Notes
Cestodes (Flatworms)
Sections
Cestodes: Flatworms
Anatomy:
No digestive systems; they absorb nutrients through their body walls.
They are hermaphroditic, with both male and female reproductive organs.
Anterior ends comprise the scolex, with suckers and a rostellum with hooklets that are used for attaching to the host.
The body is called the *strobila and comprises multiple segments called proglottids. The distal, more mature proglottids house uteruses with eggs; these are called gravid proglottids. The outer covering is tegument.
Cestodes develop from proximal to distal: as the immature proglottids develop at the anterior end, the mature proglottids move distally. The most distal segments comprise proglottids with uteruses full of eggs; thus, they are the gravid proglottids.
The gravid proglottids break free, and release their eggs.
Life Cycles:
Many cestodes have complex life cycles with one or more intermediate hosts.
When humans host the adult forms, cestode infections are often asymptomatic, although some people experience abdominal pain, diarrhea, and other gastrointestinal problems.
Larvae exit the eggs and are ingested by intermediate hosts. In these hosts, which range from insects to fish, the larvae undergo transformations that produce infective forms.
Humans ingest the infective larvae; this usually occurs when humans eat the intermediate host. In the small intestine, the parasites mature to the adult form and produce gravid proglottids.
The proglottids release eggs, which are then passed to the environment in the feces.
Diagnosis & Treatment:
Diagnosis is confirmed by the finding of eggs or proglottids in the stool.
Praziquantel can be administered as treatment.
Infections are more serious when humans host the larval stages of some cestodes, which form damaging cysts.
Diphyllobothrium latum
Largest human tapeworm: Grows up to ten meters long
Life cycle includes two intermediate hosts:
– Early larval forms are ingested by microcrustaceans, which are then eaten by fish.
– When humans eat undercooked infected fish, they become the definitive hosts.
Serious infection can cause Vitamin B12 deficiency, pernicious anemia, and intestinal obstruction.
Hymenolepis nana
Small worm with a short life span.
Infection is common, especially among children.
The life cycle of this cestode does not always involve an intermediate host:
– Humans can become infected via ingestion of arthropod intermediate hosts
– Humans can directly ingest eggs.
Autoinfection is also possible, particularly in children, and can lead to significant, long-term infection.
Taenia solium & Taenia saginata
Pork tapeworm & Beef tapeworm
Both can cause taeniasis.
Pigs (or cows) ingest vegetation contaminated with the parasites' eggs; humans then ingest the infected meat.
If the worm burden is high, infection can cause gastrointestinal problems.
Cysticercosis is a more serious infection that occurs when humans ingest Taenia solium eggs in food or water contaminated by human feces.
Once ingested, the eggs give rise to larvae that form cysts in the tissues. This is particularly serious when cysts form in the brain or spinal cord:
Neurocysticercosis occurs when Taenia solium larvae form cysts and calcified lesions in the CNS. This is a significant cause of epilepsy worldwide; other neurological manifestations depend on the location and size of the lesions.
Echinococcus granulosus
Typically passes its life cycle in canids (dogs, wolves, etc.) and sheep. Humans are accidental hosts when they ingest the eggs, which are passed in canine feces.
Large, complex cysts form in human tissues.
These cysts can rupture and cause anaphylactic shock and disseminate the parasites.
References
"CDC - DPDx - Taeniasis," December 18, 2017. https://www.cdc.gov/dpdx/taeniasis/index.html.
García, Héctor H, Armando E Gonzalez, Carlton A W Evans, and Robert H Gilman. "Taenia Solium Cysticercosis." Lancet 362, no. 9383 (August 16, 2003): 547–56. https://doi.org/10.1016/S0140-6736(03)14117-7.
Heyneman, Donald. "Cestodes." In Medical Microbiology, edited by Samuel Baron, 4th ed. Galveston (TX): University of Texas Medical Branch at Galveston, 1996. http://www.ncbi.nlm.nih.gov/books/NBK8399/.
Lee, Eun Bin, Jung Hoon Song, Nam Seon Park, Byung Kook Kang, Hyung Suk Lee, Yoon Ju Han, Hyo-Jin Kim, Eun-Hee Shin, and Jong-Yil Chai. "A Case of Diphyllobothrium Latum Infection with a Brief Review of Diphyllobothriasis in the Republic of Korea." The Korean Journal of Parasitology 45, no. 3 (September 2007): 219–23. https://doi.org/10.3347/kjp.2007.45.3.219.
Meng, Qiang, and Lingchun Liu. "Disseminated Cysticercosis." N-icm. https://doi.org/10.1056/NEJMicm1601189, December 28, 2016. https://doi.org/10.1056/NEJMicm1601189.
Murray, P. R., Rosenthal K.S., & Pfaller, M.A. Medical Microbiology. 8th ed. Elsevier, 2016.
Philips, Cyriac A., and Amrish Sahney. "Taenia Solium." New England Journal of Medicine 376, no. 4 (January 26, 2017): e4. https://doi.org/10.1056/NEJMicm1606747.
Prevention, CDC-Centers for Disease Control and. "CDC - Cysticercosis," May 2, 2017. https://www.cdc.gov/parasites/cysticercosis/index.html.