Parasitic worm

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


Parasitic worms or helminths are a division of eukaroytic parasites that, unlike external parasites such as lice and fleas, live inside their host.[1] They are worm-like organisms that live and feed off living hosts, receiving nourishment and protection while disrupting their hosts' nutrient absorption, causing weakness and disease. Those that live inside the digestive tract are called intestinal parasites. They can live inside humans as well as other animals.

Helminthology is the study of parasitic worms and their effect on their hosts.

Parasitic worms are categorized into three groups: cestodes, nematodes, and trematodes.

These are the principal morphologic differences of the different families of helminths.

Cestodes Trematodes Nematodes
Shape segmented plane plane no segmented cylindrical
Celoma NOT NOT Present
digestive tube NOT Ends in cecum Ends in anus
Sex Hermaphrodites Hermaphrodites, except Shistosoma Dioics
hook organ component oral sucker, botridias and doble Rostellar hooks Lips, teeth, filariform extreme and dentary plates

Diseases caused in humans by helminth infection include ascariasis, dracunculiasis, elephantiasis, hookworm, lymphatic filariasis, onchocerciasis, schistosomiasis, and trichuriasis.

(Images courtesy of Dr Imtiaz Ahmed Wani)


Parasitic worms are sequential hermaphrodites and reproduce depending on the species of worm, either with the presence of a male and female worm, joining sperm and eggs, producing fertile eggs, such as hookworms, or by breaking off segments that contain both male and female sex organs that are able to produce fertile eggs without the presence of a male or female. (e.g., tapeworms)

All worm offspring are passed on through poorly-cooked meat, especially pork, wild fish, and beef, contaminated water, feces, mosquitoes, and, in general, areas of poor hygiene and food regulation standards such as parts of Africa, Central and South America, and Asia. However, it is estimated that 40 million Americans are infected with the most common roundworm, the pinworm. Medical experts theorize that widespread intestinal parasite infestation is leading to the rise of colon cancer in the United States.

Worm eggs or larvae or even adults enter the human body through the mouth, anus, nose, or skin, with most species attaching themselves to the intestinal tract. With the presence of digestive enzymes, worm egg shells are dissolved, releasing a brand-new worm; unlike its egg shell, the parasitic worm is protected from the body's powerful digestive enzymes by producing a protective keratin layer.

Immune response

The immune response to worm infection in humans is a Th2 response in the majority of cases. This results in inflammation of the gut, and results in cyst-like structures forming around the egg deposits throughout the body. The host's lymphatic system is also heavily taxed the longer helminths propagate, which excrete toxins after feeding. These toxins are released into the intestines to be absorbed by the host's bloodstream. This phenomena makes the host susceptible to more common diseases such as seasonal viruses and bacterial infection.


  1. Maizels RM, Yazdanbakhsh M (2003). "Immune regulation by helminth parasites: cellular and molecular mechanisms" (PDF). Nat. Rev. Immunol. 3 (9): 733–44. doi:10.1038/nri1183. PMID 12949497.

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ar:دودة طفيلية he:הלמינתולוגיה id:Cacing parasit th:พยาธิ uk:Гельмінтологія

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