4.2 Parasitic Helminths

Learning Objectives
  • Explain why we include the study of parasitic worms within the discipline of microbiology
  • Describe the characteristics of parasitic nematodes
  • Describe the characteristics of parasitic trematodes and cestodes

Parasitic helminths are animals that are often included within the study of microbiology because many species of these worms are identified by their microscopic eggs and larvae. There are two major groups of parasitic helminths: the roundworms (Nematoda) and flatworms (Platyhelminthes). Of the many species that exist in these groups, about half are parasitic and some are important human pathogens. As animals, they are multicellular and have organ systems. However, the parasitic species often have limited digestive tracts, nervous systems, and locomotor abilities. Parasitic forms may have complex reproductive cycles with several different life stages and more than one type of host.

Nematoda (Roundworms)

Phylum Nematoda (the roundworms) is a diverse group containing more than 15,000 species, of which several are important human parasites (Figure 4.11). These unsegmented worms have a full digestive system even when parasitic. Some are common intestinal parasites, and their eggs can sometimes be identified in feces or around the anus of infected individuals. Ascaris lumbricoides is the largest nematode intestinal parasite found in humans; females may reach lengths greater than 1 meter. A. lumbricoides is also very widespread, even in developed nations, although it is now a relatively uncommon problem in the United States. It may cause symptoms ranging from relatively mild (such as a cough and mild abdominal pain) to severe (such as intestinal blockage and impaired growth).

A micrograph of the nematode Enterobius vermicularis, also known as the pinworm.
Figure 4.11 A micrograph of the nematode Enterobius vermicularis, also known as the pinworm. (credit: modification of work by Centers for Disease Control and Prevention)

Of all nematode infections in the United States, pinworm (caused by Enterobius vermicularis) is the most common. Pinworm causes sleeplessness and itching around the anus, where the female worms lay their eggs during the night. Toxocara canis and T. cati are nematodes found in dogs and cats, respectively, that can be transmitted to humans, causing toxocariasis. Antibodies to these parasites have been found in approximately 13.9% of the U.S. population, suggesting that exposure is common.[1] Infection can cause larval migrans, which can result in vision loss and eye inflammation, or fever, fatigue, coughing, and abdominal pain, depending on whether the organism infects the eye or the viscera. Another common nematode infection is hookworm. Symptoms of hookworm infection can include abdominal pain, diarrhea, loss of appetite, weight loss, fatigue, and anemia.

Trichinellosis, also called trichinosis, caused by Trichinella spiralis, is contracted by consuming undercooked meat, which releases the larvae and allows them to encyst in muscles. Infection can cause fever, muscle pains, and digestive system problems; severe infections can lead to lack of coordination, breathing and heart problems, and even death. Finally, heartworm in dogs and other animals is caused by a nematode.

  • What is the most common nematode infection in the United States?

Platyhelminths (Flatworms)

Phylum Platyhelminthes (the platyhelminths) are flatworms. This group includes the flukes and tapeworms, as medically important parasites.

The flukes (trematodes) are nonsegmented flatworms that have an oral sucker (Figure 4.12) (and sometimes a second ventral sucker) and attach to the inner walls of intestines, lungs, large blood vessels, or the liver. Trematodes have complex life cycles, often with multiple hosts. Several important examples are the liver flukes, the intestinal fluke, and the oriental lung fluke. Schistosomiasis is a serious parasitic disease, considered second in the scale of its impact on human populations only to malaria. Immature forms burrow through the skin into the blood. They migrate to the lungs, then to the liver and, later, other organs. Symptoms include anemia, malnutrition, fever, abdominal pain, fluid buildup, and sometimes death.

(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.
Figure 4.12 (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 head region (Figure 4.12). 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 that contain reproductive structures; these detach when the gametes are fertilized, releasing fertile segments 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 (Figure 4.13). Tapeworms can 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.

Life cycle of a tapeworm.
Figure 4.13 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?

Micro Connections

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.[2] 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).[3] 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.


  1. Won K, Kruszon-Moran D, Schantz P, Jones J. “National seroprevalence and risk factors for zoonotic Toxocara spp. infection.” In: Abstracts of the 56th American Society of Tropical Medicine and Hygiene; Philadelphia, Pennsylvania; 2007 Nov 4-8.
  2. Fenwick, A. “The global burden of neglected tropical diseases.” Public health 126 no.3 (Mar 2012): 233–6.
  3. de Silva, N., et. al. (2003). “Soil-transmitted helminth infections: updating the global picture”. Trends in Parasitology 19 (December 2003): 547–51.

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