Assessment of fitness for purpose of an insect bioassay using the desert locust (Schistocerca gregaria L.) for the detection of paralytic shellfish toxins in shellfish flesh

We have developed a bioassay using 5th instar desert locusts (Schistocerca gregaria L.) for the detection of saxitoxin—the paralytic shellfish poison in shellfish flesh. The bioassay procedure is to inject 10 locusts with a shellfish extract, and assess their conditions at time points up to 2 h post injection, looking for an endpoint of paralysis. From the proportion responding, the equivalent dose of pure saxitoxin could be estimated. Performance characteristics of the bioassay were assessed using shellfish samples spiked with saxitoxin, and we found the bioassay could detect and quantify toxin levels in the range of regulatory relevance. Relative toxicities of selected saxitoxin analogues differed from those reported in mammalian systems. Variation for repeatability conditions was acceptable but variation was higher under reproducibility conditions. This was related to (a) batches of insects from different suppliers, (b) different operators, and (c) different observers assessing the endpoint. We also noted adverse reactions with some shellfish species. These problems may be resolved by further refinement of the method and operator training, before formal validation. Nevertheless, we suggest the method potentially offers a simple, ethically acceptable, broad-specificity functional bioassay, which is desirable in any toxin-monitoring programme.