Hookworm Infections

Definition, Etiology, PathogenesisTop

Hookworm infection is a parasitic disease of the human small intestine, caused by nematodes belonging to soil-transmitted helminths (geohelminths)—Ancylostoma duodenale and Necator americanus.

1. Etiologic agent: A duodenale and N americanus nematodes, commonly called hookworms. In some parts of the world, invasions with A ceylanicum, a hookworm typically found in animals, are also observed in humans.

Adult hookworms are 5 to 15 mm long. Female parasites daily lay 20,000 to 30,000 delicate, oval eggs (60-80 × 35-45 microm), which are excreted with feces by 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 invade humans.

2. Pathogenesis: After direct contact with the human body, filariform larvae release substances enabling them to penetrate the skin and reach the lungs through bloodstream. Then the larvae migrate from capillaries to alveoli and ascend with respiratory secretions through bronchi and the larynx to the pharynx. When swallowed, they travel with saliva to the target infection site—the small intestine, where they mature. In infections via the oral route, the larvae directly invade the small intestine, where they molt several times and transform into adult forms. They cut the mucosal epithelium with chitinous teeth, attach to its surface, and start to feed on the host’s blood.

Skin penetration is facilitated by release of anticoagulants, proteolytic enzymes, and immunomodulators, which may cause a local allergic reaction at the penetration site. Larvae migration through the lungs may trigger a toxic-allergic reaction. Inflammatory infiltrates develop in the lungs as a result of eosinophil accumulation.

3. Reservoir and transmission: Humans are the only reservoir of N americanus and A duodenale. Hookworms are geohelminths—parasites whose invasions are transmitted through soil contaminated with human feces. Infection can be contracted 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 invasive larvae.

4. Risk factors for infection: Performing activities that promote skin contact with soil during stay in regions endemic for hookworm infections, particularly walking barefoot and wearing shoes that do not fully cover the foot.

5. Incubation and contagious period: Pulmonary manifestations develop within 10 days of skin penetration by the larvae; parasite migration to the digestive tract lasts 3 to 5 weeks. Direct human-to-human transmission is impossible, as eggs excreted with feces are noninvasive.

EpidemiologyTop

Hookworm infections most frequently occur in tropical and subtropical climate zones. Endemic areas include sub-Saharan Africa, East Asia, Southeast Asia, Middle and South Americas, and Oceania. Endemic cases have also been reported in Southern Europe. The number of infected individuals is estimated at ~500 million.

Clinical Features and Natural HistoryTop

1. Local manifestations: Papular rash and pruritus may develop at the site of skin penetration.

2. Pulmonary manifestations: Cough, dyspnea, or other signs and symptoms of pneumonia or bronchitis (only in massive invasions).

3. Intestinal manifestations: Iron deficiency anemia and hypoproteinemia are observed in the intestinal phase; massive invasions can present with nausea, vomiting, weakness, bloody diarrhea, and weight loss.

DiagnosisTop

Diagnostic Tests

1. Identification of the etiologic agent:

1) Microscopic examination of stool samples (the Kato-Katz thick smear technique should be routinely used).

2) Stool culture using the Harada-Mori technique.

2. Other: Peripheral eosinophilia.

Diagnostic Criteria

Diagnosis is established based on the identification of A duodenale or N americanus eggs in the microscopic stool examination. A duodenale or N americanus eggs look identical, therefore stool samples should be cultured using the Harada-Mori technique if hookworm infection is suspected. This technique allows for imitation of tropical conditions (high temperature and moisture), observation of rhabditiform larvae, and identification of hookworm species based on parasite morphology.

Differential Diagnosis

1. Infections with other geohelminths (trichuriasis, ascariasis, strongyloidiasis).

2. Other causes of anemia.

3. Other causes of eosinophilia.

4. Other causes of infectious diarrhea.

TreatmentTop

Antiparasitic Treatment

1. First-line treatment: A single dose of mebendazole 500 mg, a single dose of albendazole 400 mg.

2. Alternative treatment: A single dose of pyrantel 11 mg/kg, a single dose of levamisole 150 mg.

ComplicationsTop

Massive infection in young children may cause delayed psychomotor development, malnutrition, and death. In pregnant women filariform larvae can cross the placenta; this may result in fetal damage, premature birth, and low birth weight.

PrognosisTop

Generally prognosis is good, as treatment is effective in the majority of cases. In countries where malnutrition is prevalent, hookworm infections may lead to death.

PreventionTop

Specific Prevention

None.

Nonspecific Prevention

1. Avoiding direct skin contact with soil (walking barefoot, sleeping or sitting on the ground or sand, working with soil without wearing protective gloves).

2. Proper utilization of sewage.

3. Banning the use of human feces to fertilize fruit or vegetable plantations.

4. Thorough washing of fruits and vegetables.