The Lincoln Memorial Reflecting Pool in Washington, D.C. isn’t the only thing facing an algae problem. A recent report from the Centers for Disease Control and Prevention shows that foodborne toxins produced by marine algae and other sea life are routinely sickening Americans.
CDC scientists looked at surveillance data from foodborne outbreaks in the U.S. dating back over a decade. Since 2011, there have been hundreds of outbreaks tied to marine toxins, often from fish or shellfish filled with algae toxins, they found. Worse still, the risk of these outbreaks could rise over time due to expanding and more frequent harmful algal blooms, the researchers warn.
“Geographic expansion, increasing frequency, and increasing intensity of harmful algal blooms in U.S. coastal waters might increase the presence of ciguatoxin and shellfish-associated toxins in aquatic animals,” they wrote in their paper, published last week in the CDC’s Morbidity and Mortality Weekly Report.
Marine toxins
Marine toxins are the leading cause of noninfectious foodborne outbreaks. But according to the CDC authors, this is the first report to summarize the toll of these outbreaks on a national level.
These toxins usually come in two flavors. Certain marine algae can produce neurotoxins; the most common ones linked to human illness are called ciguatoxins. Small fish will eat the algae, which are then eaten by other fish and so on. Though ciguatoxins aren’t harmful to the fish themselves, they can accumulate in marine predators as the food chain goes up higher, and large fish like bass or snapper can contain enough ciguatoxin to sicken or even kill people who eat them. Shellfish that filter feed on algae can also accumulate dangerous levels of biotoxins, particularly when there’s an algal bloom going on.
The other common kind of marine toxin illness is called scombroid fish poisoning, in reference to the family of fish that usually cause it (scombroid fish include tuna and mackerel, though a few other species in other families can cause it, too). When these fish are improperly stored at too high a temperature for too long (over 40 degrees Fahrenheit (4.4 degrees Celsius)), bacteria inside will break down an amino acid called histidine into histamine and other scombroid toxins. Eating these histamine-loaded fish can then trigger an allergy-like reaction.
The CDC researchers looked at data from its Foodborne Disease Outbreak Surveillance System. They found that between 2011 and 2023 there were 402 reported foodborne outbreaks linked to marine toxins. These outbreaks resulted in at least 1,280 illnesses, 96 hospitalizations, and one death. Nearly all outbreaks (95%) involved scombroid toxins (192 outbreaks) or ciguatoxins (189 outbreaks). Shellfish toxin outbreaks were much less common, but often more severe, with at least 25% of reported illnesses leading to hospitalizations.
The researchers note that these tallies are almost certainly an underestimate. There are no readily available diagnostic tests for marine toxin poisoning in humans, and both sick people and their doctors may fail to consider these toxins as a likely source of illness, while many doctors might not know that such cases should be reported to their local health department.
What to do
Unfortunately, marine toxins are resistant to standard food safety methods like cooking. They’re also odorless and tasteless, though fish with scombroid toxins can sometimes have a “honey-combed” appearance or a metallic taste to them. And though many cases of marine toxin poisoning cause only mild or short-lasting illness, some people will experience severe or long-term symptoms that last months or even years.
That said, there are still ways to lower the risk of these outbreaks, the authors say.
“Scombroid toxin poisoning prevention relies on ensuring adequate temperature control of seafood; a better understanding of food safety practices for imported fish and food handlers in restaurants is critical for prevention efforts,” they wrote. “Most outbreaks caused by ciguatoxin and shellfish-associated toxins implicated recreationally harvested seafood. Targeted messaging about affected areas and species is essential for prevention of outbreaks from algal toxins linked to recreational fishing.”
