Common symptoms of canine illnesses include changes in appetite, changes in behavior or personality and vomiting or diarrhea, according to Pet Health Network. Changes in movement, such as difficulty standing or a stiff gait, can also indicate pain or illness.
Occasional vomiting and diarrhea is not necessarily a sign of illness, according to WebMD. However, frequent digestive upset or vomiting combined with lethargy and lack of appetite may be a sign of a more serious problem. If there is vomit in the blood or stool, it may indicate that the dog is suffering from gastric ulcers or has eaten something dangerous.
In most cases of appetite change, the dog loses interest in food and may seem lethargic, as noted by WebMD. However, some diseases, such as diabetes, cause excessive hunger and thirst. This is often paired with frequent or excessive urination.
Coughing or other signs of respiratory distress are also common symptoms, according to WebMD. While an occasional cough may not be a sign of illness, persistent coughing can indicate kennel cough or heartworms, among other diseases.
Drooling and bad breath can also be a sign of illness, notes Pet Health Network. Many dogs also suffer from skin diseases, which may take the form of hair loss, itchiness or visible redness and irritation on the skin.