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Subject 10 (Labours, 1st year DDS)


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General characteristics of Nematodes

• The common name of the nematodes - round worms.
• Nematodes are elongated, tapered at both ends.
• The nonsegmented body is covered with a noncellular cuticle that is secreted by underlying hypodermis. The cuticle is shed four-times during ontogeny.
• The digestive system of female is complete with mouth, esophagus, intestine and the anus near the posterior end.
• The digestive system of male is complete with mouth, esophagus, intestine and cloaca.
• Most nematodes are dioecious and show sexual dimorphism.
• The general life cycle involves following stages: egg – larvae - adult stages (female and male).
• Parasitic nematodes have simple or - rarely - complex life cycles.

Ascaris lumbricoides

Kind of parasite: homoxenous and monoxenous.

Host(s): human.

Infective stage: fully embryonated egg.

Transmission:†
• the hosts acquire the infection by the ingestion of infective eggs (containing developed larvae);†
• eggs are transmitted by hand contaminated with soil or ingestion of raw vegetables fertilized with human faeces;
• the larvae hatch in duodenum, enter venuls and migrate in the circulatory system via liver and right heart to the lungs, reach the trachea and pharynx and after swallowing the larvae maturate in the small intestine.

Site of infection: the small intestine.

Diagnostic method(s): microscopic identification of eggs in stool specimens.

Geographical distribution: cosmopolitan.

Remarks: A. lumbricoides is a geohelminth - part of the life cycle takes please in moist, warm soil where eggs become fully embryonated.

Trichuris trichiura

Kind of parasite: homoxenous and monoxenous.

Host(s): human.

Infective stage: fully embryonated egg.

Transmission: by the ingestion of infective eggs (containing developed larvae). After ingestion of infective eggs, larvae enter the LieberkŁhn’s crypts and after a short development, reenter the intestinal lumen, migrate to the large intestine and maturate.
Site of infection: the large intestine.

Diagnostic method(s): microscopic identification of eggs in stool specimens.

Geographical distribution: cosmopolitan.
Remarks:
• the larvae do not migrate in blood of host organism;
T. trichiura - is a geohelminth.

Enterobius vermicularis

Kind of parasite: homoxenous and monoxenous.

Host(s): human.

Infective stage: eggs and larvae.

Transmission:
• ingestion - the hosts acquire the infection by the ingestion of infective eggs (containing developed larvae);
• inhalation of dust with infective eggs;
• retroinfection - juvenile larvae hatched on the skin reenter the intestine through the anus.

Site of infection: the large intestine.

Diagnostic method(s):†
• the infection is diagnosed by microscopic examination of swabs for eggs deposited on the perianal skin;
• the eggs are sampled by using cellophane strip fixed on a glass applicator stick or by tongue depressor covered with a transparent adhesive tape.

Geographical distribution: cosmopolitan.

Remarks:
Females actively migrate during the night down the colon and outside by the anus, where the eggs are deposited on the perianal skin, embryonated eggs complete the development within 6 hours and therefore retroinfection is possible.

 Author: Piotr Nowosad date: 2020-05-15  print    back  
 
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