Roundworm or Ascaris lumbricoides: Introduction, Morphology,Life Cycle

Roundworm or Ascaris lumbricoides: Introduction, Morphology,Life Cycle, Pathogenesis, Lab Diagnosis and Treatment

Adult Roundworm

Adult roundworm or Ascaris lumbricoides in formalin bottle as shown above picture.

Introduction of Roundworm

Ascaris lumricoides common name is roundworm of humans, growing to a length of up to 35 cm and it is the most common parasitic worm in humans. This organism causes disease ascariasis, a type of helminthiasis and one of the groups of neglected tropical diseases. An estimated one-sixth of the human population is infected by A. lumbricoides and it is prevalent worldwide, especially in tropical and subtropical countries.

Scientific Classification of Roundworm

Kingdom: Animalia
Phylum: Nematoda
Class: Chromadorea
Order: Ascaridida
Family: Ascarididae
Genus: Ascaris
Species: A. lumbricoides
Binomial name Ascaris lumbricoides

Geographical distribution
Cosmopolitan (especially in the tropics, such as China, India, and South-East Asia. Present in about 25% human population.

Habitat
The lumen of the small intestine (jejunum) and maintains its position by its muscle tone

Morphology of Roundworm

Adult worm

Largest intestinal nematode
Light brown or pink in color – fresh, but gradually changes to white
Round and tapered at both ends, the anterior end being thinner than posterior

Fertilized Eggs

Round and oval in shape 60-75 um x 40-50 um
Bile stained and golden brown in color
Surrounded by the thick smooth translucent shell with an outer albuminous coat.
Sometimes outer coat is lost called decorticated egg
Contain very large conspicuous unsegmented ovum
There is a clear crescentic area at the pole
Floats in sodium chloride ( NaCl) solution
Narrow, longer ( 80 x 55 um) and more elliptical, brownish in color
Irregular coating of albumin (thinner cell)
Contain a small atrophied ovum with a mass of disorganized highly refractile granules of various size
Does not float in NaCl solution (heaviest of all helminth egg)

Life cycle of Ascaris lumbricoides

Infective stage: embryonated egg
Fertilized egg-containing unsegmented ovum are passed with the feces
Rhabditiform larva is developed from unsegmented ovum within the eggshell within 10-40 days depending on atmospherical temperature and humidity
The larva undergoes molting (1st molting in soil, called embryonated egg)
Embryonated egg when ingested with food passes down to the duodenum where the digestive juices (trypsin) weakens the eggshell and stimulates the enclosed larva.

Eggshell splits and larva are liberated in the upper part of the small intestine. The newly hatched larva burrows through the mucus membrane of the small intestine and is carried to the portal circulation to the liver. They live in the liver for 3-4 days and pass out of the liver to the right side of the heart to the pulmonary circulation. In the lungs, they grow bigger and molt twice (2nd,3rd). Breaking through the capillary wall they reach the lung alveoli (about 10-15 days). Larva crawl up bronchi and trachea and with the help of ciliated epithelium of the respiratory tract, they are propelled into the larynx and pharynx and are once more swallowed. Larva passes down the esophagus to the stomach and is localized in the upper part of the intestine. 4th molting occurs between 25-29 days of infection (in the small intestine). The larva grows and sexually matures in 6-10 weeks. The gravid female discharge within 2 months after the time of infection and the cycle repeats. Egg-laying capacity is 2,00,000/day

Moulting of Ascaris lumbricoides

The total number of moultings is four and they are-

First molting-within eggshell
Second and third moultings- in lungs
Fourth molting in the intestine

Mode of Infection

Ingestion of foods contaminated with embryonated egg
Inhalation of desiccated egg in the dust reaching the pharynx and swallowed

Pathogenesis and clinical symptoms

Infestation of Ascaris lumbricoides in man is known as ascariasis
Symptoms due to migrating larva:
Ascaris pneumonia (Loeffler’s syndrome) with symptoms of fever, cough, and dyspnea.
Larva in general circulation filtrate out to various organs where they may cause unusual clinical symptoms of the brain, spinal cord, heart, and kidney
Symptoms due to adult worm:
Incubation period 60-75 days, symptoms are manifested at this period
mostly related to the GI tract.

Lab diagnosis of Roundworm

Direct evidence
Indirect evidence
Direct evidence
Finding of adult
The adult worm may pass out spontaneously in the stool or per anum between stool, or be vomited or escape through nares.
X-ray diagnosis: The presence of A. lumbricoides has been demonstrated by radiography with barium emulsion, which is ingested by the worm 4-6 hours, casts an opaque shadow (string-like shadow).

Finding of Eggs

In stool: Direct microscopic examination of a saline emulsion of the stool
Concentration methods may be used.
Note: The fertilized egg floats in a salt solution.
Unfertilized eggs do not float.
In the bile: microscopic examination

Indirect Evidence

Blood examination:
Eosinophilia at the early stage of invasion but if present in the intestinal phase suggest strongyloidiasis or toxocariasis
Dermal reaction (allergic):
scratch test-variable result
Traditional allergy tests utilize the Skin Prick method (also known as a Puncture or Scratch test) to determine whether an allergic reaction will occur by inserting possible triggers into your skin using a needle prick. If you are allergic to the substance, you are forced to suffer through your body’s reaction to the allergen.
Serological tests
The epidemiological study, also useful in the diagnosis of extra-intestinal ascariasis (Loeffler’s syndrome). Larva may be found in the sputum during the stage of migration.

Treatment of ascariasis

Pyrantel pamoate
Thiabendazole
Mebendazole
Albendazole
Piperazine citrate

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