A genetic background
Chihuahuas have fewer genetic defects than most breeds. They are a healthy, hardy breed in general. A Chihuahuas have been known to live as long as 15 to 20 years or longer!
One of the major problems Chihuahuas can have is patellar luxation,(which is a loose kneecap, or slipped stifles) They can be born with this, which will usually show up under 7 months of age, making it genetic or they can get it later usually making it an environmental cause. A Chihuahua born without defects can jump and land incorrectly and damage their knees. Chihuahuas are little jumpers, sometimes it is just too much stress on the knees that can cause this condition. They can also get a slipped knee cap from being overweight. Just as humans can cause disabilities from excessive weight ( grinding down the knee joints). Sometimes the slipped kneecap can become painful and debilitating and surgery may be the only option. Milder cases show little, if no signs at all of the condition.
Hydrocephalus can also be a problem in Chihuahuas. It is a build up of fluids around the brain, caused by genetics or environmental causes or injuries. There is no cure for hydrocephalus. Severe cases have eyes that point outward, not forward, they have mental retardation, puppies born with this usually die at birth. Many Chihuahua are able to live with milder forms through adulthood. Milder cases can be treated with prednisone and lasix.
Chihuahuas are prone to a painful disease known as goniodysgenesis, which can lead to blindness. Excessive fluid develops, putting pressure on the inside of the eye, from abnormal tissue developing preventing proper drainage of the eye. This damages the retina and optic nerves. It is treated with medication and or surgery.
Mitral Valve Disorder
The atrialventricular valve or mitral valve, is a genetic condition, the valve does not pump blood correctly, it back flows into the valve. Symptoms include coughing, shortness of breath, inability to exercise for extended periods, can lead to weakness and fainting. Do not be confused with a what is called a backward sneeze, that Chihuahuas often have. It leads to heart failure. It is treated with low sodium diets, minimal exercise, and medication, prolonging the dog's life.