There are many causes of death in Chihuahuas, but the most common is probably heart disease. Heart disease can be caused by a number of factors, including genetics, obesity, and stress. Stress can be caused by a number of things, including exercise, loud noises, and changes in routine. Obesity is often caused by overfeeding and lack of exercise. Genetics plays a role in many diseases, including heart disease. Some breeds are more prone to heart disease than others. The best way to prevent heart disease is to keep your Chihuahua at a healthy weight, free from stress, and with regular checkups from a veterinarian.