Answer to Case 818

Answer to the Parasite Case of the Week 818: Anisakid larva

As noted by Daniel Tanzola, Sarah Sapp, Antoine, Idzi Potters, HCLM fan, and others, the size and characteristic morphologic features of this worm - including the anterior perforant ("boring") tooth and slightly rounded caudal end with a small mucron - together with the fishy story, all point to this being an L3 anisakid larva.

Several readers wished bon appétit to Florida Fan, to which he replied that he "preserved the worm in formalin for examination and caramelized the worm free fillet with chopped scallion, ground black pepper, garlic, cayenne pepper and fish sauce in a clay pot for lunch with rice. It tasted fantastic and never felt any gastrointestinal discomfort." Now that's dedication to parasitology - and an excellent reminder that finding one worm doesn't necessarily mean the rest of the fish needs to go to waste!

Anisakiasis is a zoonotic infection caused by third-stage (L3) larvae of ascaridoid nematodes, most commonly species in the genera Anisakis, Pseudoterranova, and Contracaecum. Humans become accidental hosts when they consume raw or inadequately cooked fish or squid containing viable larvae. Since humans are not suitable definitive hosts, the larvae cannot mature to adulthood, but they may invade the gastric or intestinal mucosa and cause disease.

Anisakid larvae recovered from clinical specimens are typically several centimeters long (~20–35 mm) and can be provisionally identified by their gross and microscopic morphology. At the anterior end is a small, triangular boring tooth that the larva uses to penetrate host tissues. Note the developing lips; adult ascaridoids have 3 well-defined fleshy lips. An excretory pore is also present near the anterior end and its location can be useful for identification. The boring tooth may be difficult to appreciate and can be damaged or lost when the worm is extracted from the mucosa during upper endoscopy.

The posterior end provides another useful clue. Some anisakid L3 larvae possess a small terminal projection called a mucron, which is readily visible in a well-preserved specimen such as the one in this case. The presence or absence of a mucron, together with features of the digestive tract and the location of the excretory pore, can help distinguish among the major anisakid groups. A mucron is typically present in Pseudoterranova and Anisakis Type I larvae, whereas it is absent in Anisakis Type II larvae and Contracaecum.

Species-level identification based on larval morphology alone can be challenging, however, and molecular methods are generally required for definitive identification. Fortunately, for the purposes of this case, "anisakid larva" is a perfectly satisfying diagnosis - particularly since this particular worm ended up in formalin rather than in Florida Fan.