CT findings of gastrointestinal anisakiasis are summarized in three characteristic observations: 1. severe submucosal edema of the involved gastrointestinal area, 2. ascites and fluid collection in the intestinal distal to the involved area, and 3. lack of obstruction of the bowel at the site of severe mucosal edema.
The pathological characteristics of anisakiasis consist of a combination of two different mechanisms: direct tissue damage and allergic reaction by Anisakis larvae [3, 4]. Direct tissue damage due to the invasion of Anisakis larvae involves development of an eosinophilic granuloma or perforation . Takei et al.  reported the pathological findings of intestinal anisakiasis: the small intestine showed transmural edema, congestion, and diffuse infiltration of neutrophils as well as eosinophils. The typical inflammatory infiltrate, rich in eosinophils, is seen as an eosinophilic granuloma or an eosinophilic abscess. Those pathophysiological findings are directly reflected on the characteristic CT findings in our study.
The clinical differential diagnosis of gastric anisakiasis includes acute gastric mucosal lesion (AGML) or gastric ulcer, eosinophilic gastritis, and caustic ingestion. The clinical differential diagnosis of intestinal anisakiasis includes bacterial or viral enteritis, eosinophilic gastroenteritis, and Crohn’s disease.
The clinical diagnosis of gastric anisakiasis is usually easy as the patients commonly provide a typical clinical history and the Anisakis larvae are often proved by endoscopic examination. Therefore, the role of the CT in gastric anisakiasis is to rule out other fatal abdominal diseases that show similar clinical symptoms. Intestinal anisakiasis is often clinically under-recognized due to the long interval (commonly 1 week) from the intake of contaminated food to the onset of symptoms. A specific serum blood test for anisakiasis is not routinely undertaken unless it is suspected based on the clinical history. In fact, many patients were clinically diagnosed with other abdominal diseases before the CT findings suggested the possibility of anisakiasis in our study. Therefore, CT is a useful tool to suggest the possibility of the under-recognized diagnosis of intestinal anisakiasis to clinicians. In this study, anisakiasis is more frequent in men than in women. Yasunaga et al.  also reported a male predominance of anisakiasis. Although the reason for the male predominance of anisakiasis remains unknown, this may be because men consume raw fish or sushi more frequently based on food habits.
Our study has some limitations. First, a small number of patients were included in this study, although our study includes a larger number of patients than previous studies. Second, our study was performed retrospectively. Third, the patient selection in our study may have been biased. Since the antigen test was performed only for the patients who were suspected of having intestinal anisakiasis, the patients who were not suspected of having intestinal anisakiasis by CT scan may have been missed in this study. Finally, we did not compare the CT findings of gastrointestinal anisakiasis with those of other abdominal diseases.
In conclusion, our study demonstrated characteristic CT findings of gastric and intestinal anisakiasis: severe submucosal edema of the involved gastrointestinal tract, fluid collection, and ascites without showing severe bowel obstruction. An understanding of the characteristic CT findings of gastrointestinal anisakiasis is useful for suggesting the possibility of this clinically under-recognized diagnosis to clinicians.