• Clinical science

Helminth infections (Helminthiasis…)

Abstract

Helminth infections are caused by worms of various species and are typically transmitted in one of three ways: ingestion of parasitic eggs or larvae in contaminated food, water, or feces; penetration and entry of worms through the skin; or entry via the bite of a vector species, such as a fly. Helminths are classified based on their macroscopic appearance as nematodes (= roundworms, e.g., Toxocara, Enterobius, Ascaris, Trichinella, Strongyloides, Ancylostoma, Necator), cestodes (= tapeworms, e.g., Taenia, Diphyllobothrium, Echinococcus), or trematodes (= flukes, e.g., Schistosoma). Common gastrointestinal complaints associated with helminth infection include abdominal pain, nausea, and diarrhea. The larvae of several worms, such as Ascaris and Ancylostoma, migrate from the intestines via the portal vein to the lungs and can cause asthma-like symptoms (e.g., dry cough, wheezing). Pulmonary symptoms may occur as a result of the "lung phase" of these particular worms, but systemic eosinophilia induced by helminth infection is also believed to be a contributing factor. Other worms, such as Taenia solium, can potentially colonize human tissues, leading to severe and life-threatening complications (e.g., neurocysticercosis). Echinococcosis and schistosomiasis are discussed in separate learning cards. 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 food poisoning.

Toxocariasis

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

References:[1][2][3][4]

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][5][6][7][8]

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][9][10]

Trichinellosis

  • Pathogen
  • Mode of transmission: fecal-oral; consumption of undercooked meat (especially pork) containing encysted larvae
  • Life cycle: ingestion of meat containing cysts → larvae invade the small bowel mucosa and develop into adult worms → larvae released by adult worms migrate to muscles and encyst
    • Sylvatic cycle: Wild animals contract Trichinella by scavenging/predation (consumption of infected meat).
    • Domestic cycle: Domestic animals contract Trichinella by consuming infected meat scraps.
  • Incubation period: 7–30 days
  • Clinical features
    • Intestinal phase : abdominal pain, diarrhea, nausea, and vomiting
    • Muscle phase
      • Myositis: myalgia, muscle swelling, weakness
      • Periorbital edema
      • Other symptoms include: fever, rash, splinter hemorrhages, retinal and conjunctival hemorrhages, chemosis
  • Diagnosis
  • Treatment: bendazoles
  • Complications

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

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

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

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 under microscope of 3 consecutive samples → detection of eggs and/or worms (2–3 months after initial infection)
  • Initial tests: as with intestinal taeniasis
  • Additional testing
    • Imaging: MRI/CT showing multiple, small (< 1 cm) cystic ring-enhancing lesions in the brain with a membranous wall
      • + often 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
  • 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][14][15][16][17][18]

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][19][20][21]

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 skin; migration 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-livinginfectious 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][22][23][24]

Diphyllobothriasis (Diphyllobothrium latum or fish tapeworm infection)

Diphyllobothrium causes B12 deficiency!

References:[1][25][26]