Necator Americanus: Biological Characteristics, Life Cycle, Symptoms and Treatment

Necator americanus is a species of parasitic worm belonging to the group of helminths, in which the elongated and soft worms that cause infections are found. The habitat of the adult parasite is the small intestine of humans, dogs and cats.

The term necatoriasis is used to indicate the condition of being infected with N. americanus, and it is considered a type of helminthiasis. This parasite is closely related to another similar species, called Ancylostoma duodenale , which belongs to the same family (Ancylostomidae) and has a similar life cycle.

In fact, infections caused by both parasites are collectively called hookworm or hookworm. This is because in some places they confuse the species of these worms and commonly they are known as hookworms.

Hookworm infection is the second helminth infection most common in humans, after ascariasis. It is also one of the most common chronic infections in the world, affecting billions of people in tropical and subtropical areas, particularly in China and sub-Saharan Africa.

The geographical distribution of these parasites is global; however, they are found mainly in regions with humid and warm climates. The presence of both species has been recorded, N. americanus Y A. duodenale , in the African, Asian and American continents.

Infections by N. americanus they can be effectively treated with anthelminthic drugs. However, in endemic areas, reinfection returns quickly. The larvae of N. americanus They possess key physicochemical properties that allow a successful infection of the host.

Hookworms are so common that they exceed the conditions caused by diabetes and lung cancer. Necator americanus It is the most frequent species of human parasite and, therefore, the most important from the point of view of public health.

Biological characteristics

Morphology

Necator americanus It is a worm of cylindrical shape and whitish color. It has a cuticle with three layers made of collagen and other compounds secreted by the epidermis. The cuticle layer protects the nematode so that it can invade the digestive tract of animals.

The females have a vulvar opening in the posterior part of the body and the males have a widening in the posterior end of their body, called copulatriz bursa.

Both the females and the males have a buccal structure with two pairs of cutting plates: one ventral and one dorsal. They also have glands that secrete important substances for the life cycle of the parasite, such as protease enzymes that degrade the proteins of the host's skin.

Its size ranges between 0.8 and 1.5 centimeters; however, in adulthood the females are a little larger than the males. On the other hand, the eggs vary in size from 65-75 micrometers x 36-40 micrometers and are practically indistinguishable from those of Ancylostoma duodenale .

The rhabditiform larva has a large bulb in the esophagus, separated from the rest of the esophagus by a region called the isthmus. On the other hand, the filariform larva does not have a bulb in the esophagus.

Habitat

The adults of N. americanus they are found exclusively in tropical and temperate regions, since eggs require a humid, warm and shaded environment to hatch. The optimal temperatures for juveniles to mature are between 23 to 30 degrees Celsius.

The eggs and juveniles die below the freezing point and also by the drying of the soil. Heavy rains and warmer temperatures seem to have a high positive correlation with the transmission rate. Necator americanus seems to prefer male guests to female ones.

However, this may be due to the division of labor in areas of high infestation. The type of soil also plays an important role in the habitat of these worms. Ideal soil conditions are where the water drains but not too fast.

Lifecycle

- The eggs come from the feces of an infected host. If the environmental conditions of light, temperature, humidity and nutrients are favorable, the eggs hatch.

- Approximately in two days the rhabditiform larva, which measures around 275 millimeters in length, matures. It feeds on bacteria and soil organic matter and doubles its size in five days.

- After two molts, it becomes a filariform larva, which has a protective cuticle and is infectious. In this state, the larva can survive up to six weeks.

- The infection occurs by direct contact with the skin of the host, usually through the hair follicles of the feet or legs.

- The larva travels through the bloodstream to the lungs, where it penetrates the alveoli, ascends to the pharynx and is swallowed by the host. This period of migration from the entrance of the parasite lasts approximately 1 week.

- After being swallowed, the larvae reach the wall of the small intestine, where they adhere and mature to become adult worms. These can live for years in the host's intestine, where each female can produce thousands of eggs daily, which will pass to the feces and repeat the cycle.

symptom

The symptomatology of necatoriasis can be divided into three phases. Initially, invasion of the larva produces irritation, inflammation and itching of the host's skin. These are reactions of the immune system that try to protect the organism that is being infected.

During the migration of the larva from the bloodstream to the lungs and throat, hemorrhages occur and the host presents a dry cough and sore throat.

Finally, when the larva is well established in the intestine of the host, abdominal pain, lack of appetite and, in some cases, desires to eat soil (geophagy) can occur.

It is believed that this need is due to the lack of minerals, particularly iron. Severe infections include severe anemia, protein deficiency, dry skin and hair, delayed development and learning (in children), and heart failure.

Diagnosis

The diagnosis of necatoriasis based on symptomatology can be misleading because the same symptoms can be the result of nutritional deficiencies or a combination of infection and these deficiencies.

The identification of the eggs in the feces is required for the diagnosis to be positive. In light infections, diagnostic techniques of the concentration type are used, such as flotation with zinc sulphate or several modifications of the formalin-ether method.

However, as the eggs of Necator americanus they are very similar to those of Ancylostoma duodenale, it is necessary to make a meticulous identification of the larvae, especially of the feces that have several days, since the rhbititiform larvae of hookworms are also very similar.

Treatment

The treatment of necatoriasis consists in the oral administration of benzimidazoles; for example: albendazole 400mg in a single dose, or mebendazole 100mg 2 times a day for 3 days. This is what is recommended by the World Health Organization.

However, as the eggs of Necator americanus they are present in contaminated soil, reinfection is frequent and there is concern that the parasites may develop resistance to the drugs.

Efforts have been made to develop vaccines against hookworm in order to avoid constant reinfections. Vaccines containing a mixture of adult proteins and infectious larvae of Necator americanus they are currently being tested.

References

1. Bethony, J., Brooker, S., Albonico, M., Geiger, S.M., Loukas, A., Diemert, D., & Hotez, P.J. (2006). Soil-transmitted helminth infections: ascariasis, trichuriasis, and hookworm. Lancet , 367 (9521), 1521-1532.
2. Becerril, M. (2011). Medical Parasitology (3rd ed.). McGraw-Hill.
3. Bogitsh, B., Carter, C. & Oeltmann, T. (2013). Human Parasitology (4 ^th ). Elsevier, Inc.
4. de Silva, N.R., Brooker, S., Hotez, P.J., Montresso, A., Engeles, D., and Savioli, L. (2003). Soil transmitted helminth infection: updating the global picture. Trends in Parasitology , 19 (12), 547-51.
5. Georgiev, V. S. (2000). Expert Opinion on Investigational Drugs Necatoriasis: treatment and developmental therapeutics. Expert Opinion on Investigational Drugs , 1065-1078.
6. Hotez, P.J., Bethony, J.M., Diemert, D.J., Pearson, M., & Loukas, A. (2010). Developing vaccines to combat hookworm infection and intestinal schistosomiasis. Nature Reviews Microbiology , 8 (11), 814-826.
7. Keizer, J., & Utzinger, J. (2009). Efficacy of Current Drugs Against Soil-Transmitted Helminth Infections. Clinician 'S Corner , 293 (12), 1501-1508.
8. Phosuk, I., Intapan, P.M., Thanchomnang, T., Sanpool, O., Janwan, P., Laummaunwai, P.,... Maleewong, W. (2013). Molecular detection of Ancylostoma duodenale, Ancylostoma ceylanicum, and Necator americanus in humans in northeastern and southern Thailand. Korean Journal of Parasitology , 51 (6), 747-749.