Diphyllobothriasis pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Enterobius vermicularis is usually transmitted via the feco-oral route to the human host. It reproduces in the small intestine of humans only. The gravid female worm lays eggs in the perianal area usually at night and causes pruritus. In addition to the fingernail contamination, the infective eggs can be transmitted via the dust and fomites.

Pathophysiology

Pathogenesis

• It is postulated that Enterobius vermicularis triggers an inflammatory response which is associated with the low-grade eosinophilia. Allergic response to the worm protein is considered the cause of pruritus; usually pruritus ani (perianal pruritus).

Lifecycle

The lifecycle of Enterobius vermicularis is completed in the human host and comprises of the following stages:

• The infective eggs of E. vermicularis are ingested via contaminated hands or fomites (clothing, toys, bed, furniture, animals' fur etc). The eggs mature into adult Enterobius in 15-40 days.
• Male adult larvae fertilize the female (in lower ileum) and are passed out in stools without producing symptoms. Adult females settle in the lower ileum, caecum, appendix and ascending colon. The gravid worm migrates from the colon to the rectal area. At night eggs laid on the perianal skin by the uterine contraction(s) of the gravid worm.
• At oviposition, the eggs are immature and non-infective. They become infective within 6 hours at body temperature.
• In cool, moist environment with little ventilation eggs remain viable for up to three weeks. However, the infectivity decreases with time. Eggs do not tolerate heat that well which might be the reason that enterobiasis is more common in temperate than tropical climates.
• In rare cases, pinworms are found in the vagina and even more rarely in the uterus, fallopian tubes, liver, and peritoneum, but the worms cannot survive long in these places.

The image shown below depicts the life cycle of the pinworm.

Enterobius Lifecycle

Mode of Transmission

E. Vermicularis is transmitted via these four modes:

1. Direct infection from the anal and perianal regions by infected fingernails.
2. Exposure to viable eggs on fomites (clothing, toys, bed, furniture, fur of pets etc).
3. By dust contaminated with embryonated(infected) eggs.
4. Retroinfection; migration of the larvae into sigmoid colon & caecum after hatching on the anal mucosa.

Life cycle of Diphyllobothrium latum

Immature eggs are passed in feces 1 . Under appropriate conditions, the eggs mature (approximately 18 to 20 days) 2 and yield oncospheres which develop into a coracidia 3. After ingestion by a suitable freshwater crustacean (the copepod first intermediate host) the coracidia develop into procercoid larvae 4 . Following ingestion of the copepod by a suitable second intermediate host, typically minnows and other small freshwater fish, the procercoid larvae are released from the crustacean and migrate into the fish flesh where they develop into a plerocercoid larvae (sparganum) 5 . The plerocercoid larvae are the infective stage for humans. Because humans do not generally eat undercooked minnows and similar small freshwater fish, these do not represent an important source of infection. Nevertheless, these small second intermediate hosts can be eaten by larger predator species, e.g., trout, perch, walleyed pike 6 . In this case, the sparganum can migrate to the musculature of the larger predator fish and humans can acquire the disease by eating these later intermediate infected host fish raw or undercooked 7 . After ingestion of the infected fish, the plerocercoid develop into immature adults and then into mature adult tapeworms which will reside in the small intestine. The adults of D. latum attach to the intestinal mucosa by means of the two bilateral groves (bothria) of their scolex 8 . The adults can reach more than 10 m in length, with more than 3,000 proglottids. Immature eggs are discharged from the proglottids (up to 1,000,000 eggs per day per worm) 9 and are passed in the feces 10 . Eggs appear in the feces 5 to 6 weeks after infection. In addition to humans, many other mammals can also serve as definitive hosts for D. latum.

Gallery

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  Illustration of the life cycle of Diphyllobothrium spp., the causal agents of Diphyllobothriasis. From Public Health Image Library (PHIL).

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  Diagram depicts the various stages in the life cycle of the tapeworm Diphyllobothrium latum, a cestode. From Public Health Image Library (PHIL).