''Necator americanus'' is a species of

hookworm Hookworms are intestinal, blood-feeding, parasitic roundworms that cause types of infection known as helminthiases. Hookworm infection is found in many parts of the world, and is common in areas with poor access to adequate water, sanitation, ...

(a type of

helminth Parasitic worms, also known as helminths, are large macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Other parasitic worms such as ...

) commonly known as the New World hookworm. Like other hookworms, it is a member of the phylum

Nematoda The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...

. It is an obligatory parasitic

nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant- parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a bro ...

that lives in the small intestine of human hosts. Necatoriasis—a type of

helminthiasis Helminthiasis, also known as worm infection, is any macroparasitic disease of humans and other animals in which a part of the body is infected with parasitic worms, known as helminths. There are numerous species of these parasites, which are ...

—is the term for the condition of being host to an infestation of a species of ''Necator''. Since ''N. americanus'' and ''

Ancylostoma duodenale ''Ancylostoma'' is a genus of nematodes that includes some species of hookworms. Species include: : ''Ancylostoma braziliense'', commonly infects cats, popularly known in Brazil as ''bicho-geográfico'' : ''Ancylostoma caninum'', commonly infect ...

'' (also known as Old World hookworm) are the two species of hookworms that most commonly infest humans, they are usually dealt with under the collective heading of " hookworm infection". They differ most obviously in geographical distribution, structure of mouthparts, and relative size. ''Necator americanus'' has been proposed as an alternative to ''

Trichuris suis ''Trichuris suis'' is a whipworm; the variations in thickness of the anterior and posterior segments give the parasite the characteristic "whip-like" appearance. Adult females measure 6 to 8 cm and adult males 3 to 4 cm. ''T. suis'' egg ...

'' in helminthic therapy.

Morphology

This

parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson h ...

has two

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal c ...

and two

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

cutting plates around the anterior margin of the buccal capsule. It also has a pair of subdorsal and a pair of subventral teeth located close to the rear. Males are usually 7–9 mm long, whereas females are about 9–11 mm long. The typical lifespan of these parasites is 3–5 years. They can produce between 5,000 and 10,000 eggs per day.

Habitat

''N. americanus'' is primarily found in tropical and temperate areas. This parasite thrives in warmer climates because to hatch, the eggs require a moist, warm, and shaded environment. The thin, smooth shells of this species cause the eggs and juveniles to die in freezing temperatures or with soil desiccation. Therefore, the type of soil where the parasite resides is also very important for their ideal living conditions. Ideal soil conditions tend to be in areas where water is able to drain at a standard pace, and the size of the soil particles is neither too large nor too small. That way, the degree of dampness, as well as the openings in the soil, allow the parasites to burrow to the surface and attach to the skin of their next host. High transmission rates seem to be congruent with the heavy rains and warm temperatures that are characteristic of tropical climates. One anomaly of this species is that it appears to prefer male hosts to female hosts, likely because of the division of labor in regions of its prevalence.

Lifecycle

This worm starts out as an unembryonated egg in the soil. After 24–48 hours under favorable conditions, the eggs become embryonated and hatch. This first juvenile stage 1 is known as 'rhabditiform'. The rhabditiform larvae grow and molt in the soil, transforming into a juvenile stage 2. The juvenile stage 2 molts once more until reaching the juvenile 3 stage, which is also called 'filariform'; this is also the infective form. The transformation from rhabditiform to the filariform usually takes 5–10 days. This larval form is able to penetrate human skin, travel through the blood vessels and heart, and reach the lungs. Once there, it burrows through the

pulmonary alveoli A pulmonary alveolus (plural: alveoli, from Latin ''alveolus'', "little cavity"), also known as an air sac or air space, is one of millions of hollow, distensible cup-shaped cavities in the lungs where oxygen is exchanged for carbon dioxide. Al ...

and travels up the

trachea The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air- breathing animals with lungs. The trachea extends from t ...

, where it is swallowed and carried to the small intestine. There, it attaches to the intestinal wall, and matures into an adult and begins reproduction. Adults live in the lumen of the intestinal wall, where they cause blood loss to the host. The eggs produced by the adults end up on the soil after leaving the body through the feces; female hookworms produce up to 30,000 eggs per day. On average, most adult worms are eliminated in 1–2 years. The ''N. americanus'' lifecycle only differs slightly from that of ''A. duodenale''. ''N. americanus'' has no development arrest in immune hosts and it must migrate through the lungs.

Pathogenesis and symptoms

The pathology of ''N. americanus'' is divided into two stages – larvae and adults. The larvae penetrate the uninfected skin and travel through various organs, including the

respiratory tract The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa. Air is breathed in through the nose to ...

and

lymph node A lymph node, or lymph gland, is a kidney-shaped organ of the lymphatic system and the adaptive immune system. A large number of lymph nodes are linked throughout the body by the lymphatic vessels. They are major sites of lymphocytes that includ ...

s. Once in the lymph nodes, the larvae start entering the blood, lungs, and

intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans an ...

s. Some larvae cannot readily enter the dermis and remain trapped in the skin, causing skin irritation and cutaneous larva migrans. Other symptoms include excessive coughing and

dyspnea Shortness of breath (SOB), also medically known as dyspnea (in AmE) or dyspnoea (in BrE), is an uncomfortable feeling of not being able to breathe well enough. The American Thoracic Society defines it as "a subjective experience of breathing di ...

(short of breath) during larval migration. Once attached to the intestinal wall, ''N. americanus'' resides and matures into adults, penetrates blood vessels, and sucks blood. The incubation process of the larvae begins once entered into the small intestine; therefore, symptoms may not arise for up to 40 days, but this is variable from person to person. Blood loss from sites of intestinal attachment may cause

iron-deficiency anemia Iron-deficiency anemia is anemia caused by a lack of iron. Anemia is defined as a decrease in the number of red blood cells or the amount of hemoglobin in the blood. When onset is slow, symptoms are often vague such as feeling tired, weak, shor ...

and protein loss. One individual ''N. americanus'' can cause 30 μl of blood loss per day. Iron-deficiency anemia can cause intellectual disability and growth insufficiency in children. Further, infected patients experience abdominal pain (exacerbated by meals) with diarrhea, bloating, and nausea.

Epidemiology

In the United States, 95% of human hookworm cases are caused by ''N. americanus'', primarily in young school children in economically deprived rural areas. Historically, there have been high rates of infection among children in the American South. Juveniles cannot survive freezing temperatures, so the highest prevalence occurs in areas with warmer temperatures and greater rainfall. The greatest incidence of infections occurs in Asia and sub-Saharan Africa, especially in poverty-stricken areas with poor sanitation. ''A. duodenale'' infections occur at a lesser rate and are seen primarily in Europe and the Mediterranean.

Genome

A draft assembly of the genome of ''Necator americanus'' has been sequenced and analyzed. It comprises 244 Mbp with 19,151 predicted protein-coding genes; these include genes whose products mediate the hookworm's invasion of the human host, genes involved in blood feeding and development, genes encoding proteins that represent new potential drug targets against hookworms, and expanded gene families encoding likely

immunomodulator Immunotherapy or biological therapy is the treatment of disease by activating or suppressing the immune system. Immunotherapies designed to elicit or amplify an immune response are classified as ''activation immunotherapies,'' while immunotherap ...

proteins, whose products may be beneficial in treating inflammatory diseases and

asthma Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, co ...

Diagnostics

The most common method for diagnosing ''N. americanus'' is through identification of eggs in a fecal sample using a

microscope A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisi ...

. ''N. americanus'' eggs have a thin shell and are oval shaped, measuring roughly 56–74 by 36–40 μm.

Treatments

The most common treatment for ''N. americanus'' infection are benzimidazoles, specifically albendazole and

mebendazole Mebendazole (MBZ), sold under the brand name Vermox among others, is a medication used to treat a number of parasitic worm infestations. This includes ascariasis, pinworm infection, hookworm infections, guinea worm infections, hydatid disea ...

. Benzimidazoles kill adult worms by binding to the nematode’s β-tubulin and subsequently inhibiting microtubule polymerization within the parasite. The efficacy of single-dose treatments for hookworm infections were: 72% for albendazole, 15% for mebendazole, and 31% for pyrantel pamoate. A current concern with this parasite is the increase in drug resistance, such as benzimidazoles and mebendazoles. Pregnant women should not be treated within their first trimester. During the 1940s, the treatment of choice was

tetrachloroethylene Tetrachloroethylene, also known under the systematic name tetrachloroethene, or perchloroethylene, and many other names (and abbreviations such as "perc" or "PERC", and "PCE"), is a chlorocarbon with the formula Cl2C=CCl2 . It is a colorless li ...

, given as 3 to 4 cc in the fasting state, followed by 30 to 45 g of

sodium sulfate Sodium sulfate (also known as sodium sulphate or sulfate of soda) is the inorganic compound with formula Na2SO4 as well as several related hydrates. All forms are white solids that are highly soluble in water. With an annual production of 6 mil ...

. Tetrachloroethylene was reported to have a cure rate of 80 percent for Necator infections, but 25 percent in Ancylostoma infections, and often produced mild intoxication in the patient.

Prevention and control

Infection and

transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission ** ...

of others can be prevented by not defecating outdoors or using human feces as fertilizer. This parasite is not transmittable directly from person to person. Pigs may be an intermediate host for ''N. americanus.''

Economic burden

''N. americanus'' causes hookworm diseases, which are associated with

blood loss Bleeding, hemorrhage, haemorrhage or blood loss, is blood escaping from the circulatory system from damaged blood vessels. Bleeding can occur internally, or externally either through a natural opening such as the mouth, nose, ear, urethra, ...

and anemia. Patients who are infected with around 25 to 100 worms experience symptoms such as fatigue, weight loss, and slight headaches. As the infestation number reaches 100 to 500 worms, the patient experiences extreme

fatigue Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...

, iron deficiency, and

abdominal pain Abdominal pain, also known as a stomach ache, is a symptom associated with both non-serious and serious medical issues. Common causes of pain in the abdomen include gastroenteritis and irritable bowel syndrome. About 15% of people have a m ...

. The symptoms worsen and result in possible death when the infestation reaches over 500 hookworms. Children and pregnant women affected by ''N. americanus'' are at greater risk due to anemia and the greater need for dietary iron and protein. The demand is high for an improvement of

sanitation Sanitation refers to public health conditions related to clean drinking water and treatment and disposal of human excreta and sewage. Preventing human contact with feces is part of sanitation, as is hand washing with soap. Sanitation syste ...

to reduce fecal contamination in regions with high

prevalence In epidemiology, prevalence is the proportion of a particular population found to be affected by a medical condition (typically a disease or a risk factor such as smoking or seatbelt use) at a specific time. It is derived by comparing the number o ...

of ''N. americanus'' infections. The current control strategies include a combination of mass drug administration for children at age 4–6 years to prevent or eliminate ''N. americanus'' infections.