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Necator americanus

By Harlen Hays

Geographic Range

Necator americanus is found in Africa, Asia, and Europe but is predominately found in the Americas and in Australia. In the United States, the largest concentration is found in the southern and southwestern United States. In the rest of the world, N. americanus is found in tropical climates. (Behnke, et al., 2000; Romstad, et al., 1998)

Habitat

Adult N. americanus are found exclusively in tropical and temperate regions. Eggs require a moist, warm and shaded environment to hatch. Optimal temperatures for juveniles to mature are from 23 to 30 deg C. Eggs and juveniles die below freezing or with soil dessication. Heavy rains and warmer temperatures appear to have a high positive correlation with the rate of transmission. Necator americanus appears to prefer male hosts to female hosts. However, this may be due to the division of labor in areas of high infestation. Soil type also plays a major part in the habitat for these worms. Ideal soil conditions are where the water drains but not too quickly. The pore size of the soil particle is a major factor. (Appleton, et al., 1999; Behnke, et al., 2000; Roberts and Janovy, Jr, 2000)

  • Aquatic Biomes
  • lakes and ponds
  • temporary pools

Physical Description

As a nematode, Necator americanus has a cylindrical body, and a cuticle with three main outer layers made of collagen and other compounds, secreted by the epidermis. The cuticle layer protects the nematode so it can invade digestive tracts of animals.

Eggs range in size from 65-75 micrometers x 36-40 micrometers and are virtually indistinguishable from those of Ancylostoma duodenale, another common hookworm. Necator americanus has four larval stages. The first stage is referred to as rhabditiform larvae because the esophagus has a large bulb separated from the rest of the esophagus by a region called the isthmus. The third stage is referred to as filariform larvae because the esophagus has no bulb. Adult females range in size from 9 mm to 11 mm while the smaller males range in size from 7 mm to 9 mm. The mouth of the adults has two pair of cutting plates, one dorsal and the other ventral. The males of the species are characterized by fused spicules found on the bursa. The common name "hookworm" comes from the dorsal curve at the anterior end. (Barnes, 1987; Brusca and Brusca, 2003; Roberts and Janovy, Jr, 2000)

  • Sexual Dimorphism
  • female larger
  • sexes shaped differently
  • Range length
    7 to 11 mm
    0.28 to 0.43 in

Development

Necator americanus adults are obligate internal parasites of humans. Both the first and second stage rhabditiform larvae are free-living. Eggs are passed out through the feces of humans. These eggs will hatch within 2 days, and the first rhabditiform larva emerges. This larva will molt twice within 10 days to become a third stage filariform larva. The filariform larva will come into contact with the host's skin, and burrow into it. Travelling through the circulatory system to the lungs, the third stage larva will either be coughed up and swallowed or will migrate up the bronchi to the throat area. Once swallowed, the larva will make its way to the intestine. Upon reaching the intestine, the larva will molt twice and an adult will emerge. This adult will use its hooks to attach itself to the inner wall of the intestine and will cause hemorrhaging. The worm will feed from this blood. Mating occurs in the intestine. The eggs are the passed out with the feces. (Liu, et al., 1999; Roberts and Janovy, Jr, 2000)

Reproduction

Sexual maturity is reached at the final molt. Egg production in females occurs five weeks or more after the female matures. Mating occurs in the intestine of the host.

Males are required to find females and inject their sperm into the females. Females may produce a pheromone to attract males. The male coils around a female with his curved area over the female genital pore. The gubernaculum, made of cuticle tissue, guides spicules which extend through the cloaca and anus. Males use spicules to hold open the genital opening on the female to allow transfer of sperm, which are amoeboid-like and lack flagella. The fertilized females then lay eggs in the surrounding areas. Females are capable of producing 10,000 eggs per day. The eggs are the passed out with the feces. (Barnes, 1987; Brusca and Brusca, 2003; Roberts and Janovy, Jr, 2000)

There is no parental investment beyond egg laying.

  • Parental Investment
  • pre-fertilization
    • provisioning

Lifespan/Longevity

The adult worms can live up to 5 years. (Liu, et al., 1999; Roberts and Janovy, Jr, 2000)

Behavior

Nector americanus larva can moves different areas within the body of its host. The filariform larva will come into contact with the host's skin, and burrow into it. Travelling through the circulatory system to the lungs, the third stage larva will either be coughed up and swallowed or will migrate up the bronchi to the throat area. Once swallowed, the larva will make its way to the intestine. Upon reaching the intestine, the larva will molt twice and an adult will emerge. (Liu, et al., 1999; Roberts and Janovy, Jr, 2000)

Communication and Perception

Nematodes within the Secernentea have phasmids, which are unicellular glands. Phasmids likely function as chemoreceptors. Females may produce pheromones to attract males.

Nematodes in general have papillae, setae and amphids as the main sense organs. Setae detect motion (mechanoreceptors), while amphids detect chemicals (chemoreceptors). (Barnes, 1987; Brusca and Brusca, 2003)

Food Habits

Adult N. americanus feed from the blood of their hosts. The worm will attach itself to the intestinal wall and use its cutting plates to cause bleeding. The worm feeds from this blood, possibly causing anemia to the host. Necator americanus does not permanently attach itself to the wall. This allows movement to new sites for feeding and reproduction within the host. Previous sites continue to bleed, adding the host's blood loss. (Blaxter, 2000)

  • Animal Foods
  • blood
  • body fluids

Predation

These parasites are probably not preyed on directly, but are ingested from host to host. Larval mortality is high as most of the parasites do not reach appropriate hosts.

Ecosystem Roles

Necator americanus mainly infects humans and appears to prefer male hosts to female hosts. However, this may be due to the division of labor in areas of high infestation. (Appleton, et al., 1999; Behnke, et al., 2000)

Species Used as Host

Economic Importance for Humans: Positive

None

Economic Importance for Humans: Negative

As a strict parasite of humans, N. americanus has played a major role in the development of the New and Old worlds. Large numbers of worms cause what is known as "hookworm disease".

Nutrition and blood loss are the major contributors to ill effects from the interaction between host and parasite. The severity of the disease is directly related to the number of worms in the host's body. Each worm is capable of ingesting .03 ml of blood per day. When small amounts of worms have infected a host, the patient will be asymptomatic. Once infections reach 25 to 100 worms, the patient will experience light symptoms that include fatigue, slight weight loss, and possible headaches. Once infestations reach between 100-500 the patient will experience fatigue, iron deficiency leading to anemia, loss of appetite and abdominal pains. Should the infestation reach over 500, the patient will experience anemia and, depending on the diet of the person, possibly death. Children are at greater risks for lifelong damage and death due to their smaller size and greater need for nutrition. Generally infestation does not lead to death, but many cause permanent damage. The mortality rate for N. americanus is around .005% while the morbidity rate is 12%.

Treatment of N. americanus infections is simple, but resistance has been detected to usual treatments. Preventive measures include proper waste disposal, clean water, and proper footwear. Since the larvae need direct skin contact to invade the host, protective clothing is a key to maintaining low incidence rates. (Behnke, et al., 2000; Blaxter, 2000; Roberts and Janovy, Jr, 2000)

Conservation Status

None

Other Comments

At present, research is being done on the possibility that pigs are intermediate hosts for N. americanus. Research has shown that the approximately 15% of eggs ingested by a pig host will pass through in the feces. (Steenhard, et al., 2000)

Contributors

Renee Sherman Mulcrone (editor).

Harlen Hays (author), University of Michigan-Ann Arbor, Barry OConnor (editor), University of Michigan-Ann Arbor.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

agricultural

living in landscapes dominated by human agriculture.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

causes disease in humans

an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).

chemical

uses smells or other chemicals to communicate

cosmopolitan

having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

marsh

marshes are wetland areas often dominated by grasses and reeds.

motile

having the capacity to move from one place to another.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

parasite

an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

sanguivore

an animal that mainly eats blood

scrub forest

scrub forests develop in areas that experience dry seasons.

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

suburban

living in residential areas on the outskirts of large cities or towns.

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

References

Appleton, C., M. Maurihungirire, E. Gouws. 1999. The distribution of helminth infections along the coastal plain of Kwazulu-Natal province, South Africa.. Annals of Tropical Medicine & Parasitology, 93 (No.8): 859-868.

Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.

Behnke, J., D. Clercq, M. Sacko, F. Gilbert, D. Ouattara. 2000. The epidemiology of human hookworm infections in the southern region of Mali.. Tropical Medicine and International Health, 5 (No 5): 343-354.

Blaxter, M. 2000. Genes and genomes of Necator americanus and related hookworms. International Journal of Parasitology, 30: 347-355.

Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..

Liu, C., Z. Xiaorong, Q. Dongchuan, X. Shuhua, P. Hotez. 1999. Epidemiology of human hookworm infection among adults villagers in Hejiang and Santai Counties, Sichuan Province, China. Acta Tropica, 73: 243-249.

Roberts, L., J. Janovy, Jr. 2000. Gerald D. Schmidt & Larry S. Roberts’ Foundations of Parasitology, Sixth Edition. Boston: McGraw-Hill Companies.

Romstad, A., R. Gasser, P. Nansen, A. Polderman, N. Chilton. 1998. Necator americanus (Nematoda: Ancylostomatidae) from Africa and Malaysia have different ITS-2 rDNA sequences. International Journal of Parasitology, 28: 611-615.

Steenhard, N., P. Storey, L. Yelifari, D. Pit, P. Nansen. 2000. The role of pigs as transport host of the human helminths Oesophagostomum bifurcum and Necator americanus. Acta Tropica, 76: 125-130.

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