In young animals worms can stop your pet from thriving, and even kill them. Older animals become fairly resistant to the effects of intestinal worms (but not lungworm) so, especially with round worms, you will often have no idea that they are producing eggs. Although they may not harm your dog these eggs can infect people, especially children and in extreme cases can cause blindness in children. Worm eggs can be present on your pets fur so can be transmitted to people just by stroking an infected dog's fur.
Tapeworms in this part of the country tend to be less of a risk to people but can still be a nuisance for your pet. In other parts of the U.K., such as Wales, The Lake District and the West Country moors, tapeworms can pose a serious risk to human health so treating for tapeworm is advisable after a visit.