Necator americanus

This article is about the organism. For the infection, see Necatoriasis.

Necator americanus
Scientific classification
Kingdom:	Animalia
Phylum:	Nematoda
Class:	Secernentea
Order:	Strongylida
Family:	Ancylostomatidae
Genus:	Necator
Species:	N. americanus
Binomial name
Necator americanus

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 a parasitic nematode that lives in the small intestine of hosts such as humans, dogs, and cats. 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 three to five years. They can produce between 5000 and 10,000 eggs per day.

Life cycle

Life cycle of Necator 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 five to 10 days. This larval form is able to penetrate human skin, travel through the blood vessels and heart, and reach the lungs. Once there, they burrow through the pulmonary alveoli and travel up the trachea, where they are swallowed and are carried to the small intestine, where they mature into adults and reproduce by attaching themselves to the intestinal wall, causing an increase of blood loss by the host. The eggs end up on the soil after leaving the body through the feces. On average, most adult worms are eliminated in one to two years. The N. americanus life cycle 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 Necator americanus is divided into two stages: larvae stage and adult stage. The larvae penetrates the uninfected skin and travels through various organs, including the respiratory tract and lymph nodes. Once in the lymph nodes, the larvae starts entering the blood, lungs, and intestines. Some larva 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 (short of breath) during larvae migration. Once attached to the intestinal wall, N. americanus reside and mature into adults, penetrate blood vessels, and suck blood. Blood loss from sites of intestinal attachment may cause iron-efficiency leading to anemia. Studies have shown that a 30 microliters of daily blood loss is due to one individual N. americanus. Iron deficiency anemia can cause mental retardation and growth insufficiency in children. Further, infected patients will 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. 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.

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 mm.

Treatments and medications

Anthelmintic drugs

The most common treatment for N. americanus are benzimidazoles, specifically albendazole and mebendazole. Benzimidazoles kill adult worms by binding to the nematde’s Beta-tubulin and subsequently inhibiting microtubule polymerization within the parasite. Keiser and Utzinger conducted a study in 2008, Efficacy of Current Drugs Against Soil–Transmitted Helminth Infections: Systematic Review and Meta-analysis, which found that the efficacy of single-dose treatments for Hookworm infections were as followed: 72% for albendazole, 15% for mebendazole, and 31% for pyrantel pamoate.

Patients infected with hookworm N. americanus may also consider cryotherapy as a treatment option.

Prevention and control

The most effective prevention technique is to not walk barefoot in areas where hookworm is common and where there may be contamination of the soil. Locations for outdoor activities should be considered if there will be skin-to-soil contact.

Infection and transmission of others can be prevented by not defecating outdoors or using human feces fertilizer.

Economic burden

N. americanus has played a significant role in the development of New and Old Worlds. N. americanus cause hookworm diseases, which are associated with nutrition and blood loss. Patients who are infected with approximately 25 to 100 worms will experience symptoms such as fatigue, weight loss, and slight headaches. As the infestations number reach 100 to 500 worms, the patient will experience extreme fatigue, iron deficiency, and abdominal pain. The symptoms worsen and result in possible deaths when the infestation reach over 500. Children and pregnant women affected by N. americanus are at greater risk due to anemia and the greater need for nutrition. There is a high demand for an improvement of sanitation to reduce fecal contamination in regions with high prevalence of N. americanus infections. The current control strategies include a combination of mass drug administration (MDA) for children at age 4–6 years to prevent or eliminate N. americanus infections.

Further reading

• Hotez, P.J., and D.I. Pritchard. 1995. Hookworm infection. Sci. Am. 272:68-74.
• Murray, P.R., K.S. Rosenthal, and M.A. Pfaller. 2009. Medical Microbiology, 6th ed. Elsevier/Mosby Publishing Company, Philadelphia, U.S.A.,865 p.
• Schmidt, G.D., and L.S. Roberts. 2009. Foundations of parasitology, 8th ed. McGraw-Hill Companies, New York, p. 472-473.