Table of Contents
Ascaris lumbricoides is a parasitic nematode roundworm that inhabits the small intestine of humans. These worms belong to the Ascarididae family, the Secernentea class, and the Oxyurida order. These parasites are commonly discovered in pigs. They are large roundworms measuring up to 40 centimeters in length, with a mouth surrounded by three lips. Ascariasis is an infection caused by Ascaris roundworms in the small intestine. This infection has no symptoms, but large infestations can cause digestive problems, malnutrition, and stunted growth in children. Ascariasis affects between 807 million and 1.2 billion persons worldwide.
Ascariasis is caused by the massive parasitic worm Ascaris lumbricoides. The Ascaris roundworm is the most prevalent parasitic worm in humans. An estimated one-sixth of the human population is infected by a roundworm such as A. lumbricoides at some point; those residing in tropical and subtropical regions are at an increased risk of infection.
Ascaris lumbricoides and Ascaris suum (pig roundworm) have been suggested to be the same species.
Habitat of Ascaris lumbricoides
- Ascaris lumbricoides is a free-living endoparasite in the lumen of the human small intestine. It has existed in man since the beginning of time. It has a worldwide distribution.
- It is more prevalent among adolescents than adults. Sometimes it migrates from the intestine to the stomach and then exits through the host’s mouth or nose.
- A single host may be inhabited by 1,000 to 5,000 adult worms. Its mode of nutrition is holozoic, as it feeds by sucking partially digested material from its host through its pharynx.
- It produces anti-enzymes to defend itself against the host enzyme’s activity.
- Sexual dimorphism is clearly distinguished; only sexual reproduction occurs; asexual reproduction is nonexistent. Simple and monogenetic life cycle; no secondary host.
External Morphology of Ascaris lumbricoides
Shape and Size
- Ascaris lumbricoides is elongated and cylindrical with tapering extremities. It is a large nematode with sexual dimorphism, meaning the sexes are distinct.
- The female is 20-41 cm (8-16 inches) long and 4—6 mm in diameter, whereas the male is 15-31 cm (6-12 inches) long and 2-4 mm in diameter, with a ventrally curved posterior end (tail).
- However, some nematodes, such as Ascaris, have a distinct reddish tint due to the presence of haemoglobin in their external cuticle, which otherwise lacks coloration.
- The front ends of both sexes are built similarly. A smooth, robust, and elastic cuticle, striated transversely, creates the illusion of segments on the worm’s body.
- There are four longitudinal epidermal chords on the exterior of the cylinder: a thin one in the mid-dorsal region, a thick one in the mid-ventral region, and two thick ones on the sides.
- The laterals are brown and the dorsal and ventral chords are white. Six lips or labia normally surround the front mouth of worms, but in Ascaris only three remain (one elliptical mid-dorsally and two oval latero-ventrally). Ascaris’s mouth, thus, is a triradiate orifice.
- While most nematodes have six papillae in the outer labial circle, nematodes only have four: two on the dorsal lip and one on each of the latero-ventral lips.
- The nematode Ascaris, as with most parasitic nematodes, lacks the inner labial circle papillae that are present in other worms.
- In parasitic nematodes, the cuticular excavation known as an amphid on the latero-ventral lips is diminished. Amphibians have the ability to smell chemical odours. There are little teeth in the lips. There are two cervical papillae, one on each side of the nerve ring, located behind the lips. A papilla is a sensory organ.
- The male has a cloaca from which two chitinous spicules or penial setae extend, whereas the female has a transverse anus with thick lips near the posterior end.
- There are cuticular elevations ventrally near the cloaca in males that are involved in copulation; they include roughly 50 pairs of pre-anal papillae and 5 pairs of post-anal papillae.
- The female has a rather straight post-anal tail, whereas the male’s is severely curled. About a third of the way back from the front is the gonopore, often known as the vulva or female genital opening. An excretory pore can be found in the mid-ventral region of the lip area.
Body Wall of Ascaris Lumbricoides
Ascaris has three distinct layers of skin: the outside cuticle, the sub-cuticle (also known as the epidermis or hypodermis), and the inner muscle layer.
The cuticular lining of the pharynx and rectum are continuous with the epidermis, which is the thick, rough, wrinkled, and translucent outermost layer of the body wall. Because it lacks cells, it can withstand the host’s digestive juices while letting through salts and water. While egg shells contain true chitin, the cuticle does not and is soluble in KOH.
However, when viewed through a light microscope, cuticle can be broken down into four distinct layers with their own unique chemical composition and structural arrangements:
- Cortical or cortex layer, composed of keratin and resistant to the digestive enzymes of the host.
- Matrix layer, which has a spongy consistency and contains sulphur-rich protein matricin; it is elastic and contains numerous fine filaments.
- Fibre layer, it is the innermost layer of the cuticle, but it is not the outermost layer; it has extensive connective tissue with intertwining collagen fibres, and
- Basement membrane, constituting the cuticle’s innermost layer.
Recent electron microscopic examinations of the cuticle of Ascaris disclosed the presence of a fifth layer in addition to the four layers previously identified.
However, under an electron microscope, the cuticle appears to be composed of the following layers:
- Lipoid layer, which is approximately 1000 A° thick and consists of a thin membrane layer;
- Cortical or cortex layer consists of (a) an outer cortical layer and an inner cortical layer; (b) a cortical sulcus layer;
- Matrix layer, which includes an outer fibrillar layer and a boundary layer;
- Fibre layer, made up of collagen fibres arranged in three layers; and
- This membrane is a thin layer that surrounds the epidermis or hypodermis.
The cuticle sheds four times over the course of its lifecycle, and only during periods of growth.
- Below the cuticle is a syncytial epidermis with numerous nuclei but no cell walls; the nuclei are only present in the longitudinal epidermal chords; and the number of epidermal cells is minimal.
- The epidermis secretes the cuticle and forms the epidermal chords, which are four longitudinal thickenings. Two are thicker lateral lines or chords, while the other two are thinner dorsal and ventral lines.
- The dorsal and ventral lines contain nerves, while the lateral lines contain excretory canals. Free-living nematodes have unicellular epidermal structures in their epidermis. The epidermis contains copious reserves of fat and glycogen.
3. Muscle Layer
- The body wall musculature consists of a single layer of longitudinal fibres extending along the length of the body and is located internally to the epidermis and between the epidermal chords.
- Each muscle cell or fibre has two zones: an outer fibrillar zone of longitudinally striated, spindle-shaped, contractile muscle that lies towards the epidermis; and a second protoplasmic zone of club-shaped, bladder-like protoplasm with a nucleus and a network of supporting fibrils that form a fibrous process or muscle tail.
- In the upper half, muscle tails are inserted into the dorsal line and connected to the dorsal nerve, whereas in the lower half, muscle tails are inserted into the ventral line and connected to the ventral nerve. Dobell (1965) emphasised that muscle tails are cellular extensions that create synapses on the dorsal and ventral motor nerves of the nerve cords.
- The muscles are divided into four quadrants by longitudinal chords. Each quadrant has about 150 muscle cells. In order to counteract the peristaltic activity of the host’s intestine, the contractions of these elongated muscles cause the body to rotate and bend, resulting in undulating movement of the Ascaris.
- When there are numerous muscle cells in each quadrant and they extend deeply into the body cavity, as in Ascaris, this condition is known as polymyarian.
- When only two or three muscles are present in each quadrant, the condition is referred to as meromyarian, as in Oxyuris. But when the muscles are small and densely compacted to form a complete layer, as in Trichuris, this type is holomyarian.
- In a transverse section, longitudinal muscles exhibit a U-shaped fibrillar zone surrounding a club-shaped protoplasmic zone, from which muscle ends extend. There are specialised muscles in the pharynx, the vagina of females, and the spicules of men.
Body Cavity or Pseudocoel of Ascaris Lumbricoides
- The space between the body wall and the alimentary canal is a pseudocoel, not a coelom, because it is circumscribed by muscles on the outside and the intestine’s cuticle on the inside.
- It results from the disintegration of connective tissue cells. Organs are surrounded by parenchyma in young animals, but almost all of it disappears in adults, leaving the organs to dangle loosely.
- Pseudocoel consists of fibrous tissue and fixed cells called coelomocytes or pseudocoelocytes or giant cells; there are four such cells occupying fixed positions along the lateral chords; they are branched and giant-sized, so as to fill the body cavity; the confluent vacuoles of these giant cells represent the pseudocoel; therefore, the pseudocoel is an intracellular space.
- The pseudocoel is filled with a clear, protein-rich fluid known as pseudocoelomic fluid or perienteric fluid, which distributes and transports various metabolites.
- It consists of 93 percent water and compounds such as protein, glucose, non-protein, nitrogenous substances, sodium chloride, phosphate, etc. In the pseudocoel, reproductive organs are allowed to move. In free-living organisms, pseudocoelocytes are tiny and abundant.
- Due to the fact that cell division ceases after hatching (except in the reproductive organs), nematodes have a constant number of cells in the body at the time of hatching; as a result, with growth the cells elongate and become giant-sized, especially the muscle cells, coelomocytes in the pseudocoel, and excretory canal cells; a cell can be longer than 1 centimetre.
- There is a tendency towards the formation of syncytia, likely due to the multiplication of nuclei through fragmentation or amitosis, and the disintegration of cell walls.
Digestive System of Ascaris Lumbricoides
1. Alimentary Canal
- It includes the mouth, a brief pharynx or oesophagus forming the foregut, a long tubular intestine or the midgut, and a short rectum or hindgut.
- As mentioned previously, the mouth is a triradiate opening located at the anterior tip and encircled by three lips or labia.
- The terminal mouth leads to a cylindrical, thick-walled pharynx or oesophagus with a posterior enlargement known as the end bulb, which is equipped with valves. The pharynx has muscular walls with radial muscle fibres that dilate the lumen.
- Internally, it is lined with cuticle, which is continued at the mouth margin by the cuticle of the body wall. These are pharyngeal or oesophageal glands. The pharynx has three large branching gland cells that open through cuticular ducts into the lumen.
- The pharynx has three deep longitudinal grooves lined by cuticle, and in a transverse section the lumen appears triradiate.
- Connective tissue fibres arise from each of the three internal grooves and travel to the cuticle lining the pharynx, maintaining the triradiate shape of the lumen. This much is the stomodaeum or the foregut.
- The pharynx opens posteriorly into a dorsoventrally compressed, thin-walled intestine or midgut that extends nearly the entire length of the body. It is composed of a single layer of columnar epithelial cells surrounded by a thin cuticle layer. The microvilli are finger-like projections generated along the free inner margin of each cell (Kessel et al., 1961).
- They create a border of densely packed foliage that increases the surface area. The intestinal wall is devoid of musculature.
- Following the intestine is the hindgut or rectum, which is also dorsoventrally flattened. Its wall is composed of tall columnar cells that are internally lined by cuticle and externally lined by a few muscle fibres. In males, the rectum opens via the cloaca, which receives the ejaculatory duct, whereas in females, the rectum opens via the anus, a transverse slit-like aperture.
- The anal aperture is protected by the anterior and posterior lips, as well as a few depressor ani muscles that extend from the rectum to the body wall. Periodically, their contractions result in the expulsion of faeces. The rectum also contains three in females and six in males, large unicellular rectal glands.
6. Food, Feeding and Digestion
- Ascaris Lumbricoides consumes blood, tissue exudates, and partially or completely digested host sustenance. The suctorial action of the pharynx draws in food.
- The digestive process is extracellular and occurs in the intestinal lumen; digestive enzymes such as proteases, amylase, and lipase are secreted by the gland cells of the pharynx. The intestinal wall absorbs the digested nutrients, which are then distributed by the pseudocoelomic fluid.
- In the intestinal wall, muscles, and syncytial epidermis, excess food is typically stored as reserve glycogen and fat.
- It has also been reported that intestinal wall cells undergo intracellular digestion as they ingest solid particles for digestion. If there are any undigested residues, they are eliminated by the contraction of special rectum muscles through the anus or cloaca.
Excretory System of Ascaris
- The excretory system of marine nematodes consists of one or two large renette gland cells lying ventrally in the pseudocoel near the junction of the pharynx and intestine. A duct arises from each renette cell, and these ducts unite and open by an excretory pore in the mid-ventral region.
- Numerous pieces of evidence support the notion that this glandular system gave rise to a tubular excretory system that resembles the letter H, with two longitudinal excretory canals connected by a bridge and a transverse canal from which a common excretory canal leading to an excretory orifice arises.
- In Ascaris Lumbricoides, there is a longitudinal excretory canal in each lateral line, the anterior limbs of the H are reduced, and the transverse canal is branched to form a network from which a short common excretory canal arises to open by a minute ventral excretory pore just behind the lips. Left side canals are more developed than right side canals.
- The canals are intracellular excavations in the solitary giant cell whose nucleus lies on the transverse canal; they are lined by a firm membrane and covered with a layer of cytoplasm. There are no internal openings, cilia, flame cells, or nephridia in the excretory system.
- Very little is known about the excretory physiology of Ascaris Lumbricoides. However, nitrogenous waste consists primarily of urea, which diffuses into the pseudocoelomic fluid.
- It is believed that the excretory canals secrete the urea that is eliminated through the excretory pore; some ammonia and urea are also expelled along with the faeces. There are also observations that suggest Ascaris Lumbricoides excretes more urea when there is a lack of water.
Nervous System of Ascaris Lumbricoides
- As with the excretory system, the nervous system of Ascaris Lumbricoides is located in the body wall, or hypodermically. However, it is made up of the circumpharyngeal ring or nerve ring, as well as nerves that originate from it.
- Circumpharyngeal ring composed of nerve fibres and some diffusely arranged nerve cells encircles the pharynx. Numerous ganglia are associated with this ring, including an unpaired dorsal ganglion and a pair of sub-dorsal ganglia. Each side of the ring contains a lateral ganglion composed of six ganglia. On the underside of the ring are two ventral ganglia of considerable size.
- Every ganglion contains a constant number of nerve cells. Six small nerves emerge anteriorly from the circumpharyngeal ring, each with a ganglion; they are distributed radially and travel to the anterior end’s sense organs (papillae and amphids).
- Six long nerves arise posteriorly from the ring and extend to the posterior end; one of these nerves is a mid-dorsal nerve and one is a mid-ventral nerve; the former lies in the dorsal line and the latter in the ventral line. Since the mid-ventral nerve is ganglionated along its anterior extent, it may be referred to as the nerve cord. It has an anal ganglion near the anus that transmits nerves to the tail.
- The remaining four posterior nerves are thinner, consisting of a pair of dorsolateral nerves and a pair of ventrolateral nerves that lie adjacent to the excretory canals on the sides. Numerous transverse commissures connect the dorsal and ventral nerves, while numerous ventrolateral commissures connect the ventral nerve to the lateral nerves. Males have a more convoluted posterior innervation than females.
Sense Organs of Ascaris Lumbricoides
Due to its parasitic lifestyle, Ascaris Lumbricoides has developed extremely basic sense organs. There are either minute elevations or depressions in the body’s cuticle. However, they include the following:
- Labial Papillae: There are four labial papillae, two on the dorsal lip and one on each of the ventrolateral lips. Each labial papillae is a double sense organ. Each labial papilla is composed of a thin sensory nerve fibre surrounded by a large number of supporting cells. These are flavour or gustatory organs.
- Amphids: There are two amphids, each located on the ventrolateral lips. These are small pockets containing nerve and glandular cells that are supplied by the amphidial nerve from the lateral or amphidial ganglia. These are the chemoreceptors of the nose.
- Phasmids: Phasmids are unicellular structures located one on each side of the tail behind the anus. Figure 46.10 E depicts pit-like chemoreceptors.
- Cervical Papillae: The cervical papillae are a pair of tiny pits located on the lateral sides of the body just behind the oral lips. These are nerve endings with supporting cells that resemble bulbs (Fig. 46.10 F). These likely serve a tactile purpose.
- Cephalic Papillae: The cephalic papillae are also pit-like, consisting of nerve fibres surrounded by supporting cells, but the nerve fibre has a lens-like expansion just beneath the cuticle and then narrows to form a canal that widens before opening on the surface.
- Genital Papillae: Only males have genital papillae. There are approximately fifty pairs of preanal papillae and five pairs of postanal papillae. Also composed of 1-3 nerve fibres embedded within a supporting cell. These also have a tactile function and aid in copulation.
Reproductive System of Ascaris Lumbricoides
- As with other nematodes, the sexes of Ascaris Lumbricoides are dioecious and readily distinguishable externally; that is, sexual dimorphism is pronounced.
- The male is smaller than the female; its tail is curved, whereas the female’s tail is straight; it has a cloaca and a pair of spicules or penial setae, whereas the female has an anus and the spicules are absent; and the male has pre- and post-anal papillae, whereas the female does not. The male system consists of a single conduit, whereas the female system has two.
Male Reproductive Organs
These include the testis, vas deferens, seminal vesicle, ejaculatory duct, and penial setae, which are confined to the posterior portion of the body.
- In Ascaris Lumbricoides, the testis is monorchic, or solitary, but in some nematodes, it may be diorchic, or comprised of two testes. In contrast, the testis of Ascaris Lumbricoides is a long, coiling, thread-like tube.
- Its cell wall consists of a single layer of cuboidal cells surrounded by a basement membrane. It has a solid cytoplasmic rachis as its central axis, which is surrounded by clusters of amoeboid cells in various phases of development. These are, in fact, maturing sperms.
2. Vas Deferens
- The testis continues distally into the vas deferens, a short, thick, coiled conduit with the same diameter. However, it differs from the testis in that it has a central lumen instead of a distinct rachis.
3. Seminal Vesicle
- The vas deferens connects posteriorly to the seminal vesicle, a much thicker, wider, somewhat muscular, and straight conduit. It is located in the posterior one-third of the pseudocoel, below the intestine.
4. Ejaculatory Duct
- At its posterior end, the seminal vesicle narrows to create a short, muscular ejaculatory duct that opens into the rectum to form the cloaca. This duct contains several prostatic glands whose secretions facilitate copulation. The cloacal aperture provides access to the cloaca.
4. Penial Setae
- A pair of muscular sacs, the penial sacs or spicule pouches, are located dorsal to the cloaca; the two spicule pouches unite and join the cloaca. The chambers contain a pair of cuticular spicules or penial setae with a cytoplasmic core. Specialized groups of protractor and retractor muscles allow the spicules to be protruded and retracted through the cloacal aperture.
- They aid in sperm transfer by opening the female genital pore during copulation; their function is assisted by a chitinous plate on the cloacal wall called the gubernaculum.
Female Reproductive Organs
Ovaries, oviducts, uteri, and vagina are contained within the posterior two-thirds of the female body. In fact, Ascaris Lumbricoides, like the majority of nematodes, has two parallel tracts of female reproductive organs, i.e. an ovary, oviduct, and uterus in a single tract; this condition is termed didelphic, although monodelphic (one tract) and polydelphic (many tracts) conditions are also observed.
- Ascaris Lumbricoides’s paired ovaries are long, thread-like, and highly contorted tubes that terminate blindly in the pseudocoel.
- A single layer of cuboidal epithelial cells, a cytoplasmic central rachis, and a basement membrane surround the epithelial cells on the inside. The rachis is surrounded by a cluster of developing ova.
- The ovaries continue posteriorly into somewhat broader oviducts with similar structure to the ovaries, but with a lumen in lieu of the solid rachis.
- Each oviduct continues into a uterus that is even larger, thicker, and more muscular; it has a dense inner layer of circular muscles and a thin outer layer of oblique muscles.
- The first portion of the uterus functions as a seminal receptacle where sperms are stored after copulation and where fertilization occurs. The remainder of the uterus stores fertilized eggs, and its cells produce egg yolk and egg shell material.
- The union of the two uteri produces a slender gonad. Maturation of the median, muscular, cuticle-lined vagina. The Va***a, but not the gonad. A transverse gonopore or vulva rests approximately one-third of the way along the length from the anterior end.
- In some nematodes, the end of the uterus or the end of the Va***a, if present, creates muscular ovejectors that, through peristaltic movement, force individual eggs through the gonopore.
Formation of Gametes in Ascaris Lumbricoides
- Gonads could be either hologonic or telogonic. In gonads that are homologonic, germ cells develop along their entire length.
- The gonads of Ascaris Lumbricoides are telogonic, with germ cells arising only at the proximal end, known as the germinal zone or the zone of proliferation.
- The subsequent portion of the gonad is a growth zone where gametogonia expand. The developing eggs are distributed radially around a central cytoplasmic rachis in the ovary.
- In the testis, amoeboid sperms in the process of development are crowded around the central rachis.
- In the final stage of development of the gonads, gametocytes separate from the rachis and endure maturation and division to form eggs or sperm. The final portion of gonads where developing gametes mature is known as the maturation zone. As indicated, sperms have an amoeboid shape, whereas ova are elliptical.
Life cycle of Ascaris Lumbricoides
Ascaris completes its life cycle in a single host. Human.
- Stage I: Eggs in faeces: A sexually mature female can produce up to 200,000 unfertilized eggs per day, which are expelled alongside feces. They are not contagious.
- Stage II: Development in soil: Embryonation occurs in soil at optimal temperatures of 20 to 25 degrees Celsius with adequate moisture and oxygen. Infectious larva develops within the egg in approximately 3-6 weeks.
- Stage III: Human infection and liberation of larvae: Food and water contaminated with embryonated egg cause infection in humans. In the egg’s embryonated state, first-stage larvae transform into second-stage larvae. This second-stage larva is referred to as a Rhabtitiform larva. The presence of alkaline pH in the small intestine and the solubilization of the outer layer of the second-stage larvae by bile stimulate the hatching of the larvae.
- Stage IV: migration of larvae through lungs: The hatchlings penetrate the intestinal wall and are transported to the liver via portal circulation. Within four to seven days after infection, it travels through the blood to the heart and lungs via pulmonary circulation. The larvae in the lungs undergo double metamorphosis, enlargement, and fragmentation into alveoli.
- Stage V: Re-entry to stomach and small intestine: From alveoli, larvae ascend through the bronchi and trachea before being ingested. The larvae travel from the oesophagus to the stomach and then reach the small intestine again. Ascaris’s natural habitat is the small intestine, and it colonizes there. Twice within the intestine, the parasite develops into an adult worm. Six to ten weeks after sexual maturity, the mature female releases ova into the intestinal lumen, which are then excreted with feces, thereby perpetuating the life cycle. Parasites have a lifespan of 12 to 18 months
Parasitic Adaptations of Ascaris Lumbricoides
As with other helminth intestinal parasites, Ascaris Lumbricoides possesses a number of adaptive characteristics that allow it to exist comfortably and successfully. However, the following are a few of them:
- The body is elongated and cylindrical in shape, with pointed extremities.
- The body is covered externally with a cuticle that is resistant to digestion, and it also secretes antienzymes to protect it from the digestive enzymes of the host.
- Ascaris lacks adhesive organs; the absence of such organs is compensated for by a very weak power of locomotion, as the parasite maintains its position in the intestine of the host and counteracts the intestinal peristalsis with slight movements.
- Cilia are extremely desiring.
- As the parasite feeds on the host’s predigested and partially digested food, its alimentary canal is simple and inadequately developed, but the muscular pharynx facilitates ingestion by suction.
- Since there is no circulatory system, the distribution of digested food to the body tissues and the transfer of waste substances to the excretory canals are carried out by pseudocoelomic fluid.
- The organism’s mode of respiration is anaerobic because it inhabits an essentially oxygen-free environment.
- Its sense organs are extremely underdeveloped due to its sheltered lifestyle.
- The large number of eggs produced compensates for the loss that occurs during their migration from one host to another.
- The egg’s protective exterior shields developing young from environmental dangers.
- Due to their diminutive size, the eggs are readily dispersed.
- Due to the lack of a secondary host in the life cycle, direct infection has increased the likelihood of juveniles reaching a new human host.
Pathogenicity of Ascaris
Mode of transmission
Infection may result from ingesting fully developed, encased Ascaris eggs, consuming fresh vegetables, or consuming contaminated water. Additionally, it may be transferred directly into the mouth via dirty fingertips. Ascariasis is more prevalent in minors than in adults.
In humans, it takes sixty to seventy-five days from the time of infection exposure for the mature female to deposit eggs, and during this time, symptoms manifest.
The adult worm produces the pathogenic effects listed below:
- Splendid conduct: By depriving the host of nourishment.
- Toxic effect: Ascaris’ bodily fluid is toxic and produces an anaphylactic reaction.
- Mechanical effect: By obstructing tubular passages, such as the appendix, bile duct, pancreatic duct, and bronchus, and perforating the gastrointestinal tract with lesions.
Life History of Ascaris Lumbricoides
Ascaris Lumbricoides has a monogenetic life cycle because it has only one host, namely man.
Nevertheless, the life cycle of Ascaris Lumbricoides can be examined as follows:
1. Copulation and Fertilization
- Copulation occurs in the host’s intestines. During copulation, the male Ascaris Lumbricoides positions itself so that its cloacal aperture confronts the female’s vulva, and then thrusts its penial setae to open the female’s vulva.
- Then, shortly thereafter, the cloacal wall of the male contracts, causing the transfer of sperms into the vagina of the female, where they rest in the seminal receptacle of the uterus, awaiting the release of eggs through the oviduct for fertilization.
- During fertilization, the sperm penetrates the egg in its entirety. Soon after fertilization, the egg’s glycogen globules migrate to the surface to create the fertilisation membrane, which quickly transforms into a thick, transparent chitinous inner shell. Soon after, the egg’s fat globules form a lipoid layer beneath the chitinous carapace.
- Now, as the fertilized egg descends, the uterine wall secretes a thick, yellow or brown albuminous (protein) sheath or outer shell with a distinctively undulating surface or rippling’s. These eggs are now known as mammiliated eggs; they measure 60-70 by 40-50 and have an elliptical shape.
- Therefore, the zygote has a dense, transparent inner shell, a lipoidal layer, and a warty, yellow or brown outer shell.
- The female Ascaris lays fertilized eggs (zygotes) in the small intestine of the host, and they are expelled with the feces; they are unsegmented when they leave the host. One female can lay between 15,000 and 200,000 eggs per day; the egg production of Ascaris is astonishing; a mature female can produce as many as 27 million eggs.
- In the intestine, the embryos turn yellowish or brownish. Eggs that land on the ground can survive for months in moist soil, but complete dehydration destroys them. To develop, they require oxygen, hydration, and a temperature below that of the human body; 85°F is the optimal temperature. They require an incubation period outside of the human organism.
3. Early Development (Outside the Host)
- Beginning in the soil are the stages of early embryonic development, such as cleavage or segmentation, etc. The cleavage pattern is spiral and determined.
- The fertilized egg undergoes two cleavages to form four cells or blastomeres; the first cleavage produces a dorsal cell AB and a ventral cell P1, the second cleavage causes AB to divide into an anterior cell A and a posterior cell B, and the ventral cell P1 divides into a dorsal cell EMST and a ventral cell P2.
- In Ascaris, these four cells are initially arranged in the shape of a T, but they subsequently assume a rhomboid arrangement as P2 moves posterior to EMST, as is typical of nematodes. These four cells are now referred to as A, B, P2 and S2 or EMST. These cells endure additional cleavage to form blastomeres of a smaller size.
- However, during the subsequent cleavage, A and B cells divide into A1, A2, and B1 B2 cells, respectively, P2 cells divide into P3 and C, and EMST cells divide into MST and E. P3 and E then separate into P4 and D and E1 and E2 respectively. The P4 is subdivided further into G1 and G2.
- Thus, the descendants of A and B will give rise to the entire ectoderm, with the exception of the posterior end, MST form the mesoderm of the body wall, pseudocoel cells, and the lining of the stomodaeum, the descendants of E (E1 and E2) give rise to the entire endoderm of the intestine, the descendants of P4 (G1 and G2) will give rise to the germ cells, and C and D will participate in the
- Thus, the cleavage of embryonic cells continues, resulting in the formation of a blastula with a cavity, the blastocoel, at the 16-cell stage. Epiboly or overgrowth of ectodermal cells over endodermal cells, and invagination of stomodaeum and endodermal cells then form the gastrula.
- After approximately 10-14 days from the onset of segmentation, a juvenile is born. A juvenile possesses an alimentary canal, a nerve ring, and a lateral excretory system from a structural standpoint.
- This adolescent closely resembles Rhabditis (a soil nematode), and is therefore also known as rhabditiform larva or rhabditoid. This stage of the life cycle of Ascaris is infectious to the host; the larva undergoes a seven-day metamorphosis within the eggshell to become the second stage juvenile or second stage rhabditoid.
- Under optimal conditions of oxygen, moisture, and temperature, the ova of Ascaris lumbricoides with infective juveniles may remain viable in the soil for approximately six years.
4. Infection to Host
- As there is no secondary host in the life cycle of Ascaris Lumbricoides, infection of the host (humans) occurs when they consume contaminated food or water containing infective Ascaris ova.
- Thus, when the infective eggs reach the small intestine of the host, the egg shells are dissolved by the digestive fluids of the host, releasing the infective second-stage juveniles. These juveniles are approximately 0.2-0.3 mm in length and 13-15 in diameter and possess all of the adult structures besides the reproductive organs.
5. Later Development in New Host
- They do not develop in the intestine but instead embark on a ten-day wandering excursion.
- They pass through the intestine wall and enter the mesenteric circulation before entering the liver via the hepatic portal vein. From there, they enter the hepatic vein and, via the postcaval vein, reach the right side of the heart, where they are carried several times through the body via the bloodstream before entering the lungs via the pulmonary arteries.
- In the lungs, juveniles rupture the capillaries and enter the alveoli, where they live for a few days, where they develop and moult to become 3rd and 4th stage larvae, respectively.
- The fourth-stage juveniles travel from the alveoli of the lungs through the bronchioles and bronchus to the trachea and then to the pharynx, where they are swallowed into the oesophagus and reach the small intestine for the second time.
- During this 10-day journey, the embryos have grown approximately tenfold and now measure between 2 and 3 millimeters in length. The fourth and final moult occurs in the intestine, and in 60 to 75 days, males and females reach sexual maturity and adulthood. The average lifespan of the pathogen in its host is only nine to twelve months.
However, the life cycle of Ascaris lumbricoides can be summarized as follows:
Adults → fertilised eggs pass out → larvae develop in egg shell and moult twice → swallowed by man → intestine where juveniles hatch → bore through intestine → mesenteric veins → hepatic portal vein → liver → hepatic vein → postcaval vein → right side of heart → pulmonary artery → lungs → alveoli where third moulting occurs → bronchioles → bronchus → trachea → glottis → oesophagus → intestine where fourth moulting occurs → grow into adults.
What is Ascaris lumbricoides?
Ascaris lumbricoides is a parasitic roundworm that lives in the small intestine of humans. It is one of the most common parasitic infections worldwide, particularly in areas with poor sanitation.
How is Ascaris lumbricoides transmitted?
Ascaris lumbricoides is transmitted through the ingestion of eggs that have been passed in the feces of infected individuals. These eggs can contaminate soil, food, or water, and can remain infectious for many years.
What are the symptoms of Ascaris lumbricoides infection?
Many people with Ascaris lumbricoides infection have no symptoms, but when symptoms occur, they can include abdominal pain, nausea, vomiting, diarrhea, and weight loss. In severe cases, the worms can cause intestinal obstruction, which can be life-threatening.
How is Ascaris lumbricoides diagnosed?
Ascaris lumbricoides infection is diagnosed by identifying the eggs in a stool sample. Sometimes, adult worms can be seen in the stool or vomit of infected individuals.
How is Ascaris lumbricoides treated?
Ascaris lumbricoides infection is usually treated with medication such as albendazole or mebendazole. In severe cases, surgery may be necessary to remove the worms.
How can Ascaris lumbricoides infection be prevented?
Ascaris lumbricoides infection can be prevented by practicing good hygiene, such as washing hands frequently with soap and water, and avoiding eating raw or undercooked vegetables that may be contaminated with soil. It is also important to treat infected individuals and properly dispose of their feces.
Who is at risk of Ascaris lumbricoides infection?
Ascaris lumbricoides infection is more common in areas with poor sanitation, particularly in developing countries. Children are also at a higher risk of infection, as they are more likely to play in contaminated soil and put their hands in their mouths.
Can Ascaris lumbricoides infection be fatal?
In most cases, Ascaris lumbricoides infection is not fatal. However, in severe cases where the worms cause intestinal obstruction, it can be life-threatening.
Can pets transmit Ascaris lumbricoides to humans?
No, Ascaris lumbricoides is a human parasite and cannot be transmitted by pets.
How long does Ascaris lumbricoides infection last?
Ascaris lumbricoides infection can last for years if left untreated. However, with proper treatment, the infection can be cleared within a few days to a few weeks.
- Walker, M., Hall, A., & Basáñez, M.-G. (2013). Ascaris lumbricoides. Ascaris: The Neglected Parasite, 155–201. doi:10.1016/b978-0-12-396978-1.00007-0
- de Lima Corvino DF, Horrall S. Ascariasis. [Updated 2023 Jan 31]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK430796/