If not controlled, internal parasites can cause illness in animals. Symptoms of parasitism include weight loss, loss of appetite, depression, weakness, lagging behind or separating from the flock, and possibly anemia, bottle jaw, or diarrhea. Livestock producers want to raise healthy animals, and so they may treat the sick animals with dewormers. But that is a temporary fix at best because unless there is a management change, animals will soon be reinfected. Also, most dewormers are no longer working because the internal parasites have developed resistance to the chemicals.