Anisakiasis is a parasitic infection caused by the accidental ingestion of the larvae of the Anisakis simplex roundworm, a common nematode found globally in marine environments. This condition is directly linked to the increasing consumption of raw, undercooked, or improperly processed seafood, particularly fish and squid. The parasite’s presence in marine life makes it a significant public health concern. Humans act as a dead-end host for the parasite, meaning the larvae cannot complete their development.
The Parasite and How Infection Occurs
The life cycle of Anisakis simplex involves several marine hosts. Eggs hatch in seawater and release larvae that are ingested by small crustaceans. These infected crustaceans are then consumed by larger marine fish and cephalopods, such as squid, which serve as intermediate hosts. Within these hosts, the larvae migrate to the muscle tissue and visceral organs, developing into the infective third-stage larvae (L3).
The natural cycle is completed when these infected fish or squid are eaten by a marine mammal, the definitive host. The larvae mature into adult worms in the mammal’s stomach, reproduce, and release eggs back into the water. Humans become accidental hosts when they consume the L3 larvae in raw or inadequately prepared seafood.
Because the human gastrointestinal tract is not the parasite’s natural environment, the larvae are unable to complete their development and eventually die. Before dying, the ingested larvae attempt to burrow into the wall of the esophagus, stomach, or intestine. This penetration triggers the human disease and is associated with culinary traditions involving raw fish, such as sushi, sashimi, and ceviche.
Recognizing Anisakiasis Symptoms
Anisakiasis symptoms manifest in two distinct forms: gastrointestinal disease and allergic reaction, often presenting within hours of consuming contaminated seafood. The most common presentation is acute gastric anisakiasis, where the larvae penetrate the stomach lining shortly after ingestion. This penetration causes intense upper abdominal pain, often accompanied by nausea, vomiting, and sometimes a mild fever. Symptoms typically appear within 8 to 12 hours.
If the larvae pass into the small intestine, it leads to intestinal anisakiasis, which is usually delayed and can take up to a week to cause symptoms. Intestinal infection can mimic other serious conditions, such as Crohn’s disease or intestinal obstruction, due to the inflammation and mass formation around the embedded worm.
A separate, frequently overlapping manifestation is the IgE-mediated allergic reaction to the parasite’s proteins, sometimes called gastroallergic anisakiasis. This reaction can occur even if the larvae are dead, as the allergenic proteins remain in the fish tissue. Sensitized individuals may experience hives, skin rashes, swelling, or difficulty breathing, which can escalate to severe anaphylaxis.
These allergic symptoms can occur alongside or independently of the digestive symptoms, sometimes within minutes to a few hours of consumption. The presence of the parasite triggers a strong immune response, characterized by elevated levels of Anisakis-specific immunoglobulin E (IgE) antibodies.
Clinical Management and Treatment
When anisakiasis is suspected following raw seafood consumption, endoscopy is the primary diagnostic and therapeutic tool for acute gastric anisakiasis. This procedure involves inserting a flexible tube with a camera to visually inspect the lining of the stomach and upper intestine.
During endoscopy, the physician can directly visualize the small, thread-like worm burrowing into the mucosal wall. The most effective treatment for gastric cases is the immediate removal of the parasite using specialized biopsy forceps attached to the endoscope. Removing the larva typically results in the prompt resolution of the patient’s severe pain and acute symptoms.
For intestinal anisakiasis, where the worm is lodged too far for endoscopic removal, conservative management is pursued. The larvae eventually die, and the body manages the remaining inflammation. For these non-removable cases, the anti-parasitic drug albendazole may be administered orally for several days to treat the infection.
Surgery is reserved for rare, complicated scenarios, such as when intestinal inflammation leads to obstruction, perforation, or peritonitis. Allergic manifestations, including anaphylaxis, require immediate, standard anti-allergy treatment, such as epinephrine, antihistamines, and corticosteroids. Serological testing for Anisakis-specific IgE is often employed to confirm allergic sensitization.
Essential Prevention Strategies
Preventing anisakiasis relies on eliminating the infective third-stage larvae in seafood before consumption. The two most reliable methods for killing the parasite are freezing and cooking to specific temperature and time parameters. This is particularly important for fish intended for raw consumption, such as sushi and sashimi.
Commercial-grade freezing treatments ensure the larvae are completely inactivated. Effective freezing protocols include:
- Lowering the temperature throughout the fish to at least -20°C (-4°F) and maintaining it for a minimum of 7 days.
- Using a faster, ultra-low temperature method requiring a core temperature of -35°C (-31°F) for at least 15 hours.
For cooking, the larvae are killed when the fish reaches an internal temperature of 63°C (145°F) or higher. This temperature must be sustained for at least one minute throughout the thickest part of the fillet. Processing methods like cold smoking, salting, or marinating in acidic solutions, such as ceviche preparation, are often insufficient to reliably kill all larvae.
Purchasing raw seafood only from reputable suppliers who adhere to strict commercial freezing regulations is the most reliable consumer-level prevention strategy. Domestic freezers often do not reach the necessary low temperatures to kill the larvae effectively.