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 hookworms or hookworms. This is because in some places they confuse the species of these worms and are commonly known as hookworms.
Hookworm is the second most common helminth infection in humans, after ascariasis. It is also one of the most common chronic infections in the world, affecting billions of people in the tropics and subtropics, particularly in China and sub-Saharan Africa.
The geographical distribution of these parasites is global; however, they are found mainly in regions with hot and humid climates. Both species, N. americanus and A. duodenale, have been recorded on the African, Asian and American continents.
N. americanus infections can be effectively treated with anthelmintic drugs. However, in endemic areas reinfection returns quickly. N. americanus larvae possess key physicochemical properties that allow successful host infection.
Hookworms are so common that they exceed the conditions caused by diabetes and lung cancer. Necator americanus is the most common species of human parasite and therefore the most important from a public health point of view.
Biological characteristics
Morphology
Necator americanus is a whitish cylindrical worm. It has a three-layered cuticle 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.
Females have a vulvar opening at the back of the body, and males have a widening at the rear end of their body, called a copulatory bursa.
Both females and males have a buccal structure with two pairs of cutting plates: one ventral and one dorsal. They also have glands that secrete substances important to the parasite's life cycle, such as protease enzymes that break down the host's skin proteins.
Its size ranges from 0.8 to 1.5 centimeters; however, as adults, females are slightly larger than males. For their part, the eggs vary in size from 65-75 microns x 36-40 microns and are practically indistinguishable from those of Ancylostoma duodenale.
The rhabditiform larvae have a large bulb in their esophagus, separated from the rest of the esophagus by a region called the isthmus. For its part, the filariform larva does not have a bulb in the esophagus.
Habitat
N. americanus adults are found exclusively in tropical and temperate regions, as the eggs require a humid, warm and shady environment to hatch. Optimum temperatures for juveniles to mature are between 23 to 30 degrees Celsius.
Eggs and juveniles die below freezing and also from drying out of the soil. Heavy rains and warmer temperatures appear to have a high positive correlation with the transmission rate. Necator americanus appears to prefer male hosts to female hosts.
However, this may be due to the division of labor in high infestation areas. Soil type also plays an important role in the habitat of these worms. Ideal soil conditions are where the water drains but not too quickly.
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 will hatch.
- The rhabditiform larva matures in approximately two days, measuring about 275 millimeters in length. It feeds on bacteria and organic matter in the soil and doubles in 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, generally through the hair follicles of the feet or legs.
- The larva travels through the bloodstream to the lungs, where it penetrates the alveoli, ascends towards the pharynx and is swallowed by the host. This period of migration from the entry 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 into the feces and repeat the cycle.
Symptoms
The symptoms of necatoriasis can be divided into three phases. Initially, the invasion of the larva causes irritation, inflammation and itching of the host's skin. These are reactions of the immune system that tries to protect the organism that is being infected.
During migration of the larvae from the bloodstream to the lungs and throat, hemorrhages occur and the host develops a dry cough and sore throat.
Finally, when the larvae are well established in the host's intestine, abdominal pain, lack of appetite and, in some cases, a desire to eat dirt (geophagia) may occur.
This need is believed to be due to a lack of minerals, particularly iron. In the most serious infections, severe anemia, protein deficiency, dry skin and hair, delayed development and learning (in children), and heart failure occur.
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.
For the diagnosis to be positive, the identification of the eggs in the feces is required. In mild infections, diagnostic techniques of the concentration type are used, such as flotation with zinc sulfate or various modifications of the formalin-ether method.
However, since the eggs of Necator americanus are very similar to those of Ancylostoma duodenale, careful identification of larvae is necessary, especially from feces that are several days old, as hookworm rhabditiform larvae also look very similar.
Treatment
The treatment of necatoriasis consists of the oral administration of benzimidazoles; for example: 400mg albendazole in a single dose, or 100mg mebendazole 2 times a day for 3 days. This is recommended by the World Health Organization.
However, as Necator americanus eggs are present in contaminated soil, reinfection is common and there is concern that parasites may develop drug resistance.
Efforts have been made to develop hookworm vaccines to prevent constant reinfections. Vaccines containing a mixture of proteins from infectious Necator americanus adults and larvae are currently being tested.
References
- Bethony, J., Brooker, S., Albonico, M., Geiger, SM, Loukas, A., Diemert, D., & Hotez, PJ (2006). Soil-transmitted helminth infections: ascariasis, trichuriasis, and hookworm. Lancet, 367 (9521), 1521–1532.
- Becerril, M. (2011). Medical Parasitology (3rd ed.). McGraw-Hill.
- Bogitsh, B., Carter, C. & Oeltmann, T. (2013). Human Parasitology (4 th). Elsevier, Inc.
- de Silva, NR, Brooker, S., Hotez, PJ, Montresso, A., Engeles, D., and Savioli, L. (2003). Soil transmitted helminth infection: updating the global picture. Trends in Parasitology, 19 (12), 547–51.
- Georgiev, VS (2000). Expert Opinion on Investigational Drugs Necatoriasis: treatment and developmental therapeutics. Expert Opinion on Investigational Drugs, 1065–1078.
- Hotez, PJ, Bethony, JM, Diemert, DJ, Pearson, M., & Loukas, A. (2010). Developing vaccines to combat hookworm infection and intestinal schistosomiasis. Nature Reviews Microbiology, 8 (11), 814–826.
- Keizer, J., & Utzinger, J. (2009). Efficacy of Current Drugs Against Soil-Transmitted Helminth Infections. Clinician 'S Corner, 293 (12), 1501–1508.
- Phosuk, I., Intapan, PM, 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.