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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Tissue Nematodes

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Toxocara canis : Parasitology

T. canis is a large, intestinal ascarid of canines, including dogs, foxes, and wolves.Each female worm discharges approximately 200,000 thick-shelled eggs daily into the fecal stream.

TOXOCARA

Toxocara canis : PARASITOLOGY

T. canis is a large, intestinal ascarid of canines, including dogs, foxes, and wolves.Each female worm discharges approximately 200,000 thick-shelled eggs daily into the fecal stream. After reaching the soil, these eggs embryonate for a minimum of 2 to 3 weeks. Thereafter, the eggs are infectious to both canines and humans and, in moist soil, may remain so for months to years. When ingested by a young dog, the larvae exit from the eggshell, penetrate the intestinal mucosa, and migrate through the liver and the right side of the heart to the lung. Here, like the offspring of Ascaris lumbri-coides, they burst into the alveolar airspaces and are coughed up and swallowed;thereafter, they mature in the small bowel. In fully grown dogs, most of the migrating larvae pass through the pulmonary capillaries and reach the systemic circulation.

These larvae eventually are filtered out and encyst in the tissues. Hormonal changes and/or diminished immunity in the pregnant bitch stimulate the larvae to resume de-velopment, migrate across the placenta, and infect the unborn pups. Larvae may also pass to the newborn puppies in their mother’s milk. Approximately 4 weeks after par-turition, both the puppies and the lactating mother begin to pass large numbers of eggs in their stools. The mother may be superinfected by ingesting the newly passed eggs and can redevelop clinical symptoms.

When humans ingest infectious eggs, the liberated larvae are small enough to pass through the pulmonary capillaries and reach the systemic circulation. Rarely does the or-ganism break into the alveoli and reach the intestine to complete its maturation to adult-hood. Larvae in the systemic circulation continue to grow. When their size exceeds the diameter of the vessel through which they are passing, they penetrate its wall and enter the tissue. The larvae induce a TH2-type CD4+ response characterized by eosinophilia and IgE production.


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