(THE COMMON ROUND WORM)
CLASS : PHASMIDIA.
Ascaris lumbricoides is comnly called the common round worm. It is the ne of the most commn enterozoic parasites of man living in the small ntestine (jejunum), more frequently in children. It is monogenetic, dimorphic , pseudocoelomate parasite. It is cosmopolitan in distribution but prevalent in tropical countries like China, India and Southeast Asia. Its mde of infection s through contaminated food and water. Infective stage is the embryonated egg with te 2nd stage rhabditiform larva. Structure:
Ascaris lumbricoides is the largest intestinal parasite. The anerior end is thinner compared to posterior end. The parasite resembles an ordinary earthworm. In shape the parasite is rounded and elongatede tapering at the ends . The mouth occurs at the anterior end which is guaded b three toothed lips. Of these , one is dorsal and two are ventrolateral. Excretory pore is located ventrally at a short distance from the anterior end of the body. The digestive and reproductive systems occur freely floating in the body cavity. Sexes are separate and the sexual dimorphism is distinct.
MALE: The male worm measure 12-23 cms in length and 3-4 mm in diameter. The tail ( posterior end ) is curved ventrally in the form of a hook , with a conical tip. On the ventral side of the tail region occurs a single aperture , the cloacal aperture, through which , the male and female reproductive systems open to the side. Cloacal aperture also contains a pair of copulatory spicules which are equal in length.
Female worm is longer and stouter than males measuring 25-49cms in length and 5mm in diameter. The tail ( posterior end ) is straight and blunt . The female genital aperture , or vulva is located mid ventrally at abut 1/3 the length from muth. The anus is present a little in front of the tail end. Egg laying capacity of Ascaris is very high. It lays about 2,00,000 per day.
EGGS OF ASCARIS:
1) Eggs are oval in shape measuring 60- 75 micrometers in length and 40- 50 micrometers in breadth.
2) Eggs are surrounded by an outer thick protein coat that rippled into rugosities , or mamillations.
3) Below the outer coat occurs a chitinous shell and below it a lipid layer.
4) The egg contains an unsegmented ovum.
LIFE CYCLE OF ASCARIS :
Ascaris lumbricoides is a monogenetic parasite passing the life cycle in a single host. Copulation takes place in the small intestine of man. After copuation , the female releases approximately two lakh eggs per day. Each egg is surrounded by a protein coat with rippled surface. Hence the eggs of Ascaris are described as mamillated eggs. The protein coat is followed by a chitinous shell and a lipid layer internally. These eggs come out along with faecal matter. In the moist soil, development takes place inside the egg so that the 1st stage rhabditiform larva is produced. It undergoes the 1st moulting and becomes the 2nd stage infective to man. They reach the alimentary canal of man through contaminated food and water. In the small intestine , the shell gets dissolved so that the 2nd stage larva is released. Now it undergoes extra intestinal migration. First it reaches the liver through the hepatic portal vein. From there it reaches the heart through the post caval vein . It goes to the lungs through the pulmonary arteries . In the alveoli of luns it undergoes the 2nd moulting to produce the 3rd stage larva . It undergoes the 3rd moulting so that the 4th stage larva is produced in the alveoli only. It leave the alveoli and reaches the small intestine again , through bronchi , trachea , larynx , glottis , pharynx , oesophagus and stomach. In the small intestine , it undergoes the 4th and 5th the final moulting to become a young one which attains sexual maturity within 8 – 10 weeks. PATHOGENECITY: The disease caused by Ascaris lumbricoides is called asariasis . It is more common in children than in adults . In the intestine , the worms consumes the food material and deprive the host of its nutritious food , causing emaciation. The Machanical damage to organs like heart , lungs, brain , muscles etc., is caused during larval migration. The ault causes intestinal pain , appendicitis , tumours , indigestion , diarrhea and eosiniphilia.
Prevention of spread of A. lumbricoides includes two aspects viz.,
1) Personal prophylaxis and 2) Community prophylaxis.
1) Personal prophylaxis :
A)Use of clean drinking water.
B) Covering of food and drinks so that they are beyond the reach of house flies , cockroaches etc.,
C)Proper washing of vegetables and fruits before they are consumed.
D)Washing of hands properly before takin meals.
E)Maintaining personal cleanliness like trimming of nails etc.,
2) Community prophylaxis:
A) Proper disposal of sewage water.
B) Supply of protected drinking water.
C) Identification and isolation of vectors.
D) Abstaining from eating of snacks from local vendors.
E) Avoiding the use of human night soil as manure.