Seafood products present unique food safety concerns beyond the usual pathogens and other risks—the marine environment from which they are harvested creates conditions that can threaten consumer health in different ways from the most common forms of food poisoning. Shellfish can be contaminated with a number of marine biotoxins, and certain species of fish are prone to building up damaging levels of histamine toxin in their systems as they decompose. Proper sanitation and traditional kill-steps cannot prevent these food safety threats, so testing becomes the key way for producers to protect consumer health.
The principal way for shellfish producers to safeguard consumers from the damaging effects of shellfish toxins, and to meet regulatory limits for those toxins, is a stringent testing program. Shellfish toxins are heat stable, meaning there is no cooking kill-step to eliminate them in food. They are also invisible to the naked eye, making scientific testing the only way to identify their presence.
Shellfish toxins are produced naturally by marine micro-algae, and they reach problematic numbers only when large algal blooms form in the water. Bivalve shellfish such as mussels and oysters are filter feeders, feeding on small particles in the water including toxin-containing micro-algae. The toxins from the algae can bioaccumulate to levels that can harm any organism that consumes the shellfish, including humans; the toxins do not harm the shellfish themselves, however.
“Because these blooms tend to form in warmer waters, shellfish producers often ramp up their testing in summer months,” says Neogen’s Kevin Mullholland. “However, in the face of gradually warming ocean temperatures, blooms have been popping up more frequently during the rest of the year in traditionally cooler waters around the globe. Experts have noted the possibility of more frequent algal bloom events in the future, requiring toxin testing more often across a wider area.”
Well-known Types of Poisoning
Amnesic shellfish poisoning (ASP). This condition is caused by domoic acid, which is produced by Pseudo-nitzschia spp. diatoms (a type of microscopic algae). Razor clams are most commonly associated with ASP, but mussels, crabs, and oysters can also be contaminated with domoic acid. In addition to nausea, vomiting, cramps, and diarrhea, ASP can cause neurological symptoms: confusion, dizziness, headaches, seizures, cardiac arrhythmia, and short-term memory loss that can become permanent. Initial symptoms usually occur within a day, and neurological symptoms take closer to 48 hours. Severe cases can lead to death.
Diarrhetic shellfish poisoning (DSP). Okadaic acid produced by the dinoflagellate Dinophysis causes DSP. The symptoms of DSP are generally more mild than other forms of shellfish poisoning, and include abdominal cramps, nausea, vomiting, and diarrhea.
Neurotoxin shellfish poisoning (NSP). Breve-toxins or their analogs cause NSP, which can trigger nausea, vomiting, and slurred speech when consumed.
Paralytic shellfish poisoning (PSP). An unusually high mortality rate is associated with PSP. The condition is caused by any of about 20 toxins derived from the neurotoxin saxitoxin. It is most often associated with molluscan shellfish, gastropods like moon snails, and crabs that feed on other shellfish. It takes usually under two hours for symptoms to appear in an infected person. Symptoms include tingling mouth, fingers, and toes, followed by a loss of motor control in the arms and legs. If enough toxin is consumed, a person might experience difficulty breathing or even paralysis of respiratory and chest muscles, causing suffocation. For these reasons, PSP can quickly become deadly.
“The issue producers face, then, is finding a consistent, accurate, and easy-to-use method for testing their shellfish for the toxins that produce these conditions,” says Neogen’s Brooke Roman. “Testing methods can be qualitative, meaning they simply screen for the presence of any given toxin, or they can be quantitative, meaning they provide a precise value that can be used to determine where a product’s toxin level is at relative to regulatory limits.”
Toxin Testing Methods
There are multiple ways to test for shellfish toxins, and the method a producer uses depends on their capabilities and needs.
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.
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.
Hammerly is a writer for Neogen. She can be reached at firstname.lastname@example.org.