|MadSci Network: Microbiology|
There are two large groups of "worms" that can be found in the intestine of animals, including man: roundworms (scientifically known as nematodes) and flatworms or tapeworms ( known as cestodes). Together, the two groups are also called "helminths". Many of these helminths need to spend some time in the soil as part of their life cycle.
Cestodes: eggs are passed out the affected animalīs feces and they remain in the soil for many months. When another animal eats soil that contains these eggs, the egg will reach the intestine where it can "hatch" and and a larval (immature) form of the worm is freed. Then one of two things can happen: 1). the larva penetrates the gut wall and goes to the animalīs tissues and stays there as a larval form or 2) the larva will mature into an adult worm and will produce eggs that will be eliminated with the feces. It all depends on the species of cestode.
Roundworms: Depending on the family to which the worm belongs, animals can acquire the infection either by eating eggs that are contaminating soil (just like cestode eggs) or because the soil contains infective larvae that will penetrate through the animalīs skin. These larvae come from eggs that have hatched in the soil.
Usually worms are very host specific, that is worms that infect humans will only infect other humans while worms that infect dogs stick to dogs. However a few species of worms that infect animals will occasionally infect humans. One of these dog worms is Toxocara canis which is acquired by eating eggs that are in the soil (the soil of course is contaminated with dog feces). In the case of the dog, the egg hatches in the intestine , the larva penetrates the gut wall, passes to the blood, goes to the lungs where it will mature, then ascends via the bronchii to the trachea, and passes from there to the esophagus and returns this way to the intestine where it becomes an adult worm. If a human swallows one of these eggs, it will hatch , the larva passes the gut wall and into the bloodstream. In the majority of the cases, the immune system will kill the larva at this point and nothing happens to the human. However, sometimes the larva escapes the immune system and starts migrating and will produce a disease known as Visceral Larva Migrans that will eventually dissappear because it s a self limiting disease. Note. this larva will NEVER become an adult worm in humans! Humans then, can acquire dog worms when they handle soil that is infected with dog feces that contains eggs or larvae. So it is very important to wash your hands very well as well as all raw vegetables!
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