Necator americanus

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)

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)

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)

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)

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.

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

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)

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)

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)

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.

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)

None

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)

None

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)