What is the most likely diagnosis?
a) Hook worm
This is an Anisakis simplex worm which can infect humans who eat undercooked, fresh fish caught in the sea. The infection is most common in areas of the world where fish is consumed raw or undercooked. Freezing the fish will also kill the worm and for this reason some countries insist that all fish must be frozen at -20°C or below for at least seven days before consumption.
Soon after ingestion, the patient develops abdominal pain. If the larvae pass into the small bowel, a severe eosinophilic granulomatous response may develop mimicking Crohn's disease. Less commonly, patients present with urticaria and anaphylaxis.
The diagnosis is endoscopic as the 2 cm worms can be seen attached to the mucosa. Ascaris worms (roundworms) are much longer (12-40cm), hookworms are smaller (2-3mm) and are attached to the duodenal and/or small bowel mucosa - not the gastric mucosa. Threadworms are commony found in the colon and tapeworms are huge.
Surprisingly, there is no medical treatment although there have been reports of response to albendazole. If found at endoscopy, the worm should be physically removed by biopsy.
In marine mammals, eggs are excreted in the faeces. The eggs are eaten by crustaceans which in turn are eated by fish or squid. In these, the worm burrows through the wall of the gut and encysts outside of the viscera. The infected fish or squid is then eaten by a marine mammal (whale, seal, or dolphin). Humans are of course "dead end hosts" but as marine mammals have similar mucosa to that of humans, the worm can feed from the human gut.