# The Eukaryotes of Microbiology

## Parasitic Helminths


### Learning Objectives

By the end of this section, you will be able to:
1. Explain why we include the study of parasitic worms within the discipline of microbiology
2. Compare the basic morphology of the major groups of parasitic helminthes
3. Describe the characteristics of parasitic nematodes, and give an example of infective eggs and infective larvae
4. Describe the characteristics of parasitic trematodes and cestodes, and give examples of each
5. Identify examples of the primary causes of infections due to nematodes, trematodes, and cestodes
6. Classify parasitic worms according to major groups

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. Some are monoecious, having both male and female reproductive organs in a single individual, while others are dioecious, each having either male or female reproductive organs.

### Nematoda (Roundworms)

Phylum Nematoda (the roundworms) is a diverse group containing more than 15,000 species, of which several are important human parasites (). 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.  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).
Of all nematode infections in the United States, pinworm (caused by ) is the most common. Pinworm causes sleeplessness and itching around the anus, where the female worms lay their eggs during the night.  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.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. 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, which is caused by  (the New World or North American hookworm) and  (the Old World hookworm). Symptoms of hookworm infection can include abdominal pain, diarrhea, loss of appetite, weight loss, fatigue, and anemia.
Trichinellosis, also called trichinosis, caused by , 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 the nematode  which is transmitted by mosquitoes. Symptoms include fatigue and cough; when left untreated, death may result.

### Platyhelminths (Flatworms)

Phylum Platyhelminthes (the platyhelminths) are flatworms. This group includes the flukes, tapeworms, and the turbellarians, which include planarians. The flukes and tapeworms are medically important parasites ().
The flukes (trematodes) are nonsegmented flatworms that have an oral sucker () (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 ( and ), 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. The parasites  mansoni, S. haematobium, and S. japonicum, which are found in freshwater snails, are responsible for schistosomiasis (). 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.
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) (). 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 proglottids 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 ().  (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.  is the largest human tapeworm and can be ingested in undercooked fish. It can grow to a length of 15 meters. , the dog tapeworm, can parasitize humans and uses dogs as an important host.

### Key Concepts and Summary

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

### Multiple Choice


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### Short Answer


### Critical Thinking
