Necator americanus
A. Classification
Phylum: Nematoda
Necator americanus
Common name: “Hookworm"
Its literal meaning is “American Murderer” because of the large amounts of blood it robs
from the human host’s body.
B. Morphology
The male adult measures 7 to 9 mm long, while the female is slightly larger measuring 9 to 11 mm.
The anterior of the worm curves in the opposite direction of the body curvature resulting in a hooked
appearance. The mouthparts of hookworms have buccal capsules with cutting plates on the ventral
surface and small teeth inside the capsule on the dorsal surface.
The posterior end of the male has a copulatory bursa, which is used to catch and hold the female
during mating. The females have a vulva near the center of the body, slightly anterior.
Above: Female vs. Male
Above left: mouth with cutting plates
Left: Copulatory bursa of male
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The eggs of Necator americanus range 57-76 m by 35-47m. They are oval or ellipsoidal with a thin
shell. The eggs cannot be distinguished from the eggs of Ancylostoma duodenale.
Above left: Eggs in stool
C. Lifecycle and Epidemiology
The adult Necator americanus worm can be found in the lumen of human small intestines.
The eggs are passed in the feces and usually hatch in 1 to 2 days. The eggs need a shady, moist, and
warm environment in order to survive. The eggs release the rhabdidiform larvae or J1 which can
grow in feces or soil. After 5 to 10 days, the J1 will molt twice and become the filariform or J3
larvae. This is the infective stage of the worm and it can survive for 3 to 4 weeks in favorable
environmental conditions. Once the worm makes contact with a potential human host, the J3 can
penetrate the skin. It then migrates through the veins to the heart and then lungs. The J 3 then
penetrates into the alveoli and ascend into the throat until swallowed. The J3 will continue on until it
reaches the small intestine, where it attaches to the intestinal wall and matures into an adult worm.
The usual life span for the adult worm is from 1 to 2 years.
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D. Geographic Distribution
The Necator americanus is the second most common human helmith infection. It infects
over 1 billion people worldwide. It thrives in warm, moist climates and is said to predominate in the
Americas and Australia. As you can see in the map below, it basically infects much of the area
called the tropics. This is the area between the Tropic of Cancer and the Tropic of Capricorn.
Geographic distribution of Necator americanus
<http://tmcr.usuhs.mil/tmcr/chapter12/geographical.htm>
E. Pathology and Symptoms
The pathology of Nector americanus can be divided into two areas: larvae and adult worms. The
larvae penetrate the skin and travel through the lymph nodes. Once in the lymph nodes, the larvae
migrate between the bloodstream, lungs, and small intestine. The most common symptoms of this
infective stage are itching due to skin penetrations and respiratory problems during the pulmonary
migration of the larvae. The adult worms attach to the villi of the small intestines. The villi are tiny,
fingerlike projections on the surface of the small intestine that help absorb nutrients. Once attached
to the wall of the small intestinal, the hookworm’s mouthparts penetrate blood vessels and suck
blood. One Necator americanus will take approximately 30 µl of blood daily. Light affections
appear to be asymptomatic. Heavier infections include abdominal discomfort, diarrhea, cramps,
anorexia, and weight loss. The loss of nutrition in the blood can lead to iron deficiency and anemia.
In more severe cases, iron deficiency anemia can cause physical and mental retardation in children.
Death is rare in humans.
Adult attached to the small intestine.
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Migrating larvae in tissue
.
F. Diagnosis
The most common method for diagnosing hookworm is through identification of eggs in a fecal
sample.
G. Treatment
The chosen drug for treating Necator americanus is Albendazole. A study of Albendazole in
treating Necator americanus in Sri Lanka has shown a 77.9% cure rate and a 94.5% reduction in egg
count. An alternative drug is Mebendazole which has shown a 95% cure rate and a 99.9% reduction
in egg count. However, Mebendazole is more costly and less convenient than Albendazole. Iron
supplements should be included in the treatment of individuals with Necator americanus.
H. Public Health Strategies and Eradication
Eradication is difficult or impossible due to the nature of Necator americanus. Like other worms we
have studied, N. americanus locates itself in the human body where it is not easily controlled by the
human immune system. In addition, poorer countries do not have the funds to improve sewage
treatments. Therefore, it is important to educate a community on the importance of proper sanitation
and investing in footwear. Some communities have invested in community-wide albendazole
chemotherapy every 18 months.
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Works Cited
Blaxter, McCarter, “Necator americanus,” Human Hookworms,
<http://nema.cap.ed.ac.uk/nematodeESTs/Necator/Necator.html>, 22 February 05
Center for Disease Control. 21 February 2005. “Laboratory Identification of Parasites and Public
Health Concern: Hookworm”. 20 February 2005.
<http://www.dpd.cdc.gov/dpdx/HTML/Hookworm.asp?body=Frames/GL/Hookworm/body_Hookworm_page1.htm>
Center for Disease Control, 22 February 05, “Hookworm,” Parasites and Health,
<http://www.dpd.cdc.gov/DPDx/HTML/Hookworm.htm>, 22 February 05
Haburchak, David R. 7 June 2002. “Hookworms.” 21 February 2005.
<http://www.emedicine.com/med/topic1028.htm>.
“Hookworm.” Center for Disease Control. 21 February 2005.
<http://www.dpd.cdc.gov/DPDx/HTML/Hookworm.asp?body=Frames/GL/Hookworm/body_Hookworm_page1.htm>.
“Material Safety Data Sheet: Infectious Substances.” Public Health Agency of Canada.
21 February 2005. <http://www.phac-aspc.gc.ca/msds-ftss/msds107e.html>.
Merck Manual of Medical Information: “Hookworm Infection”. 20 February 2005.
<http://www.merck.com/mmhe/sec17/ch196/ch196g.html>
“Necator americanus and Ancylostoma duodenale.” Cam.University Schistosome
Research Group. 21 February 2005.
<http://www.path.cam.ac.uk/~schisto/Nematodes/hookworm.html>.
Public Health of Canada. 23 May 2001. “Material Safety Data Sheet: Infectious Diseases”. 20
February 2005. <http://www.phac-aspc.gc.ca/msds-ftss/msds107e.html>
University of Cambridge. 5 October 1998. “Helminthology and General Parasitology Pages
Nematode Parasitic Infections: Necator americanus”. 20 February 2005.
<http://www.path.cam.ac.uk/~schisto/Nematodes/hookworm.html>
Springer, “Geographic Distribution,” Tropical Medicine Central Resource,
<http://tmcr.usuhs.mil/tmcr/chapter12/geographical.htm>, 22 February 05
Erin Hoffman
Carrie Graham
Carrie Barker
February 2005
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