Intestinal Nematodes

Intestinal Nematodes

Jorge Cardenas-Alvarez, MD

Department of Medicine, Division of Infectious Diseases, Columbia University Irving Medical Center, New York, United States

Author: Jorge Cardenas-Alvarez

Peter Hotez, MD, PhD, FASTMH

Professor of Pediatrics and Molecular Virology & Microbiology. Dean, National School of Tropical Medicine. Baylor College of Medicine. Texas, United States.

Editor: Peter Hotez

Section Contents

  1. Soil-transmitted helminths
    1. Ascaris lumbricoides (roundworm)
      1. General
      2. Life Cycle
      3. Clinical Presentation
      4. Diagnosis
      5. Treatment
    2. Hookworms (Ancylostoma duodenale, Necator americanus)
      1. General
      2. Life Cycle
      3. Clinical Presentation
      4. Diagnosis
      5. Treatment
    3. Trichuris trichiura (whipworm)
      1. General
      2. Life Cycle
      3. Clinical Presentation
      4. Diagnosis
      5. Treatment
  2. Enterobius vermicularis (pinworm)
    1. General
    2. Life Cycle
    3. Clinical Presentation
    4. Diagnosis
    5. Treatment & Management
  3. Other intestinal nematodes of minor medical importance
    1. Anisakis spp.
    2. Oesophagostomum spp.
    3. Capillaria spp.
  4. Assessment: Did I Get It? (DIG-IT)
  5. Other Media Resources (Optional)
  6. References

Soil-transmitted helminths

Soil-transmitted helminths (STHs) or Geohelminthiasis are a group of intestinal nematodes that are transmitted by either ingestion or skin penetration of infective parasitic stages, and requires fecally contaminated soil as part of its life cycle. They are all found primarily in tropical and subtropical areas, and are highly associated with poverty, poor sanitary conditions, and low socioeconomic status.

In this module we will review: Ascaris lumbricoides (roundworm), Ancylostoma duodenale & Necator Americanus (hookworms), and Trichuris trichiura (whipworm). Strongyloides stercoralis (threadworm) is also a STH, but it has a lot of unique features, so we gave it a separate module.

Ascaris lumbricoides (roundworm)

General

Ascaris is the most common intestinal parasitosis worldwide. In 2021, Global Burden of Disease data from the Institute for Health Metrics and Evaluation (IHME) estimated about 294 million infections worldwide. Ascariasis is transmitted via fecal-oral contamination with infective eggs. Children are disproportionately affected. They are unique in that adult females can reach up to 30-40 cm in length and live in the small intestine.

Life Cycle

Please click below for a video on the life cycle:

Clinical Presentation

Most infections are asymptomatic +/- eosinophilia! When symptoms are present, they depend on the stage of the infection:

  • Migratory stage (due to larval migration) → 1-2 weeks after initial exposure, individuals with a high parasitic burden -usually ingestion of hundreds of eggs-can experience interstitial pneumonitis, Loeffler’s syndrome (cough, fever, eosinophilia), migratory pulmonary infiltrates, and an asthma-like syndrome. Most symptoms improve after 1-2 weeks.
  • Intestinal stage (due to adult parasites) → dyspepsia-like symptoms (e.g., abdominal pain, bloating, nausea), diarrhea. Depending on the parasitic burden, worms can be released through natural orifices (e.g. with cough, with stool).
    • Complications: due to mechanical obstruction of the intestinal or the biliary lumen.
      • Biliary obstruction - cholecystitis, cholangitis, pancreatitis.
      • Appendicitis.
      • Intestinal obstruction (only in heavy infections).
Do you know which STHs are associated with Loeffler's syndrome?

Answer

Loeffler’s syndrome is seen in Ascaris spp., Strongyloides spp., and Hookworms (Ancylostoma duodenale, Necator americanus)

Diagnosis

Ascariasis can be diagnosed through:

  • Direct visualization of adult worms: if expelled during coughing spells or in stool.
  • Stool microscopy: may show Ascaris eggs - concentration techniques (e.g., Kato-Katz) are preferred. Note that infections with only male worms will not produce any eggs.

For more information on the morphology of the eggs, larvae, and adults, please visit the Parasites Wonders Blog.

Treatment

Albendazole x 1 (mebendazole is an alternative).

  • If Loeffler’s syndrome: consider steroids before giving anti-helminthic therapy.
  • If intestinal/biliary obstruction: surgical or endoscopic therapy may be needed.

Hookworms (Ancylostoma duodenale, Necator americanus)

General

Unlike Ascaris, hookworms are transmitted via skin penetration of infectious larvae. In 2021, Global Burden of Disease data from IHME estimated that approximately 207 million people were affected by hookworms worldwide. Female worms can reach up to 9-13 mm (very different from 30-40 cm in Ascaris) and live in the small intestine. Necator americanus is responsible for roughly 3/4 of all hookworm cases.

Life Cycle

Please click below for a video on the life cycle:

Clinical Presentation

Most infections are asymptomatic +/- eosinophilia! When symptoms are present, it depends on the stage of the infection:

  • Migratory stage (due to larval migration) →
    • Cutaneous symptoms: “Ground itch”: mild pruritic papular rash at the site of larval penetration.
    • Respiratory symptoms (less common than Ascaris): 1-2 weeks after initial exposure, individuals with a high parasitic burden -usually infection with hundreds of larvae- can experience interstitial pneumonitis, Loeffler’s syndrome (cough, fever, eosinophilia), migratory pulmonary infiltrates, and asthma-like syndromes. Most symptoms improve after 1-2 weeks.
  • Intestinal stage (due to adult parasites) → individuals can have non-specific GI symptoms (e.g., abdominal pain, nausea, vomiting, diarrhea). Intestinal symptoms are often due to concurrent infections.
    • Complications:
      • Iron deficiency anemia.

Diagnosis

Hookworm infection can be diagnosed through:

  • Direct visualization of adult worms: can be seen incidentally during endoscopy.
  • Stool microscopy: may show hookworm eggs (indistinguishable amongst each other).  Concentration techniques (e.g., Kato-Katz) are preferred.
Note: It is very common for individuals to be co-infected with multiple STHs, because most of them share similar mechanisms of transmission and risk factors

Treatment

Albendazole (mebendazole is an alternative). Although one dose is recommended in most textbooks, for U.S. patients, some experts favor three daily doses of Albendazole.

  • If Loeffler’s syndrome: consider steroids before giving anti-helminthic therapy.
  • If anemia: rule out other causes, but consider iron supplementation.

Trichuris trichiura (whipworm)

General

Similar to Ascaris, whipworms are transmitted via fecal-oral contamination with infective eggs. In 2021, Global Burden of Disease data from IHME estimated that approximately 204 million people worldwide were affected by whipworm. Children are disproportionately affected. Female worms can reach up to 40-50 mm, and live in the colon.

Life Cycle

For a video on the life cycle:

Clinical Presentation

Most infections are asymptomatic +/- eosinophilia! Eosinophilia, if present, is typically mild. When symptoms are present, it can include:

  • Intestinal stage (due to adult parasites) → abdominal pain, tenderness, diarrhea (can be dysenteric).
    • Complications: (rare)
      • Anemia: long-standing damage to the mucosa can lead to chronic small blood losses.
      • Trichuris dysentery syndrome (massive infantile trichuriasis): associated with chronic mucoid diarrhea, rectal bleeding, and finger clubbing.
      • Rectal prolapse: if prolonged rectal tenesmus.

Diagnosis

Trichuriasis can be diagnosed through:

  • Direct visualization of adult worms: can be seen incidentally during colonoscopy.
  • Stool microscopy: looking for Trichuris eggs. Concentration techniques (e.g., Kato-Katz) are preferred.

For more information on the morphology of the eggs, and adults, please visit the Parasites Wonders Blog.

Treatment

Albendazole x 3 days (mebendazole is an alternative).

Enterobius vermicularis (pinworm)

General

Enterobius is sometimes considered a STH; however, unlike the other parasites we have reviewed so far, eggs do not necessarily need soil exposure to become infectious - so we decided to separate it. Children are disproportionately affected. Female worms can reach up to 10 mm, and live in the colon.

Life Cycle

Clinical Presentation

Most infections are asymptomatic! When symptoms are present, it can include:

  • Anal pruritus: typically nocturnal (when adults lay eggs) and can cause sleep disturbances.
    • Complications:
      • Secondary bacterial infections: from scratching in the perianum.
      • Vaginitis: with scratching, worms can aberrantly migrate to the vagina.
What are some causes of anal pruritus?

Answer

Parasitic causes (Enterobius vermicularis, and less frequently, Taenia saginata). Others causes include hemorrhoids, anal fissures and neoplasias.

Diagnosis

Enterobiasis can be diagnosed through:

  • Graham test (tape test): should be done first thing in the morning before hygiene or bowel movement. Apply a wooden paddle covered with the adhesive side of tape outward to the margin of the anus. Place tape onto the slide. Place the slide under a microscope at 10X.
  • Direct visualization of adult worms: can be seen in the perianal region only in heavy infections.

For more information on the morphology of the eggs, and adults, please visit the Parasites Wonders Blog.

Treatment & Management

Pyrantel pamoate, repeat at 2 weeks (albendazole/mebendazole are alternatives).

  • Anti-helminthic therapy only kills adults. Therefore, treatment after 2 weeks is needed to kill new adult worms that were not killed while they were eggs.
  • Because eggs can persist in linen or clothing, please advise your patients to shower first thing in the morning (to avoid contamination of linen or clothing with eggs during the day).
  • Ask them to remove linens, clothing and underclothing and to thoroughly wash in hot water (to kill all eggs).
  • Avoid shaking linen, clothing, and underclothing as eggs can be (rarely) airborne.
  • Discuss hand hygiene, and nail picking & perianal scratching avoidance.
Incubation Period Adult lifespan Time needed for eggs/larvae to mature in the environment
Ascaris lumbricoides <1 month 1-2 years Weeks - eggs
Necator americanus & Ancylostoma duodenale <1 month 1-2 years Days - larvae
Trichuris trichiura <1 month -can be longer ~1 year Weeks - eggs
Enterobius vermicularis 1-2 months <1 year Hours - eggs
Strongyloides stercoralis <1 month Decades Days to weeks - larvae

Table 1. Important timeline of most clinically relevant intestinal nematodes (use this for study reference)

Most Common Presentation Hallmarks of clinical presentation Transmission Infective Stage Definitive habitat Drug of choice
Ascaris lumbricoides Asymptomatic Intestinal or biliary obstruction Fecal-oral Eggs (embryonated) Small intestine Albendazole
Necator americanus & Ancylostoma duodenale Asymptomatic Iron deficiency anemia Cutaneous L3 larvae Small intestine Albendazole
Trichuris trichiura Asymptomatic Rectal tenesmus & prolapse Fecal-oral Eggs (embryonated) Large intestine Albendazole
Enterobius vermicularis Asymptomatic Anal pruritus Fecal-oral Eggs (embryonated) Large intestine Pyrantel pamoate or Albendazole
Strongyloides stercoralis Asymptomatic Hyperinfection & Disseminated syndrome Cutaneous
Autoinfection		 L3 larvae Small intestine Ivermectin

Table 2. Summary of most clinically relevant intestinal nematodes

Other intestinal nematodes of minor medical importance

We wanted to give an honorable mention to three intestinal nematodes, that unfortunately did not make the cut for a complete module, but we still want you to know they exist. They may show up in your CTropMed/DTM&H examinations!

Assessment: Did I Get It? (DIG-IT)

Assessment: Did I Get It? (DIG IT)

DIG ITs are online modules designed to reinforce key learning points for you! Please choose the best answer, then check all of the answer choices for more learning pearls

Begin Assessment

Other Media Resources (Optional)

References

This lesson was last updated May 23 2025


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