Necator americanus is a species of hookworm (a type of helminth) commonly known as the New World hookworm. Like other hookworms, it is a member of the phylum Nematoda. It is an obligatory parasitic nematode that lives in the small intestine of human hosts. Necatoriasis—a type of helminthiasis—is the term for the condition of being host to an infestation of a species of Necator. Since N. americanus and Ancylostoma duodenale (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 in helminthic therapy.
Morphology
This parasite has two dorsal and two ventral 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
thumb|Life cycle of N. americanus inside and outside of the human body
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 and travels up the trachea, 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. Blood loss from sites of intestinal attachment may cause iron-deficiency anemia and protein loss. 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 proteins, whose products may be beneficial in treating inflammatory diseases and asthma.
Diagnostics
The most common method for diagnosing N. americanus is through identification of eggs in a fecal sample using a microscope. 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. 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.
<!-- Patients infected with hookworm N. americanus may also consider cryotherapy as a treatment option.
-->Pregnant women should not be treated within their first trimester. given as 3 to 4 cc in the fasting state, followed by 30 to 45 g of sodium sulfate. 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 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.