Anisakids
Anisakids
Anisakids are nematodes (round worms) in the family Anisakidae. The main anisakids affecting humans are Anisakis species (including Anisakis simplex), Pseudoterranova species (including Pseudoterranova decipiens), and Contracecum species. In nature, they are parasites of fish and marine mammals; however, when ingested by humans, they cause a condition called anisakiasis. Humans become infected when ingesting live larvae in raw or undercooked fish. Once ingested, the larvae are unable to mature into adults, but may cause symptoms in a variety of manners; they may be 'coughed' or vomited up, or may cause gastric or intestinal pain by burrowing into the stomach or intestinal mucosa respectively. Rarely, they may penetrate the mucosa and enter the peritoneal cavity. Some individuals also develop an allergic reaction to anisakids and are therefore at risk for future reactions, even when dead worms are ingested. Anisakids are commonly found in a variety of fish including salmon and cod. Therefore, they are easily acquired for teaching purposes and very popular in medical school and global health labs! Shown here are a variety of photos of anisakids in fish - discovered in both food designed for consumption and specimens obtained for teaching. Click HERE for a (somewhat disturbing) video of a live anisakid discovered in a salmon dish. Anisakids resemble immature Ascaris lumbricoides but can be differentiated by the presence of a posterior spicule called a mucron. Presence and characteristics of a cecum can be used to differentiate the various anisakids commonly found in humans.
Anisakiasis, Pseudoterranova decipiens, Adult
Pseudoterranova decipiens complex are nematodes usually found in cold-water coastal environments. Adult worms have three anterior “lips” (like Ascaris lumbricoides, from which it must be differentiated). However, unlike A. lumbricoides, it also possesses a boring tooth. In the second image below, you can make out the “lips.” The labeled image by Idzi Potters indicates the intestinal cecum, ventricle, and esophagus. To visualize the intestinal cecum and/or esophagus, clearing of the organism by lactophenol is necessary.
These images were donated by Idzi Potters of the Institute of Tropical Medicine Antwerp.
Anisakiasis, tissue
Anisakiasis is an infection with nematode larvae of the genera Anisakis, Pseudoterranova, and Contracaecum. Normally, definitive hosts are marine mammals (e.g. whales, sea lions, seals, dolphins) which shed embryonated eggs in their feces into the water. L3 larvae hatch from the shed eggs and are consumed by planktonic crustaceans which serve as the paratenic host. These crustaceans are consumed by fish or squid (another paratenic host) which might then be eaten by the definitive host (or other fish, again serving as paratenic host). Humans become accidental hosts upon consuming inadequately cooked fish or squid with the infective larvae.
Clinically, symptoms may be present if worms migrate up the esophagus to be dispelled from the nose or mouth. Alternatively, the worms may invade the mucosa of the stomach or intestine, causing acute abdominal pain. If the worm is not removed, it can cause chronic pain or perforation of the intestine.
In tissue, there is an eosinophilic inflammatory response around the worms. Morphologically, the worm has a thick multilayered cuticle, uninucleate intestinal cells with a brush border, a muscular esophagus, polymyarian/coelomyarian musculature, and prominent lateral cords with excretory cells. Of note, these features are all also seen in Ascaris lumbricoides. Therefore, differentiating between the two can be challenging. When found in the intestinal wall like in this case, they must also be differentiated from the nematode Angiostrongylus costaricensis.
Finally, while it is not necessary for treatment, it is possible to differentiate the genera of nematodes causing Anisakiasis based upon the morphology of their lateral cords. Pseudoterranova spp. will have a “butterfly-shaped” lateral cord, while Anisakis spp. will have a “Y-shaped” lateral cord.
You can view a WSI HERE. Notice the lateral cords and their “butterfly shape” indicating Pseudoterranova species.
