Those strands are usually collagen-rich connective tissue found naturally in beef. During slow cooking, the collagen softens and breaks down, sometimes appearing as pale, stringy or gelatin-like pieces between the meat fibers.
This is a normal part of cooking tougher cuts of meat and often means the roast has become tender. If the meat was stored properly, smells normal, and reached a safe cooking temperature, the strands are generally harmless.
True parasites in inspected beef are very rare and would not survive proper cooking temperatures. So those “worm-like” pieces are most likely just melted connective tissue — a normal result of low and slow cooking.
