Anisakiasis is a rare parasitic disease in humans who eat raw marine fish and squid. It is caused by Anisakis nematodes when their larvae invade the gastrointestinal mucous membrane to cause inflammatory symptoms. Upper gastrointestinal endoscopy along with radiological investigations like ultrasonography and computed tomography help confirm the diagnosis.
Anisakiasis is a rare parasitic disease which occurs when the larvae of the nematode Anisakis are ingested by humans through raw or undercooked fish or squid [1] [2]. The larvae invade the gastrointestinal lining leading to the symptoms of anisakiasis. The highest incidence of the illness has been reported to be in winter [3] due to the high consumption of raw fish in that season. The symptoms depend on the site invaded by the larvae and can be classified as gastric, intestinal, and ectopic anisakiasis [1] [4] [5]. A majority of the patients have gastric anisakiasis.
Symptoms of gastric anisakiasis typically start within 12 hours of ingestion of infected raw fish or squid [3] while those of intestinal anisakiasis have been reported to occur up to a week after ingestion [1] [6]. The commonest symptom is sudden onset severe abdominal or epigastric pain resembling an acute abdomen. This is accompanied by nausea and vomiting. Intestinal anisakiasis can present with symptoms resembling Crohn's disease with an inflammatory mass. Other reported presentations include severe hematemesis in patients with gastric ulcer and hemoptysis [3] [7] [8] [9] [10]. The Anisakis larvae do not survive for long in the human gastrointestinal tract and therefore the symptoms subside in a couple of weeks with medical management [11]. However, a few cases of intestinal strangulation and obstruction requiring surgical intervention have been reported [12] [13].
Clinical suspicion is very vital in the work up for anisakiasis. The first step in the diagnosis is to obtain a history of ingestion of raw or undercooked fish or squid, if possible, as patients may forget details about their meals. After a thorough physical examination, the next step is to order routine laboratory tests like complete blood count, erythrocyte sedimentation rate, serum electrolytes and stool test for larvae, although stool test is not useful in most cases.
Currently, upper gastrointestinal endoscopy forms the mainstay of the diagnosis and requires accurate interpretation of the clinical images to identify the causative worm sticking to the stomach wall. Intestinal anisakiasis is difficult to diagnose as the small intestine cannot be accessed with the endoscope [14]. Capsule endoscopy or double balloon endoscopy may not be available universally. Another method to diagnose anisakiasis is by examining Anisakis-specific immunoglobulin (Ig) A, IgG, and IgE. These tests have a high sensitivity [15] [16] but do not provide results immediately and are therefore not useful in practice.
Radiological tests can help to confirm the diagnosis of anisakiasis. On ultrasonography, features of anisakiasis include severe localized Kerckring’s fold edema (corn sign), and dilatation of the proximal small intestine with the accumulation of fluid [14]. Computed tomography scan findings show partial small bowel edema and dilatation of the small intestine with fluid collection on the cranial side of the lesion [12] [17] [18].