Anisakiasis is a parasitic infection caused by the larvae of nematodes, most commonly Anisakis simplex. It occurs when a person consumes raw or undercooked marine fish or cephalopods containing the live third-stage larvae (L3). The larvae attempt to burrow into the gastrointestinal tract lining, causing inflammation. As humans are accidental hosts, the larvae cannot complete their life cycle, but their presence leads to acute and chronic health issues.
Sources of Infection and Prevention Strategies
The Anisakis nematode life cycle involves multiple marine hosts. Marine mammals serve as definitive hosts. Larvae are consumed by crustaceans, and then fish and squid become infected. The larvae migrate to the muscle tissue, becoming infective to humans.
Common marine sources that harbor infective larvae include cod, herring, salmon, mackerel, and squid, especially when prepared as sushi, sashimi, ceviche, or lightly pickled fish. The infection is preventable through proper seafood preparation.
Cooking fish to 145°F (63°C) for one minute inactivates the parasites. Freezing is also highly effective for fish intended to be eaten raw. Health authorities recommend freezing fish at -4°F (-20°C) or below for seven days, or at -31°F (-35°C) for at least 15 hours. This protocol ensures the larvae are non-viable, reducing the risk.
Acute Symptoms and Clinical Presentation
Symptoms begin shortly after ingesting infected seafood, depending on the site of larval penetration. Gastric Anisakiasis is the most common presentation, with symptoms appearing one to twelve hours after eating contaminated food. This form is characterized by the sudden onset of severe upper abdominal pain, nausea, vomiting, and sometimes a low-grade fever.
The intense pain results from the larva attempting to burrow into the stomach’s mucosal lining. The resulting tissue damage and inflammatory response can mimic conditions like a peptic ulcer or acute gastroenteritis. Some infected persons experience “tingling throat syndrome” as the worm moves, sometimes leading to the larva being coughed up or expelled.
Intestinal Anisakiasis is less common, with symptoms appearing five to seven days after ingestion. When larvae reach the small intestine, inflammation can lead to symptoms resembling appendicitis, Crohn’s disease, or intestinal obstruction. The inflammatory response causes marked submucosal edema, a feature seen on imaging. This delayed presentation complicates diagnosis.
Diagnostic Procedures and Immediate Treatment
Diagnosis is suspected based on acute gastrointestinal symptoms and recent raw seafood consumption. The definitive method for Gastric Anisakiasis is an upper endoscopy, which allows visualization of the upper gastrointestinal tract. The larva, appearing as a small, thread-like worm, can often be seen attached to the gastric lining.
Endoscopy serves as both a diagnostic and therapeutic tool. The physician uses specialized biopsy forceps to grasp and remove the parasite immediately. This endoscopic removal is curative for Gastric Anisakiasis and results in a rapid resolution of symptoms. For Intestinal Anisakiasis, where endoscopic access is difficult, diagnosis relies on imaging techniques.
CT scans and ultrasound can reveal characteristic signs, such as localized wall thickening and severe inflammation. In many cases of Intestinal Anisakiasis, the larvae die within a few weeks, and symptoms resolve spontaneously with supportive care. Surgical intervention is reserved for rare, complicated cases involving intestinal obstruction, peritonitis, or bowel perforation. Standard antiparasitic drugs, such as albendazole, are ineffective against the Anisakis larvae.
Chronic Effects and Potential Complications
Although the acute phase resolves, the infection can lead to chronic effects. Chronic allergic sensitization is a significant long-term consequence, caused by the immune response to parasite antigens. A person can remain sensitized and develop allergic reactions upon subsequent seafood consumption.
These persistent allergic symptoms, sometimes called gastroallergic anisakiasis, manifest as recurrent urticaria (hives) or angioedema (swelling beneath the skin). The condition can range from mild to severe, occasionally progressing to anaphylaxis requiring immediate medical attention.
For patients with severe intestinal involvement, the inflammatory process can lead to granuloma formation. These dense inflammatory structures wall off the dead or dying parasite and can persist for months or years. Chronic inflammation and scarring can lead to serious complications, including intestinal obstruction or the narrowing of the bowel (ileal stenosis). These complications may necessitate surgical procedures.