Ascaris Lumbricoides: Parasitic Adaptions | Zoology Optional Notes for UPSC PDF Download

Introduction

External Features

• Ascaris has a distinctive appearance. It is slender, elongated, cylindrical, and vermiform.
• The worm is typically light yellowish-brown, giving it a lustrous appearance due to its glistening cuticle.
• There is sexual dimorphism, with females being larger than males. Males measure about 15-31 cm in length and 2-4 mm in diameter, while females are usually 20-40 cm long and 6-8 mm wide.
• The body of Ascaris is rounded and slender, tapering at both ends, resulting in its vermiform shape.
• The surface of the worm exhibits a pseudo-segmented appearance due to transverse striations in the thick and elastic cuticle.
• Four longitudinal lines run along the length of the body, primarily on the lateral sides, and are known as lateral lines.
• The anterior end of the body features a terminal mouth with a triangular opening. The mouth is surrounded by three rounded lips: one mid-dorsal and two ventro-lateral.
• The mid-dorsal lip is elliptical, with a central forbed and a fleshy core. It possesses four sensory papillae.
• The two ventro-lateral lips have forked fleshy cups and bear two rows of sensory papillae.
• The jaws are equipped with sensory structures known as amphids, which function as chemoreceptors.
• The sensory papillae serve as tangoreceptors.
• The inner edges of the lips are toothed.
• The excretory pore is located mid-ventrally slightly behind the anterior end.
• In females, the gonopore is situated at about the anterior 1/3 of the body, while the posterior part shows sexual dimorphism.
• In females, the posterior part is straight, whereas in males, it is sharply curved.
• The female has an anus as a transverse slit in the posterior part of the body, while males have a cloacal opening.
• Sometimes, two penial setae protrude from the cloacal opening.
• The part of the body behind the anus is referred to as the tail.

Body Wall

1. Cuticle: The transparent, elastic, and thick cuticle covers the entire body. It also lines the buccal cavity, esophagus, rectum, cloaca, excretory pore, and vagina. The cuticle protects the worm against digestive enzymes and is secreted by the epidermis. It consists of five layers: lipoid layer, cuticle (cortex layer), matrix layer (comprising fibrillar layer, homogeneous layer, and boundary layer), fiber layer, and basement membrane.
2. Epidermis: The epidermis is separated from the cuticle by a basement membrane. It is a thin, syncytial ectodermal layer found along the mid-dorsal, mid-ventral, and lateral lines and forms chords. Lateral chords contain the excretory canal and lateral nerves, while dorsal and ventral nerves run along dorsal and ventral chords, respectively.
3. Muscle Layer: Circular muscles are absent in Ascaris. Instead, longitudinal muscles form a single layer composed of spindle-shaped cells located below the epidermis. These muscles line the body cavity and are divided into four longitudinal columns. Each column contains 150 muscles and consists of a muscular portion and a protoplasmic portion.
4. Pseudocoel: The body cavity of Ascaris represents a pseudocoel, bounded externally by the muscle cells and internally by the lining of the gut. This pseudocoel is filled with a pseudocoelomic fluid containing pseudocoelomocytes.

Digestive System of Ascaris

• Mouth and Buccal Cavity: The mouth is located anteriorly and has a terminal aperture. It is surrounded by three lips: one median-dorsal and two ventro-laterals. The mouth leads into the buccal cavity, a narrow muscular chamber connected to the pharynx.

• Esophagus/Pharynx: The pharynx is a small cylindrical chamber with a muscular bulb. Its walls are composed of radially arranged muscles. The pharyngeal cavity has a tri-radiate appearance and is lined by a cuticle. The walls of the pharynx are embedded with two branched sub-ventral esophageal glands and a branched dorsal esophageal gland, which secrete their contents into the pharyngeal cavity. The pharynx acts as a suction tube and leads to the intestine.

• Intestine: The intestine is a straight, thin-walled, dorso-ventrally flattened tube devoid of muscles. Its wall consists of a single layer of columnar cells lined by a basement membrane and a thin layer of cuticle. The inner intestinal layer is equipped with finger-like microvilli that increase the surface area for absorption.

• Rectum: The rectum is the distal, narrow part of the intestine and has a thick wall. In females, the rectum opens via a transverse slit known as the anus. In males, it opens into the cloaca, which also receives the ejaculatory duct. The anus is guarded by thick anterior and posterior muscles and a sphincter muscle. In males, there are six large unicellular rectal glands, while females have three.

Physiology of Digestion: Ascaris feeds on semi-digested food from the host, as well as tissues and blood from the host's mucous membrane, using its muscular pharynx to perform a sucking action. Digestion in Ascaris is both partially extracellular and partially intracellular. The digested food is absorbed by intestinal cells, and excess food is stored as reserve glycogen and a small amount of fat in the epidermal cells of the body wall.

Respiratory System of Ascaris

• Excretory System: Ascaris possesses a simple H-shaped excretory system. Each lateral side contains a longitudinal excretory canal, with the anterior limbs of the "H" being reduced. The transverse canal is branched and forms a network. From this network, a short common excretory canal leads to a minute excretory pore on the ventral side. The canals are more developed on the left side than on the right. The excretory system lacks internal openings, nephridia, flame cells, and cilia. Ascaris excretes nitrogenous waste in the form of urea, which diffuses into the pseudocoelomic fluid. Urea is then secreted through the excretory pore. Some urea and ammonia are eliminated with fecal matter. When water is scarce, Ascaris excretes more urea.
• Central Nervous System: The nervous system of Ascaris is epidermal and includes a circumpharyngeal nerve ring. The nerves arise from this ring and innervate various parts of the body. There are various types of cephalic ganglia associated with the nerve ring, and both anterior and posterior nerves are present, innervating different regions of the body.

Reproductive System in Ascaris

• Male Reproductive System: It includes a single testis, unpaired vas deferens, seminal vesicle, ejaculatory duct, and penial sac with penial setae. The testis is a long, coiled structure with telogonic germ cells that develop only in the proximal part. The vas deferens opens into the seminal vesicle, which, in turn, leads into the ejaculatory duct. The cloaca opens to the exterior, and penial sacs with penial setae help in sperm transfer during copulation.

• Female Reproductive System: It comprises a pair of ovaries, paired oviducts, paired uteri, and a vagina. The ovaries are long, coiled structures in the posterior part of the body. The paired oviducts are thin ducts that open into the uteri, which are long and coiled muscular walls. The uteri unite to form a short muscular tube known as the vagina, which opens to the exterior through the female gonopore or vulva.

Life Cycle of Ascaris lumbricoides

1. Single Host Life Cycle: Ascaris lumbricoides has a simple life cycle that involves only one host, which is the human.

2. Reproductive Process: Within the host's intestine, male and female Ascaris worms copulate, with males transferring sperm into the female's vagina. The fertilization occurs in the upper part of the female's uterus.

3. Egg Formation and Deposition: After fertilization, the eggs move downwards within the female. They are enclosed in a chitinous eggshell that is highly resistant and also covered with an albumen layer.

4. Egg Laying: The eggs are laid within the host's intestine and are subsequently excreted outside the host's body in feces. Ascaris females can produce a substantial number of eggs, estimated to be as many as 27,000,000 with a daily deposition of 20,000 eggs.

5. Egg Characteristics: Ascaris eggs are elongated, elliptical, or oval, typically measuring 40-75 microns. They are protected by a tough, transparent chitinous eggshell and an albuminous layer. These layers make the eggs resistant to environmental changes, allowing them to remain dormant for extended periods.

6. Development Outside the Host: The development of Ascaris eggs initiates outside the host's body. The process includes spiral and determinate cleavage, leading to the formation of a blastula. Eggs undergo gastrulation by invagination and eventually develop into a rhabditoid larval stage.

7. Host Infection: To initiate infection, an embryonated egg must be accidentally ingested by a human host. This can occur through various means, such as contaminated water, soil, or uncooked vegetables. Once ingested, the egg hatches within the host's duodenum, releasing a second-stage rhabditoid larva.

8. Larval Migration: The second-stage larva, measuring about 0.2 to 0.3 micrometers and equipped with a developed system, penetrates the mucous membrane of the small intestine. It enters the bloodstream, travels to the liver, heart, lungs, and finally crawls back up the intestine.

9. Maturation to Adults: Within 6-10 weeks, the larvae mature into adult worms, reaching a size of about 1-2 millimeters. This maturation process occurs inside the host's intestine, allowing the parasites to become sexually mature.

Pathogenicity and Treatment of Ascaris Infection

1. Pathogenic Effects: Larvae and adult Ascaris worms can cause various health issues when pursuing their migratory course. The presence of larvae in the lungs can lead to inflammation, severe pneumonia, fever, anemia, eosinophilia, and leucocytosis.

2. Gastrointestinal Symptoms: Mature worms, especially when present in large numbers, can cause abdominal pain, acute colic, diarrhea, and even appendicitis by blocking the intestine. Damaging the intestinal wall may lead to peritonitis.

3. Neurological Symptoms: The toxins released by Ascaris can result in convulsions, coma, delirium, and nervousness, affecting the nervous system.

4. Growth and Cognitive Impact: Ascaris infection can lead to stunted growth and poor memory in affected individuals.

5. Treatment Options: To treat Ascaris infection (ascariasis), various antihelminth drugs are used, such as piperazine citrate or hydrate, hetrazan, dithiazanine, and tetramezole. Additionally, a mixture of chenopodium oil and tetrachloroethylene, along with fasting before and after treatment followed by a purgative, can effectively remove Ascaris infection.

Parasitic Adaptations of Ascaris

1. Body Structure: Ascaris exhibits specific adaptations to its parasitic lifestyle, including a long, cylindrical body shape with pointed ends.

2. Thick Cuticle: The body wall of Ascaris is covered with a thick, tough, and multi-layered cuticle. This cuticle provides protection against the digestive enzymes of the host's intestine.

3. Absence of Cilia: Ascaris lacks cilia, which are often found in free-living organisms.

4. Simplified Alimentary Canal: As a parasite, Ascaris has a simplified and poorly developed alimentary canal because it primarily receives semi-digested food from the host's intestine. The muscular pharynx is modified for sucking food.

5. Underdeveloped Nervous System: Ascaris has a poorly developed nervous system and lacks sense organs and eyes. However, it possesses sensory papillae on its lips, which function as tangoreceptors and chemoreceptors.

6. Anaerobic Respiration: Ascaris predominantly relies on anaerobic respiration and lacks specialized respiratory organs. The organism stores fatty acids that are essential for anaerobic glycolysis of stored glycogen, providing the energy required for vital functions.

7. Reproductive Adaptations: Ascaris exhibits a highly developed reproductive system, with females laying numerous eggs daily. The eggs are encased in a thick chitinous shell, protecting them from host enzymes and environmental factors.

Summary of Ascaris lumbricoides

1. Parasite in Human Intestine: Ascaris lumbricoides is a nematode parasite that resides in the small intestine of children and adult humans.

2. Body Structure: Ascaris has a narrow, elongated, cylindrical, and vermiform body, with the female being larger than the male.

3. Alimentary Canal: Its alimentary canal is simple and poorly developed, primarily receiving semi-digested food from the host's intestine.

4. Respiration: Ascaris respires anaerobically through glycolysis and can also consume free oxygen in the host's intestine.

5. Excretory System: The excretory system of Ascaris is simple, featuring an H-shaped structure with a network of canals. It eliminates urea as waste.

6. Nervous System: Ascaris has an epidermal nervous system with a circumpharyngeal nerve ring.

7. Reproductive System: Both sexes are separate, and their reproductive organs are morphologically distinguishable. Males have a single testis, while females have a pair of ovaries.

8. Life Cycle: Ascaris follows a life cycle with a single human host, involving the laying of eggs in the host's intestine, egg deposition in feces, and the potential for ingestion by a new host.

9. Pathogenicity: Ascaris can cause various health issues in the host, affecting the respiratory, gastrointestinal, and neurological systems.

10. Treatment: Treatment of ascariasis includes the use of antihelminth drugs and specific therapeutic mixtures.

11. Parasitic Adaptations: Ascaris exhibits multiple adaptations for its parasitic lifestyle, including body structure, cuticle protection, absence of cilia, simplified alimentary canal, underdeveloped nervous system, anaerobic respiration, and reproductive adaptations. These adaptations enable Ascaris to thrive in its specific environment