General Causes of Canine Lethargy
Anemia (low red blood cell count)
Other blood disorders
Cardiovascular (heart and vessels) and pulmonary (respiratory) disorders
Chronic inflammation or infection
Drug or medication related
Electrolyte abnormalities
Endocrine (hormonal) or metabolic disorders
Gastrointestinal diseases
Urinary tract disorders
Cancer
Immune diseases
Certain severe skin diseases
Certain eye diseases, particularly those associated with blindness
Neurologic and neuromuscular disorders
Nutritional disorders
Behavioral disorders
Skeletal diseases
Infectious diseases
Exposure to certain toxins
Physical trauma
A trip to the vet is warranted to identify what is going on