A more recent technological development in the field of shellfish testing is the lateral flow immunoassay. Often compared to home pregnancy tests, these test strips screen for the presence of specific marine biotoxins and offer results in just minutes.
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Explore this issueFebruary/March 2019
An example of how these tests work is as follows: A shellfish sample, prepared for testing by a simple extraction process, is absorbed into a test strip, and travels upwards toward a reagent (a substance that provokes chemical reactions). This reagent zone contains antibodies specific for the targeted toxin. If the toxin is present in the shellfish extract, a chemical reaction will occur, resulting in lines being displayed on the test strip that indicate a negative or positive result—no toxin is present, or toxin is present above a predetermined level. Most tests take under 10 minutes to complete from start to finish.
Some test strips can be read visually. However, visual interpretation of the results between different people can vary. Problems can arise when differentiating between low positives and high negatives. In light of this, some companies now offer electronic readers that remove user subjectivity when reading the test strips. The electronic readers can be networked and can also store test data, making recordkeeping easier for the user.
“These tests are easy for anybody to conduct, and don’t require specialist training,” says Roman. “They’re portable, making them easy to use right on the boat or elsewhere onsite or in the field. They’ve made testing easier and more affordable for operations that use them.”
For testers processing a large number of samples, immunoassays are available in microwell formats, which allow the testing of up to 96 samples at the same time. These formats are often known as ELISAs (enzyme-linked immunosorbent assays) or EIAs (enzyme immunoassays).
“For those unable to test onsite, sending samples to independent laboratories is another method,” says Mulholland. “These methods are highly scientific and are run by experienced chemists. Most lab tests for shellfish toxins are liquid chromatography methods, including liquid chromatography-mass spectrometry (LC-MS) and high-performance liquid chromatography (HPLC). These are quantitative methods that can identify toxins in a wide range of shellfish from around the world.”
Both HPLC and LC-MS work by deconstructing the compounds that make up shellfish toxins. The sample and a solvent are pumped through a column with packing material that makes molecules with certain properties travel faster, while others travel more slowly. A reader analyzes the separated compounds to determine the substances that made up the original sample.
A chemical compound, histamine is released by cells when damaged (or as part of an allergic reaction). High levels of histamine may develop in a variety of fish species as they decompose, especially when they are not kept at suitably cold temperatures. These species include tuna, mahi-mahi, marlin, bluefish, sardines, anchovy, bonito, herring, and mackerel. Histamine poisoning is also sometimes known as “scombroid poisoning” because some of the earliest fish associated with the condition were members of the suborder Scombridae.
When it affects humans, histamine poisoning can cause red blotches to appear on the skin, nausea, a burning sensation in the mouth, headaches, muscle weakness, abdominal pain, diarrhea, wheezing, and swelling of the face and mouth. Symptoms can appear within a half-hour of consumption and usually last a few hours.
“In rare cases, histamine poisoning has been deadly, and so histamine testing of fish products is generally considered an important part of a Hazard Analysis and Critical Control Point plan for certain fish species,” says Roman.
Both lateral flow and microwell tests are available for histamine testing and are just as easy to run as similar tests for shellfish toxins. Laboratory testing methods, such as HPLC and LC-MS, can also be used to determine histamine levels.