Ancylostoma duodenale- An Overview
Ancylostoma duodenale is a parasitic roundworm that infects the small intestine of humans and causes a disease called hookworm infection. Hookworm infection is one of the most common neglected tropical diseases, affecting more than 500 million people worldwide, especially in poor and rural areas. Hookworm infection can cause anemia, malnutrition, growth retardation, and cognitive impairment in children and adults.
Ancylostoma duodenale belongs to the phylum Nematoda, which comprises of roundworms that have a cylindrical body with a complete digestive system and a pseudocoelom. Nematodes are the most diverse and abundant animals on Earth, with about 25,000 described species and an estimated 1 million undescribed species. They can be found in almost every habitat, from marine to freshwater, from soil to plants, and from animals to humans.
Habits and Habitat of Ancylostoma duodenale
- The adult worms are endoparasitic and live in the small intestines of infected persons, mostly in the jejunum, less often in the duodenum, and infrequently in the ileum.
- The infective juveniles enter the human host percutaneously from the soil contaminated by the feces in which they abound.
- It can also be transmitted orally, and probably transplacentally.
- They flourish under primitive conditions where people go barefoot, modern sanitary conditions do not exist and human feces are deposited on the ground.
- They are common in the tropical, subtropical, and temperate region of Asia, Africa, Europe, the Pacific Islands, and the Southern States of America.
Morphology of Ancylostoma duodenale
Ancylostoma duodenale is a small cylindrical worm with a curved body and a whitish or grayish color. The anterior end of the worm is narrow and bent dorsally, forming a hook-like shape. The posterior end of the worm is blunt and tapered. The body is covered by a thin cuticle with longitudinal ridges.
The mouth of the worm is located dorsally and surrounded by a prominent buccal capsule that has six teeth. Four of these teeth are hook-like and located ventrally, while two are knob-like or lancet-shaped and located dorsally. The teeth are used to attach the worm to the intestinal mucosa of the host and to suck blood.
The digestive system consists of a muscular pharynx, a short esophagus, and a long intestine that ends in a rectum and an anus. The excretory system consists of paired lateral canals that open at the excretory pore near the anterior end.
The reproductive system is sexually dimorphic. The male worm is smaller than the female worm, measuring 8-11 mm in length and 0.4 mm in thickness. The male worm has a copulatory bursa at the posterior end, which is an umbrella-like structure with three lobes supported by chitinous rays. The bursa surrounds the cloaca, which opens ventrally and contains two long spicules that are used for copulation. The male worm also has two testes, two vas deferens, and two seminal vesicles.
The female worm is larger than the male worm, measuring 10-13 mm in length and 0.6 mm in thickness. The female worm has a vulva at the junction of the middle and posterior thirds of the body, which opens ventrally. The vulva leads to a vagina, which connects to two coiled ovarian tubes that occupy most of the body cavity. The female worm also has two uteri, two oviducts, and two ovaries.
The eggs of Ancylostoma duodenale are oval or elliptical, measuring 60 μm by 40 μm. They have a thin transparent shell and contain an unsegmented ovum or a developing embryo with 2-8 cells. They are colorless and not bile-stained.
Life cycle of Ancylostoma duodenale
The life cycle of Ancylostoma duodenale is completed in a single host, hence it is called monogenetic. Humans are the only natural host for this parasite. No intermediate host is required like other helminths. The infective form is the third-stage filariform larva.
The life cycle can be summarized as follows:
- Eggs are passed in the stool of infected persons, and under favorable conditions (moisture, warmth, shade), they hatch in 1 to 2 days and become free-living rhabditiform larvae in the contaminated soil.
- The rhabditiform larvae grow and molt twice in the feces and/or the soil, and after 5 to 10 days, they become infective filariform larvae. These larvae can survive 3 to 4 weeks in favorable environmental conditions.
- On contact with the human host, typically bare feet, the filariform larvae penetrate the skin and enter the blood vessels. They are carried to the heart and then to the lungs, where they break into the alveoli and ascend the bronchial tree to the pharynx. They are then swallowed and reach the small intestine.
- In the jejunum of the small intestine, the larvae mature into adults and attach to the intestinal wall with their buccal capsule, causing blood loss by the host. The adult worms are 8 to 13 mm long and have a characteristic hook-like shape.
- The adult worms mate and produce eggs, which are passed in the stool. The female worm can lay up to 28,000 eggs per day. The cycle repeats.
Copulation and fertilization
Copulation occurs in the host`s intestine, where the male worm attaches its copulatory bursa to the vulva of the female worm. The copulating pair forms a Y-shaped appearance due to the position of the genital openings. The male worm transfers sperms to the female worm through two long retractile spicules. The sperms are stored in the seminal receptacles of the female worm, where fertilization takes place. The fertilized eggs are then pushed into the uteri and laid through the vagina and the gonopore. The female worm can produce about 10,000 to 20,000 eggs per day.
The female worms of Ancylostoma duodenale lay eggs in the intestine of the human host, where they are passed out with the feces. The females can lay 10,000 to 30,000 eggs per day. The eggs are oval or elliptical, measuring 60 μm by 40 μm, and have a thin transparent shell. They are colorless and not bile stained. When released by the worm in the intestine, the egg contains an unsegmented ovum. Freshly-excreted eggs contain a developing embryo in the early stages of cleavage (2-8 cells). There is a clear space between the segmented ovum and eggshell.
The eggs of Ancylostoma duodenale are not infective for humans when they are freshly passed in the feces. They need to develop in a warm, moist and shady environment in the soil for about 48 hours before they hatch into first-stage larvae. The eggs can also be ingested by animals such as dogs, cats and pigs, which can serve as reservoir hosts for the parasite. The eggs of Ancylostoma duodenale can also be transmitted through the ingestion of raw or undercooked meat containing larvae that have migrated to the muscle tissue. However, these modes of infection are less common than skin penetration by infective larvae.
Development in soil
The eggs of Ancylostoma duodenale are passed out in the feces of the infected person and reach the soil. The eggs are not infective for humans when they are freshly excreted. They need to undergo development in the soil to produce infective larvae.
The development of the eggs depends on the temperature and moisture of the soil. The optimal temperature range is 25-30°C and the soil should be moist but not waterlogged. The eggs hatch in about 48 hours, releasing first-stage larvae or rhabditiform larvae. These larvae are about 250 µm long and feed on organic matter and bacteria in the soil.
The rhabditiform larvae molt twice in the soil, shedding their cuticles and becoming second-stage and then third-stage larvae or filariform larvae. The filariform larvae are about 500-600 µm long, with a sharp-pointed tail and a slender body. They are non-feeding and can survive in the soil for 5-6 weeks, waiting for a suitable host. They can also climb on vegetation or other objects, using capillary water films on their surface.
The filariform larvae are the infective stage of Ancylostoma duodenale. They can penetrate the skin of humans who come into contact with contaminated soil, especially through bare feet, hands, arms or legs. The time taken for the development from eggs to filariform larvae is on average 8 to 10 days.
The development in soil is an essential part of the life cycle of Ancylostoma duodenale. It allows the parasite to multiply and spread to new hosts. It also exposes the parasite to environmental factors that can affect its survival and infectivity. Therefore, improving sanitation and hygiene practices can reduce the transmission of this parasitic infection.
Mode of infection
The main mode of infection by Ancylostoma duodenale is through the skin contact with soil contaminated with infective filariform larvae. The larvae can penetrate the skin, usually of the feet, legs, hands, or arms, by using their oral spears. This can cause a skin reaction known as cutaneous larva migrans or ground itch, which is characterized by itching, burning, redness, swelling, and blisters.
Another possible mode of infection is through the oral ingestion of filariform larvae. This can occur when people eat food or drink water that is contaminated with soil containing the larvae. Ancylostoma duodenale can also infect humans by the transmammary route, where the larvae can pass from the mother`s milk to the infant. Additionally, some Ancylostoma duodenale larvae can become dormant in the human tissues and reactivate later to cause infection.
Necator americanus, another species of hookworm that infects humans, has a similar mode of infection as Ancylostoma duodenale, except that it does not seem to be transmitted by oral or transmammary routes. Ancylostoma ceylanicum and Ancylostoma caninum are other species of hookworm that can infect humans, usually by oral ingestion. Ancylostoma caninum can also cause eosinophilic enteritis, a condition where the larvae partially develop in the human intestine and cause inflammation.
Hookworm infection can be prevented by avoiding walking barefoot or having direct skin contact with contaminated soil. It is also important to practice good hygiene and sanitation, and to avoid using human feces as fertilizer. Hookworm infection can be diagnosed by examining stool samples for eggs under a microscope, and treated with anti-parasitic drugs prescribed by a health care provider. Hookworm infection can cause anemia, abdominal pain, diarrhea, weight loss, fatigue, and impaired growth and development in children, so it is important to seek medical attention if symptoms occur.
The larvae of Ancylostoma duodenale migrate through the human body during their life cycle. After penetrating the skin, the larvae travel through the blood vessels to the lungs, where they break into the alveoli and ascend the bronchial tree to the pharynx. They are then swallowed and reach the small intestine, where they mature into adult worms and attach to the intestinal wall. This migration can cause various symptoms and reactions, including cutaneous larva migrans, ground itch, and inflammation.
During larval migration, the larvae can also enter a dormant state (hypobiosis) in the intestine or muscle of the host. These dormant larvae can reactivate later and cause patent infection. Larval migration is an important aspect of the life cycle of Ancylostoma duodenale and contributes to the spread and persistence of the parasite in the human population.
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