Seafood Poisoning from Marine Toxins: U.S. Outbreaks, 2011–2023

Marine toxins are consistently the most commonly reported cause of noninfectious foodborne disease outbreaks in the United States. They are tasteless, odorless and survive cooking and freezing, so once fish or shellfish are contaminated, illness is hard to prevent. They can cause a confusing mix of gastrointestinal, neurologic and neuropsychologic symptoms, among them nausea, vomiting, diarrhea, flushing, numbness and tingling, and cold allodynia (pain from cold). Severe cases can involve the heart and lungs, leading to hospitalization or death, and symptoms occasionally last months or years.

The toxins come from different places:

• Scombroid toxin forms when fish are not kept cold after being caught. Held at 40°F (4.4°C) or warmer, fish can develop histamine and other scombroid toxins produced by bacteria with high histidine carboxylase activity. It is preventable.
• Algal toxins, such as ciguatoxin and the shellfish toxins, are made by algae and build up in fish and shellfish through the food chain. Blooms of these algae have occurred on both U.S. coasts, and the range of some species, such as Gambierdiscus in tropical and subtropical waters, has expanded. Nutrient pollution and climate-related changes such as warming water have made blooms longer, more frequent and more intense.
• Other toxins occur naturally in certain fish, or have unknown sources, such as the one behind Haff disease.

Americans have been eating more fish and shellfish since the 1980s, and an estimated 80% to more than 90% of seafood eaten in the United States is imported. Some links in the import supply chain, such as vessels that harvest or carry but do not process fish, are exempt from the Food and Drug Administration's hazard-control rules, leaving room for improper handling. Yet illness from marine toxins has been poorly measured nationally: case surveillance for illnesses from harmful algal blooms began only in 2016 and does not exist for other marine toxins. CDC's report is the first national summary of all foodborne outbreaks caused by these toxins.

The data

Health departments voluntarily report foodborne outbreaks, defined as two or more cases of the same illness from a common food, to CDC's Foodborne Disease Outbreak Surveillance System, since 2009 through the National Outbreak Reporting System. The analysis covered every finalized marine toxin outbreak with first illness in 2011–2023.

The big picture

From 2011 to 2023, health departments reported 402 outbreaks from 10 marine toxins, causing 1,280 illnesses, 96 hospitalizations and one death. The median was 30 outbreaks a year (range 11–52) and two illnesses per outbreak. The national rate was 1.2 outbreaks per million people. Outbreaks were reported by 32 states, Washington, DC, and Puerto Rico, plus one outbreak spanning 11 states.

Those four accounted for 56% of outbreaks and 54% of illnesses.

Reported foodborne outbreaks from marine toxins by state, territory or district, number and rate per million people, 2011–2023. CDC, MMWR.

Scombroid toxin or ciguatoxin caused 95% of outbreaks. A food was identified in 396 outbreaks: fish in 379 (96%), shellfish (mussels, clams and sea snails) in 12, and other foods (sushi, roe, octopus and seafood pasta) in 5. Where importation status was known (313 outbreaks), 219 (70%) involved food that was not imported. Where a single place of preparation was reported (377 outbreaks), 193 (51%) were private homes and 147 (39%) restaurants, 130 of them sit-down restaurants.

Scombroid toxin: a cold-chain problem

Scombroid caused 192 outbreaks, 597 illnesses and 6 hospitalizations. New York (43 outbreaks, 103 illnesses) and Florida (38 outbreaks, 93 illnesses) reported the most, followed by California (23) and Hawaii (18). One multistate outbreak from contaminated yellowfin tuna sickened 50 people in 11 states. Tuna was implicated in 76% of the outbreaks with a known food. Where importation status was known, 53% involved imported fish, and none of the fish had been caught recreationally. Where a single preparation site was known, 57% were sit-down restaurants.

Because histamine builds up whenever fish get warm, anywhere from boat to plate, prevention means keeping seafood below 40°F (4.4°C) from catch to consumption. The authors call for more attention to handling of imported fish and to food safety in restaurants that serve fish commonly linked to scombroid.

Ciguatoxin: reef fish caught close to home

Ciguatoxin caused 189 outbreaks, 619 illnesses and 67 hospitalizations. Florida reported 88 outbreaks (274 illnesses), Puerto Rico 55 (185 illnesses) and Hawaii 18 (54 illnesses), all places where the algae that make it are endemic. The fish most often blamed were barracuda (31%), grouper (13%) and amberjack (12%). Where known, 87% of the fish were caught domestically, and 60% of reports with sourcing information said the fish were caught recreationally. Private homes were the place of preparation in 80%.

Ordinary food safety does not remove algal toxins. The authors suggest a One Health approach: monitoring water and satellite data to detect blooms or toxins before harvest, discouraging harvest of reef fish and shellfish from high-risk areas during and just after blooms, and reaching recreational fishers with targeted education. Some outbreaks were also reported from states where the algae are not native, mostly from domestically caught reef fish not usually found wild there, so processors and consumers there need information too.

Shellfish toxins: rare but more severe

Shellfish toxins caused 13 outbreaks, 40 illnesses and 9 hospitalizations:

Florida reported five and Alaska four. Mussels and sea snails were each implicated in four outbreaks and clams in three; one paralytic shellfish poisoning outbreak was traced to puffer fish. None involved imported shellfish, six of the seven reports with sourcing information said the shellfish were caught recreationally, and 73% were prepared at home. Commercial shellfish are routinely tested for paralytic and neurotoxic toxins, which has cut poisonings from commercial sources, but there is no simple test for recreational harvesters. Many state health departments monitor public shorelines and publish shellfish safety maps.

Other toxins

Eight more outbreaks came from other toxins: four from an unidentified toxin behind Haff disease (11 illnesses, 10 hospitalizations and the study's one death), three from tetrodotoxin in puffer fish (9 illnesses, 4 hospitalizations) and one from an unspecified fish toxin (4 illnesses). Implicated foods were puffer fish, buffalo fish, roe and carp.

Limitations

The numbers are likely an undercount. There are no readily available tests for marine toxins in people, diagnosis relies on food history and complex symptoms, many clinicians do not know these outbreaks are reportable, and where poisonings are common people may not seek care. The COVID-19 response also likely reduced outbreak investigation and reporting in 2020–2022. Trends over time may reflect changes in reporting, and outbreak data say little about individual patients or sporadic cases.

Sources

• Hartley CM, Hoang ST, Roberts VA, Chard AN. "Foodborne Disease Outbreaks Associated with Marine Toxins — Foodborne Disease Outbreak Surveillance System, United States, 2011–2023." MMWR Surveillance Summaries 75(SS-3). https://www.cdc.gov/mmwr/volumes/75/ss/ss7503a1.htm
• Supplementary figures: https://stacks.cdc.gov/view/cdc/256286; Foodborne Disease Outbreak Surveillance System: https://www.cdc.gov/nors/about/fdoss.html
• The report's discussion describes shellfish toxin hospitalizations as 25% of illnesses; its counts are 9 hospitalizations among 40 illnesses, which are given here.