Loss of appetite is a tricky symptom, potentially pointing in so many directions. Some causes are quite serious, so consult your vet to get to the bottom of what's going on. He relies on information you provide, so pay attention. Note what and when your dog does and doesn't eat, details about her bowel movements, when she first started decreasing her intake, other behavioral changes or possible symptoms and anything else that stands out.
If your dog has recently been vaccinated or is taking a new medication or supplement, it may deter her from eating. Many products depress the canine appetite as a side effect. This isn't a reason to avoid veterinarian-prescribed vaccinations or medications. It's a short-lived adverse reaction to vaccination, and it often fades fairly quickly when associated with medication or supplements as your pet's digestive tract adjusts to the new substance. Always mention side effects to your vet, though. If decreased appetite continues to be a problem with a treatment regimen, talk to him about the possibility of decreasing the dosage or finding an alternative.
Many infections and illnesses sometimes cause a decrease in appetite as a symptom. It may just be a cold or flu or other viral or bacterial infection. Conditions directly relating to the gastrointestinal tract, such as intestinal worms or irritable bowel disorders, are obvious suspects too. However, more serious problems with the kidneys, liver or heart are also possibilities, as are hormonal imbalances, autoimmune disorders, neurological problems, respiratory ailments and other afflictions. Periodontal disease, a broken tooth, acid reflux or another condition triggering pain during or after eating also can deter dogs from eating.
Some dogs have food allergies or intolerances, and they can become worse over time or develop later in life; in other words, don't rule them out just because your dog's eating the same foods as always. Beef, wheat, dairy, egg, chicken, lamb, pork, soy, fish and rabbit are common potential allergens, but that's by no means a comprehensive list. While allergies are an immune response, intolerances are digestive problems, such as the shortage of an enzyme required to break down certain foods. If you're feeding your dog something she has an allergy or intolerance to, it causes symptoms that may deter her from wanting to eat. Signs may include rash, hives, itching, swelling, diarrhea and vomiting.
A number of other explanations may account for why your dog doesn't want to eat. If you've recently changed foods, perhaps she doesn't care for the new stuff. Dogs are notoriously picky, especially if they've been fed a limited diet for a long time. Stress also depresses the appetite. Recent travel, illness, boarding, grooming, veterinary attention, environmental changes and other stressors may all leave your dog not wanting food for a while. Many dogs also start eating a lot less as they enter their senior years. Particularly hot weather or a decrease in physical activity also tend to diminish a dog's appetite.