• 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 learning card on food poisoning. Echinococcosis and schistosomiasis are discussed in separate learning cards.

Ascariasis

  • Pathogen
  • Epidemiology: most common helminth infection worldwide (mainly affecting children in tropical and developing countries)
  • Mode of transmission: fecal-oral
  • Life cycle: oral ingestion of eggs → eggs hatch to release larvae → larvae invade intestinal walls migration to lungs via portal vein → migration into alveoli, trachea (“tracheal migration”), larynxlarvae are expectorated and swallowed back into the intestine → larvae returning to the intestine mature into adult worms, which then lay new eggs.
  • Clinical features
  • Diagnosis
    • Initial test: complete blood cell (CBC) count showing eosinophilia
    • Confirmatory test: stool sample showing the presence of worms; visible eggs with a knobby appearance under microscope
  • Treatment: bendazoles and pyrantel pamoate

You could get sick if you EATTT Enterobius, Ascaris, Toxocara, Trichinella, Taenia!

References:[1][1][2][3]

Enterobiasis

  • Pathogen
  • Epidemiology
    • Most common helminthic infection in the U.S.
    • Prevalence in the US: ∼ 40 million
    • Primarily 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, particularly in children
    • Occasionally symptoms of intestinal infection (nausea, vomiting, and abdominal pain, sometimes severe enough to mimic appendicitis)
  • Diagnosis: tape test
  • Treatment: bendazoles or pyrantel pamoate
    • During pregnancy: pyrantel pamoate is the drug of choice
      • Should only be administered if pregnancy is compromised (e.g., weight loss, sleeplessness)
      • If indicated, preferably not start until the 3rd trimester, when risk to the fetus is likely reduced

Enterobius, or pinworm, can cause perianal pruritus!

You could get sick if you EATTT Enterobius, Ascaris, Toxocara, Trichinella, Taenia!

References:[1][4][5][6][7]

Trichiuriasis

  • 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

Toxocariasis

You could get sick if you EATTT Enterobius, Ascaris, Toxocara, Trichinella, Taenia!

References:[1][8][9][10]

Trichinellosis

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


References:[11][1][12][13]

Hookworm infections (ancylostomiasis; necatoriasis)

In patients with microcytic anemia and recent travel to the tropics, consider hookworm infection!

Strongyloides, Ancylostoma, and Necator get into your feet from the SANd!

References:[1][14][15][16]

Strongyloidiasis

  • Pathogen
  • Mode of transmission: Larvae excreted in feces penetrate the skin, e.g., while a potential host is walking barefoot.
  • Life cycle: penetration and migration into the skinmigration of larvae via the bloodstream to the lungs → migration via the alveoli and bronchial system to the pharynxswallowing and autoinfection → larvae mature into adult, egg-producing worms in the intestine → eggs develop into “free-living” infectious larvae and are excreted in feces
  • Incubation period: 1–4 weeks
  • Clinical features
  • Complications: hyperinfection syndrome, possibly leading to organ dysfunction and septic shock, and occurring especially in immunosuppressed individuals (e.g., AIDS, steroid therapy)
  • Diagnosis
  • Treatment: ivermectin or bendazoles

Strongyloides, Ancylostoma, and Necator get into your feet from the SANd!

References:[1][17][18][19]

Onchocerciasis

  • Pathogen: Onchocerca volvulus
  • 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.

References:[20][21][22]

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 fly during a blood meal → Microfilariae mature into larvae, thus completing the cycle.
  • Clinical features
  • Diagnosis
  • Treatment
    • Diethylcarbamazine (DEC): drug of choice
    • Ivermectin: used in areas where onchocerciasis is prevalent, as DEC can worsen onchocercal eye disease
    • Elevation of the affected extremity, exercise, and wearing therapeutic shoes: recommended for those with lymphedema
    • Surgery for hydrocele

Taeniasis

Taeniae are tapeworms. Tapeworms are a group of worms also referred to as cestodes.

Intestinal taeniasis Cysticercosis
Pathogen
Mode of transmission
  • Ingestion of larvae (cysticerci) in raw or undercooked beef/pork
  • Fecal-oral ingestion of eggs from feces of humans with intestinal taeniasis
Clinical features
  • Often asymptomatic
  • Symptoms caused by adult worms in the intestinal tract: abdominal pain, anorexia, weight loss, nausea, and vomiting
Diagnosis
  • Initial test: complete blood cell (CBC) count may show eosinophilia (in only ∼ 45% of patients)
  • Confirmatory test: stool examination → detection of eggs and/or worms
  • Initial tests: as with intestinal taeniasis
  • Additional testing
  • Confirmatory test: serum enzyme-linked immunotransfer blot (EITB) assay
Treatment
  • Praziquantel
Prevention
  • Avoid raw pork meat and inspect for cysticerci
  • Adequately freeze and cook meat to destroy viable cysticerci
  • Proper disposal of human feces
  • Hand washing before meal preparation


Cysticercosis can cause cysts in the brain and seizures!

You could get sick if you EATTT Enterobius, Ascaris, Toxocara, Trichinella, Taenia!

References:[11][1][23][24][25][26][27]

Diphyllobothriasis (Diphyllobothrium latum or fish tapeworm infection)

Diphyllobothrium causes B12 deficiency!

References:[1][28][29]

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
    • Microscopic examination: detection of eggs in the stool or bile sample
    • CBC count: eosinophilia (marked in the acute phase)
    • Enzyme-linked immunosorbent assay (ELISA)
    • Endoscopy: adult flukes in the biliary tract
  • Treatment
    • Triclabendazole
    • ERCP in case of biliary obstruction

References:[30]