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This article is about the organism. For for the infection, see Helminthiasis.
Hookworms attached to the intestinal mucosa

Helminths (/ˈhɛlmɪnθs/) are large multicellular organisms, which when mature can generally be seen with the naked eye. An infection by a Helminth is known as helminthiasis.

They are worm-like organisms living in and feeding on 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 and other animals.

Helminthology is the study of parasitic worms and their effects on their hosts. The word helminth comes from Greek hélmins, a kind of worm.


Helminths is a polyphyletic group of morphologically similar organisms, consisting of members of the following taxa: monogeneans, cestodes (tapeworms), nematodes (roundworms), and trematodes (flukes). The following table shows the principal morphological distinctions for each of these helminth families:

Tapeworms (Cestodes) Flukes (Trematodes) Roundworms (Nematodes)
Shape Segmented plane Unsegmented plane Cylindrical
Body cavity No No Present
Body covering Tegument Tegument Cuticle
Digestive tube No Ends in cecum Ends in anus
Sex Hermaphroditic Hermaphroditic, except schistosomes which are dioecious Dioecious
Attachment organs Sucker or bothridia, and rostellum with hooks Oral sucker and ventral sucker or acetabulum Lips, teeth, filariform extremities, and dentary plates
Example diseases in humans Tapeworm infection Schistosomiasis, swimmer's itch Ascariasis, dracunculiasis, elephantiasis, enterobiasis (pinworm), filariasis, hookworm, onchocerciasis, trichinosis, trichuriasis (whipworm)

Note: ringworm (dermatophytosis) is actually caused by various fungi and not by a parasitic worm.

Use in medicine[edit]

Main article: Helminthic therapy

Parasitic worms have been used as a medical treatment for various diseases, particularly those involving an overactive immune response.[1] As humans have evolved with parasitic worms, proponents argue they are needed for a healthy immune system.[1] Scientists are looking for a connection between the prevention and control of parasitic worms and the increase in allergies such as hay-fever in developed countries.[1] Parasitic worms may be able to damp down the immune system of their host, making it easier for them to live in the intestine without coming under attack.[1] This may be one mechanism for their proposed medicinal effect.

One study suggests a link between the rising rates of metabolic syndrome in the developed worlds and the largely successful efforts of Westerners to eliminate intestinal parasites. The work suggests eosinophils (a type of white blood cell) in fat tissue play an important role in preventing insulin resistance by secreting interleukin 4, which in turn switches macrophages into "alternative activation". Alternatively-activated macrophages are important to maintaining glucose homeostasis (i.e., blood sugar regulation). Helminth infection causes an increase in eosinophils. In the study, the authors fed rodents a high-fat diet to induce metabolic syndrome, and then injected them with helminths. Helminth infestation improved the rodents' metabolism.[2] The authors concluded:

Although sparse in blood of persons in developed countries, eosinophils are often elevated in individuals in rural developing countries where intestinal parasitism is prevalent and metabolic syndrome rare. We speculate that eosinophils may have evolved to optimize metabolic homeostasis during chronic infections by ubiquitous intestinal parasites….[2]


Public health campaigns to reduce helminth infections in the US may be traced as far back as 1910, when the Rockefeller Foundation began the fight against hookworm – the so-called “germ of laziness” – in the American South.[3] This campaign was enthusiastically received by educators throughout the region; as one Virginian school observed: “children who were listless and dull are now active and alert; children who could not study a year ago are not only studying now, but are finding joy in learning...for the first time in their lives their cheeks show the glow of health.”[4] From Louisiana, a grateful school board added: "As a result of your treatment...their lessons are not so hard for them, they pay better attention in class and they have more energy...In short, we have here in our school-rooms today about 120 bright, rosy-faced children, whereas had you not been sent here to treat them we would have had that many pale-faced, stupid children."[4]

Similar (albeit somewhat more imperialist) reports emerged from various other regions of the developing world at the time; for example, two scholars in Puerto Rico found that: "Over all the varied symptoms with which the unfortunate jibaro [peasant], infected by uncinaria [hookworm], is plagued, hangs the pall of a drowsy intellect, of a mind that has received a stunning blow...There is a hypochondriacal, melancholy, hopeless expression, which in severe cases deepens to apparent dense stupidity, with indifference to surroundings and lack of all ambition.’[3]

Such observations made an intuitive connection between worm burden and intellectual performance, but even today this link is anything but well-established. While it seems that worms may impair cognition in some way, the mechanisms driving this relationship are still hotly debated.[5]



  1. ^ a b c d "Eat worms - feel better". BBC News. 3 December 2003. Retrieved 13 July 2011. 
  2. ^ a b Wu, Davina; et al. (8 April 2011). "Eosinophils Sustain Adipose Alternatively Activated Macrophages Associated with Glucose Homeostasis". Science 332 (6026): 243–247. doi:10.1126/science.1201475. PMC 3144160. PMID 21436399. Retrieved 18 April 2011. 
  3. ^ a b Watkins WE & Pollitt E (1997). "'Stupidity or Worms': Do Intestinal Worms Impair Mental Performance?". Psychological Bulletin 121 (2): 171–91. doi:10.1037/0033-2909.121.2.171. PMID 9100486. 
  4. ^ a b Bleakley, Hoyt (2007). "Disease and Development: Evidence From Hookworm Eradication in the American South". The Quarterly Journal of Economics 122: 73. doi:10.1162/qjec.121.1.73. 
  5. ^ "Deworming drugs for soil-transmitted intestinal worms in children: effects on nutritional indicators, haemoglobin and school performance.". Cochrane Database of Systematic Reviews: CD000371.pub5 (Orig. rev.). 2012 Jul 11. doi:10.1002/14651858.CD000371.pub5.

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