Hookworm Infections

Intestinal hookworm infections that affect humans are caused mainly by Necator americanus and Ancylostoma duodenale. Millions of people are infected around the world, mainly in tropical regions, where warm and moist environments facilitate larva survival in the soil. Transmission is via dermal penetration by the larvae. From entry, the parasite undergoes a transpulmonary passage, reaching the trachea and pharynx, where it is swallowed. In the small intestine, worms mature and attach to the duodenum. Diarrhea, nausea, and vomiting are GI symptoms. Blood loss (leading to anemia) and subsequent malnutrition are complications. Diagnosis is by stool microscopy showing the hookworm eggs and by PCR. Management targets prevention through proper sanitation and regular deworming of high-risk groups. Treatment involves the use of anti-parasitic medications, with iron supplements for anemia.

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Overview

General characteristics

  • Hookworms:
    • Average length of adult worm: 10 mm
    • Head is often curved in the opposite direction of the body: worms have a “hooked” appearance.
    • Male worms have a unique copulatory bursa instead of the curved, pointed tail seen in other intestinal nematodes.
  • Major species of hookworms that cause human infections:
    • Ancylostoma duodenale:
      • 4 sharp tooth-like structures
      • Life span: 1 year
    • Necator americanus (New World hookworm):
      • Dorsal and ventral cutting plates
      • Life span: 3–5 years
  • Other species: 
    • A. ceylanicum: 
      • Infects dogs and cats
      • Cause of zoonotic infections in Asian and South Pacific regions
    • A. caninum
    • A. braziliense

Epidemiology

  • Approximately 500 million people are infected by hookworm around the world every year.
  • N. americanus is the causative agent worldwide.
  • A. duodenale is usually found in:
    • Mediterranean region
    • Northern India
    • China
  • Preference for tropical climates

Host risk factors

  • Children
  • Pregnancy
  • Low socioeconomic background
  • Walking barefoot
  • Poor sanitation
  • Poor personal hygiene

Pathogenesis

Reservoirs and transmission

  • Reservoirs: 
    • A. duodenale and N. americanus: human
    • A. ceylanicum: dogs and cats
    • A. caninum: dogs
  • Modes of transmission:
    • Direct penetration of skin
    • Ingestion of larvae (e.g., contaminated water supply)
  • Factors facilitating transmission:
    • Fecal contamination of the soil
    • Moist, warm, and shady environment in which the larvae survive
    • Human skin contact

Life cycle

  • External environment:
    • Hookworm eggs contaminate soil.
    • Eggs hatch in moist soil → rhabditiform larvae (live for 3–4 weeks) → mature into filariform larvae (can penetrate skin)
  • Humans:
    • Larvae penetrate the skin (typically on the feet) and enter the bloodstream.
    • Enter right side of the heart → pulmonary vessels → alveoli → bronchial tree → trachea → pharynx → swallowed into the intestine
    • Attach onto duodenal mucosa → adult hookworms mature
    • Adult females → lay up to 30,000 eggs per day → fecal ejection

Disease process

  • Dermal penetration and migration by proteolytic enzymes:
    • N. americanus → protease → break down elastin and collagen
    • A. duodenale → Ancylostoma–secreted proteins (development of parasite) + hyaluronidase enzyme (breach dermal integrity)
    • Penetration leads to skin irritation.
  • Transpulmonary passage: can cause respiratory symptoms including pharyngeal/airway irritation and eosinophilic inflammation
  • Duodenal attachment and blood consumption:
    • Teeth or cutting plates on buccal capsule → attaches to the host’s intestinal mucosa
    • Metalloprotease and anticoagulant peptides (inhibit activated factor X and factor VIIa/tissue factor complex) aid in digesting blood.
    • Blood consumption by worms and blood leakage → anemia (blood loss) and hypoalbuminemia/malnutrition (loss of albumin)
      • N. americanus consumes 0.3 mL/day.
      • A. duodenale consumes 0.5 mL/day.
  • Immunosuppression: 
    • Parasite protease inhibitors → neutralize host proteolytic enzymes and reduce host nutritional absorption
    • Parasite induces apoptosis of T lymphocytes → T cell hyporesponsiveness

Clinical Presentation

Dermal penetration

  • At time of penetration → localized reaction (“ground itch”)
    • Often affects area between the toes
    • Resolves within a few days
  • Cutaneous larvae migrans: erythematous papule → characteristic serpiginous pattern (tunnels beneath the skin)

Pulmonary stage

  • Symptoms:
    • Can be asymptomatic
    • Cough or sneezing
    • Bronchitis
    • Hemoptysis
    • Loeffler syndrome: eosinophilic pneumonia
  • Usually self limiting

Gastrointestinal stage

  • Abdominal cramps or pain
  • Nausea, vomiting, diarrhea
  • Flatulence
  • GI bleeding in severe cases (melena or fecal occult blood)

Nutritional impairment

  • Failure to thrive (mental and physical development of children adversely affected)
  • Iron deficiency anemia (dependent on worm burden)
  • Edema/anasarca (due to hypoalbuminemia)
  • Chronic infection in pregnant women: low birth weight

Diagnosis and Management

Diagnosis

  • Stool microscopy: 
    • Identify eggs.
    • Fecal eggs are detectable about 8 weeks (in N. americanus infection) and up to 38 weeks (in A. duodenale infection) after skin penetration.
    • Prior to the GI stage, stool studies are not helpful.
  • PCR of stool
  • CBC:
    • Eosinophilia: a major finding suggestive of parasitic infection
    • Microcytic anemia (from iron deficiency)
    • Low albumin

Diagnostic characteristics of hookworm eggs include a thin shell, which is oval or ellipsoidal in shape.

Image: “4825” by CDC. License: Public Domain

Management

Medical management:

  • Antiparasitic medication:
    • Mebendazole (multiple dose)
    • Albendazole (single dose)
    • Pyrantel pamoate
  • Anemia
    • Iron supplement (especially for pregnant or lactating women)
    • Blood transfusion when indicated
  • Nutritional support

Prevention:

  • Key, as reinfection is common
  • Hygiene (proper handling of food, safe drinking water, handwashing)
  • Use of appropriate footwear
  • Sanitary disposal of human feces
  • Regular deworming of high-risk groups

Differential Diagnosis

  • Malabsorption: the inability of the intestinal wall to absorb the broken-down products of food. Manifestations include weight loss, diarrhea, weakness, anemia, and fatigue. Hookworm infections cause malabsorption, usually presenting with nutritional deficiencies due to iron and protein loss. Other malabsorptive disorders that should be considered include celiac disease and inflammatory bowel disease. Differentiation of other causes is important, as treatment is specific to the underlying cause.
  • Ascariasis: an infection caused by the parasitic roundworm Ascaris lumbricoides. Transmission occurs via ingestion of contaminated water or food. Most patients are asymptomatic. If symptoms do occur, they can be mild, with only abdominal discomfort, or severe, causing intestinal obstruction. Other symptoms, such as cough, are due to the migration of the worms through the body.
  • Trichinellosis: caused by a Trichinella infection, commonly due to T. spiralis (found in pigs). Transmission is through the ingestion of undercooked meat. Once ingested, the parasite moves to the intestine. Patients may have GI symptoms similar to hookworm infections. Spread through the bloodstream follows, and larvae reach striated muscles. Systemic symptoms include fever, chills, myalgias, and periorbital edema. Diagnosis is by serological examination and is confirmed by muscle biopsy. Systemic disease is treated with antiparasitic medications and corticosteroids.
  • Strongyloidiasis: a disease caused by the roundworm (nematode) Strongyloides. Strongyloidiasis has various clinical manifestations including GI symptoms and eosinophilia. Diagnosis is via serology.

References

  1. Feldmeier, H., Schuster, A. (2012). Mini review: Hookworm-related cutaneous larva migrans. Eur J Clin Microbiol Infect Dis. 31(6),915–918. https://pubmed.ncbi.nlm.nih.gov/21922198/
  2. Ghodeif, A.O., Jain, H. (2021). Hookworm. StatPearls. Retrieved April 3, 2021, from: https://www.ncbi.nlm.nih.gov/books/NBK546648/
  3. Loukas, A., et al. (2016). Hookworm infection. Nat Rev Dis Primers. 2(16088). https://pubmed.ncbi.nlm.nih.gov/27929101/
  4. Jiraanankul, V., et al. (2011). Incidence and risk factors of hookworm infection in a rural community of central Thailand. Am J Trop Med Hyg. 84(4),594–598. https://pubmed.ncbi.nlm.nih.gov/21460016/
  5. Riedel, S., et al. (Eds.). (2019). Jawetz, Melnick, & Adelberg’s Medical Microbiology, 28e. McGraw-Hill.
  6. Speich, B., et al. (2016). Efficacy and reinfection with soil-transmitted helminths 18-weeks post-treatment with albendazole-ivermectin, albendazole-mebendazole, albendazole-oxantel pamoate and mebendazole. Parasit Vectors. 9(123). https://pubmed.ncbi.nlm.nih.gov/26935065/
  7. Weller, P., Leder, K. (2019). Hookworm infection. UpToDate. Retrieved April 3, 2021, from https://www.uptodate.com/contents/hookworm-infection

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