Hookworms are soil-transmitted nematode parasites that can reside for many years in the small intestine of their human hosts, where they suck blood and can cause iron deficiency anaemia (IDA) in individuals who harbour moderate and high numbers of adult worms, which is known as hookworm disease
Microscopic view of the mouth of a live, adult A. caninum worm, showing three teeth on either side of the ventral rim of the buccal cavity. | Necator americanus, the New World hookworm (Handout courtesy of Severine Navarro)
Contents
Introduction
Hookworms are soil-transmitted nematode parasites that can reside for many years in the small intestine of their human hosts, where they suck blood and can cause iron deficiency anaemia (IDA) in individuals who harbour moderate and high numbers of adult worms, which is known as hookworm disease
Epidemiology
Soil-transmitted helminths (STHs):
Ascaris lumbricoides
Trichuris trichiura
Hookworms
Hookworms:
Hookworms are nematodes belonging to the family Ancylostomatidae, a part of the superfamily Strongyloidea. The two major genera that affect humans, Necator and Ancylostoma, are characterized by the presence of either teeth or cutting plates that line the adult parasite buccal capsule
Necatur americanus (predominant human hookworm globally)
Ancylostoma duodenale (focally endemic in Mediterranean region, in northern India & China and in North Africa)
Ancylostoma ceylanicum
Ancyclostoma braziliense “Dog hookworm” (parasite adapted for dogs and hence cannot enter human circulation and wander in skin causing creeping eruptions)
Presumed current world distribution of human hookworm infections, derived from numerous sources. Zones where A. duodenale predominates are indicated in red, whereas areas where N. americanus is primarily endemic are cream. Note that (i) the outer limits of regions of endemic infection are indicated, although focal infection prevalences and intensities vary widely according to local geographic, socioeconomic, climatic and other factors; (ii) in southern Europe and northern Africa, A. duodenale is virtually the exclusive species; (iii) in sub-Saharan Africa, both species occur together, although N. americanus predominates generally and is the exclusive species in many foci; (iv) in China, A. duodenale occurs exclusively in the northern range whereas N. americanus predominates in the south, with a broad overlapping of the two species in the intermediate zone; (v) recent data are not available from the southeastern United States; (vi) in Australia, A. duodenale is the only species now present, although in the past, both A. duodenale and N. americanus were endemic, to a much more extensive zone than indicated here, with frequent mixed infections; (vii) A. ceylanicum has been found in southern India, Sri Lanka, Indonesia, Malaysia, and neighboring countries in Southeast Asia and western New Guinea. | Loukas, A., & Prociv, P. (2001). Immune responses in hookworm infections. Clinical microbiology reviews, 14(4), 689–703. https://doi.org/10.1128/CMR.14.4.689-703.2001
Life cycle:
Briefly, humans become infected with hookworms when third-stage infective larvae (L3) penetrate the skin and then migrate into subcutaneous venules and lymphatics before traveling via the afferent circulation to the pulmonary capillary bed. From there, L3 enter the respiratory tree through the alveolae and ascend the bronchioles and the bronchi, pass over the epiglottis, and enter the gastrointestinal tract. In the small intestine, L3 molt twice to become adult male and female hookworms where they can live for five years or more. The adult worms, approximately one centimeter in length, attach to the mucosa and submucosa, rupture capillaries and arterioles, and then feed on host blood and mucosal tissues. The hookworms mate and female worms then produce thousands of eggs that exit the body in the feces. The eggs hatch in soil with adequate moisture and high temperature and then molt twice into L3 that seek higher ground to come into contact with human skin.
Hookworm eggs hatch in soil and rhabditiform (early) larvae moult twice (first-stage larvae (L1) and L2) before becoming infective (L3). L3 accumulate in soil or on grass awaiting exposure to human skin (often the hands, feet or buttocks), which they can penetrate. L3 then make their way to the peripheral vasculature, where they are passively swept within the bloodstream, first to the right side of the heart and then to the pulmonary vasculature. In the lungs, L3 exit from the alveolar capillaries into the bronchial tree, which they ascend to reach the pharynx, from which they enter the gastrointestinal tract to finally complete their migration to the small bowel. Once in the duodenum, immature L5 hookworms use ‘teeth’ (Ancylostoma spp.) or cutting plates (Necator spp.) that line their buccal capsule to lacerate the mucosa and anchor themselves in position to facilitate feeding and avoid being ejected by gut peristalsis. As they begin to feed on blood, juvenile worms mature into sexually dioecious adult parasites. Mature adult male and female hookworms mate, and female hookworms produce as many as 10,000 eggs per day. Eggs are evacuated from the host via the faecal stream. The process from L3 invasion to patency (egg production) takes approximately 6–8 weeks for Necator americanus and possibly a similar period of time for Ancylostoma duodenale. | Loukas, A., Hotez, P., Diemert, D. et al. Hookworm infection. Nat Rev Dis Primers 2, 16088 (2016). https://doi.org/10.1038/nrdp.2016.88
Clinical features
Ground itch
Although initial skin invasion might go unnoticed, it can result in a local pruritic, erythematous rash, referred to as ‘ground itch’.
Local pruritic, erythematous rash
Typically appears on hands and feet (main sites of L3 percutaneous entry; via hair follicles, which are the path of least resistance).
a | A few days after Necator americanus third-stage larvae (L3) infect the host, a rash known as ‘ground itch’ develops at the site of skin penetration. b | A longitudinal histological section of an adult Ancylostoma caninum attached to the duodenal wall of an infected dog. c | A scanning electron micrograph of A. caninum, with its ‘teeth’ clearly visible. d | An N. americanus female (arrow) and male in copula in the small bowel of a human volunteer. e | Egg of N. americanus in human faeces. | Loukas, A., Hotez, P., Diemert, D. et al. Hookworm infection. Nat Rev Dis Primers 2, 16088 (2016). https://doi.org/10.1038/nrdp.2016.88
Intestinal blood loss
Main clinical manifestation of human hookworm infection. Blood loss can be as high as 9·0 mL/day in heavy infections, with hookworm burdens of 40–160 worms sufficient to cause anemia
Iron deficiency anemia (IDA): Occurs on blood host with poor nutritional background
Accompanying hypoalbuminemia
Hookworm infections in the patient’s small bowel, as observed with endoscopy. (A) Most hookworms frequently appeared between the distal duodenum and the distal jejunum. (B, C) Hookworms appeared with reddish bodies attached to the intestinal mucosa. (D) Hookworms feeding on the intestinal mucosa with a hemorrhagic spot. (E) Blood oozing was found at the hookworms’ site of attachment. | Wei, K. Y., Yan, Q., Tang, B., Yang, S. M., Zhang, P. B., Deng, M. M., & Lü, M. H. (2017). Hookworm Infection: A Neglected Cause of Overt Obscure Gastrointestinal Bleeding. The Korean journal of parasitology, 55(4), 391–398. https://doi.org/10.3347/kjp.2017.55.4.391
Other features:
Systemic eosinophilia: Often asymptomatic, although flatulence is common for the first 12 weeks and gastrointestinal disturbance is sometimes reported within the first 3–15 weeks
Case study:
A day of fun at the beach turned into a nightmare for one teen. Michael Dumas, 17, was buried in the sand by his buddies. He didn’t know it at the time, but lurking in the sand were tiny parasites known as hookworms. They bit into Michael’s foot and crawled beneath his skin. As the hookworms made a home there, Michael developed painful sores that became infected. The hookworms most likely came from an infected dog.
Diagnosis
Faecal examination
As hookworms are parasites of the digestive system, the diagnostic examinations use samples of fresh, fixed or frozen faeces, in which hookworm eggs (ova), larvae and whole or parts of the parasites can be detected.
Hook Worm Ova. Contributed by Centers of Disease Control and Prevention (Public Domain)
Management
Water, Sanitation and Hygiene (WASH) programme
Diverse and include a range of improvements in water conditions (such as quality, quantity and access), sanitation (such as improved latrines and sewer maintenance) and hygiene (for example, adopting good personal hygiene habits, such as hand washing before eating and after defaecation, wearing shoes and safely storing water)
Benzimidazole anthelmintic therapies:
These drugs act by inhibiting microtubule polymerization in invertebrates, therefore killing adult worms.
Albendazole > mebendazole
Ivermectin
Manamgent of complications:
Antihitamines
Iron supplementation
Summary:
Hookworms are soil-transmitted human parasites that can survive and multiply in the small intestine for years. | Hookworm infection. Nat Rev Dis Primers 2, 16089 (2016). https://doi.org/10.1038/nrdp.2016.89
