Anisakiasis: Key Symptoms, Diagnostics, and Treatment

Anisakiasis is a parasitic infection caused by consuming raw or undercooked seafood contaminated with Anisakis larvae. Though relatively rare, it can cause significant gastrointestinal distress and severe allergic reactions. With the growing global popularity of sushi, sashimi, and other raw fish dishes, awareness of this condition is increasingly important.

Recognizing symptoms early and understanding diagnostic techniques and treatment options can help manage the infection effectively.

Transmission and Life Cycle

Anisakis species follow a complex life cycle involving multiple hosts. Adult nematodes reside in the gastrointestinal tracts of marine mammals such as whales, dolphins, and seals, which excrete Anisakis eggs into the water through their feces. The eggs hatch into free-swimming larvae, which are ingested by small crustaceans like krill, serving as the first intermediate hosts. The larvae develop further before being consumed by larger fish and squid, which act as paratenic hosts, where the parasite remains viable but does not mature.

As predatory fish consume smaller infected fish, the larvae migrate into muscle tissues and encyst in a dormant state. Human infection occurs when contaminated raw or undercooked seafood is consumed. Once inside the digestive system, the larvae attempt to penetrate the stomach or intestinal lining, triggering inflammation. Since humans are not natural hosts, the larvae cannot mature and eventually die, but not before causing significant tissue damage and symptoms.

Common Symptoms

Symptoms typically appear within hours of ingesting contaminated seafood as the larvae attempt to burrow into the stomach or intestinal lining. The most immediate sign is sudden, severe epigastric pain, often accompanied by nausea and vomiting. In some cases, hematemesis (vomiting blood) may occur if tissue damage is severe. These symptoms can resemble peptic ulcers or acute gastritis, leading to potential misdiagnosis if dietary history is not considered.

If the larvae migrate to the small intestine, intestinal anisakiasis develops, causing severe abdominal cramping, diarrhea, and, in some cases, obstruction due to inflammation. This condition can mimic appendicitis or Crohn’s disease, sometimes leading to unnecessary surgical intervention. Unlike bacterial or viral foodborne illnesses, anisakiasis does not cause systemic symptoms like fever or chills, as the infection remains localized.

Some individuals experience allergic reactions, ranging from mild hives and itching to severe anaphylaxis, including hypotension and airway compromise. Repeated exposure to infected seafood increases the likelihood of hypersensitivity, even without live larvae. This allergic response complicates diagnosis when gastrointestinal symptoms are absent.

Risk Factors and Populations

Dietary habits strongly influence the likelihood of anisakiasis, particularly in regions where raw fish is a staple, such as Japan, Korea, and coastal Spain. Japan, where sushi and sashimi are widely consumed, reports thousands of cases annually. In Western countries, the growing popularity of raw seafood dishes like ceviche and carpaccio has led to more infections.

Occupational exposure also increases risk. Fish handlers, seafood processors, and commercial fishermen may ingest viable larvae through accidental contamination. Studies indicate that individuals in the seafood industry often develop antibodies to Anisakis, suggesting frequent exposure even without symptoms.

Certain fish species are more likely to harbor Anisakis larvae. Large predatory fish such as cod, mackerel, herring, and hake are common reservoirs. Improper freezing or insufficient cooking allows larvae to remain viable, emphasizing the need for strict food handling protocols. Even restaurant-prepared dishes can pose a risk if seafood is not properly treated.

Diagnostic Methods

Diagnosis relies on clinical history, symptom presentation, and confirmatory testing. Physicians assess recent dietary intake, as symptoms typically appear within 24 to 48 hours of consuming raw or undercooked seafood. Because symptoms mimic other gastrointestinal conditions, misdiagnosis is common, especially in regions where anisakiasis is less prevalent.

Endoscopy is the most effective diagnostic tool for gastric anisakiasis, allowing direct visualization and removal of larvae embedded in the stomach lining. During an upper gastrointestinal endoscopy, a flexible scope with a camera identifies the parasites, which appear as thin, whitish worms burrowing into the mucosa. Endoscopic removal confirms the diagnosis and provides immediate relief. For intestinal anisakiasis, capsule endoscopy can detect larvae, though it lacks extraction capability.

If endoscopy is inconclusive, imaging techniques like computed tomography (CT) or ultrasound may reveal intestinal wall thickening, indicating inflammation. However, these methods are less specific. Stool analysis is generally unhelpful, as Anisakis larvae do not reproduce in humans. Serological tests detecting anti-Anisakis antibodies have limited sensitivity in acute infections.

Treatment Options

Treatment focuses on symptom relief and, when possible, removal of larvae. Since the parasite cannot survive long in humans, most cases resolve within weeks as the larvae die and are expelled. However, inflammation can cause significant discomfort, requiring medical intervention.

Endoscopic removal is the most effective treatment for gastric anisakiasis, eliminating the source of inflammation and preventing further tissue damage. Proton pump inhibitors and antacids may be prescribed to reduce gastric irritation. If endoscopic retrieval is not possible, supportive care with analgesics and antiemetics helps manage symptoms until the parasite dies naturally.

For intestinal anisakiasis, treatment is more complex. Since larvae are not easily accessible, management focuses on reducing inflammation and preventing complications like bowel obstruction. Mild cases require hydration and pain control, but severe cases may necessitate surgical intervention. Unlike bacterial or protozoan infections, antiparasitic medications like albendazole or ivermectin are ineffective, as Anisakis larvae do not establish a systemic infection.

Preventive Measures

Preventing anisakiasis requires proper seafood handling and preparation. Anisakis larvae are resistant to salting and pickling, making complete inactivation essential. Regulatory agencies like the U.S. Food and Drug Administration (FDA) and the European Food Safety Authority (EFSA) provide guidelines for safe seafood preparation.

Freezing fish at -20°C (-4°F) for at least seven days or at -35°C (-31°F) for 15 hours effectively kills Anisakis larvae. Many countries mandate these freezing protocols for commercially sold raw fish. Cooking remains the most reliable method, with temperatures of at least 60°C (140°F) ensuring complete inactivation. Despite these measures, improper handling in restaurants and home kitchens can still lead to infections.

Consumers should source seafood from reputable suppliers following freezing protocols. Home cooks should avoid relying on acidic marinades like those used in ceviche, as they do not kill larvae. Inspecting fish for visible parasites can help, though some larvae may be embedded deep in muscle tissue. Strengthening seafood inspection policies and enforcing compliance in the food industry can further reduce contamination risks.

Global Distribution and Epidemiology

Anisakiasis prevalence varies worldwide, depending on dietary habits and seafood consumption. Japan reports the highest number of cases, primarily from infected mackerel, squid, and salmon. Despite strict seafood freezing regulations, cases persist due to the popularity of fresh, unfrozen fish.

In Europe, Spain has the highest incidence, particularly in coastal regions where marinated anchovies (boquerones) are widely eaten. Unlike Japan, where gastric anisakiasis is more common, European cases often involve the intestines, leading to frequent misdiagnosis as inflammatory bowel disease or appendicitis. Studies have found Anisakis larvae in over 30% of commercially sold fish in Spain, highlighting ongoing risks despite regulations. Other European countries, including Italy and the Netherlands, have also reported cases, particularly among raw herring consumers.

North America sees fewer cases, but infections have risen with the growing popularity of sushi and ceviche. The U.S. Centers for Disease Control and Prevention (CDC) has identified sporadic outbreaks linked to improperly handled seafood, particularly in coastal states. South America, particularly Peru and Chile, has also reported cases due to raw seafood dishes like tiradito and ceviche. While still underdiagnosed in many regions, increased awareness and improved diagnostics have led to more frequent case identification.