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. 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).
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. 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 Necator americanus (the New World or North American hookworm) and Ancylostoma 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 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 the nematode Dirofilaria immitis, which is transmitted by mosquitoes. Symptoms include fatigue and cough; when left untreated, death may result.
Parker, N., Schneegurt, M., Thi Tu, A.-H., Forster, B. M., & Lister, P. (n.d.). Microbiology. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/microbiology