The average consumption of fish per person has increased dramatically over the last four decades. People are eating more fish because of its high nutritional value, particularly omega-3, and as a result, seafood restaurants are becoming more popular. As the consumption of fish grows, so do cases of scombroid and methylmercury fish poisoning. Fish is the most common cause of scombroid and methylmercury poisoning in humans. Restaurant associated scombroid poisonings are caused by improper handling of fish in restaurants but in the case of methylmercury poisoning, customer eating habits, such as excessive fish consumption, expose humans to methylmercury, resulting in methylmercury poisoning.
Restaurant operators, have a social responsibility to protect the health and well-being of their customers, and therefore play an important role in preventing scombroid poisoning and lowering methylmercury exposure. This is accomplished by carefully selecting the seafood to be served and following standard purchasing, receiving and storage procedures.
Before delving into the preventative measures that restaurant operators can take to prevent Scombroid poisoning and reduce methylmercury exposure, it is important to understand what these two fish poisonings are all about. Let’s get started.
What is Scombroid Fish Poisoning
Scombroid Fish Poisoning, also known as Mahi Mahi Flushes or Histamine Fish Poisoning, is an allergic reaction caused by eating fish that have high levels of histamine. This poisoning is most commonly associated with fish from the Scombroid family, such as Salmon, Mahi Mahi, Herring, Sardines, Tuna, Mackerel, Skipjack and Bonito, though many other types of fish can also cause scombroid poisoning.
Histamines can form when fish are not handled properly from the time they are caught until they are consumed. If fish is not handled properly, histamines can form at any time, from the time it is caught to the time it is stored and transported. Natural bacteria on fish skin and intestine, such as Organella Morganii, E. coli, and Pseudomonas Aeruginosa, break down the flesh and convert histidine, which occurs naturally in fish, to histamine, resulting in histamine formation (also called scombrotoxin). Typically, histamine levels in fish that exceed 20-25 mg per 100 grams are usually cause for concern.
Risk factors causing histamine formation
Some risk factors for histamine formation include leaving fish in the hot sun for an extended period of time and failing to refrigerate at the recommended temperature. If fish is not refrigerated immediately after being caught, decomposition and microbial spoilage occur more quickly, resulting in the formation of histamine. Holding fish in the sun, even on a bed of ice, can result in histamine formation because one side of the fish warms up faster than the other. Once histamine has been formed in the fish, it cannot be removed by freezing or cooking. Hence, proper fish handling is essential from the time the fish is caught until it is cooked.
The appearance, taste and odour of the fish are not reliable indicators of histamine presence. Histamine-laced fish usually have a peppery or metallic flavour and a foul odour. Furthermore, histamine-laced fish may have a honeycomb appearance, though this is not always seen. Even if they do not appear spoiled or emit odours, they can be laced with histamine. Also, histamine accumulates at varying levels in various parts of a fish. It has also been observed that after eating the same histamine-laced fish, some people experience symptoms of histamine poisoning while others do not. However, in some diners, even minor histamine exposure, such as sharing a small portion of fish can cause histamine poisoning.
Symptoms of Scombroid Fish Poisoning
The most common symptoms of scombroid poisoning are rash, a burning sensation in the mouth and throat, swelling of the face and tongue, abdominal cramps, dizziness, headache and vomiting. The vast majority of people experience symptoms within minutes to two hours of eating histamine-laced fish. Most people will feel better within eight to twelve hours, but some may experience symptoms for several days. Most people recover on their own and do not require treatment for histamine poisoning.
People with underlying medical conditions may experience severe symptoms that necessitate medical attention. Scombroid poisoning can be caused not only by eating contaminated fresh fish and shellfish but also by eating contaminated canned or smoked fish.
What is Methylmercury Fish Poisoning
Methylmercury Fish Poisoning is another source of concern for restaurant operators. Methylmercury is a highly toxic neurotoxin that can cause kidney failure, neurological complications and even death. Let's start by looking at how methyl mercury accumulates in fish tissues.
Mercury is released into the air and soil by industry smoke emissions, and it is then washed into bodies of water. Mercury is converted to methylmercury by naturally occurring toxins and bacteria in water bodies. When fish drink methylmercury-laced water, it builds up in their body tissues. According to research studies, methylmercury can be found in nearly all fish and shellfish. Some fish and shellfish contain more methylmercury than others. Methylmercury levels in fish and shellfish vary depending on the amount of mercury they are exposed to, their age and the environment in which they live.
Which fish have high mercury levels
High-mercury-level fish include King Mackerel, Swordfish, Orange Roughy, Shark, Marlin, Tilefish, Mahi-Mahi and Bigeye Tuna. Many of these fish are used to make sushi and other seafood dishes in restaurants. Humans are mostly exposed to methylmercury through fish and shellfish. Large fish that feed on small fish, crabs and prawns have higher methylmercury levels. The risk of mercury poisoning from fish is not a health concern for the majority of people. Different people react to methylmercury differently.
The majority of people have trace amounts of methylmercury in their bodies that do not pose a serious health risk. However, a number of factors will influence how severe the effects of mercury exposure on humans will be. These include the quantity and type of fish consumed, duration of exposure, the route of exposure, the person's age and overall health. Pregnant and nursing mothers are particularly vulnerable, as consuming mercury-laced fish can have serious consequences for the foetus and child's development. Even if they do not exhibit symptoms, their children may be born with disabilities.
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About Dhruv Kishore Bole
MBA - Hotel Management | Preventive Controls Qualified Individual (PCQI)
ISO 9001: 2015 (QMS) | FSSC 22000 V5 (FSMS) | cGMP | HACCP
Lean Six Sigma | Train The Trainer | Revenue Management | CGSP
Food Handler | FOSTAC | OHSMS
Dhruv Kishore Bole is a hospitality and food safety specialist with qualifications in hotel management, food safety and quality management system. He has extensive experience spanning over twelve years in operational and training roles. His expertise centers on hospitality operation, food and beverage services and food safety. He has attended numerous workshops and conferences on customer service, leadership and food safety and quality and is certified by Food Safety and Standards Authority of India in food safety competencies. He is currently offering services in the capacity of Faculty, Food & Beverage service at State Institute of Hotel Management, Siddhpur, India. He is an empanelled trainer with Hero Mindmine and IL&FS Skills. He is a member of Quality Council of India and an instructor and proctor with ServSafe for India region.
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Editor’s Note
According to the Western Cape Government website, the Tygerberg Poisons Information Centre reports an increase in seafood poisoning calls to their centre in autumn (March to May). In South Africa, red tides usually occur in late summer and autumn. They posit that the high occurrence of scombroid poisoning at this time could be linked to the uniquely South African tradition of serving pickled fish on Good Friday, which usually occurs during March/April over Easter. With an increase in demand for fish over this time, there is an associated likelihood of cold chain issues between vendors, or as a result of poor temperature control from the consumer, due to a lack of refrigeration.
The site also states that the Cape yellowtail (which is an example of a red-muscled fish) is usually the culprit when it comes to scombroid poisoning in South Africa.
https://www.westerncape.gov.za/news/beware-seafood-poisoning
Recommended reading:
Fish and Fishery Products Hazards and Control Guidance
https://www.fda.gov/media/80637/download
Watch out for more information coming soon on legislation relating to fish and seafood.
