An Example of Phylum Nematoda: Ascaris

In this article we will discuss about Ascaris:- 1. Habit and Habitat of Ascaris 2. Structures of Ascaris 3. Body Wall 4. Body Cavity 5. Digestive System 6. Excretory System 7. Nervous System 8. Reproductive System 9. Life History 10. Pathogenicity 11. Control Measures of Disease 12. Treatment of Disease 13. Parasitic Adaptations.

Contents:

1. Habit and Habitat of Ascaris
2. Structures of Ascaris
3. Body Wall of Ascaris
4. Body Cavity of Ascaris
5. Digestive System of Ascaris
6. Excretory System of Ascaris
7. Nervous System of Ascaris
8. Reproductive System of Ascaris
9. Life History of Ascaris
10. Pathogenicity of Ascaris
11. Control Measures of Disease of Ascaris
12. Treatment of Disease of Ascaris
13. Parasitic Adaptations of Ascaris

1. Habit and Habitat of Ascaris:

It resides as an endoparasite in the small intestine of man. The worm may migrate into other neighbouring channels. The para­sites are most frequent in tropical and sub­tropical countries.

2. Structures of Ascaris:

It is milk-white in colour but presents a reddish-yellow shade when alive. Female worms are larger than the males and are further distinguished by the presence of sepa­rate and independent genital aperture situ­ated on the ventral surface at about one-third of the body length from the anterior (Fig. 15.3).

Sexes can also be distinguished exter­nally by the shape of the post-anal portion of the body. The post-anal portion in the male is sharply curved downwards while in the female it is nearly straight. Females are usu­ally 20-25 cm in length and 5 mm in diam­eter while the males are 15-17 cm in length and 3 mm in diameter.

The body is cylindrical and tapering at both ends. The mouth aperture is anterior and terminal in position and is bounded by three lips, one median and dorsal and two ventrolateral in position.

A little upon the ventral side of the posterior end is situated the anus (Fig. 15.4D) which is transverse in position, and in males it serves also as the reproductive aperture. In males a pair of needle-like chitinoid bodies project from this aperture which are called penial setae.

A little down the anterior tip and on the ventral surface is situated an excretory pore (Fig. 15.4B). The female genital aperture or vulva is situated on the ventral surface at about one-third of the body length from the ante­rior end. There is no male genital aperture.

The body is marked with four longitudinal streaks or lines running along the entire length of the body. Of these four streaks one is dorsal, one is ventral and two are lateral in position. The dorsal and ventral streaks are narrow and pure white in living condition while the lateral ones are thick and brown in colour.

There are small cuticular elevations at the anterior and posterior regions of body, called papillae. In males about 55 pairs of papillae are found around the anal region, of which 50 pairs are pre-anal and 5 pairs are post­anal. These are also called genital papillae (Fig. 15.4C), which help during capulation. On the outer surface of each lip there are sensory papillae (15.4B).

A pair of papillae, one on each side of ventrolateral lip, are blind, pouch-like cuticular invaginations, lined by specialized non-motile cilia, open by a small pore, called amphids (Fig. 15.4A).

These are chemosensory in function and connected with amphidial nerves. Another paired papillae, situated on the caudal re­gion, one on each side and unicellular glan­dular structure, called phasmids and are also thought to be chemosensory (Fig. 15.4C).

3. Body Wall of Ascaris:

The outer surface of the body is furnished by cuticle which is thin, transparent, delicate and wrinkled transversely (Fig. 15.5). The cuticle is secreted by the ectoderm. The ectoderm underneath the cuticle forms a syncytial protoplasmic layer. Below the ectoderm is a single layer of longitudinal muscles.

The individual cells of the longi­tudinal muscles are very peculiar in appear­ance (Fig. 15.6). Each cell is spindle-shaped, straightened longitudinally and bears at the middle a bladder-like mass of protoplasm containing the nucleus which faces the side of the body. This peculiar appearance is due to the differentiation of the cell into a con­tractile and a nuclear part.

The longitudinal muscle layer is not continuous and is ar­ranged into four longitudinal bands, two dorso-lateral and two ventro-lateral. This is due to the fact that the ectoderm projects inwards at the dorsal, ventral and two lateral sides between the muscles to give rise to the streaks on the outer surface of the body.

4. Body Cavity of Ascaris:

The body cavity is not a coelom (= eucoelom) in true sense as it is not lined by cell layers derived from mesoderm. The body cavity is called pseudocoel (false coelom) because its wall is not lined with mesoder­mal coelomic epithelium and has not devel­oped either by schizocoely or enterocoely. It develops as a persistent embryonic blastocoel.

It is lined externally by fibrous proc­esses of the longitudinal muscle cells and internally by the cuticle encasing the intes­tine. The cavity is filled with a fluid and contains large stellate cells called coelomocytes or pseudocoelomocytes. The fluid acts as hydrostat.

5. Digestive System of Ascaris:

The mouth is anterior and terminal in position and leads into a straight tube which runs along the entire length of the body (Fig. 15.7). The mouth is guarded by three lips. Behind the mouth is buccal cav­ity, which leads into pharynx. The pharynx is dilated and its wall is muscular.

The phar­ynx draws food from the intestinal contents of the host. It leads to the intestine. The posterior part of the intestine narrows down to form the rectum which opens to the out­side through anus. The entire alimentary canal is made up of single epithelial layer covered internally and externally by cuticle derived from ectoderm. Digestive glands of any kind are absent.

6. Excretory System of Ascaris:

The excretory system consists of two longitudinal excretory canals one through each lateral line (Fig. 15.8). The two canals unite with each other at the anterior end and open to the outside through the single excre­tory aperture situated on the ventral side. Four to six big tufts of cells with ramifying protoplasmic processes remain in close con­tact with the canals.

It is believed that these cells collect, store and pass on the waste matter in dissolved state to the excretory canal. The whole system is neither a ciliated one nor there is any flame cell. It is presumed that the canal is an intracellular tubular extension of a single enormously elongated cell.

7. Nervous System of Ascaris:

The nervous system consists of a ring of nervous matter round the pharynx (Fig. 15.7B). The ring is swollen at the ventral side and is ganglion-like. The ring gives off six nerves to the anterior and six nerves to the posterior side.

Of the posterior nerves, two are of considerable thickness and run along the dorsal and ventral lines up to the poste­rior end of the body. The dorsal and ventral longitudinal nerves are connected with each other by transverse commissures. The tip of the ventral nerve cord swells and forms a ganglion just in front of the anus.

The only sense organs are the sensory papillae situated as small elevations on the lips.

8. Reproductive System of Ascaris:

The sexes are separate (Fig. 15.8) and there exists a considerable degree of sexual dimorphism (Fig. 15.3).

Male Reproductive Organs:

Male repro­ductive organs consist of a single thread-like, much-coiled structure occupying some por­tion of the body cavity. This structure is differentiated at the anterior region as the testis, the middle region as the vas deferens and the posterior region as the seminal vesi­cle. The sperms are stored in the seminal vesicle.

The demarcations between these three parts are very poor. The wall of testis is composed of a single layer of cuboidal cells which are covered by basement membrane. The gamete formation takes place within tubular gonoducts. The seminal vesicle con­tinues as the ejaculatory duct and opens into the anus.

The opening is associated with a pair of chitinoid spicules called penial setae or spicules, each of which is provided with a muscular penial sac. The penial spicules help in the copulation by opening the vulva or gonopore and transfer the sperms during copulation.

They can be protruded and re­traction by the action of protractor and re­tractor muscles. The anus, in case of male Ascaris, is used for the elimination of faeces as well as of sperms and in some way it resembles the cloaca of toad.

The sperms of Ascaris are very peculiar as they show amoeboid movement inside the body of the female. A ripe sperm is cone- shaped in appearance having a broad base and an apex (Fig. 15.8C). The apex contains the acrosome and the broad base contains nucleus and mitochondria. The sperms remain non-motile in the male gonoduct.

Female Reproductive Organs:

Female reproductive organs consist of a pair of much coiled thread-like ovaries which pass into a uterus. The two uteri unite and form a mus­cular vagina which opens to the outside by the female genital aperture or vulva or gonopore situated on the ventral surface at about one- third of the body length from the head.

9. Life History of Ascaris:

Kuchemeister and Davaine first started the investigation of the life history of A. lumbricoides. Stewart (1916) demonstrated that the first stage-rhabditoid larva emerges from the embryonated eggs in the intestine and takes a migratory route through the different organs of the host. He also demon­strated in 1921 that one host is required to complete the life cycle of A. lumbricoides.

Ascaris is an intestinal parasite of man. It is a monogenetic parasite because its life cycle (Fig. 15.9) is completed in one host only (man).

Copulation:

The copulation or mating takes place in­side the host’s (man) intestine.

Fertilization:

The eggs are fertilized in the oviduct or upper part of uterus. When the fertilized eggs move downwards through the uteri, they are surrounded with highly resistant, chitinous egg shell and an irregular coat of albumen.

Egg Laying:

The fertilized eggs are laid in the host’s intestine. A female Ascaris is recorded to produce as many as 27,000,000 eggs in her life time with a deposition of 200,000 eggs daily.

Structure of Fertilized Egg:

After fertilisation, the fertilized eggs or zygotes are oval or elliptical in outline with an albuminous coat which are enclosed in a tough, chitinous egg shell. The chitinous shell protects the egg. The eggs measure about 60 to 70 µm in length and 40 to 50 µm in width.

The tough, thin chitinous egg shell consists of 3 layers:

(i) An inner lipoid layer,

(ii) A middle chitinous layer and

(iii) An outer quinone- tanned protein layer.

This protein layer rep­resents as wart-like structure. A fine net-like fibrous layer is also present at the outside of the protein layer.

Among the 3 layers, the inner lipoid layer and middle chitinous layer are formed from fat globules and glycogen globules of the unfertilized egg respectively and the outer proteinaceous (albunaceous) layer is formed from the secretion of the uterine wall when the fertilized egg passes down through the uteri.

Liberation of Fertilized Eggs:

The shelled eggs without un-segmented embryos (zygote) come out of the body of the host along with the faeces. Formation of embryo is completed in about 9 to 23 days.

Development of the Zygote:

Further development in the zygotes (em­bryos) occurs only if the environmental tem­perature is between 15.5 °C and 35 °C and the development of the embryos ceases be­low the temperature of 15.5 °C. Other factors such as moist soil, water and oxygen are necessary for development of embryos.

Smyth (1994) has reported that development of the embryos takes place between the tem­peratures of 22 °C-33 °C and the embryos are destroyed above the temperature of 38 °C. They can survive a few months in moist soil in the presence of O₂ and the larvae take 8- 12 weeks after ingestion to attain the sexual maturity.

Cleavage and Larval Stages:

The cleavage is of spiral type and the development is of determinate. Cleavage leads to blastula, and blastula transforms into gastrula by epiboly.

The gastrula ultimately develops into a tiny larva or juvenile. The first larval stage is called rhabditiform larva or rhabditoid. It closely resembles the genus Rhabditis; a nema­tode which is found in soil, water and fre­quently in the faeces of man.

Rhabditiform Larva:

The larva contains the following charac­ters:

1. The larva measures about 0.25 to 0.33 mm in length and 13 to 15 µm in diameter.

2. The first larval stage contains an ali­mentary canal with mouth and anus at opposite ends.

3. The oesophagus is short and a swol­len bulb-like structure at the poste­rior part of oesophagus.

4. There is a nerve ring in the anterior part of oesophagus.

5. A larval excretory organ opens on the left side of the oesophagus.

6. The rhabditiform larva develops within 9-13 days after the beginning of the cleavage.

7. The first larval stage is not infective.

8. After another week, the second stage rhabditiform larva or second stage rhabditoid or second stage juvenile develops within the egg shell after the first moult.

9. The second stage is infective, if swal­lowed by man.

10. The second stage rhabditiform remains in the coiled condition within the egg shell.

11. During the period of development within the egg shell, the water and dissolved gases pass through the egg shell but not allowed the salts to pass through the shell.

Infection to the New Host through Food:

Infection is direct, i.e., intermediate hosts are not involved. Infection results from in­gesting fully developed embryos encased in shells along with contaminated food and drink.

Hatching:

The infective eggs are not hatched until they reach the small intestine of the host. They hatch within two hours of reaching the intestine. Within the intestine the shell is dissolved by a secretion of the young worms containing the esterase, chitinase and prob­ably proteinase enzymes. Another factor which is to be considered is carbon dioxide that serves as an inducing agent for hatching of the shelled embryos.

Migration:

Sprent (1952) has reported two types of migration in Ascaroidea. These types are— tracheal and somatic. The migration is of tracheal type in A. lumbricoides. In somatic type the larvae reside permanently in the cells forming cysts.

A newly hatched larva burrows the mucosa of the intestine and enters the lymphatic ducts or veinules. From the lymphatic ducts it is carried to the me­senteries, lymph nodes and from there to the portal vein through circulation.

From the portal vein the larva comes to the liver through hepatic portal veins where it re­mains for several days. Here they pass from interlobular to intralobular veins and are carried to the right side of the heart through post caval vein from where they are pas­sively carried to the lungs through the pul­monary artery.

In the lungs it enters the alveoli and in the alveoli the larvae settle down for some-time and undergo two moultings (2nd and 3rd) after 6 and 10 days.

From the alveoli of lungs the larva passes into the trachea through broncheoles and reach the pharynx where they cause cough­ing. Finally they come down to the intestine through oesophagus and stomach. In the intestine they reach for the second time and the fourth moulting takes place which gives rise to adult worm.

During this migration the worms increase its length about ten times from its initial length of the body. The whole journey takes about ten days and adulthood is reached in about two months-time.

Moulting:

Moulting is the shedding of the cuticle of the body and moulting takes place four times in the life cycle of Ascaris. First moult­ing takes place within egg which is found in the faeces of the host and second and third moultings within lungs and fourth moult­ing takes place in the small intestine.

Factors for Moulting:

The moulting is influenced by the pres­ence of a combination of high CO₂ tension and a low oxidation-reductional potential at a pH about neutrality, and at 98 °F helps to induce a physiological change in the embryo.

Modes of Infection:

(i) When the ripe eggs or larvae of A. lumbricoides are swallowed by man with raw vegetables which are culti­vated in an area where infected hu­man excreta are employed as manure.

(ii) The damaged water supply pipes pass through the infected soil of ascarid larvae, and water may be contami­nated by the eggs and larvae and these larvae easily reach into the intes­tine with the drinking water.

(iii) In the villages or in the small towns there are no proper sanitary method. The children when play on this soil polluted areas, the eggs are accumu­lated under finger’s nail and may be conveyed to the mouth.

(iv) Sometimes infection takes place by the inhalation of the desiccated ripe eggs with dust which reach in the pharynx and are swallowed.

10. Pathogenicity of Ascaris:

The disease caused by the infection of the A. lumbricoides is called ascariasis. It is more susceptible to the children than adults. The second stage rhabditiform larvae which penetrate intestinal wall cause haemorrhage, temperature, muscular spasms and anae­mia.

When the larvae migrate from intestine to the lungs through hepatic veins there may be transient hepatitis. The migratory larvae when pass through the alveoli of the lungs, cause inflammation of the lungs, fever, eosinophilia, produce fatal pneumonia or Ascaritis pneumonia or Loffler’s pneumo­nia. The larvae may cause a number of complications in the lungs, kidneys, brain, spinal cord and muscles.

Heavy infestation by the adult worms produces the following troubles:

(i) Volvulus is caused by aggregation and intestinal obstructions.

(ii) Obstruction of eustachian tube.

(iii) Appendicitis is caused by the accu­mulation of large number of adult worms which block the intestine or appendix.

(iv) Peritonitis is caused by the damage of the intestinal wall.

(v) Delirium, a disordered state of mind with incoherent speech is caused by the toxic substances released from the worms with convulsion, abdominal tumours, gastric ulcer and vomiting, etc.

11. Control Measures of Disease Caused by Ascaris:

Prophylaxis (Preventive treatment against disease):

1. Raw vegetables which are grown in the areas using infected human ex­creta as the manure, should be thor­oughly washed and cooked properly before consumption.

2. Drinking water must be filtrate or boiled before drinking.

3. Children should be taught in the ele­mentary school or at home to wash hands with soap before eating or should regularly cut the nails to clean the ascarid eggs.

4. Every home should have sanitary toi­let and children must have knowledge to use the toilet.

12. Treatment of Disease Caused by Ascaris:

Human ascariasis can be controlled suc­cessfully by the use of antihelminth drugs such as:

1. Oil of chenopodium

2. Santonin

3. Tetrachlorethylene

4. Carbon tetrachloride

5. Hexylresorcinol crysoids in gelatin capsule will be taken with 12 hrs fasting before.

6. Piperazine citrate syrup or antepar should be used two spoonful twice a day for one week followed by an­other course after a gap of one week.

7. Hetrazan

8. Dithiazanine

9. Thiabendazole.

13. Parasitic Adaptation of Ascaris:

(A) Morphological adaptions:

1. Larger size of the body is related to increased egg production.

2. Parasitic nematodes show little morphological specialisation.

3. Locomotory organs are absent in Ascaris and mild locomotion is re­stricted to the bending of the lon­gitudinal muscles of the body.

4. The muscular pharynx draws food from the intestinal contents of the host by sucking action.

5. There are no digestive glands be­cause the ingested food is pre-digested.

6. There is a reduction in the central nervous system and sense organs. The sense organs are labial papil­lae and amphids, etc.

7. Simple type of excretory system and the excretory system excretes urea. But the alimentary canal eliminates also urea and ammonia. Neither nephridia nor flame cells are present.

8. There is no circulatory system. The pseudocoelomic fluid serves the absorption, transport and distribu­tion of the food, oxygen and wastes and acts the functions of circula­tory system.

9. There is a great elaboration of the reproductive organs and the fe­male produces a vast number of eggs than their free-living relatives because there is a maximum wast­age of eggs and a number of haz­ards during the transference of eggs to a new host.

The embryonated eggs reach to the intestine of the host when the eggs are acciden­tally ingested and swallowed, and oxygen, water and optimum tem­perature are required for develop­ment of the embryo.

10. The eggs are enclosed by a multi- layered shell which is highly resist­ant and offers protection from the unfavourable conditions.

(B) Physiological adaptations:

1. The body of Ascaris secretes some anti-enzymes which counteract and protect from the host’s digestive enzymes.

2. Metabolic rate is extremely low, thus requiring a minimum amount of O₂.