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a) A micrograph of Opisthorchis viverini; an oval cell with a projection at one end. B) A micrograph of one end of Taenia solium showing a whorl of small projections above an area labeled scolex.
(a) The oral sucker is visible on the anterior end of this liver fluke, Opisthorchis viverrini . (b) This micrograph shows the scolex of the cestode Taenia solium , also known as the pork tapeworm. The visible suckers and hooks allow the worm to attach itself to the inner wall of the intestine. (credit a: modification of work by Sripa B, Kaewkes S, Sithithaworn P, Mairiang E, Laha T, and Smout M; credit b: modification of work by Centers for Disease Control and Prevention)

The other medically important group of platyhelminths are commonly known as tapeworms ( cestodes ) and are segmented flatworms that may have suckers or hooks at the scolex (head region) ( [link] ). Tapeworms use these suckers or hooks to attach to the wall of the small intestine. The body of the worm is made up of segments called proglottid s that contain reproductive structures; these detach when the gametes are fertilized, releasing gravid proglottids with eggs. Tapeworms often have an intermediate host that consumes the eggs, which then hatch into a larval form called an oncosphere. The oncosphere migrates to a particular tissue or organ in the intermediate host, where it forms cysticerci. After being eaten by the definitive host, the cysticerci develop into adult tapeworms in the host's digestive system ( [link] ). Taenia saginata (the beef tapeworm) and T. solium (the pork tapeworm) enter humans through ingestion of undercooked, contaminated meat. The adult worms develop and reside in the intestine, but the larval stage may migrate and be found in other body locations such as skeletal and smooth muscle. The beef tapeworm is relatively benign, although it can cause digestive problems and, occasionally, allergic reactions. The pork tapeworm can cause more serious problems when the larvae leave the intestine and colonize other tissues, including those of the central nervous system. Diphylobothrium latum is the largest human tapeworm and can be ingested in undercooked fish. It can grow to a length of 15 meters. Echinococcus granulosus , the dog tapeworm, can parasitize humans and uses dogs as an important host.

Schistoma mansoni, japonicum, and haematobium are found in feces; S. japonicum and S. haematobium are also found in urine. These can be diagnosed in the water and produce eggs which hatch releasing miracidia. The miracidia penetrate snail tissues and produce sporocysts in the snail (successive generations). The Cercariae released by snail into the water are free flowing and are the infective stage which can penetrate skin. S. mansoni travels to the large intestines, S. japonicum travels to the small intestines, and S. haemotobium travels to the rectum. The cercariae lose their tails during penetration and become schistosomalae. Theese enter circulation and migrate to portal blood in liver and mature into adults. The paired adulted worms migrate to the mesenteric venules of the bowels/rectum (laying egs that circulate to the liver and are shed in stools) – for S. mansoni and S. Japonicum. S. haematobium migrates to the venous plexus of the bladder.
The life cycle of Schistosoma spp. includes several species of water snails, which serve as secondary hosts. The parasite is transmitted to humans through contact with contaminated water and takes up residence in the veins of the digestive system. Eggs escape the host in the urine or feces and infect a snail to complete the life cycle. (credit “illustration”: modification of work by Centers for Disease Control and Prevention; credit “step 3 photo”: modification of work by Fred A. Lewis, Yung-san Liang, Nithya Raghavan&Matty Knight)
Eggs or gravid proplottidis from an infected individual are passed into the environment; this is the diagnostic stage. Cattle (T. saginata) and pigs (T. solium) become infected by ingesting vegetation contaminated by eggs or gravid proglottids. Oncospheres hatch, penetrating intestinal wall and circulate to musculature. The oncospheres develop into cysticerci in muscles and become infective. Humans are infected by ingesting raw or undercooked infected meat. The scolex attaches to intestine and adults are found in the small intestine.
Life cycle of a tapeworm. (credit “illustration”: modification of work by Centers for Disease Control and Prevention; credit “step 3 micrographs”: modification of work by American Society for Microbiology)
  • What group of medically important flatworms is segmented and what group is unsegmented?

Food for worms?

For residents of temperate, developed countries, it may be difficult to imagine just how common helminth infections are in the human population. In fact, they are quite common and even occur frequently in the United States. Worldwide, approximately 807–1,221 million people are infected with Ascaris lumbricoides (perhaps one-sixth of the human population) and far more are infected if all nematode species are considered. Fenwick, A. “The global burden of neglected tropical diseases.” Public health 126 no.3 (Mar 2012): 233–6. Rates of infection are relatively high even in industrialized nations. Approximately 604–795 million people are infected with whipworm ( Trichuris ) worldwide ( Trichuris can also infect dogs), and 576–740 million people are infected with hookworm ( Necator americanus and Ancylostoma duodenale ). de Silva, N., et. al. (2003). “Soil-transmitted helminth infections: updating the global picture”. Trends in Parasitology 19 (December 2003): 547–51. Toxocara, a nematode parasite of dogs and cats, is also able to infect humans. It is widespread in the United States, with about 10,000 symptomatic cases annually. However, one study found 14% of the population (more than 40 million Americans) was seropositive, meaning they had been exposed to the parasite at one time. More than 200 million people have schistosomiasis worldwide. Most of the World Health Organization (WHO) neglected tropical diseases are helminths. In some cases, helminths may cause subclinical illnesses, meaning the symptoms are so mild that that they go unnoticed. In other cases, the effects may be more severe or chronic, leading to fluid accumulation and organ damage. With so many people affected, these parasites constitute a major global public health concern.

Eradicating the guinea worm

Dracunculiasis , or Guinea worm disease , is caused by a nematode called Dracunculus medinensis . When people consume contaminated water, water fleas (small crustaceans) containing the nematode larvae may be ingested. These larvae migrate out of the intestine, mate, and move through the body until females eventually emerge (generally through the feet). While Guinea worm disease is rarely fatal, it is extremely painful and can be accompanied by secondary infections and edema ( [link] ).

A worm is being pulled out of  a wound in the leg of an infected individual.
The Guinea worm can be removed from a leg vein of an infected person by gradually winding it around a stick, like this matchstick. (credit: Centers for Disease Control and Prevention)

An eradication campaign led by WHO, the CDC, the United Nations Children’s Fund (UNICEF), and the Carter Center (founded by former U.S. president Jimmy Carter) has been extremely successful in reducing cases of dracunculiasis. This has been possible because diagnosis is straightforward, there is an inexpensive method of control, there is no animal reservoir, the water fleas are not airborne (they are restricted to still water), the disease is geographically limited, and there has been a commitment from the governments involved. Additionally, no vaccines or medication are required for treatment and prevention. In 1986, 3.5 million people were estimated to be affected. After the eradication campaign, which included helping people in affected areas learn to filter water with cloth, only four countries continue to report the disease (Chad, Mali, South Sudan, and Ethiopia) with a total of 126 cases reported to WHO in 2014. World Health Organization. “South Sudan Reports Zero Cases of Guinea-Worm Disease for Seventh Consecutive Month.” 2016. http://www.who.int/dracunculiasis/no_new_case_for_seventh_consecutive_months/en/. Accessed May 2, 2016.

Key concepts and summary

  • Helminth parasites are included within the study of microbiology because they are often identified by looking for microscopic eggs and larvae.
  • The two major groups of helminth parasites are the roundworms (Nematoda) and the flatworms (Platyhelminthes).
  • Nematodes are common intestinal parasites often transmitted through undercooked foods, although they are also found in other environments.
  • Platyhelminths include tapeworms and flukes , which are often transmitted through undercooked meat.

Fill in the blank

Flukes are in class _________.

Trematoda

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A species of worm in which there are distinct male and female individuals is described as _________.

dioecious

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Short answer

What is the best defense against tapeworm infection?

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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