Thread worms are small roundworms that live in the intestine of dogs. They pass infectious eggs in the faeces. They usually only cause mild diarrhoea, but infection in young puppies can cause severe disease which may present as diarrhoea, inappetance, dehydration and weight loss. Your vet can do a simple faecal test to detect the presence of threadworms. Threadworms can be passed to humans. If you suspect threadworms, you should treat you animal and practice good hygiene. Remove all faeces from the yard, and wash hands after touching your dog.