• Clinical science

Helminth infections (Helminthiasis…)

Summary

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)

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
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
  • Skin snip
  • Mazotti test
  • Slit-lamp examination
  • Serology
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.

Ascariasis

  • 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
    • CBC shows eosinophilia.
    • Confirmatory test: Stool samples show the presence of worms or visible oval eggs with a knobby appearance under the microscope.
  • Treatment

Enterobiasis

  • 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.

Trichiuriasis

Toxocariasis

Trichinellosis

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

Hookworm infections (ancylostomiasis; necatoriasis)

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

Strongyloidiasis

Loiasis

  • Pathogen: Loa loa, a filarial nematode [9][10]
  • Mode of transmission: bite from Chrysops (deer fly), horse fly, mango fly
  • Life cycle: Introduction of larvae into bite wound → Larvae mature into adult worms → Adult worms reside in subcutaneous tissue, migrating through the body and causing symptoms → Adult worms produce microfilariae and release them into the bloodstream, where they are ingested by a female fly during a blood meal → Microfilariae mature into larvae, thus completing the cycle.
  • Incubation period: 3–5 months
  • Clinical features
    • Most infected individuals are asymptomatic.
    • The two cardinal manifestations are:
  • Diagnosis
  • Treatment: diethylcarbamazine
  • Complications

Onchocerciasis

  • Pathogen: Onchocerca volvulus [9][11]
  • Mode of transmission: bite of the female blackfly
  • Life cycle: Host is bitten by blackfly → Larvae are introduced into bite wound → Larvae mature into adult worms that reside in subcutaneous or intramuscular tissue → Adult worms produce microfilariae Microfilariae migrate through subcutaneous tissue, causing clinical symptoms Female fly consumes microfilariae during a blood meal → Microfilariae mature into larvae, thus, completing the cycle.
  • Clinical features
  • Diagnosis
    • Skin snips (gold standard): superficial skin biopsies are taken from an inflamed area and are incubated in saline, causing microfilariae to leave the skin sample. Microfilarial load can be determined microscopically.
      • At least two specimens are sampled and examined for motile microfilariae.
      • Specimens of subcutaneous nodules can also be sampled and examined for adult worms.
    • Slit-lamp examination: investigation of choice for ocular onchocerciasis
    • Mazzotti test: a provocative test that involves administering diethylcarbamazine (DEC) to induce microfilarial death and exacerbate symptoms. A positive test is highly suggestive of onchocerciasis.
    • Serology: an unreliable means of diagnosis as it cannot distinguish between active and past infection
  • Treatment: ivermectin

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

Treat rIVER blindness with IVERmectin.

Lymphatic filariasis

  • Pathogens
  • Mode of transmission: female mosquito bite (Aedes, Mansonia, Anopheles, and Culex)
  • Incubation period: 9–12 months
  • Life cycle: Mosquito introduces filarial larvae into host via bite wound Larvae mature into adult worms that reside in the lymphatic system Adult worms produce microfilariae Microfilariae move throughout vascular and lymphatic system → Microfilariae are consumed by a female mosquito during a blood meal → Microfilariae mature into larvae, thus completing the cycle.
  • Clinical features
  • Diagnosis
  • Treatment

Cestodes (tapeworms)

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

Taeniasis

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.

Diphyllobothriasis (Diphyllobothrium latum or fish tapeworm infection)

Diphyllobothrium causes vitamin B12 deficiency.

Hymenolepiasis

  • 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)

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

Clonorchiasis

Paragonimisias

Fascioliasis

  • 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

Treatment

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]

  • Inhibit microtubule polymerization resulting in
    • Decreased glucose uptake and