Detection of paralytic shellfish poisoning (PSP) toxins in shellfish tissue using MIST Alert™, a new rapid test, in parallel with the regulatory AOAC ® mouse bioassay

Abstract

In parallel trials with the mouse bioassay, MIST Alert™ for Paralytic Shellfish Poisoning (PSP), a rapid diagnostic test for PSP, detected 100% of the toxic extracts in over 2100 regulatory samples. Toxic extracts contained at least 80 μg saxitoxin equivalents (STX equiv.) in 100 g of shellfish tissue, or more, as measured by the regulatory AOAC ® mouse bioassay. Only one potentially toxic sample, which contained 78 and 86 μg STX equiv./100 g shellfish tissue in two different mouse bioassays, was recorded as negative in one replicate of MIST Alert™. All other toxic extracts among more than 2100 regulatory shellfish tissue samples were detected by MIST Alert™ for PSP. The MIST Alert™ for PSP also detected the majority of extracts containing PSP toxin greater than 32 μg STX equiv./100 g, which is the mouse bioassay detection limit. The MIST Alert™ for PSP gave a false positive result compared to the mouse bioassay at an average rate of about 14% over all sites, although some differences were seen between sites. Further analysis by high performance liquid chromatography (HPLC) of the (false positive) extracts showed that many contained PSP toxicity in the range of 20–40 μg STX equiv./100 g, below the level detectable by the mouse bioassay. The MIST Alert™ for PSP gave false positive results from extracts containing less than 20 μg STX equiv./100 g shellfish tissue only about 6% of the time. The PSP family of toxin analogues can occur in any combination in naturally contaminated shellfish tissue and the antibody mixture in the MIST Alert™ tests detect each of the different PSP toxin analogues with different efficacy. It is therefore impossible to provide an exact detection limit for the MIST Alert™ that would be applicable for all possible toxin profiles. Through the experience of comparison testing with the regulatory mouse bioassay in many parts of the world, with over 2100 different samples, the MIST Alert™ for PSP has proven its ability to detect all types of profiles of the PSP toxin analogues. The detection limit for MIST Alert™ for PSP was about 40 μg STX equiv./100 g for the ‘average’ profile of PSP toxin analogues. Since the detection limit depends on the toxin profile in the individual extract, it will also vary depending on the profile of analogues most commonly found at each geographic location. This was observed in our study. Over all sites in the trials, approximately 5% of samples below 40 μg STX equiv./100 g were positive, and 5% of samples between 40–80 μg STX equiv./100 g were negative. This is a reflection of the different analogue profiles found in naturally contaminated extracts, even after acid hydrolysis using the AOAC ® extraction method.