When people think of foodborne illness, they often picture bacteria like Salmonella or E. coli. But not all threats to human health come from bacteria, some come from parasites, and one of the most notorious of these in seafood is Anisakis. Anisakis is a genus of parasitic nematodes (roundworms) found in the flesh of fish and squid. These microscopic worms can cause a condition known as anisakiasis in humans, leading to gastrointestinal symptoms and allergic reactions. As raw and undercooked seafood continues to grow in popularity worldwide, awareness of Anisakis and its dangers is increasingly important.
What Is Anisakis?
Anisakis simplex, the most common species associated with human infections, is part of a complex life cycle that involves marine mammals, crustaceans, and fish. The adult worms live in the stomachs of marine mammals such as whales and seals, where they release eggs into the water through feces. These eggs hatch into larvae, which are then consumed by crustaceans like krill. When fish or squid eat the infected crustaceans, the larvae embed themselves in the muscle tissue. Humans become accidental hosts when they consume raw or undercooked fish containing these larvae.
Symptoms of Anisakiasis
In humans, Anisakis larvae cannot complete their life cycle, but they can still cause significant illness. Once ingested, the larvae can embed themselves in the lining of the stomach or intestines. Symptoms of anisakiasis usually occur within a few hours to a few days after consuming infected seafood and include:
- Severe abdominal pain
- Nausea and vomiting
- Diarrhea
- Low-grade fever
- Allergic reactions, including hives and anaphylaxis in rare cases
In some cases, the body reacts to the presence of the worm by attempting to expel it, causing intense gastrointestinal distress. Endoscopy, a procedure where a small camera is inserted down the throat, may reveal and even remove the worms lodged in the stomach lining. In other instances, surgical intervention may be required if the larvae cause a blockage or if they migrate deeper into the intestines.
Diagnosis and Treatment
Diagnosing anisakiasis can be challenging because its symptoms mimic other conditions such as appendicitis or peptic ulcers. A detailed food history is crucial, particularly any recent consumption of raw or undercooked seafood. In confirmed cases, the primary treatment is removal of the larvae through endoscopy. Since the worms do not reproduce or live long in the human body, medication is not typically required once they are removed. However, antihistamines or corticosteroids may be used to treat allergic reactions.
At-Risk Foods and Regions
The prevalence of anisakiasis varies depending on dietary habits and food preparation techniques. It is more common in regions where raw fish is a staple, such as Japan, where up to 90% of reported global cases occur. However, with the globalization of sushi, ceviche, and other raw seafood dishes, cases have been reported across Europe, North America, and South America.
High-risk foods include:
- Sushi and sashimi
- Lightly pickled or marinated fish (like herring)
- Undercooked or cold-smoked fish
- Ceviche
Prevention: How to Stay Safe
Fortunately, anisakiasis is entirely preventable with proper food handling and preparation techniques. Freezing or thoroughly cooking seafood can kill Anisakis larvae. The FDA recommends freezing fish at -4°F (-20°C) for at least 7 days, or flash freezing at -31°F (-35°C) for 15 hours, to ensure the destruction of parasites.
Restaurants that serve raw fish are expected to follow strict freezing guidelines to prevent infection. Consumers preparing raw seafood at home should only use fish labeled as “sushi-grade” or previously frozen for parasite destruction.
Other safety tips include:
- Avoid eating raw or undercooked fish from questionable sources.
- Be cautious with home-made sushi or ceviche.
- When in doubt, cook seafood to an internal temperature of at least 145°F (63°C).
A Growing Public Health Concern
With increasing demand for raw fish and global seafood trade, public health officials are paying closer attention to parasitic infections like anisakiasis. In 2020, the European Food Safety Authority noted a rise in cases in southern Europe. Likewise, the CDC has highlighted Anisakis as an emerging foodborne parasite of concern in the United States.
As knowledge spreads and food safety protocols improve, the risk of infection can be minimized. But it remains crucial that both consumers and food handlers understand the risks associated with Anisakis and take appropriate preventive measures.
Final Note: Anisakis may be invisible to the naked eye, but its impact on health can be significant. As our palates expand to include raw and exotic seafood dishes, so too must our vigilance against parasites like Anisakis. Understanding how this parasite works, where it’s found, and how to avoid it can help ensure that your next sushi night doesn’t come with an unwelcome guest.
