NAME: Necator americanus (“American murderer”)
SYNONYM OR CROSS REFERENCE: New world hookworms, necatoriasis, hookworm disease or infection Footnote 1.
CHARACTERISTICS: Necator americanus belongs to the family Ancylostomatidae and subfamily Bunostominae Footnote 2. N. americanus is an intestinal nematode parasite that infects humans. Adult male worm measures 7 to 9 by 0.4-0.5 mm, while the female is 9 to 11 mm long by 0.4-0.5 mm wide. The buccal cavity which is in the opposite orientation to the curvature of body consists of cutting plates and gives the worm a characteristic hook appearance Footnote 3, Footnote 4. Eggs have a thin shell and are oval shaped, measuring ~56-74 by 36-40 mm. A clear space is present between the cells and the shell. Eggs are passed in feces and then embryonate and hatch within 1-2 days in soil as first stage rhabditoid larvae. First stage larvae develop to third stage, infectious, filariform larvae in approximately 1 week Footnote 2, Footnote 4-Footnote 6.
PATHOGENICITY/TOXICITY: N. americanus (third-stage) larvae follow thermal gradients and are guided by heat to human skin where they may irritate or invade the skin Footnote 7.
A pruritic, erythematous, papular rash develops and produces what is known as “ground itch” around penetration sites of the infective larvae, usually on the hands and feet Footnote 2, Footnote 4. Invasive infection occurs when larvae enter the bloodstream and are carried to the lungs. A mild cough and pharyngeal irritation may occur during larval migration in the airways; however, passage through the lungs is usually asymptomatic Footnote 8. The larvae are swallowed down the oesophagus and migrate to the gastrointestinal tract. Larvae then hook onto the intestinal mucosa where they mature into adult hookworms by feeding on blood. The major-hookworm related injury occurs due to intestinal blood loss, which can lead to iron-deficiency anaemia in moderate to heavy infections. Patients with light hookworm burden (lower severity of infection) tend to be asymptomatic, while those with moderate to heavy hookworm burden may experience symptoms such as epigastric pain and tenderness, nausea, exertional dyspnea, pain in lower extremeties and in joints, sternal pain, headache, fatigue, and impotence. Fatality is relatively rare; however, infection can cause discomfort and disability Footnote 9.
EPIDEMIOLOGY: N. americanus is the most common human gastrointestinal parasite, responsible, with A. duodenale, for an estimated 740 million cases of hookworm infection in poor regions of the tropics and subtropics with the greatest incidence of hookworm infections occurring in Asia and sub-Saharan Africa Footnote 2, Footnote 4. Generally, infection rates are high in areas with warmer temperatures and greater rainfall. Mortality rate, worldwide, attributed to deaths occurring through direct hookworm infection, is estimated to be ~65,000 deaths annually Footnote 9. However, morbidity and disability caused by hookworm infection tends to be much higher than mortality rate. Prevalence of infection tends to increase with age, and is highest between the ages of 6 and 10 years, and then reaches a plateau during adulthood. Similarly, the mean intensity of infection also tends to increase progressively with age. Prevalence of infection tends to be higher in males; however, females (particularly pregnant women and mothers) and infants are most vulnerable to chronic blood loss by hookworm infection since they already have low iron stores Footnote 2, Footnote 4.
INFECTIOUS DOSE: Unknown; however, normal human volunteers infected with 10 N. americanus larvae, were positive by stools examination after 5 weeks post infection Footnote 12.
MODE OF TRANSMISSION: Transmission of N. americanus larvae occurs through exposed skin, by environmental acquisition from soil, feces, and other contaminated surface Footnote 4. Vertical transmission among neonates is also possible Footnote 1.
INCUBATION PERIOD: Following penetration, N. americanus migrates to the lungs within about 10 days Footnote 2. After 3 to 5 weeks, it passes through the gastrointestinal tract and attaches to the intestinal mucosa, where it matures into an adult worm and may stay for up to ~5 years Footnote 9. The average period between larva penetration and egg production (prepatent period) is ~ 4 to 8 weeks Footnote 2.
COMMUNICABILITY: Not directly transmitted from human to human.
RESERVOIR: Infected humans and possibly pigs Footnote 11.
DRUG SUSCEPTIBILITY/RESISTANCE: N. americanus is susceptible to benzimidazoles such as albendazole and mebendazole; however, N. americanus is suspected to have developed resistance to some degree towards mebendazole in some regions including: Mali, South Africa, New Zealand, and Australia Footnote 3. N. americanus is also resistant to ivermectin, which is effective against many helminths.
SUSCEPTIBILITY TO DISINFECTANTS: N. americanus larvae are susceptible to chlorinated hydrocarbons (tetrachloroethylene), 70% ethanol for 10 minutes, and 0.5% Dettol® for 20 minutes Footnote 13, Footnote 14. Sodium hypochlorite (1%) and glutaraldehyde (2%) are not effective Footnote 14.
PHYSICAL INACTIVATION: N. americanus larvae are susceptible to desiccation and freezing Footnote 15. They have lower survival times and physical resistance outside the human host compared to Ancylostoma duodenale (the other major hookworm, from the same family as N. americanus, that infects humans) Footnote 7.
SURVIVAL OUTSIDE HOST: N. americanus larvae can survive on a variety of surfaces such as agar, membrane filters, metal surfaces, various fabrics, paper, and human skin, as long as an adequate water film is present Footnote 7. The infective larvae can survive for several weeks under suitable conditions of moist, warm (temperatures around 30 °C), and shaded soil Footnote 16. Sandy soils (sandy loam) are most favoured by infective larvae Footnote 4.
SURVEILLANCE: Monitor for symptoms of infection/disease. The most common method of diagnosis involves stool analysis for presence of eggs which provides an indirect measure of infection Footnote 2, Footnote 4, Footnote 10. Quantitative egg counts include Kato-Katz test, concentration test, Beaver direct egg count, Stoll dilutional egg count, and McMaster techniques Footnote 17; and PCR-based tests, with PCR-based tests being the most sensitive.
Note: All diagnostic methods are not necessarily available in all countries.
FIRST AID/TREATMENT: Administer appropriate drug therapy (benzimidazole anthelminthics) Footnote 2. Pregnant women should not be treated with benzimidazole drugs during first trimester, and pyrantel can be used instead. In case of iron-deficient anaemia, administration of iron supplementation is recommended.
IMMUNISATION: No vaccine currently available; however, a vaccine, N. americanus Secreted Protein-2 (NA-ASP-2) Hookworm Vaccine, is currently under preclinical and initial clinical testing Footnote 9. NA-ASP-2 vaccine aims to provide immunity (neutralizing antibodies) against the larval stage of hookworm infection.
PROPHYLAXIS: Hygiene measures such as hand washing, drinking safe water, properly cleaning and cooking food and wearing shoes Footnote 8.
LABORATORY-ACQUIRED INFECTIONS: No cases of laboratory-acquired have been reported to date.
PRIMARY HAZARDS: Primary hazards include: ingestion of material contaminated with feces or stool, skin exposure to contaminated samples or surfaces, and/or droplet exposure.
SPECIAL HAZARDS: None
RISK GROUP CLASSIFICATION: Risk Group 2.
CONTAINMENT REQUIREMENTS: Containment Level 2 facilities, equipment, and operational practices for work involving infectious or potentially infectious materials, animals, or cultures.
PROTECTIVE CLOTHING: Lab coat. Gloves when direct contact with infected materials or animals is unavoidable. Eye protection must be used where there is a known or potential risk of exposure to splashes Footnote 18.
OTHER PRECAUTIONS: All procedures that may produce aerosols or involve high concentrations or large volumes should be conducted in a biological safety cabinet (BSC). The use of needles, syringes, and other sharp objects should be strictly limited. Additional precautions should be considered with work involving animals or large scale activities Footnote 18.
SPILLS: Allow aerosols to settle and, wearing protective clothing, gently cover spill with paper towels and apply an appropriate disinfectant, starting at the perimeter and working towards the centre. Allow sufficient contact time before clean up.
DISPOSAL: Decontaminate all wastes that contain or have come in contact with the infectious organism before disposing by autoclave, chemical disinfection, gamma irradiation, or incineration.
STORAGE: The infectious agent should be stored in leak-proof containers that are appropriately labelled.Footnote 18.
UPDATED: December 2011
PREPARED BY: Pathogen Regulation Directorate, Public Health Agency of Canada.
Although the information, opinions and recommendations contained in this Pathogen Safety Data Sheet are compiled from sources believed to be reliable, we accept no responsibility for the accuracy, sufficiency, or reliability or for any loss or injury resulting from the use of the information. Newly discovered hazards are frequent and this information may not be completely up to date.
Public Health Agency of Canada, 2011