Hookworm Infections

Etiology and Pathogenesis

1. Etiologic agent: Hookworm infection is predominantly caused by 2 species of helminths, Ancylostoma duodenale and Necator americanus. In addition, A ceylanicum, a zoonotic species, contributes to hookworm infections primarily in Southeast Asia. Hookworms are soil-transmitted helminths (STHs), as transmission occurs through contact with contaminated soil. Adult hookworms are 5 to 15 mm long. Female parasites lay eggs, which are excreted with feces of an infected individual. At temperatures of 20 to 30 degrees Celsius, rhabditiform larvae hatch from eggs deposited in moist, shaded soil. After 5 to 10 days and subsequent molts, they transform into filariform larvae, which infect humans through skin invasion or ingestion. After penetrating tissues, the larvae enter the bloodstream and are carried to the lungs, where they penetrate the alveoli, ascend to the larynx, and are swallowed to eventually be deposited in the small intestine, where adult worms are found. Worms have cutting plates, or teeth, that allow them to attach to the intestinal wall and feed on blood from the host. N americanus has a lifespan of ~3 to 5 years, while A duodenale survives for ~1 year.

2. Reservoir and transmission: Humans are the only reservoir of N americanus and A duodenale. Infections are acquired through direct skin contact with soil (walking barefoot, sitting on the ground, working with soil without wearing protective gloves) or drinking water or consuming food containing larvae.

3. Risk factors: Performing activities that promote skin contact with soil in regions endemic for hookworm infections, particularly walking barefoot.

4. Incubation and contagious period: Pulmonary manifestations develop within 1 week to 3 weeks of skin penetration by the larvae. Eggs are excreted in stool months after skin penetration. Direct human-to-human transmission is impossible, as eggs excreted with feces are not infective. Infective larvae can survive in warm, moist soil for weeks but are killed by exposure to sunlight or desiccation.

Epidemiology

Hookworm infections most frequently occur in tropical and subtropical regions. N americanus accounts for the majority of infections and is found in Africa, China, Southeast Asia, and the Americas. A duodenale is endemic in the Mediterranean region, India, China, and North Africa. The number of infected individuals is estimated to be ~500 million globally, with the majority of cases being found in Southeast Asia and sub-Saharan Africa, particularly in rural areas with poor sanitation infrastructure. Locally acquired infections are rare in North America, although cases are reported in impoverished communities in the southern United States. Most cases are imported from tropical and subtropical regions.

Clinical Features and Natural History

Clinical symptoms are dependent on the stage of invasion and worm burden (the number of worms present in the host). Many individuals are asymptomatic, especially if they have a low worm burden.

1. Skin invasion: Pruritic and erythematous rash can occur at the site of skin penetration, most often on the hands and feet. The rash can have a serpiginous pattern and is often maculopapular, although vesicles can be present.

2. Larval migration: Pulmonary manifestations may occur as a result of larval migration through the lungs, with symptoms including cough, dyspnea, or other manifestations of pneumonia or bronchitis. Pulmonary symptoms are more likely to occur with higher numbers of migrating larvae. Fever and eosinophilia can occur during this phase.

3. Intestinal manifestations: Abdominal pain, malaise, diarrhea, and eosinophilia. Iron deficiency anemia is dependent on the worm burden (more common in moderate to severe infections) and is also more common in children, pregnant women, and those with preexisting nutrient deficiencies. Hypoproteinemia occurs in severe cases and can result in associated anasarca and abdominal distension with ascites.

Diagnosis

Diagnostic Tests

Standard ova and parasite examination of stool for microscopic detection of hookworm eggs in stool. For increased sensitivity ≥3 stool samples should be collected within ≤10 days (ideally every 48 h). Stool should be collected in a preservative (eg, sodium acetate-acetic acid-formalin [SAF]) to prevent eggs from hatching prior to analysis. Eggs will not be detected in cases where only adult male or developing larval stages are present.

Differential Diagnosis

Symptoms consistent with pneumonitis can be found in other migratory parasites including those causing toxocariasis, ascariasis, strongyloidiasis, schistosomiasis, and tropical pulmonary eosinophilia.

Other parasites that can cause anemia and eosinophilia include Schistosoma species (schistosomiasis), Fasciola species (fascioliasis), other liver flukes, and Strongyloides species (strongyloidiasis).

Treatment

Antiparasitic Treatment

1. First-line treatment: Oral albendazole 400 mg/d for 3 days. Single-dose regimens are less effective than 3-day regimens.

2. Alternative treatment: Oral mebendazole 100 mg bid for 3 days or oral pyrantel pamoate 11 mg/kg (max 1 g) daily for 3 days (this may not be effective for hookworms from north-western Australia).

Iron deficiency anemia should also be treated with oral iron supplementation.

Complications

Massive infection in young children may cause severe iron deficiency anemia as well as growth and cognitive delays. Hookworm infection in pregnancy, especially in resource-poor settings, can result in increased maternal morbidity and neonatal mortality.

Prevention

1. Avoiding direct skin contact with soil (wearing shoes).

2. At the community level prevention involves providing sanitation facilities, avoiding open defecation, avoiding the use of human feces as a fertilizer, and ensuring access to clean drinking water.