causing toxocariasis. Antibodies to these parasites have been found in approximately 13.9% of the U.S. population,
suggesting that exposure is common.[7]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 byNecator americanus(the
New World or North American hookworm) andAncylostoma duodenale(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 byTrichinella 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 the nematodeDirofilaria immitis, which is transmitted by
mosquitoes. Symptoms include fatigue and cough; when left untreated, death may result.
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?
Jump to thenextClinical Focus box. Go back to thepreviousClinical Focus box.
What is the most common nematode infection in the United States?Platyhelminths (Flatworms)
PhylumPlatyhelminthes(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).
Theflukes(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
andOpistorchis), 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 parasitesSchistosoma mansoni, S. haematobium, andS. japonicum, which are found in freshwater
snails, are responsible for schistosomiasis (Figure 5.22). Immature forms burrow through the skin into the blood.
Clinical Focus
- 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.
Chapter 5 | The Eukaryotes of Microbiology 201