Parasitic worms are a veterinary or medical problem but they are included here because some may be transmitted by rats, mice or certain insects. Threadworms are relatively common in dogs, cats and children. They are thin whitish wrigglers about 5mm long, typically only detected after they have passed out of the gut. They may cause intense anal itching at night in young children. The trouble starts when threadworms’ eggs are swallowed with unwashed, contaminated fruit or raw vegetables or from food handled by unwashed hands.