Necator americanus is a species of hookworm 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 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.
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.
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.
N. americanus was first discovered in Brazil and then was found in Texas. Later, it was found to be indigenous in Africa, China, southwest Pacific islands, India, and Southeast Asia. This parasite is a tropical parasite and is the most common species in humans. Roughly 95% of hookworms found in the southern region of the United States are N. americanus. This parasite is found in humans, but can also be found in pigs and dogs.
Transmission of N. americanus infection requires the deposition of egg-containing feces on shady, well-drained soil and is favored by warm, humid (tropical) conditions. Therefore, infections worldwide are usually reported in places where direct contact with contaminated soil occurs.
When adult worms attach to the villi of the small intestine, they suck on the host's blood, which may cause abdominal pain, diarrhea, cramps, and weight loss that can lead to anorexia. Heavy infections can lead to the development of iron deficiency and hypochromic microcytic anemia. This form of anemia in children can give rise to physical and mental retardation. Infection caused by cutaneous larvae migrans, a skin disease in humans, is characterized by skin ruptures and severe itching.
The most common technique used to diagnose a hookworm infection is to take a stool sample, fix it in 10% formalin, concentrate it using the formalin-ethyl acetate sedimentation technique, and then create a wet mount of the sediment for viewing under a microscope. However, the eggs of A. duodenale and N. americanus cannot be distinguished; thus, the larvae must be examined to identify these hookworms. Larvae cannot be found in stool specimens unless the specimen was left at ambient temperature for a day or more.
Education, improved sanitation, and controlled disposal of human feces are critical for prevention. Nonetheless, wearing shoes in endemic areas helps reduce the prevalence of infection.
An infection of N. americanus parasites can be treated by using benzimidazoles, albendazole, and mebendazole. A blood transfusion may be necessary in severe cases of anemia. Light infections are usually left untreated in areas where reinfection is common. Iron supplements and a diet high in protein will speed the recovery process. In a case study involving 56-60 men with Trichuris trichiura and/or N. americanus infections, both albendazole and mebendazole were 90% effective in curing T. trichiura. However, albendazole had a 95% cure rate for N. americanus, while mebendazole only had a 21% cure rate. This suggests albendazole is most effective for treating both T. trichiura and N. americanus.
Cryotherapy by application of liquid nitrogen to the skin has been used to kill cutaneous larvae migrans, but the procedure has a low cure rate and a high incidence of pain and severe skin damage, so it now is passed over in favor of suitable pharmaceuticals. Topical application of some pharmaceuticals has merit, but requires repeated, persistent applications and is less effective than some systemic treatments.
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