Roundworm passes its life cycle in single host and no intermediate host (the host that harbors larval stage of parasite) is required. Man is the only definitive host (the host in which parasite harbors its adult stage or it utilizes sexual method of reproduction) of A. lumbricoides.
The various stages in life cycle are described as follows:
Stage 1. Egg in faeces:
Fertilized egg containing unsegmented ovum are passed with the faeces. These eggs are not infective to man when freshly passed.
Stage 2. Development in the soil:
A rhabditiform larva is developed from the unsegmented ovum within the egg shell in 10 – 40days depending on atmospheric temperature and humidity in the soil. The developing embryo remains coiled up within the egg shell and undergoes one moult. The second moult may occur in the egg and get completed during early migration in the intestinal wall or liver of the definitive host. The ripe egg containing the second stage( larva coiled up embryo) is infective to man. The infective eggs may survive in loose, shady and moist soil upto 7 years in temperate zone.
Stage 3. Infection by Ingestion and Liberation of Larva:
Infection occurs by ingestion of infective embryonated eggs in contaminated food, drink or raw vegetables or faecally contaminated hands. The eggs escape the acid action of gastric juice due to presence of thick egg shell. The embryonated eggs pass down the duodenum where the digestive juices weaken the egg shell and stimulate the enclosed larvae into activity. Splitting of egg shell occurs and the rhabditiform larva measuring 250 um X 14 um are liberated in the upper part of the small intestine.
Stage 4. Migration through Lungs
The larvae liberated into the small intestine do not directly develop into mature worms. The rhabditiform larvae penetrate the gut mucosa and are carried by the portal circulation to the liver where they live for 3 to 4 days. Finally, they pass out of the liver and via right heart enter the pulmonary circulation. In the capillary of the lungs, they grow almost ten times bigger from 0.2 to 2mm and moult twice. (first on 5th or 6th day, second after the 10th day). The developed larvae then break through the capillary wall and reach the lung alveoli. The time taken for such migration is on average 10 to 15 days.
Stage 5. Re-entry into the stomach and the small intestine.
From the lung alveoli the larvae crawl up the bronchi and trachea that is aided by the current caused by the ciliated epithelium of the respiratory tract and are propelled into the larynx. The larvae leave the respiratory tract by means of cough and once more get swallowed. The larvae then pass down through esophagus to the stomach and localize in the upper part of the small intestine. Another moulting occurs between 25th to 29th days of infection.
Stage 6. Sexual Maturity and Egg liberation:
On reaching their normal habitat, the larvae grow in adult worms and become sexually mature in 6 to 10 weeks. Fertilization of the female by male initiates egg production. The gravid females begin to discharge eggs in the stool within about 2 months (60 to 70 days) from the time of infection. The cycle is again repeated. The female worms that have never been fertilized produce unfertilized eggs.
Figure: Life cycle of Ascaris lumbricoides
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