Vomiting

• Vomiting is one of the most common signs of an issue with a dog. A dog's vomit can range in color from yellow to a foamy white and be indicative of such conditions such as Addison's disease, bloat (gastric torsion,) distemper and even food allergies. Occasional vomiting is usually not a reason for concern, such as what may occur after a dog "inhales" his food. However, excessive vomiting, such as twice or more in one week, should be examined by a veterinarian as soon as possible.
  
  

Skin

• Dogs often show many issues through their skin. If you notice a change in your dog's skin, such as a thickening of the skin, lesions in any size, shape or color, as well as dandruff, change of pigmentation and so forth, you should contact your veterinarian. Common canine diseases that are indicated by a skin reaction include mange, nasal solar dermatitis (crusting of the skin of the nose), Cushing's disease (hyperadrenocorticism) and histiocytosis (a form of skin cancer).
  
  

Eyes

• The eyes are also a great indicator of a disease. Common symptoms of an eye disease include inflammation of the eyes, abnormal discharge, obvious tumors, sudden loss of balance and blindness. Dogs can develop a variety of eye conditions such as cancer of the eye, glaucoma, ulcerative keratitis (an inflammation of the cornea), pannus (an abnormal growth of tissue over the eye) and hemeralopia (daytime blindness).
  
  

Changes in Bowel Movements

• A change in bowel movements is also a cause for alarm in pets. Diarrhea, excessive urination, loose stool, lack of bowel movements and so forth can all indicate a disease or infection within the urinary tract. The urinary tract includes the kidneys, the bladder, the urethra and the ureters. Some common diseases that are indicated by a change in bowel movement include kidney disease, bladder infections, Cushing's disease, diabetes and distemper.
  
  

The Respiratory System

• Dogs that suffer from various diseases such as lung cancer or an infection of the lungs, myasthenia gravis (paralysis of the mouth or esophagus), eclampsia (hypocalcemia or low calcium levels) heartworm (which can cause what is known as heartworm cough) and more. Common signs of a disease of the respiratory system include coughing, wheezing and difficulty breathing. There may also be unseen symptoms such as lesions or parasites within the respiratory system.