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Helminth infections

Last updated: February 2, 2021

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Helminths, i.e., parasitic worms, are a group of macroparasites encompassing a variety of species that can infect their hosts in three different ways: ingestion of eggs or larvae (e.g., via contaminated food and water or fecal-oral route), direct penetration of the skin, and via the bite of vectors (e.g., certain species of flies and mosquitoes). Helminths are classified based on their macroscopic appearance as nematodes (i.e, roundworms; genera include Toxocara, Enterobius, Trichuris, Ascaris, Trichinella, Strongyloides, Ancylostoma, Necator), cestodes (i.e.,tapeworms; genera include Taenia, Diphyllobothrium, Echinococcus), or trematodes (i.e., flukes; genera include Schistosoma, Metorchis, Fasciola). Most helminth species colonize the gastrointestinal tract of their hosts, provoking symptoms such as abdominal pain, nausea, and diarrhea. The larvae of certain helminth species, such as those of the Ascaris and Ancylostoma genus, migrate from the intestines via the portal vein to the lungs, potentially causing asthma-like symptoms (e.g., dry cough, wheezing). Other species, such as Taenia solium, are capable of colonizing other human tissue, such as the brain or the liver, which can lead to life-threatening complications (e.g., neurocysticercosis). Diagnosis of helminth infection is made primarily via evidence of eosinophilia in the blood and direct detection of worms, eggs, or larvae in stool samples. Serum IgE levels are often elevated. Treatment consists of anthelmintic agents, such as albendazole or praziquantel. For helminth infection prevention, see “Food and water safety” in the article on food poisoning. Echinococcosis and schistosomiasis are discussed in separate articles.

Nematodes (roundworms) are long, thin, unsegmented, tube-like worms with a longitudinal digestive tract opening at both ends. Adult worms form separate sexes, with the males usually being smaller than the females. Filarial Nematodes are thread-like nematodes. They are transmitted by arthropod vectors.

Nematodes (roundworms)
Disease Pathogen Mode of transmission Clinical features Diagnosis Treatment
Ascariasis
  • Fecal-oral
  • Pulmonary symptoms
  • GI symptoms
    • Abdominal discomfort
    • Nausea
    • Intestinal or bile duct obstruction
Enterobiasis (pinworm)
Trichuriasis
  • Eggs in stool
Toxocariasis
Trichinellosis
  • Consumption of undercooked meat (pork)
  • Fecal-oral (rare)
Hookworm (ancylostomiasis, necatoriasis)
  • Penetration of the skin by a larvae
Strongyloidiasis
Filariasis Loiasis
  • Bite of mango fly, deer fly, or horse fly
Onchocerciasis
  • Female blackfly bite
Lymphatic filariasis
  • Female mosquito bite

eating a Toxic TrEAT: Toxocara, Trichiniella, Enterobius, Ascaris, and Trichiuris are transmitted by ingestion.

SANd on your Shins, Ancles, and Neck: Strongyloides, Ancylostoma, and Necator penetrate the skin while walking on sand.

The OWL bites: Onchocera, Loa loa, and Wucheria are transmitted by bites.

  • Pathogen
  • Epidemiology: most common helminth infection worldwide (mainly affects children in tropical countries with low standards of hygiene)
  • Mode of transmission: fecal-oral (infection occurs in the larval state following the consumption of contaminated food, especially raw vegetables that have been contaminated by human waste used as a fertilizer)
  • Life cycle: Host ingests eggs Eggs hatch and release larvae → Larvae invade intestinal walls → Larvae migrate to lungs via portal vein Larvae migrate into alveoli, trachea (“tracheal migration”), and larynx Larvae are expectorated into the mouth and swallowed back into the intestine Larvae return to the intestine → Larvae mature into adult worms, which then lay new eggs.
  • Clinical features
  • Diagnosis
  • Treatment

  • Pathogen
  • Epidemiology [2]
    • Most common helminthic infection in the U.S.
    • Prevalence in the US: ∼ 12%
    • Primarily affects children 5–10 years of age
  • Mode of transmission
    • Initial infection: fecal-oral
    • Reinfection: digital-oral after scratching anal region
  • Clinical features
    • Anal pruritus (especially at night)
    • Vulvovaginitis, especially in children
    • Occasionally, symptoms of intestinal infection (i.e., nausea, vomiting, and abdominal pain which may become severe enough to mimic appendicitis)
  • Diagnosis
    • Tape test: microscopic detection of oval eggs (ova) and/or pinworms on tape that has been pressed against the perianal region
    • Can be an incidental finding on endoscopy
  • Treatment: bendazoles OR pyrantel pamoate [3][4]
    • During pregnancy: pyrantel pamoate is the drug of choice
      • Should only be administered if pregnancy is compromised (e.g., weight loss, sleeplessness)
      • Should not be started until the 3rd trimester, after which the risk to fetus is likely to be low.

Enterobius commonly manifests with perianal pruritus.

  • Pathogen
  • Mode of transmission: fecal-oral
  • Life cycle: Host ingests eggs → Eggs hatch and release larvae in the small intestine → Larvae mature into adult worms in colon Adult worms lay eggs, which are shed in feces.
  • Clinical features
  • Diagnosis
    • Microscopic examination of stool for eggs
    • Concentration technique can be used for light infections.
  • Treatment: mebendazole OR albendazole

Consider trichinella infection in patients with myositis, periorbital edema, and eosinophilia.

Consider hookworm infection in patients who present with a history of recent travel to a tropical country and microcytic anemia.

Everything turns black in onchocerciasis: black flies, black skin nodules, black vision (blindness).

Treat rIVER blindness with IVERmectin.

Cestodes (tapeworms) are long, flat, ribbon-like worms composed of numerous segments and a single scolex at the head with which they anchor themselves to the intestine. Since they do not have a digestive tract, all nutrients are absorbed through the tegument. Cestodes are hermaphroditic (they contain both male and female organs).

Overview of cestode infections
Disease Pathogen Mode of transmission Clinical features Diagnosis Treatment
Taeniasis

Intestinal taeniasis

  • Consumption of undercooked beef or pork
  • Mostly asymptomatic
  • GI symptoms
    • Abdominal pain
    • Nausea, vomiting
    • Weight loss
Cysticercosis
  • Fecal-oral
Diphyllobothriasis
  • Consumption of raw or undercooked freshwater fish
Echinococcosis
  • Fecal-oral
  • Hoofed animals (e.g., sheep) are intermediate hosts
Hymenolepisis
  • Ingestion of eggs or cysticercoids
  • Eggs in stool

Taeniae are tapeworms. Tapeworms belong to the Cestode class.

Intestinal taeniasis Cysticercosis [12]
Description
  • An intestinal infection with adult tapeworms that causes mainly GI symptoms
  • A tissue infection with tapeworm larvae. Symptoms depend on the infected organ (e.g., muscles, brain, skin).
Pathogen
Mode of transmission
  • Ingestion of larvae (cysticerci) in raw or undercooked beef/pork
  • Fecal-oral: eggs are ingested from contaminated water or vegetables
Life cycle
  • Eggs hatch in the human intestine → Develop into adult worms → Produce proglottids which can detach from the tapeworm and are passed in the feces.
Clinical features
  • Often asymptomatic
  • Symptoms caused by adult worms in the intestinal tract: abdominal pain, anorexia, weight loss, nausea, and vomiting
Diagnosis
  • Initial test: CBC may show eosinophilia
  • Additional testing
    • Imaging: cerebral MRI/CT showing multiple, small (< 1 cm) cystic lesions with a membranous wall and an invaginated scolex (“dot sign”)
    • CT/MRI may also show
      • Cysts with an invaginated scolex during earlier stages
      • Calcified cyst remnants in later stages
    • Lumbar puncture: ↑ protein, ↓ glucose, mononuclear pleocytosis
  • Confirmatory test: serum enzyme-linked immunotransfer blot (EITB) assay
Treatment
Prevention
  • Avoid raw pork and inspect for cysticerci
  • Adequately freeze and cook meat to destroy viable cysticerci
  • Dispose of human feces properly
  • Hand washing before meal preparation

Cysticercosis can cause brain cysts and seizures.

Diphyllobothrium causes vitamin B12 deficiency.

  • Pathogen: Hymenolepis nana (dwarf tapeworm) [14]
  • Mode of transmission
    • Ingestion of eggs from contaminated food or water
    • Ingestion of cysticercoids from infected arthropods
  • Life cycle: Ingestion of eggs → Develop into cysticercoid larvae in the small intestine villus (alternatively cysticercoids can be consumed from infected arthropods) → Cysticercoids (released upon rupture of the intestinal villus) develop into an adult worm in the intestinal lumen → Eggs are passed through the stool
  • Clinical features
  • Diagnosis: stool examination for eggs
  • Treatment

Trematodes (flukes) are small, flat, oval worms with two suckers (one located at the mouth and the other ventrally) and a blind-ending gut. Most species are hermaphroditic, but some also form separate male and female adults.

Overview of trematode infections
Disease Pathogen Mode of transmission Clinical features Diagnosis Treatment
Schistosomiasis
  • Penetration of the skin by a larvae
  • Freshwater snails are intermediate hosts
Clonorchiasis
  • Consumption of raw/undercooked freshwater fish

Paragonimisias

  • Consumption of raw/undercooked seafood
  • Utilization of contaminated cooking utensils
Fascioliasis
  • Consumption of contaminated freshwater plants
  • Triclabendazole
  • ERCP in case of biliary obstruction
  • Pathogen: Fasciola hepatica, Fasciola gigantica
  • Mode of transmission
    • Consumption of contaminated freshwater plants (e.g., watercress)
    • Drinking contaminated freshwater
  • Life cycle
    • Metacercariae on freshwater plants are ingested by herbivorous mammals e.g., sheep, cattle (both definitive hosts), or humans (incidental hosts) → Migration of the organism through the intestinal wall, then liver parenchyma, and into the biliary ducts → Adult fluke release eggs into the biliary ducts, which are then passed in the stool of the host.
    • Snails serve as the intermediate hosts.
  • Clinical features
  • Diagnosis
  • Treatment
    • Triclabendazole
    • ERCP in case of biliary obstruction

Antihelminthic agents

Antihelminthics act as a vermicide and are contraindicated during pregnancy. While a single dose is usually sufficient in disease with taenia, infection with Echinococcus multilocularis may require lifelong treatment in some cases.

Drug Mechanism of action Clinical use Side effects

Bendazoles (e.g., albendazole, mebendazole) [19][20]

Pyrantel pamoate [21]
Praziquantel [22]
  • Causes vacuolization of the schistosome integument and increases cell membrane permeability to calcium resulting in paralysis, dislodgement, and death of parasite
  • A single dose is sufficient for most worms.
Ivermectin [23]
Diethylcarbamazine [24]

Parasites Meet ImPending Doom: the most important antihelminthic agents are Pyrantel pamoate, Mebendazole, Ivermectin, Praziquantel, and Diethylcarbamazine.

  1. Albendazole. https://www.drugbank.ca/drugs/DB00518. Updated: November 2, 2019. Accessed: November 4, 2019.
  2. Mebendazole. https://www.drugbank.ca/drugs/DB00643. Updated: November 3, 2019. Accessed: November 4, 2019.
  3. Pyrantel. https://www.drugbank.ca/drugs/DB11156. Updated: November 3, 2019. Accessed: November 4, 2019.
  4. Praziquantel. https://www.drugbank.ca/drugs/DB01058. Updated: November 3, 2019. Accessed: November 4, 2019.
  5. Ivermectin. https://www.drugbank.ca/drugs/DB00602. Updated: November 3, 2019. Accessed: November 4, 2019.
  6. Diethylcarbamazine. https://www.drugbank.ca/drugs/DB00711. Updated: September 2, 2019. Accessed: November 4, 2019.
  7. Parasites - Toxocariasis (also known as Roundworm Infection) - Epidemiology & Risk Factors. https://www.cdc.gov/parasites/toxocariasis/epi.html. Updated: January 10, 2013. Accessed: March 22, 2017.
  8. Parasites - Enterobiasis (also known as Pinworm Infection) - Epidemiology & Risk Factors. https://www.cdc.gov/parasites/pinworm/epi.html. Updated: January 10, 2013. Accessed: March 22, 2017.
  9. Parasites - Enterobiasis (also known as Pinworm Infection) - Treatment. https://www.cdc.gov/parasites/pinworm/treatment.html. Updated: September 23, 2016. Accessed: March 22, 2017.
  10. Parasites - Enterobiasis (also known as Pinworm Infection) - Resources for Health Professionals. https://www.cdc.gov/parasites/pinworm/health_professionals/. Updated: September 23, 2016. Accessed: March 22, 2017.
  11. Vu Thi N, Trung DD, Litzroth A, Praet N, Nguyen Thu H, Nguyen Thu H, Nguyen Manh H, Dorny P. The hidden burden of trichinellosis in Vietnam: a postoutbreak epidemiological study.. BioMed Research International. 2013 .
  12. Parasites - Zoonotic Hookworm. https://www.cdc.gov/parasites/zoonotichookworm/. Updated: October 11, 2012. Accessed: March 22, 2017.
  13. Fish tapeworm infection. https://medlineplus.gov/ency/article/001375.htm. Updated: September 10, 2015. Accessed: March 22, 2017.
  14. Parasites - Cysticercosis - Biology. https://www.cdc.gov/parasites/cysticercosis/biology.html. Updated: November 2, 2010. Accessed: March 22, 2017.
  15. Hymenolepiasis. https://www.cdc.gov/dpdx/hymenolepiasis/index.html. Updated: December 13, 2017. Accessed: June 8, 2020.
  16. Parasites - Lymphatic Filariasis. https://www.cdc.gov/parasites/lymphaticfilariasis/treatment.html. Updated: March 16, 2018. Accessed: September 23, 2020.
  17. Keiser J, Utzinger J. Food-borne trematodiases.. Clinical Microbiology Reviews. 2009 .
  18. Resources for Health Professionals. https://www.cdc.gov/parasites/clonorchis/health_professionals/index.html. Updated: February 18, 2018. Accessed: January 8, 2020.
  19. James H. Diaz. Paragonimiasis Acquired in the United States: Native and Nonnative Species. Clinical Microbiology Reviews. 2013 .
  20. World Health Organization. Investing to Overcome the Global Impact of Neglected Tropical Diseases. World Health Organization ; 2015
  21. Parasites - Strongyloides - Biology. https://www.cdc.gov/parasites/strongyloides/biology.html. Updated: April 13, 2015. Accessed: March 22, 2017.
  22. Ralston SH, Penman ID, Strachan MWJ, Hobson R. Davidson's Principles and Practice of Medicine. Elsevier ; 2018
  23. Parasites - Onchocerciasis (also known as River Blindness). https://www.cdc.gov/parasites/onchocerciasis/index.html. Updated: August 10, 2015. Accessed: November 5, 2018.
  24. Parasites - Loiasis. https://www.cdc.gov/parasites/loiasis/index.html. Updated: January 20, 2015. Accessed: November 5, 2018.
  25. Hong S-T, Fang Y. Clonorchis sinensis and clonorchiasis, an update. Parasitol Int. 2012; 61 (1): p.17-24. doi: 10.1016/j.parint.2011.06.007 . | Open in Read by QxMD