Published: January 31, 2024 | Source: Centers for Disease Control and Prevention

When Sushi Comes with an Uninvited Guest

Imagine taking a bite of fresh sashimi or ceviche and feeling a strange tingling in your throat. That sensation could be more than just nerves—it might be a tiny worm. This is the reality of anisakiasis, a parasitic infection sometimes called "herring worm disease." While the name may sound alarming, understanding the risks and simple prevention steps lets you enjoy seafood safely and confidently.

What Is Anisakiasis?

Anisakiasis is caused by microscopic nematodes (roundworms) that can attach to the lining of your esophagus, stomach, or intestines after you eat raw or undercooked fish or squid. The adult worms naturally live in marine mammals like whales and sea lions. Their eggs are released into the ocean through animal waste, hatch into larvae, and are then eaten by small crustaceans. Fish and squid consume these crustaceans, and the larvae embed themselves in the muscle or organs of the seafood.

When humans eat infected fish or squid that has not been properly cooked or frozen, the larvae can invade the gastrointestinal tract. The parasite cannot survive long in humans, but while it is alive, it can cause inflammation, pain, and allergic reactions.

Who Is at Risk?

Anyone who eats raw or undercooked fish or squid is at risk. This includes fans of:
- Sushi and sashimi
- Ceviche and crudo
- Pickled, salted, or lightly marinated fish
- Cold-smoked seafood
- Traditional dishes featuring raw marine ingredients

Anisakiasis is most commonly reported in regions where raw fish consumption is popular, such as Japan. However, as global food trends spread and undercooked seafood becomes more widely available, cases have been documented in the United States, Europe, South America, and beyond.

Symptoms: What to Watch For

Symptoms can appear within hours of eating contaminated seafood and vary from mild discomfort to more intense reactions.

Common Gastrointestinal Symptoms

Sharp or cramping abdominal pain
Nausea and vomiting
Abdominal bloating or distention
Diarrhea
Blood or mucus in stool
Mild fever

Allergic Reactions

Some people experience immune responses to the parasite, including:
- Skin rashes or hives
- Itching
- In rare cases, anaphylaxis—a severe, life-threatening allergic reaction requiring emergency care

The "Tingling" Warning Sign

A unique clue to anisakiasis is a tingling, crawling, or prickling sensation in the mouth or throat during or shortly after eating raw fish. This feeling is actually the worm moving. If you notice this:
- Stop eating immediately
- Try to cough or gently remove the worm with clean tweezers if it is visible and accessible
- Do not swallow further bites from the same dish

In some cases, vomiting shortly after exposure can expel the worm before it attaches, potentially preventing full infection.

How Does It Spread?

The life cycle of the anisakid worm is entirely marine:
1. Infected whales, seals, or sea lions release eggs into seawater through their feces
2. Eggs hatch into free-swimming larvae
3. Tiny crustaceans (like krill) eat the larvae
4. Fish and squid consume the infected crustaceans
5. Larvae encyst in the muscle or organs of the fish or squid
6. Humans become accidental hosts by eating raw or undercooked infected seafood

Importantly, anisakiasis cannot spread from person to person. You cannot catch it from someone else, and proper cooking or freezing destroys the larvae before they can cause harm.

Prevention: Simple Steps for Safe Seafood

You do not need to avoid seafood to stay safe. Follow these evidence-based precautions:

Cook It Right

Heat seafood to an internal temperature of at least 145°F (63°C)
Use a food thermometer to verify doneness, especially for thicker cuts

Freeze It Properly

If you plan to eat fish raw, freezing is essential. The FDA recommends one of the following protocols to kill parasites:
- Freeze at -4°F (-20°C) or below for at least 7 days
- Freeze at -31°F (-35°C) or below until solid, then store at -31°F (-35°C) or below for 15 hours
- Freeze at -31°F (-35°C) or below until solid, then store at -4°F (-20°C) or below for 24 hours

Note: Home freezers often do not reach these temperatures. Commercial "sushi-grade" fish is typically flash-frozen to meet these standards—ask your supplier about their freezing process.

Practice Smart Food Handling

Wash hands, utensils, and surfaces after handling raw seafood
Keep raw fish separate from ready-to-eat foods to avoid cross-contamination
When in doubt, cook it out

Diagnosis: How Doctors Identify the Infection

If you suspect anisakiasis, see a healthcare provider promptly. Diagnosis may involve:
- A detailed history of recent seafood consumption
- Physical examination and symptom review
- Endoscopy: a thin, flexible camera is used to visualize and sometimes remove the worm from the stomach or esophagus
- Imaging studies like X-rays or CT scans if the worm has migrated deeper
- In rare cases, surgical removal if the parasite causes obstruction or severe inflammation

Telling your provider exactly what you ate and when can speed up diagnosis and treatment.

Treatment and Recovery

The good news: the parasite cannot complete its life cycle in humans and will eventually die. Treatment focuses on relief and removal:

Endoscopic removal: If the worm is accessible, a doctor can often extract it during an endoscopy, providing immediate symptom relief
Supportive care: Pain relievers, anti-nausea medications, and anti-inflammatory drugs help manage discomfort while the body clears the infection
Surgery: Rarely needed, but may be required if the worm causes intestinal blockage or perforation

Most people recover fully within days to a few weeks. Allergic symptoms may require additional management, such as antihistamines or, in severe cases, epinephrine.

The Bottom Line

Anisakiasis is uncommon but entirely preventable. By choosing properly handled seafood, cooking fish thoroughly, or ensuring it has been frozen to parasite-killing standards, you can enjoy marine cuisine without worry. If you ever feel that unusual tingling after eating raw fish, act quickly—and remember, when it comes to seafood safety, a little caution goes a long way.

Stay informed, eat wisely, and savor your meals with confidence.

Content provided by Crestwood General. We share clear, practical health information to help our community make confident, informed choices about food safety and wellness.