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A case of asymptomatic incidental live anisakid worm infestation on a large rectal polyp
  1. Shimpei Matsui1,
  2. Toshio Uraoka2,
  3. Hirotoshi Hasegawa1,
  4. Yuko Kitagawa1
  1. 1Department of Surgery, Keio University, Tokyo, Japan
  2. 2Tokyo Medical Center, Tokyo, Japan
  1. Correspondence to Dr Toshio Uraoka, turaoka{at}

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A 65-year-old man who had been treated for prostate cancer was noted to have an incidental large rectal tumour on surveillance pelvic MRI. He subsequently underwent a colonoscopy that revealed a penduculated tumour in the rectum. When the tumour was inspected closely (figure 1), a white mobile tubular body was noted on the surface of the tumour. It was strongly suspected to be an anisakis worm. After carefully grasping and removing the worm using biopsy forceps, it was found to be alive and to have meandering movements (figure 2). Subsequently, successful endoscopic submucosal dissection of the tumour was completed (figure 3). Histopathology revealed an anisakiasis and a tubular adenoma with intramucosal cancer. On further interview after the procedure, the patient said he had eaten a raw mackerel for dinner the day before.

Figure 1

A white mobile tubular part of anisakis worm was noted on the surface of the rectal tumour.

Figure 2

The Anisakis worm was carefully grasped and removed using biopsy forceps.

Figure 3

After removal of the anisakis worm, endoscopic submucosal dissection of the tumour was completed.

Anisakiasis commonly involves the wall of the stomach, causing an inflammatory response and acute symptoms, such as sudden abdominal pain.1 Since the incidence of anisakiasis is associated with consumption of raw fish, it is relatively common in Japan due to eating habits. However, it is very rare that the nematode is found alive and causing infestation in the colon. In this rare case, the nematode’s body transited through the entire colon alive to finally infest the rectal polyp. To the best of our knowledge, there has been only one case report of anisakiasis of the rectum.2 Furthermore, its incidental location on the rectal tumour could possibly be explained by the size of the tumour impeding free flow of stool, resulting in entrapment of the worm on its surface.

Learning points

  • It is important to inspect and observe any tumour surface beyond assessment of surface and vascular pattern.

  • We happened to find the apparent asymptomatic state with rectal anisakid infestation incidentally.


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  • Contributors SM was involved in the study concept, data collection and writing of manuscript. TU was involved in the reviewing of manuscript and overseeing the writing of this article. HH was involved in the reviewing of manuscript. YK was involved in the intellectual contribution and reviewing of manuscript.

  • Competing interests None.

  • Patient consent Obtained.

  • Provenance and peer review Not commissioned; externally peer reviewed.