Necator americanus

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Jonah Lester
Necator americanus specimen. Source: Yale Peabody Museum of Natural History, CC0, via Wikimedia Commons

What is it Necator americanus?

Necator americanus, also known as the "new world hookworm”, Is a species of parasitic nematode that infects the intestinal tissues of human beings that live in the poorest tropical and subtropical areas of the planet, affecting almost 500 million people worldwide.

Nematodes, called "roundworms", are part of a very abundant group of invertebrate animals - the Phylum Nematoda - that includes more than 20,000 species. This is the third most important phylum of invertebrates and is comprised of both free-living worms and parasites of plants and other animals, vertebrates and invertebrates..

The parasitic species of nematodes, then, are of great importance from the sanitary, veterinary and agricultural point of view, since they usually cause relatively serious diseases in the organisms they parasitize..

Among the parasitic nematodes, the hookworms or hookworms (from the English hookworms), which are generally transmitted through the soil. Ancylostoma duodenale Y Necator americanus (Ancylostomatidae family) are two of the most important species that infect humans, responsible for conditions known as hookworm or hookworm.

Ancylostoma duodenale It is known as the “old world hookworm” and is found mainly in Europe, Africa, important parts of Asia and the Pacific Islands. Necator americanus, the “new world hookworm” mainly affects the populations of America and the Caribbean, although reports indicate that it has been found in Africa, Asia and the Pacific as well..

These parasites lodge in the digestive system, mainly in the small intestine, where they suck blood from this tissue and cause what is called “iron deficiency anemia”; some patients may even manifest other conditions related to the migration of the parasite to other tissues.

There is no vaccine against these parasites, but the infection is effectively treated with anthelmintics, although patients tend to be quickly reinfected in endemic areas where there is no sanitary control.

Characteristics of Necator americanus

Necator americanus is a parasitic worm (helminth) belonging to the phylum Nematoda and the family Ancylostomatidae.

It is a species of roundworm that proliferates in the small intestine of humans that infects and has a translucent white color..

Life forms

In the life cycle of Necator americanus at least three life forms are distinguished: adult worms, eggs, and filariform larvae.

  • Adults

Preserved specimen of Necator americanus. Source: Yale Peabody Museum of Natural History, CC0, via Wikimedia Commons

N. americanus it is a dioecious species, that is, it has two sexes: one female and one male, separated into different individuals.

Adult male worms are about 9 millimeters long and 0.5 millimeters in diameter, while female worms are 9 to 11 millimeters long and the same 0.5 millimeters in diameter as males..

Both adult worms have, in their oral cavity, two sharp plates that function as hooks and that allow them to penetrate the host's tissues and anchor in them to feed and sustain themselves..

  • Eggs

Egg of N. americanus. Source: SuSanA Secretariat, CC BY 2.0 , via Wikimedia Commons

The eggs of N. americanus They have an oval shape and a thin layer that covers and protects them. They are 50 to 80 microns long and 30 to 50 microns wide.

  • Larvae

The larvae of this parasite are of two types and are known as "rhabditiform" larvae or first stage larvae and "filariform" or third stage larvae..

Rhabditiform larvae have a maximum length of 300 microns and a tail with a sharp, narrow terminal end..

The filariform larvae, on the other hand, have a maximum length of 500 microns, they do not have the plates or hooks that adult worms have, but they do have a significant capacity for movement. Their body appears more robust than that of rhabditiform larvae, but they are not easily distinguishable from each other..

Under certain conditions, these larvae can be encapsulated or "sheathed", forming resistance structures that are difficult to eliminate..

 Geographical distribution

This parasite is considered to have a worldwide distribution, although the most important endemic areas correspond to those of the American continent (North America, Central America and South America) and the Caribbean islands, especially those with the lowest socioeconomic conditions and with the hottest and humid climates..

It has also been found in Southeast Asia, India, China, some islands in the Southwest Pacific and in the sub-Saharan region of the African continent..

Hosts and reservoirs

It is considered that N. americanus It is a nematode that exclusively infects humans, but various reports agree that it can also affect pigs.

The main natural reservoirs for this parasite are consequently such farm animals, humans, the soil, bodies of water and their related vegetation..

Life cycle of Necator americanus

The life cycle of N. americanus it is a direct life cycle, which means that it does not require any intermediate host or vector to infect its definitive hosts: humans. Let's see the steps that are completed during the life cycle of this parasite:

  1. Inside an infected human being, the adults produce eggs that are released with the feces. In the right environmental conditions (humidity, shade and heat) after 1 or 2 days, the eggs hatch, releasing the free-living rhabditiform larvae that grow between the feces and the soil contaminated with them.
  2. After 5 to 10 days in the soil, going through two molts, the rhabditiform larvae turn into filariform larvae or stage 3 larvae, which represent the infective stage of the parasite. These larvae can live up to 4 weeks in the soil, if environmental conditions allow it..
  3. When they come into contact with a human being, which generally occurs when an individual walks barefoot on soil, water, or vegetation contaminated with feces from infected humans, filariform larvae penetrate the skin of the feet and travel through the bloodstream to the heart. and the lungs. Once there, they penetrate the pulmonary alveoli, ascend through the bronchial tree and pharynx and are then swallowed, reaching the digestive system.
  4. In the jejunum region of the small intestine, the larvae mature into adult forms, male and female, that live and feed in the lumen of the small intestine, attached to said tissue thanks to their mouth hooks, which they use for feeding. and that cause blood loss in the host.
  5. Adult worms can live for periods of 1 to 2 years in the gut of their host, but this time can be much longer..
  6. In the intestine, adults reproduce and release their eggs into the small intestine, which are expelled with the feces, thus completing the life cycle. Some of these eggs can hatch directly in the digestive system and make their way to other organs, where there may be clinical manifestations as well..

Caused diseases and their symptoms

The pathological condition produced by Necator americanus is known as necatoriais, hookworm or hookworm infection and is almost always asymptomatic. The most common patients include both school-age children and adults, although the latter less frequently.

Cutaneous penetration (through the skin) of filariform larvae is usually imperceptible, but discomfort such as hives, burning and rash may occur in the first instance. When the larvae migrate into the respiratory system, a mild cough and irritation of the pharynx can be observed, but this is in very few cases.

Infection of Necator americanus can cause rashes

The main symptom of an abundant parasitosis is the development of iron deficiency anemia, due to the loss of blood caused by adults attached to the intestinal walls when feeding on said blood.

Other symptoms include:

  • Headache.
  • Sickness.
  • Pain in the extremities and joints.
  • Abdominal pain.
  • Dyspnoea.
  • Chronic fatigue.

The diagnosis of this condition generally consists of microscopic observation of the feces in search of eggs or larvae of the parasite..

Treatment

Millions of people in the world are at permanent risk of contracting the disease caused by the nematode Necator americanus, since this is one of the most common intestinal parasites that exist.

Treatment to eliminate adult worms from the body is usually effective and involves the administration of antimicrobial or anthelmintic drugs such as albendazole, nitazoxanide, mebendazole, etc..

There is no preventive vaccination for this parasitosis, so disinfection and rigorous sanitary control of both water and food consumed daily are usually recommended, especially in endemic regions..

The correct disposal and inactivation of human feces is one of the most effective preventive mechanisms, since it "cuts" the life cycle of the parasite, reducing the possibility of contact of a human being with infective larvae..

References

  1. Brusca, R. C., & Brusca, G. J. (2003). Invertebrates (No. QL 362. B78 2003). Basingstoke.
  2. Hossain, M., & Bhuiyan, J. U. (2016). Hookworm infection: A neglected tropical disease of mankind. Journal of Advanced Veterinary and Animal Research, 3 (4), 297-320.
  3. Kucik, C. J., Martin, G. L., & Sortor, B. V. (2004). Common intestinal parasites. American family physician, 69 (5), 1161-1168.
  4. Loukas, A., Hotez, P. J., Diemert, D., Yazdanbakhsh, M., McCarthy, J. S., Correa-Oliveira, R.,… & Bethony, J. M. (2016). Hookworm infection. Nature Reviews Disease Primers, 2 (1), 1-18.
  5. Silva-Díaz, H. (2018). Morphological differences relevant for the specific identification of hookworm larvae and Strongyloides stercoralis. Revista Medica Herediana, 29 (4), 211-216.

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