Keeping your dog's coat at its best means keeping a regular check on internal parasites. Worms can sap essential nutrients from your dog, which would cause the hair coat to lose its luster and quality, not to mention causing other serious health problems. Your veterinarian will recommend a fecal exam during wellness exams to check for parasites. Watch for fleas and ticks too. These creatures can make your pet feel miserable and itchy and cause severe scratching, which could damage the coat.