Anisakiasis is a foodborne illness resulting from the accidental ingestion of parasitic roundworms found in marine fish and squid. This infection is a zoonotic concern, meaning it is transmitted from animals to humans. The rising global popularity of raw and lightly cured seafood dishes, such as sushi, sashimi, and ceviche, has contributed to an increasing number of reported human cases worldwide. While most prevalent in areas with high raw fish consumption, like Japan, cases are now reported across North America and Europe.
The Parasite and How Infection Occurs
The disease is caused by the larval stage of nematodes, primarily belonging to the Anisakis simplex complex. These larvae are part of a marine life cycle where marine mammals, such as whales and dolphins, serve as the definitive hosts. Adult worms reside in the mammal’s stomach, releasing eggs that hatch in the water.
The free-swimming larvae are ingested by small crustaceans, which are then consumed by larger fish and squid, which act as intermediate or transport hosts. Within the flesh of these fish and cephalopods, the larvae develop into the third-stage (L3) form, which is infective to mammals. When humans consume raw or undercooked seafood containing these live L3 larvae, the infection is acquired.
Humans are considered accidental or dead-end hosts because the larvae cannot complete their life cycle in the human gastrointestinal tract. The parasites attempt to burrow into the stomach or intestinal lining but eventually die after a period. This burrowing attempt initiates the symptoms of Anisakiasis.
Identifying Symptoms and Diagnosis
The presentation of Anisakiasis can vary depending on where the parasite attempts to attach itself within the digestive system. In the most common form, gastric anisakiasis, symptoms typically begin within hours of consuming the contaminated fish. This acute reaction often involves severe upper abdominal pain, nausea, and forceful vomiting.
A less common but more delayed form is intestinal anisakiasis, where the larvae migrate past the stomach and attempt to penetrate the small or large intestine wall. Symptoms in these cases, which include intense abdominal pain and inflammation, may not appear until one to two weeks after ingestion. Additionally, some individuals develop an allergic reaction to the parasite antigens, which can manifest as hives, swelling, or, in rare instances, life-threatening anaphylaxis.
Diagnosis begins with a thorough patient history. The most definitive diagnostic method for gastric cases is an upper endoscopy, where a physician can visually locate the larvae attached to the stomach lining. For intestinal forms, diagnosis may rely on imaging techniques like a computed tomography (CT) scan, which can show characteristic wall thickening and inflammation in the affected bowel segment.
Treatment Options for Anisakiasis
For patients presenting with acute gastric anisakiasis, treatment is the physical removal of the larvae via endoscopy. Using specialized forceps, the physician grasps and extracts the worm, which leads to the immediate resolution of symptoms in most cases.
In cases of intestinal anisakiasis, where the larva is lodged beyond the reach of an endoscope, symptoms often resolve spontaneously within a few weeks. Supportive care, including pain management, is the usual course of action for these self-limiting infections.
While standard anti-parasitic medications are often ineffective against Anisakis larvae, the drug Albendazole may be considered in certain situations, such as when endoscopic removal is not possible or for confirmed intestinal cases. When allergic reactions occur, treatment focuses on supportive therapy, which can include antihistamines or corticosteroids, and epinephrine in the event of anaphylaxis.
Safe Preparation and Prevention
Preventing Anisakiasis relies on eliminating the live larvae in the seafood before it is consumed. The two commercially accepted methods for parasite inactivation are adequate heating and proper freezing.
To ensure safety through heat, seafood must be cooked to an internal temperature of at least 145°F (63°C) for a minimum of one minute. Methods like frying, baking, or grilling are effective as long as the temperature is verified at the product’s core.
The alternative, used for raw preparations, is freezing under specific commercial conditions. The United States Food and Drug Administration (FDA) recommends freezing fish at -4°F (-20°C) or below for a minimum of seven days. Alternatively, a more rapid, deep freeze is effective, such as lowering the temperature to -31°F (-35°C) or below until solid, and then storing it at that temperature for 15 hours.