Hookworm disease or infection is caused by hookworm’s (intestinal nematodes) that invade the intestinal walls of humans. Intestinal worms or hookworms cause disease by attaching to the intestines of their human host, and they cause loss of blood in the process. The infection is usually characterized by a loss of blood (anaemia) in the infected individual- especially in cases where there is heavy and prolonged hookworm infection in the affected individual. Hookworm disease is a widespread parasitic infection especially in the tropical and subtropical parts of the world where the disease is endemic and affects millions of the population. Human infection with hookworms is usually via the soil-borne route in which the soil becomes heavily contaminated with the parasite. Hookworms gain entry into the body via skin penetration especially when susceptible human hosts work or walk barefoot on hookworm-infected soil.
The name “hookworm” was actually derived from the morphological appearance of the worms i.e. the anterior end of the body of the worm is curved dorsally; and this gives the parasite a hook-like look – thus the name hookworm. Adult hookworms are cylindrical and they constrict anteriorly and have a curved-like appearance. Hookworms are usually well adjusted to their human hosts; and in large numbers they colonize the intestines of their hosts to cause growth retardation, malnutrition and severe anaemia (due to the blood-sucking activity of their curved anterior ends) especially in children living in endemic regions.
Hookworm disease is caused by two types of intestinal worms:
- Ancylostoma duodenale – This hookworm specie is commonly found in North Africa, Asia, South America, Southern Europe, Japan, China and parts of the Mediterranean and India.
- Necator americanus – This hookworm species is found in North America, Africa, India, Brazil, Pacific Islands, China, South-east Asia and Central and Southern America.
Other species of hookworms in the genus Ancylostoma and Necator that parasitize humans also exist; but A. duodenale and N. americanus are often the main important hookworms responsible for hookworm disease in humans.
Type and morphology of hookworms
Hookworms (i.e. A. duodenale and N. americanus) exist in various morphological forms within and outside their human hosts. They exist as filariform larva (the infective form); as rhabditiform larvae (the first stage larva that hatches from the egg); and as adult A. duodenale and N. americanus worms which produce numerous eggs within the body of infected human hosts and even outside the body.
Vector, reservoir and habitat of hookworms
There are no intermediate hosts for hookworms. The lifecycle of the parasite is usually a direct one. Humans are the only definitive hosts for hookworms. Outside the human body, the soil is the normal habitat of hookworms, and human infection occurs via the penetration of the skin by the infective form of the parasite. Hookworms have no animal or insect vector as other parasites.
Clinical signs and symptoms of hookworm infection
Hookworm infections are marked by intense skin reaction (known as the ground itch) at the site where the infective stage of the parasite penetrated the skin. Anorexia, nausea and abdominal cramp are some non-specific symptoms of hookworm disease. In some severe cases, there is usually loss of blood or anaemia in infected individuals, and this is due to the bloodsucking potential of the hookworms in the intestines.
Pathogenesis of hookworm infections
Hookworm infection occurs in humans following skin penetration by filariform larva from faecally contaminated or polluted soil (Figure 1). Drinking contaminated water containing the eggs or larva of the worms especially those of A. duodenale can also lead to hookworm infection. People who walk or work in feacally-contaminated soil without foot wears are more prone to hookworm infection than those who do. After skin penetration the filariform larva migrate to the trachea and are swallowed. They become mature worms in the small intestine and attach to the intestinal walls with their hook-like head which enables them to suck blood. Heavy hookworm infection can lead to chronic loss of blood or anaemia in infected individuals; and this development is often terminal in people who have low intake of iron in their daily meal (e.g. pregnant mothers) or the malnourished. The adult worms in the small intestine produce eggs which are passed out via the feaces of infected persons. In the soil, the egested eggs transform into rhabditiform larva and later hatch into the infective form (i.e. filariform larva) to continue the life cycle of the parasite.
Figure 1: Life cycle of Ancylostoma duodenale. 1. Eggs of A. duodenale (known as rhabditiform larvae) are passed in the stool of a human host. 2. The released rhabditiform larvae grow in the feces and/or the soil under favorable conditions (such as optimal moisture, warmth, and shade). The larvae hatch in 1 to 2 days. 3. After 5 to 10 days (and two molts) of hatching they become filariform (third-stage) larvae that are infective. 4. Filariform (infective larvae of hookworms) can survive 3 to 4 weeks in favorable environmental conditions. On contact with the human host, the larvae penetrate the skin and are carried through the blood vessels to the heart and then to the lungs. They penetrate into the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are eventually swallowed. 5. The larvae reach the small intestine, where they reside and mature into adult worms. Adult worms live in the lumen of the small intestine, where they attach to the intestinal wall with resultant blood loss by the human host. Most adult worms are eliminated in 1 to 2 years after infection, but the longevity of the disease may reach several years. Some A. duodenale larvae, following penetration of the host skin, can become dormant (in the intestine or muscle). In addition, infection by A. duodenale may probably also occur by the oral and transmammary route (i.e. via breastfeeding). However, certain Ascaris species such as Necator americanus, however, requires a transpulmonary migration phase to initiate an infection. CDC
Laboratory diagnosis and treatment of hookworm infections
Hookworm infection is diagnosed in the laboratory by direct microscopical examination of fresh stool specimens for the eggs of A. duodenale and N. americanus (Figure 2). Hookworm infection can be treated using mebendazole, albendazole and pyrantel pamoate.
Figure 2: Eggs of hookworm. The eggs of A. duodenale and N. americanus are usually morphologically indistinguishable i.e. they cannot be differentiated microscopically. Hookworm eggs are thin-shelled and colorless and they measure 60-75 µm by 35-40 µm. CDC
Control and prevention of hookworm infection
Hookworm disease is a public health disease that is preventable through improvement in environmental sanitation and the health of the general public. Good toilets should be provided at public places to avoid indiscriminate defaecation in the environment. The disease is no longer in existence in countries where public health and environment sanitation has been greatly improved upon. People who work in farms and in occupations where there is constant contact with the soil should always wear protective footwear’s to avoid infection by the filariform larva of hookworms in the soil. Infected individuals should be properly treated; and the general public should be educated about on the disease so that appropriate caution should be taken especially in endemic areas.
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