Necator americanus

Necator americanus is a nematode that is one of several nematodes causing “hookworm” infections. Necator americanus is described as the New World hookworm. The name necator means “killer’ or “murderer”.

Disease / Pathology

Necator americanus initially produce an intense itch or papule at the site of skin penetration. Though bronchial pneumonitis can occur in the lung stage of the parasite, this is uncommon and only seen in very heavy infections. The penetration of the worm into the intestinal wall will cause host blood loss by two causes. The first is by the ingestion of blood by the worm(s) and the other is by blood loss due to hemorrhaging at the site of attachment. This will manifest itself as a microcytic / hypochromic anemia in the host when infections are heavy and will lead to a chronic infection that produces symptoms such as pallor, listlessness, facial and pedal edema. In children infections can result in mental and physical development to be impaired.

Intestinal symptoms include flatulence and diarrhea. This may lead to weight loss, dizziness, loss in strength. The loss of blood chronically may also lead to iron deficiency. Nutritional supplementation may be needed to counteract the effects of the infection as the patients may also suffer from hypoproteinemia. In heavy infections not even nutritional supplementation can negate the losses.

In areas where infections are high there is a significant loss in human productivity due to hookworm infections.

Location in the Host

The adult Necator americanus is located attached to the mucosa of the small intestine. Developing larva are in the alveoli of the lung.

Geographic Distrubution

Necator americanus can be found throughout the southern United States, Mexico, Central and South America, the West Indies. It is also found in Africa where it is thought to have originated from during the slave trade, northern Australia, and southern India. Unlike Ancylostoma, Necator americanus eggs will not survive temperatures below 7 degrees centigrade and thus will not be found in geographically temperate regions where temperatures may reach these lows.

Life Cycle

The filariform larva is the infective form. This developmental form penetrates the skin (usually the feet since the form will be found in moist soil). Therefore the use of footwear will dramatically act as protection from infection with the filariform larva.

The larva remain inactive in the epidermis for about 40 hours before they begin to move into the cutaneous blood vessels and migrate to the heart and then to the lungs where further development occurs in the alveoli.

Eventually they ascend up the respiratory tree to the epiglottis to then descend down to the upper levels of the small intestine. Here they develop into adult worms. The worms copulate and begin to shed eggs (about 5,000 eggs daily per female). Host penetration to production of eggs occurs in about 6 weeks and the adults can live in the host for up to 14 years.

Morphology & Diagnosis

Necator americanus eggs found in the feces is diagnostic.

Images

Hookworm egg in Lugols iodine prep – note the thin shell and translucent interior with developing ova – photo by W. Vientos