Microbiology, 2021

194 5 • The Eukaryotes of Microbiology
may result.
Clinical Focus
Part 2
The physician explains to Sarah’s mother that ringworm can be transferred between people through touch.
“It’s common in school children, because they often come in close contact with each other, but anyone can
become infected,” he adds. “Because you can transfer it through objects, locker rooms and public pools are
also a potential source of infection. It’s very common among wrestlers and athletes in other contact sports.”
Looking very uncomfortable, Sarah says to her mother “I want this worm out of me.”
The doctor laughs and says, “Sarah, you’re in luck because ringworm is just a name; it is not an actual
worm. You have nothing wriggling around under your skin.”
“Then what is it?” asks Sarah.
• What type of pathogen causes ringworm?
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• What is the most common nematode infection in the United States?
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
(Figure 5.20).
The flukes (trematodes) are nonsegmented flatworms that have an oral sucker (Figure 5.21) (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 (Clonorchis and Opisthorchis), the intestinal fluke (Fasciolopsis buski), and the oriental lung fluke
(Paragonimus westermani). Schistosomiasis is a serious parasitic disease, considered second in the scale of its
impact on human populations only to malaria. The parasites Schistosoma mansoni, S. haematobium, and S.
japonicum, which are found in freshwater snails, are responsible for schistosomiasis (Figure 5.22). 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.
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