Actually, you need to give fenbendazole (Safeguard) for treatment. You do give a shot of Ivomec too but only 1 shot. Safeguard goes past the blood brain barrier while Ivomec doesn't. So Ivomec is the prevention and Safeguard is the treatment. For alpacas and goats, the dosage for Safeguard is 1cc per 7 lbs (liquid) for 5 days in a row. Not sure what the dosage on sheep would be but it is definitely higher than normal dosage.
And that is the key. Catching it early. If they honestly didn't have M Worm than the worst you have done is thoroughly wormed them. If you did catch M Worm, then you saved a life.
First sign is back end problems. At first they just seem wobbly on the back end, then they are limping on the back end. Once you get past that, then it is very hard to treat successfully.