HOW TO DEAL WITH INTERNAL PARASITES

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The snail, however, crawls merrily around and hardly notices the presence of the worm larva. Then along comes your friendly lamb or kid—who isn't a particularly fussy eater—and he or she munches on the tasty grass and inadvertently swallows the tiny snail.

Now that the larva is back in the kind of critter that makes it happiest (in this example the sheep or goat is the final or '' definitive'' host), it will do one of several things. For instance, the larval worm may move into the animal's stomach, develop into an adult parasite, and begin passing more eggs onto your pasture. Or, for some unknown reason, the larva might decide to take a detour on the way to its host's stomach. This "side trip" is called larval migration, and—since it could take the pest through muscles, blood vessels, lungs, or liver—can be more harmful to your animal than the adult worm would be. (This is, of course, not a particularly appealing or appetizing picture of the worm's life cycle, but internal parasites aren't particularly appealing or appetizing creatures.)

On the other hand, the parasite larva may find the environment inside your critter not to its liking. When this happens, the larva will form a cocoonlike substance around itself (this is called "encysting") and can—in this form—lie dormant until a change in the internal environment causes it to emerge. Unfortunately, the environmental changes that make life easier for the encysted larva can further compound your parasite problems. When your ewe or doe gets pregnant—for example—its internal environment may change enough to cause the parasite to emerge from its cyst and begin another migration. If this happens, the traveling larva might find its way directly into the unborn offspring or into the mother's milk . . . which explains why many animals are infested with parasites at birth.

This example should convince you that worms are pretty difficult to eliminate. You could, for instance, try to get rid of all the eggs . . . but—unless you're capable of (and interested in) picking up every little, black sheep or goat nugget in your pasture—this will be a nearly impossible task.

Or, of course, you can try to kill the adult parasites by giving your beast the proper medication . . . but the medicine that will destroy the adults will, usually, not kill the larvae. Because of this, you must administer worm medicine at least twice (once to destroy the adults and once to kill the larvae after they've become adults), and it's better still to administer wormers periodically throughout the host critter's lifetime.

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