Nematoda: Roundworms and Characteristics

• Class is Nematoda.
• Roundworms are Ascaris lumbricoides.
• Nemathelminthes is the Phylum of Nematoda.
• Nematoda are cylindrical worms.

General Characteristics

• Nematodes are elongated, cylindrical, unsegmented worms with tapering ends.
• The name 'nematode' means 'thread-like' and is derived from the word 'nema.'
• Nematodes are bilaterally symmetrical.
• Adults vary in size from about a millimeter to a meter in length.
• A pseudocoele body cavity suspends the viscera.
• The digestive system has an anterior mouth that leads to an esophagus, which varies in shape and structure among different groups.
• Nematodes have simple excretory and nervous systems.
• Sexes are separate.
• The male reproductive system has a single delicate tubule differentiated into testis, vas deferens, seminal vesicle, and ejaculatory duct that opens into the cloaca.
• The female reproductive system has the ovary, oviduct, seminal receptacle, uterus, and vagina.
• Nematodes may produce eggs (oviparous) or larvae (viviparous).
• Some lay eggs containing larvae that immediately hatch (ovoviviparous).
• The life cycle has four larval stages and an adult form.
• The cuticle is shed when passing from one stage to another.
• Most nematodes are free-living forms in soil and water, unlike parasitic trematodes and cestodes.
• Several nematode species are plant parasites and have great economic importance.

Ascaris lumbricoides

• This is the largest nematode (roundworm) that parasitizes the human intestine.
• As giant intestinal roundworms, they cause the disease Ascariasis.
• Adult worms live in the small intestines of infected persons, and the larva live the lung.
• Transmission occurs mainly through the ingestion of water or food contaminated with embryonated eggs.
• Children playing in contaminated soil may acquire the eggs from their hands.
• Morphology of worms are large cylindrical with tapering ends, the anterior end is more pointed than the posterior.
• The worms are pale pink or flesh-colored when freshly passed in stools and become white outside the body.
• The mouth has three finely denticulate lips at the anterior end: one dorsal and two ventro-lateral.

Sex differences

• Male worms measure 15 to 30 cm in length and 2 to 4 mm in thickness.
• The posterior end of the male worm is curved ventrally to form a hook and carries two copulatory spicules.
• Female worms are larger, 20 to 40 cm long and 3 to 6 mm thick.
• The female posterior extremity is straight and conical.
• The vulva is situated in the middle third of the body and leads to a single vagina.
• The genital tubules of the gravid female worm contain an enormous number of eggs, as many as 27 million at a time.
• A single female worm lays up to 200,000 eggs per day.

Eggs

• Eggs can survive for prolonged periods and are resistant to low temperatures and strong chemicals, but are killed by boiling.
• The eggs can remain viable for up to 10 years and are resistant to usual methods of chemical water purification.
• Eggs embryonate but do not hatch until ingested.
• Two types of eggs are passed by the worms: fertilized and unfertilized.

Fertilized Eggs

• Fertilized eggs are embryonated and develop into infective eggs.
• They are spherical or ovoid, bile-stained to a golden brown color.
• Fertilized eggs measure 60 to 75 μm in length and 40 to 50 μm in breadth.
• The eggs are fulled and have a thick external layer.

Unfertilized eggs

• Unfertilized eggs are non-embryonated and cannot become infective.
• They are elongated and larger than fertilized eggs.
• The shell is thinner with a scanty and irregular outer mammillary coat.
• Egg development depends on the nature of the soil and various environmental factors.
• Heavy clayey soil and moist, shady locations, with temperatures between 20° and 30°C, are optimal for rapid development of the embryo.
• Development takes 10 to 40 days, the embryo molts twice and becomes the infective rhabditiform larva, coiled up within the egg.

Life Cycle

• Infection occurs through the ingestion of the egg containing the infective rhabditiform larva.
• Frequent transmission occurs through fresh vegetables grown in fields manured with human feces.
• Infection may also be transmitted through contaminated drinking water.
• When swallowed eggs reach the duodenum, the larvae hatch out.
• Rhabditiform larvae are about 250 μm in length and 14 μm in diameter and are actively motile.
• The larvae penetrate the intestinal mucosa, enter the portal vessels, and are carried to the liver.
• The migration continues via the hepatic vein, inferior vena cava, and the right side of the heart.
• After about 4 days, they reach the lungs, where they grow and molt twice.
• After 10-15 days in the lungs, the larvae pierce the lung capillaries and reach the alveoli.
• They crawl up or are carried up the respiratory passage to the throat and are swallowed.
• The larvae molt to finally develop into adults in the upper part of the small intestine.
• They become sexually mature in about 6-12 weeks.
• Gravid females start laying eggs to repeat the cycle.
• The adult worm has a lifespan of 12-20 months.

Pathogenicity and Clinical Features

• Ascariasis is the name of the disease.
• Migrating larvae or adult worms are caused by The pathogenicity.

Migration of Larvae

• The pathogenic effects are due to allergic reactions, not the presence of larvae.
• Initial exposure is asymptomatic unless the larval load is very heavy.
• Re-infection may cause an intense cellular reaction to the migrating larvae in the lungs.
• Larvae destroy capillaries in the lungs, causing hemorrhage.
• Heavy infections can lead to pools of blood that block air sacs.
• Migration of white blood cells leads to more congestion, this condition is known as Ascaris pneumonitis or Loeffler's pneumonia.
• Lung tissue destroyed and bacterial infections may be fatal.

Adult Worms

• Overcrowding leads to wandering.
• If worms migrate to the stomach, acid irritates them, leading to nausea, abdominal pain, and allergic reaction.
• Intestinal penetration can lead to peritonitis, which is often fatal.
• If worms migrate to the lungs, they can cause extensive damage and possible death.

Diagnosis

• Macroscopic examination can be performed by seeing the adults passed in stool or throat the mouth or nose.
• Larva stage can be identified via sputum.
• The eggs are demonstrated in faces.

Laboratory Diagnosis

• Ascaris lumbricoidses is done by detection of parasite
• Definitive diagnosis of ascariasis is made via demonstration of eggs in feces.
• Rarely is diagnosis in light infestations possible; eggs are demonstrable only by concentration methods.
• In early infection stages, when migrating larva cause Loeffer's syndrome, diagnosis is possible via demonstration the larvae in sputum or in gastric washings.
• On occasion, adult worms are seen by the naked eye in stool or sputum
• X-rays, barium meals, and ultrasound imaging may help in diagnosis.
• Ascaris antibodies can be detected by ELISA, IHA, or IFA in serodiagnosis
• Serodiagnosis is helpful in extra-intestinal ascariasis, like Loeffler's syndrome.
• Possibly examine blood examination in early stages of infection.