According to several sources, including veterinary data and studies, the leading cause of death in Chihuahuas is heart disease, specifically heart failure. This is often caused by valve deterioration, which accounts for 75% of heart disease in Chihuahuas. Other common causes of death in Chihuahuas include lower respiratory tract infections, traumatic injuries, and brain disorders. It is important to note that many of these health issues are genetic and related to the breed, but not all Chihuahuas will experience them. Regular check-ups with a veterinarian can help detect and manage any potential health issues.