BlueBee; Some of the parasites in a goat are passed on through the poop. When the eggs hatch in the poop the parasite climbs up a blade of grass 4 or 5 inches. The goats come through and eat the low grass thus ingesting the parasite, who takes up host in the intestinal tract. The parasite can only survive either 30 or 60 days before it has to find a host. If no host is available it dies. By having larger grazing areas it increases the likelihood that the parasite won't complete it's cycle. Some goat ranchers have enough paddocks (fenced in pastures within a pasture) that by the time they move them back to the first paddock the parasites there didn't complete their didn't survive.
If your goat is stressed, i.e., relocation, pregnancy/giving birth, or other factors, the parasites will have a population explosion inside the goat. I try to worm mine at least once a year and after they give birth. You can monitor their worm load by doing a fecal test using a microscope or just check the inside of their bottom eyelid. You're looking for a healthy red. Anything else suggests that the parasites are sucking the blood out of them. However, I have a couple of goats whose eyelids have never appeared to be a healthy red. But they're fine. Sometimes it's just the goat. My Kiko nanny's eyelids are always a bright red.
When I first started getting goats I got a "deal" on a herd. Averaged out about $22 a goat for around a dozen. That was my enrollment into parasite school. Anywho, after I lost a couple, and doctored the heck out of them, I had one that was always skin and bones. I mean her rib bones were very pronounced. I poured the feed to her too. Never got fat. Most likely what happened was she had had such a heavy parasite load all her life they damaged her intestinal tract and/or stomach lining. When they take up residence in a host they attached themselves to the inner lining of the stomach or intestines. (goats have 4 stomachs). When you kill them they release and scar tissue forms. This scar tissue prevents nutrients from being absorbed into the body.
You have to be careful when treating a goat (or any animal) with a heavy load. If you kill too many at one time the goat will bleed to death on the inside. I isolate the goat and give them a mild treatment the first week. By the third treatment I put it to them, making sure no "resistant" parasites survive.