Hookworm infections

Morphology and differences between species

The diseases are caused by two morphologically similar nematode species: Ancylostoma duodenale (Old World hookworm, causes ancylostomiasis) and Necator americanus (New World hookworm, causes necatoriasis). Both helminths are pinkish-yellow with faint transverse cuticular striations. The esophagus is cylindrical, and the intestine is typically filled with blood.

Several key features are used for differential diagnosis:

• Buccal capsule: In Ancylostoma, it contains chitinous teeth (4 ventral and 2 dorsal). In Necator, teeth are replaced by two semilunar cutting chitinous plates. At the base of the buccal capsule are glands that secrete anticoagulants preventing blood clotting.
• Body shape and size: Ancylostoma is larger (females 10–13 mm, males 8–11 mm), with its anterior end curved ventrally. After death, the parasite takes the shape of the letter "C". Necator is smaller (females 9–10 mm, males 5–8 mm), curved dorsally, and the dead body forms a hook-like shape resembling the letter "S".
• Female reproductive system: In Ancylostoma, the genital opening is located at the border of the middle and posterior thirds of the body, and the female produces 10–25 thousand eggs per day. In Necator, the opening is shifted anteriorly, and productivity is lower—5–10 thousand eggs per day. The tail end is tapered in both, but Ancylostoma has a characteristic spine.
• Male reproductive system: Males differ in the structure of the copulatory bursa at the tail end. In Ancylostoma, it is wide, three-lobed, with two spicules. In Necator, the bursa is narrower, and two whip-like spicules about 1 mm long end in a hook shape.

Life cycle and migration in the body

Both species are geohelminths, meaning their development proceeds without an intermediate host. The lifespan of Ancylostoma in the intestine is 7–8 years, and that of Necator is up to 15 years.

Development in the external environment: Eggs of Ancylostoma and Necator are morphologically indistinguishable. They are passed with feces and mature in the soil in the presence of oxygen and at a temperature of 14–40 °C. The optimal humidity for Ancylostoma is 85–100%, and for Necator 70–80%.

1. In 1–2 days, a non-infective rhabditiform larva hatches from the egg (the esophagus has 2 bulbs) and feeds on organic matter.
2. After two molts, in 7–10 days, it turns into a filariform larva. This is the infective stage. It does not feed, has a cylindrical esophagus, and is capable of active movement in the soil.

Infection and migration: Penetration into the body is possible percutaneously (through the skin, more common for Necator) and orally (more common for Ancylostoma). If larvae penetrate through the skin, they undergo complex migration: via blood vessels they reach the right heart, then the pulmonary capillaries, alveoli, bronchioles, trachea, and pharynx. From there they are swallowed with saliva. Entering the small intestine, they molt twice and reach sexual maturity in 5–6 weeks. It is important to note that with oral infection, migration through the bloodstream does not occur.

Epidemiology

Ancylostomiasis and necatoriasis are widespread diseases, infecting more than 900 million people worldwide. Human susceptibility to these helminths is extremely high. The source of infection is always an infected person excreting eggs with feces.

Main endemic foci are formed in countries with a subtropical and tropical climate (between 45° N and 30° S), with about 1000 mm of precipitation per year and humidity maintained at 70%. At the same time, necatoriasis tends to occur more in humid subtropics. A unique feature of ancylostomiasis is the ability to form local foci in deep mines where a stably high temperature and humidity are artificially maintained. In addition to contact with contaminated soil, transplacental and transmammary (via milk) transmission routes are possible.

Class and exam questions

What else to study on this topic

• Pathogenic action and mechanism of anemia development (the role of hematophagy and anticoagulants).
• Symptoms of the migration phase: skin manifestations and eosinophilic pneumonia.
• Harada-Mori method: the principle of larva cultivation for species-specific diagnosis.
• Prevention of ancylostomiasis and sanitary soil protection in endemic regions.

Other topics in the "Medical Helminthology" section

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