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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.
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.
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] ).
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.
A species of worm in which there are distinct male and female individuals is described as _________.
dioecious
What is the best defense against tapeworm infection?
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