Helminthic Diseases of the Digestive System

Tapeworms

Tapeworms are helminths capable of targeting the digestive system, but require various hosts for successful growth and transmission.

Learning Objectives

Compare and contrast the routes of transmission for various tapeworms and describe their life cycles

Key Takeaways

Key Points

  • There are numerous types of tapeworms capable of infecting humans including beef, pork, and fish tapeworms.
  • Taenia solium, the pork tapeworm, are passed via infected feces to the pig host but requires ingestion of under cooked pork by a human to ensure transmission.
  • Taenia saginata, the beef tapeworm, are passed to humans by ingesting under cooked or raw beef that has been infected with the cyst stage in the life cycle.
  • Diphyllobothrium spp, the fish tapeworm, requires multiple hosts to successfully infect a human host.

Key Terms

  • scolex: The “head” of the tapeworm that contains suckers and rows of hooks by which it attaches itself to the host.

Tapeworms are characterized as adult parasitic flatworms that target and infect the digestive tract. Typically, transmission occurs by ingestion of a live tapeworm larva which is found in undercooked or contaminated food. Once inside the digestive tract, the larvae can then grow and develop into a large tapeworm. There are various types of tapeworms identified to date that are capable of infecting humans. A few common tapeworms include the pork tapeworm (Taenia solium), the beef tapeworm (Taenia saginata), and the fish tapeworm (Diphyllobothrium spp. ).

Taenia solium

Taenia solium, the pork tapeworm, infects both pigs and humans. Humans are typically infected by ingesting infected pork. Taenia solium will target the intestinal area in humans. Using the four suckers and two rows of hooks present on its scolex, it lodges itself against the intestinal wall. Once anchored, the tapeworm continues to grow in the intestine and will pass its eggs through the feces. When Taenia solium is in its larval stage, it is referred to as a cysticercus and can cause cysticercosis. Cysticercosis occurs when the cysticerci reach the central nervous system and cause neurological issues.

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Scolex of the Taenia solium: An image of the scolex of the Taenia solium.

Taenia saginata

Taenia saginata, the beef tapeworm, is capable of infecting both cattle and humans. Infection by Taenia saginata is referred to as taeniasis in humans and occurs by ingestion of contaminated meat that has been improperly handled. Taeniasis is caused by the ability of the tapeworm to lodge itself in the small intestine of the human and release fertilized eggs through the feces. The eggs can then infect cattle upon ingestion. Upon infection of the cattle, the fertilized eggs will develop into zygotes which will exit the digestive tract and enter into the circulatory system. The larval stages will then form cysts, referred to as Cysticercus bovis, within the muscular system of the cattle. Therefore, if humans ingest under prepared meat with cysts, the cysts will break open into the digestive system and develop into an adult tapeworm in the human host.

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Tapeworm: An adult Taenia saginata.

Diphyllobothrium spp.

Diphyllobothrium spp, the fish tapeworm, encompasses various species that can infect humans upon ingestion of under cooked or raw fish. For example, these tapeworms include those found on broad fish and salmon. Interestingly, only a few species of these tapeworms are found to infect humans on a frequent basis. The tapeworms found in fish exhibit the ability to also infect canines, felines, bears, and mussels.

The life cycle for fish tapeworms includes movement through numerous hosts. The mammal host is considered the definitive host as this is the site of worm reproduction. The immature eggs are passed through the feces of the mammal host and then infect a freshwater host. This freshwater host is considered to be the intermediate host. The intermediate host is then ingested by a second intermediate host which includes the fish. The larvae, which developed in the first intermediate host, then migrate out into the flesh of the fish (the second intermediate host). The larvae develop into a more mature form and constitute the infective stage for the definitive host. It is important to note that many second intermediate hosts are ingested by larger predator species that are utilized as a food source for humans. Therefore, it is not necessary for humans to directly eat the second intermediate host in order to be infected.

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Diphylloborthrium latum life cycle: Overview of the life cycle of Diphylloborthrium latum. Note the various hosts which are necessary for successful transmission.

Hydatid Disease

Hydatid disease, also referred to as cystic echinococcosis, is characterized by the slow growth of large cysts within various organs.

Learning Objectives

Outline the life cycle of Echinococcus granulosus

Key Takeaways

Key Points

  • Echinococcus granulosus causes hydatid disease, or cystic echinococcus and causes slowly enlarging cysts to develop in vital organs.
  • Echinococcus multilocularis results in alveolar echinococcosis and Echinococcus vogeli can cause polycystic echinococcosis.
  • Individuals infected with Echinococcus species who develop hydatid disease may not develop symptoms for many years after infection.

Key Terms

  • echinococcosis: a parasitic disease of both animals and humans, also known as hydatid disease.

Hydatid disease, commonly referred to as echinococcosis, is a result of infection by the tapeworm included in the Echinococcus spp.

Echinococcus multilocularis results in alveolar echinococcosis and Echinococcus vogeli can cause polycystic echinococcosis. In individuals that develop alveolar echinococcosis, which is extremely rare, cysts develop and grow within the alveolar, liver, lungs, and brain. In the cases of alveolar echinococcosis, the parasite responsible, Echinococcus multilocularis, is associated with dogs, cats, wolves and most commonly, foxes, serving as the definitive hosts. The intermediate hosts include small rodents. In individuals that develop polycystic echinococcosis, there is the development of slow growing cysts within the body that exhibit the capability to infiltrate surrounding areas. Cysts are typically found in the liver and in the thorax or abdominal cavity. The parasite responsible for polycystic echinococcosis, Echinococcus vogeli, uses dogs or bush dogs as a definitive host and rodents as an intermediate hosts.

Echinococcus granulosus is a tapeworm found in dogs, who function as the definitive host, as well as sheep, cattle, goats, and pigs who serve as intermediate hosts. Echinococcus granulosus are capable of infecting humans, resulting in hydatid disease, and cause slowly enlarging cysts that develop in vital organs such as the liver or lungs, also referred to as cystic echinococcosis. However, the growth of these cysts are slow and may go unnoticed for a significant duration of time. This form of echinococcosis is more prevalent and commonly found in humans. The adult Echinococcus granulosus targets the small bowel of the definitive hosts and release eggs through the feces. After ingestion by the intermediate hosts, the eggs will hatch and penetrate the intestinal wall. The exiting of the intestinal wall results in circulation of oncospheres that will target various organs, including the liver and lungs. The oncospheres undergo further growth and form cysts. The definitive host will then become infected upon ingestion of the cyst-containing organs of the infected intermediate host. It is within the definitive host that the cyst will develop into the adult stage and the cycle continues and repeats.

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Overview of the Echinococcus life cycle: A general overview of the various hosts and steps involved in the life cycle of Echinococcus species.

Hydatid disease is characterized by the growth of these cysts into the adult stage for the tapeworm. Individuals infected with Echinococcus species that develop hydatid disease may not develop symptoms for many years after infection. However, if symptoms develop, it may present in the form of pain or discomfort due to the growing cysts in the upper abdominal regions. Additionally, rupture of fluid within the cysts can result in various medical issues including allergic reactions or death.

Nematodes

Nematodes, or roundworms, are capable of exhibiting parasitic behavior.

Learning Objectives

Compare and contrast mechanisms of infection for the parasitic nematodes: Ascaris lumbricoides and Enterobius

Key Takeaways

Key Points

  • Parasitic nematodes often contain specific body structures which promote parasitic behaviors such as ridges, rings or bristles that allow for attachment.
  • Two major types of roundworms that commonly infect humans include Ascaris and Enterobious.
  • Ascariasis is a disease that is caused by the parasitic roundworm Ascaris lumbricoides and is transmitted by ingesting food contaminated with Ascaris eggs.
  • Enterobius, referred to as pinworm, causes enterobiasis and is commonly found in the intestine of children. The entire life cycle of the pinworm occurs within the human gastrointestinal tract.

Key Terms

  • visceral: of or relating to the viscera – the internal organs of the body
  • nematode: A small invertebrate animal of the phylum Nematoda.

Nematodes, or roundworms, are the most diverse phylum of pseudocoelomates and one of the most diverse animal phyla. Nematodes are characterized by the presence of a tubular digestive system with openings at both ends. They can be found in various ecosystems ranging from the polar regions to the tropics, fresh to marine water and various terrestrial environments ranging from mountains, deserts and oceanic floors. Nematodes are also capable of exhibiting parasitic behavior that contribute to digestive system diseases. Analysis of parasitic nematodes reveals the presence of specific body structures which promote parasitic behaviors such as ridges, rings or bristles that allow for attachment. Parasitic nematodes that commonly infect humans include ascarids (Ascaris), pinworms (Enterobius), whipworms (Trichuris trichiura), filarias and hookworms. The following is a brief overview of two of these types of roundworms, including Ascaris and Enterobious.

Ascariasis

Ascariasis is a disease that is caused by the parasitic roundworm Ascaris lumbricoides. Ascariasis is commonly found in tropical and sub-tropical regions and is transmitted by ingesting food contaminated with Ascaris eggs that are typically present in the feces of an infected individual. Upon ingestion of Ascaris eggs, the larvae continue development and hatch within the intestine of the host. The larvae burrow through the intestinal wall and begin circulation in the system and migrate to the lungs. Once they have migrated to the lungs, the larvae break into the alveoli and continue to move through the trachea and esophagus where they are eventually coughed up and swallowed. For a second time, the larvae enter into the intestine and mature into adult worms. Individuals affected with ascariasis can be asymptomatic or suffer from visceral damage due to the travel of the larvae through the body. The presence of the worms within the intestine may also result in malabsorption or intestinal blockage.

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Ascariasis Life Cycle: An overview and summary of the life cycle leading to Ascariasis.

Enterobius

Enterobius, referred to as pinworm, is a type of parasitic nematode that is commonly found in the intestine of children. This infection is often called enterobiasis. The entire life cycle of the pinworm occurs within the human gastrointestinal tract. The life cycle begins with ingestion or insertion of eggs in the anus that have been transmitted via human-to-human contact. The life cycle of the Enterobious ends with eggs laid in the anus and transfered to surfaces that come in contact with this region, such as fingernails, hands, night-clothing and bed linens. Infection by pinworm is more prevalent in children due to their behaviors. Upon infection with eggs, they travel to the small intestine and hatch. The pinworm larvae will then migrate towards the colon and undergo molting which allows for further development. After two molt cycles, the larvae have developed into adults. The pinworms exhibit the ability to mate in the small intestine. Shortly after mating, the male worms die and are passed out via the feces. However, the female pinworms will attach to the mucus and wait for the egg-laying process to begin. The egg-laying process begins with the migration of the females towards the rectum. The eggs, which are covered with a sticky covering, are then released and deposited on the outside or near the anus. The cycle begins again once eggs are ingested and infect an additional host.

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Overview of the pinworm life cycle: Overview of the pinworm life cycle, specifically Enterobius vermicularis.