that have historically experienced high PSP levels but would reduce the risk of non-detection from a maximum of 26% under the current scheme to 5% under a more targeted scheme.
- Under the current sampling scheme, fortnightly sampling is limited to April-September. This appears insufficient for DSP in mussels as DSP levels continued to peak in October-December. As a consequence, October had the highest number of sites having a large risk (exceeding 20%) of a field being unknowingly toxic. For several sites, weekly sampling during June-November would be required to reduce this risk to 10% or less.
- For ASP in scallops, toxin levels were high, particularly during August-December. Under the current scheme, the risk of non-detection is 20-57% during these months. Weekly sampling would be required during July-December in order to reduce this risk to 10% or less.
- Sampling frequencies may have to be adjusted if toxin levels start to change. For example, if continued monitoring indicates increased toxin levels at a particular site compared to those observed in the data used in this study, it would be advisable to increase sampling frequency at this site.
- The alternative schemes devised during this study were designed to reduce the risk of a site being unknowingly toxic to 10% or 5%. However, it should be emphasised that this is an arbitrary choice. Before the findings of this study are used to develop future monitoring schemes, the FSAS should first determine acceptable risk levels for each of the toxin groups.
Project Code: S01026