Necator americanus - MegaMicro

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Necator americanus

Microbiology > Parasitology > Nematodes
Human Hookworms:  Necator americanus and Ancylostoma duodenale
Structure: Helminth, Nematode (hookworms)

Epidemiology:
• Hookworm infections are common in the tropics and subtropics. There are two species of hookworm that cause human infections: Ancylostoma duodenale in Mediterranean countries, Iran, India, Pakistan and the Far East and Necator americanus in North and South America, Central Africa, Indonesia and South Pacific Islands.
• The prevalence of hookworm in rural SE United States was widespread in the 20th century, but has since been significantly reduced with extensive control measures.

Pathobiology:
• The hookworm life cycle begins with the passage of eggs from a host into the feces. On contact with soil, the eggs release  rhabditiform larvae that mature after 2 weeks into filariform larvae (infectious form). Infection is transmitted when the filariform larvae penetrate the skin of the human host (often occurring when walking barefoot).  Infection by A. duodenale may also occur by the oral route.
• From the skin, the larvae migrate to the blood vessels and are carried to the lungs.  They penetrate the pulmonary alveoli, ascend to the pharynx and are swallowed.  
• In the small intestine, the larvae mature into adult worms and attach to the intestinal wall.  Gravid female adult worms lay eggs within the bowel which are released into the feces.

Disease Manifestations:
• Skin penetrating larvae may produce itching and a rash at the site of entry.  
• Larvae migrating in the lungs generally produce no symptoms, but may result In pneumonitis and eosinophilia.
• Adult worms in the intestine may produce GI symptoms of nausea, vomiting and diarrhea.
• With heavy worm burdens, a microcytic hypochromic anemia may develop with blood loss from the site of intestinal attachment.
• In severe, chronic infections, emaciation, severe physical and mental retardation may occur due to blood and nutritional losses.

Laboratory Diagnosis:
• Stool exam for the presence of characteristic hookworm eggs (thin walled, enclosed developing larvae).  The eggs of N. americanus and A. duodenale are morphologically indistinguishable.
• Larvae rarely seen unless the stool specimen is left at ambient temperature so that the eggs hatch.
• Larvae are distinguishable:  A. duodenale larvae mouth parts contain chitinous teeth and N. americanus has shearing chitinous plates.
• Mild, unexplained eosinophilia.
• No reliable serology tests.

Differential Diagnosis:  Skin entry component: cutaneous larval migrans (infection with dog or cat hookworm), other intestinal parasites

Treatment:
• The drug of choice is albendazole or mebendazole for 1-3 days; alternatively, pyrantel pamoate.
• In severe cases of anemia, iron therapy may be necessary.

Prevention and Control:
• Improved sanitation and controlled disposal of human feces.
• Wearing shoes in endemic areas to prevent skin penetration by larvae.


Related concepts
Helminth, nematode, hookworm, Necator americanus, Ancylostoma duodenale
 
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