There are many culprits for skin problems in dogs, such as allergies, parasites and bacteria. Many skin problems are caused by a combination of these causes.
Allergies
Environmental, food or parasite allergies can cause skin problems. Dogs exhibit allergy symptoms through their skin, rather than their sinuses.
Of the three, the easiest to cure is parasite allergies, which can easily be prevented with flea and tick repelling medication. Even though you haven't seen any parasites on your dog, an allergic dog can experience an allergic reaction weeks after only one flea bite.
Food can also cause skin allergies, which cause your dog to chew on his skin and excessively lick himself. Diagnosing a skin allergy usually requires a food trial, during which you remove your dog from all food except one novel protein and vegetable source, such as duck or venison and potato. Food items are gradually added back in until the allergens are identified.
If treating parasite and food allergies doesn't solve the problem, it could be environmental allergies, which can be determined through a skin test. There is often no cure for environmental allergies, but regularly bathing your dog with an oatmeal shampoo or wiping him down each time he comes in from outside can ease symptoms.
Parasites
Many different types of parasites can infect your dog and cause skin problems. Fleas and ticks can often cause allergic reactions that lead to skin problems, but they're not the only threat to your dog's skin.
Common types of mites include Cheyletiella mites, Sarcoptic mites and Demodex mites, all of which cause dry, flaky skin, which looks like dandruff, hair loss, scaly skin and red, itchy skin. Of these, Sarcoptic mites, also known as scabies, are the most difficult to diagnose because they burrow into the skin and can often not be detected by a skin scraping.
If your older dog contracts Demodex mites, also known as mange, it can be an indicator of an underlying problem such as stress, poor diet or a low immune system.
All of these mites can be passed to humans, so you will often be experiencing the same symptoms as your dog if he has contracted a parasite.
Bacteria and Yeast
The skin of every healthy dog contains large amounts of bacteria and comes in contact with yeast on a daily basis. Thus, bacterial and yeast infections are usually secondary infections. These are caused by an allergy or parasite infection causes your dog to excessively lick and chew on his skin, giving the bacteria or yeast a chance to enter the body and cause more problems.
Bacterial infections often cause hair loss in particular patches, which can become inflamed and red, while yeast infections cause greasy or scaly skin with a stronger odor. These can be easily treated with antibiotic or anti-fungal medication from your vet, but the underlying cause should be investigated as well.
Skin problems are extremely uncomfortable for your dog and should be treated as soon as possible. There are many different sources for skin infections, so consult your veterinarian on what is causing your dog's symptoms.