How long do dogs live is a common question. The answer requires you to consider many factors, which include breed mix and size, and environmental influences. In general, smaller dogs live longer, from 14-22 years, with medium sized dogs living 11-14 years and large dogs living 8-12 years. Mixed breed dogs tend to live longer as many purebreds suffer from breed specific genetic problems that may shorten lifespan.
Diseases of the neurological system can also cause lethargy, and examples of these include fibromyalgia and Parkinson's disease. Diseases such as these have joint discomfort or stiffness, which may cause those affected to move less. Since they cannot move well without pain, this may increase their lethargy.
Full Definition of LETHARGIC. 1. : of, relating to, or characterized by laziness or lack of energy: feeling or affected by lethargy: sluggish. 2.