Hookworm infection is an infection by a type of intestinal parasite known as a hookworm. Initially, itching and a rash may occur at the site of infection. Those only affected by a few worms may show no symptoms. Those infected by many worms may experience abdominal pain, diarrhea, weight loss, and tiredness. The mental and physical development of children may be affected. Anemia may result. Two common hookworm infections in humans are ancylostomiasis and necatoriasis, caused by the species Ancylostoma duodenale and Necator americanus respectively. Hookworm eggs are deposited in the stools of infected people. If these end up in the environment, they can hatch into larvae (immature worms), which can then penetrate the skin. One type can also be spread through contaminated food. Risk factors include walking barefoot in warm climates, where sanitation is poor. Diagnosis is by examination of a stool sample with a microscope. The risk of infection can be reduced on an individual level by not walking barefoot in areas where the disease is common. At a population level, decreasing outdoor defecation, not using raw feces as fertilizer, and mass deworming are effective. Treatment is typically with the medications albendazole or mebendazole for one to three days. Iron supplements may be needed in those with anemia. Hookworms infected about 428 million people in 2015. Heavy infections can occur in both children and adults, but are less common in adults. They are rarely fatal. Hookworm infection is a soil-transmitted helminthiasis and classified as a neglected tropical disease.
Signs and symptoms No symptoms or signs are specific to hookworm infection, but they give rise to a combination of intestinal inflammation and progressive iron-deficiency anemia and protein deficiency. Coughing, chest pain, wheezing, and fever sometimes result from severe infection. Epigastric pains, indigestion, nausea, vomiting, constipation, and diarrhea can occur early or in later stages, as well, although gastrointestinal symptoms tend to improve with time. Signs of advanced severe infection are those of anemia and protein deficiency, including emaciation, cardiac failure, and abdominal distension with ascites. Larval invasion of the skin (mostly in the Americas) can produce a skin disease called cutaneous larva migrans also known as creeping eruption. The hosts of these worms are not human and the larvae can only penetrate the upper five layers of the skin, where they give rise to intense, local itching, usually on the foot or lower leg, known as ground itch. This infection is due to larvae from the A. braziliense hookworm. The larvae migrate in tortuous tunnels between the stratum basale and stratum corneum of the skin, causing serpiginous vesicular lesions. With the advancing movement of the larvae, the rear portions of the lesions become dry and crusty. The lesions are typically intensely itchy.
Incubation period The incubation period can vary between a few weeks to many months and is largely dependent on the number of hookworm parasites an individual is infected with.
Cause
Hookworm infections in humans include ancylostomiasis and necatoriasis. Ancylostomiasis is caused by Ancylostoma duodenale, which is the more common type found in the Middle East, North Africa, India, and (formerly) in southern Europe. Necatoriasis is caused by Necator americanus, the more common type in the Americas, sub-Saharan Africa, Southeast Asia, China, and Indonesia. Other animals such as birds, dogs, and cats may also be affected. A. tubaeforme infects cats, A. caninum infects dogs, and A. braziliense and Uncinaria stenocephala infect both cats and dogs. Some of these infections can be transmitted to humans.
Morphology A. duodenale worms are grayish-white or pinkish with the head slightly bent in relation to the rest of the body. This bend forms a definitive hook shape at the anterior end for which hookworms are named. They possess well-developed mouths with two pairs of teeth. While males measure approximately one centimeter by 0.5 millimeter, the females are often longer and stouter. Additionally, males can be distinguished from females based on the presence of a prominent posterior copulatory bursa. N. americanus is very similar in morphology to A. duodenale. N. americanus is generally smaller than A. duodenale with males usually 5 to 9 mm long and females about 1 cm long. Whereas A. duodenale possesses two pairs of teeth, N. americanus possesses a pair of cutting plates in the buccal capsule. Additionally, the hook shape is much more defined in Necator than in Ancylostoma.
Life cycle
The hookworm thrives in warm soil where temperatures are over 18 °C (64 °F). They exist primarily in sandy or loamy soil and cannot live in clay or muck. Rainfall averages must be more than 1,000 mm (39 in) a year for them to survive. Only if these conditions exist can the eggs hatch. Infective larvae of N. americanus can survive at higher temperatures, whereas those of A. duodenale are better adapted to cooler climates. Generally, they live for only a few weeks at most under natural conditions and die almost immediately on exposure to direct sunlight or desiccation. Infection of the host is by the larvae, not the eggs. While A. duodenale can be ingested, the usual method of infection is through the skin; this is commonly caused by walking barefoot through areas contaminated with fecal matter. The larvae can penetrate the skin of the foot, and once inside the body, they migrate through the vascular system to the lungs, and from there up the trachea, and are swallowed. They then pass down the esophagus and enter the digestive system, finishing their journey in the intestine, where the larvae mature into adult worms. Once in the host gut, Necator tends to cause a prolonged infection, generally 1 to 5 years (many worms die within a year or two of infection), though some adult worms have been recorded to live for 15 years or more. Ancylostoma adults are short-lived, surviving on average for only about 6 months. However, the infection can be prolonged because dormant larvae can be "recruited" sequentially from tissue "stores" over many years, to replace expired adult worms. This can give rise to seasonal fluctuations in infection prevalence and intensity (apart from normal seasonal variations in transmission).
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