A 28-year-old Japanese man had been clear for 21 months after surgery for testicular cancer when a routine surveillance scan turned up something new. A 20-millimeter low-density lesion, oddly dumbbell-shaped, had appeared in segment 7 of his liver. Three earlier scans had shown nothing.
His team at Mie University Hospital in Tsu ran everything they had. Tumor markers were normal. Ultrasound identified an irregular mass with indistinct margins and no blood flow signal. MRI showed the pattern you would expect from a tumor. A PET scan showed no abnormal glucose uptake, which was reassuring but not conclusive. Because he had a history of cancer, they could not exclude metastasis, so they removed part of the liver laparoscopically.
Inside the specimen was a white nodule 12 millimeters in diameter with a tiny pinhole.
There was no cancer. Under the microscope was a granuloma with a dead center, ringed by eosinophils and other inflammatory cells, and inside it a collapsed nematode larva with the Y-shaped lateral cords that identify the anisakid family. PCR run on DNA extracted from the paraffin-embedded slide identified it as Pseudoterranova decipiens, a parasite acquired from raw or undercooked seafood. The case was published in BMC Infectious Diseases.
The Imaging Pattern That Should Prompt A Second Question
The Mie case is not isolated. Surgeons at Kobe City Medical Center General Hospital described a 60-year-old man seven months past surgery for stage III rectal cancer whose follow-up CT found a 10-millimeter low-density area at the segment 4/8 margin of the liver. This time, PET did light up, with a standardized uptake value of 2.8. The lesion was resected as a suspected metastasis and was found to contain Anisakis simplex.
Their conclusion was practical: clinicians should ask about diet and investigate further when a liver margin tumor under 20 millimeters is not positively malignant, because hepatic anisakiasis is a possible answer.
The numbers explain why almost nobody thinks of it. Extra-gastrointestinal anisakiasis accounts for about 0.45% of anisakiasis cases in Japan, and liver involvement is rarer still. When the Mie team searched the literature, they found only eight detailed hepatic cases in full-text reports, including their own. All came from Japan. The median tumor size was 15 millimeters, with a range of 4 to 20, and most sat on the surface of the liver.
The confusion is not confined to one organ. A 2016 report in BMC Medical Imaging described two women whose lesions were suspected on PET-CT to be recurrent gynecological cancer and turned out to be anisakiasis, one of them a 2-centimeter liver lesion found 19 months after surgery for endometrial cancer.
A 2023 report identified Anisakis pegreffii in a 71-year-old man with a 20-millimeter lesion in liver segment 6. Ultrasound, CT, and contrast-enhanced MRI could not settle it, and the differential ran through hepatocellular carcinoma, metastatic disease, cholangiocarcinoma, and inflammatory pseudotumor before a biopsy answered the question.
The mechanism is the same each time. Larvae swallowed with raw fish burrow into the wall of the stomach or intestine, occasionally traveling beyond the digestive tract. The body walls them off in a granuloma, and the larva itself often collapses over time, which is why genetic testing has become the reliable route to a final diagnosis. On a scan, a walled-off dead worm and a small metastasis look much alike.
What Changed in the Science This Summer
A review published on July 20, 2026, in the International Journal of Molecular Sciences summarizes a decade of rapid change. Researchers from the Complutense University of Madrid, Hospital Universitario de La Princesa, and the University of Würzburg report that taxonomic revision has reshuffled the Anisakis genus, moving several species into the genera Skrjabinisakis and Peritrachelius.
The diagnostic advances are the practical part. The review describes using the IgA/IgG4 antibody ratio and antibody avidity profiling to distinguish gastroallergic anisakiasis from ordinary chronic hives, a distinction that has historically been difficult. It also flags something unexpected: the identification of alpha-Gal epitopes in the larvae, which the authors suggest points to a possible route into alpha-Gal syndrome, the red meat allergy usually associated with tick bites. That is a proposed link, not an established one.
The review also notes that current endoscopy guidelines already list Anisakis as a potential cause of pseudo-tumors, and that dead larvae in tissue outside the gut can form granulomas that are difficult to distinguish from tumors on PET-CT.
The Freezer Rule Standing Between Americans and This Parasite
American diners are not defenseless, and the protection is regulatory rather than medical. Under section 3-402.11 of the FDA Food Code, fish served or sold raw, raw-marinated, or partially cooked must be frozen for parasite destruction.
Three protocols are accepted, and any one is sufficient: freezing and storing at minus 4 degrees Fahrenheit or below for at least 168 hours, freezing at minus 31 degrees or below until solid and holding at that temperature for at least 15 hours, or freezing at minus 31 degrees until solid and then holding at minus 4 degrees for at least 24 hours. Molluscan shellfish, several tuna species, and farmed fish raised on formulated feed containing no live parasites are exempt.
Thorough cooking also kills the larvae. Freezing does not address bacteria, and marinating in citrus is not a substitute.
The reported hepatic cases overwhelmingly come from Japan, where raw seafood consumption is highest and clinicians know to look for it. Nothing in this literature establishes how often the liver form occurs in the United States. Anyone with unexplained abdominal pain after eating raw fish, or an unexplained liver lesion, should be evaluated by a clinician and should mention raw seafood in their diet history.
Key Questions Answered
What is anisakiasis? An infection acquired by swallowing larvae of anisakid nematodes present in raw or undercooked seafood. Humans are accidental hosts in which the parasite cannot complete its life cycle.
How does a parasite end up looking like cancer? Larvae can penetrate the stomach or intestinal wall and travel to other organs, where the immune system walls them off in a granuloma. On CT, these appear as small, low-density nodules that resemble metastases, particularly in patients already under cancer surveillance.
How common is the liver form? Rare. Anisakiasis outside the digestive tract accounts for roughly 0.45% of cases in Japan, and one literature search identified only eight detailed hepatic cases in full-text reports, all of which were Japanese.
What should tip clinicians off? Reported hepatic lesions have clustered at the liver surface, with a median size of 15 millimeters and none above 20. One surgical team recommends asking about raw fish consumption before assuming a small liver margin lesion is malignant.
Does freezing sushi fish actually work? The FDA Food Code sets specific freezing time and temperature combinations for fish served raw, designed to kill parasites. Thorough cooking also kills them. Citrus marinades do not.
What symptoms follow eating an infected fish? Gastric and intestinal forms can cause abdominal pain, nausea, and vomiting. Allergic reactions ranging from hives to anaphylaxis have also been reported. Many infections cause no symptoms at all. Anyone with severe symptoms after raw seafood should seek medical care.
