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Hookworm infection, also known as hookworm disease, is an infection by a parasitic bloodsucking roundworm. Hookworm infections include ancylostomiasis and necatoriasis. These worms live in the small intestine of their host, which may be a bird or a mammal such as a dog, cat, or human. Hookworm infection in pregnancy can cause retarded growth of the fetus, premature birth and a low birth weight. Hookworms in children can cause intellectual, cognitive and growth problems.

Two species of hookworms commonly infect humans: Ancylostoma duodenale and Necator americanus. A. duodenale predominates in the Middle East, North Africa, India and (formerly) in southern Europe, while N. americanus predominates in the Americas, Sub-Saharan Africa, Southeast Asia, China, and Indonesia. A. tubaeforme infects cats, A. caninum infects dogs and A. braziliense and Uncinaria stenocephala infect both cats and dogs. Hookworms are much smaller than the giant roundworms Ascaris lumbricoides and so cause less tissue damage and obstruction. The most significant risk of hookworm infection is anemia, secondary to loss of iron (and protein) in the gut. The worms suck blood voraciously and damage the mucosa. However, the blood loss in the stools is not visibly apparent. Hookworm infection affects over half a billion people globally. It is a leading cause of maternal and child morbidity in the developing countries of the tropics and subtropics. In developed countries, hookworm infection is rarely fatal, but anemia can be significant in a heavily infected individual. Hookworm infection is a soil-transmitted helminthiasis and therefore classified as a neglected tropical disease.[2] Ancylostomiasis is the disease caused when Ancylostoma duodenale hookworms, present in large numbers, produce an iron deficiency anemia by sucking blood from the host's intestinal walls. There are no specific symptoms or signs of 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 will 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 advancing movement of the larvae, the rear portions of the lesions become dry and crusty. The lesions are typically intensely itchy. 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. See the image for the biological life cycle of the hookworm where it thrives in warm earth where temperatures are over 18 °C. They exist primarily in sandy or loamy soil and cannot live in clay or muck. Rainfall averages must be more than 1000 mm (40 inches) a year for them to survive. Only if these conditions exist can the eggs hatch. Infective larvae of Necator americanus can survive at higher temperatures, whereas those of Ancylostoma 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 are able to 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 1–5 years (many die within a year or two of infecting), though some adult worms have been recorded to live for 15 years or more. On the other hand, Ancylostoma adults are short lived, surviving on average for only about 6 months. However, infection can be prolonged because dormant larvae can be recruited sequentially from tissue stores (see Pathology, above) 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). They mate inside the host, females laying up to 30,000 eggs per day and some 18 to 54 million eggs during their lifetime, which pass out in feces. Because it takes 5–7 weeks for adult worms to mature, mate and produce eggs, in the early stages of very heavy infection, acute symptoms might occur without any eggs being detected in the patient's feces. This can make diagnosis very difficult. N. americanus and A. duodenale eggs can be found in warm, moist soil where they will eventually hatch into first stage larvae, or L1. L1, the feeding non-infective rhabditoform stage, will feed on soil microbes and eventually molt into second stage larvae, L2. L2, which is also in the rhabditoform stage, will feed for approximately 7 days and then molt into the third stage larvae, or L3. L3 is the filariform stage of the parasite, that is, the non-feeding infective form of the larvae. The L3 larvae are extremely motile and will seek higher ground to increase their chances of penetrating the skin of a human host. The L3 larvae can survive up to 2 weeks without finding a host. While N. americanus larvae only infect through penetration of skin, A. duodenale can infect both through penetration as well as orally. After the L3 larvae have successfully entered the host, the larvae then travel through the subcutaneous venules and lymphatic vessels of the human host. Eventually, the L3 larvae enter the lungs through the pulmonary capillaries and break out into the alveoli. They will then travel up the trachea to be coughed and swallowed by the host. After being swallowed, the L3 larvae are then found in the small intestine where they molt into the L4, or adult worm stage. The entire process from skin penetration to adult development takes about 5–9 weeks. The female adult worms will release eggs (N. americanus about 9,000–10,000 eggs/day and A. duodenale 25,000–30,000 eggs/day) which are passed in the feces of the human host. These eggs will hatch in the environment within several days and the cycle will start anew. Diagnosis depends on finding characteristic worm eggs on microscopic examination of the stools, although this is not possible in early infection. Early signs of infection in most dogs include limbular limping and anal itching. The eggs are oval or elliptical, measuring 60 µm by 40 µm, colourless, not bile stained and with a thin transparent hyaline shell membrane. When released by the worm in the intestine, the egg contains an unsegmented ovum. During its passage down the intestine, the ovum develops and thus the eggs passed in feces have a segmented ovum, usually with 4 to 8 blastomeres. As the eggs of both Ancylostoma and Necator (and most other hookworm species) are indistinguishable, to identify the genus, they must be cultured in the lab to allow larvae to hatch out. If the fecal sample is left for a day or more under tropical conditions, the larvae will have hatched out, so eggs might no longer be evident. In such a case, it is essential to distinguish hookworms from Strongyloides larvae, as infection with the latter has more serious implications and requires different management. The larvae of the two hookworm species can also be distinguished microscopically, although this would not be done routinely, but usually for research purposes. Adult worms are rarely seen (except via endoscopy, surgery or autopsy), but if found, would allow definitive identification of the species. Classification can be performed based on the length of the buccal cavity, the space between the oral opening and the esophagus: hookworm rhabditoform larvae have long buccal cavities whereas Strongyloides rhabditoform larvae have short buccal cavities. Recent research has focused on the development of DNA-based tools for diagnosis of infection, specific identification of hookworm, and analysis of genetic variability within hookworm populations. Because hookworm eggs are often indistinguishable from other parasitic eggs, PCR assays could serve as a molecular approach for accurate diagnosis of hookworm in the feces. School-based mass deworming programs have been the most popular strategy to address the issue of hookworm infection in children. School-based programs are extremely cost effective as schools already have an available, extensive, and sustained infrastructure with a skilled workforce that has a close relationship with the community. With little training from a local health system, teachers can easily administer the drugs which often cost less than US$0.50 per child per year. Recently, many people have begun to question if the school-based programs are necessarily the most effective approach. An important concern with school-based programs is that they often do not reach children who do not attend school, thus ignoring a large amount of at-risk children. A 2008 study by Massa et al. continued the debate regarding school-based programs. They examined the effects of community-directed treatments versus school-based treatments in the Tanga Region of Tanzania. A major conclusion was that the mean infection intensity of hookworm was significantly lower in the villages employing the community-directed treatment approach than the school-based approach. The community-directed treatment model used in this specific study allowed villagers to take control of the child's treatment by having villagers select their own community drug distributors to administer the antihelminthic drugs. Additionally, villagers organized and implemented their own methods for distributing the drugs to all children. The positive results associated with this new model highlight the need for large-scale community involvement in deworming campaigns.

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