Which of the following diseases is not known to affect dogs?

Black Leg is a disease that primarily affects cattle and is caused by the bacterium Clostridium chauvoei. This illness is characterized by sudden death in affected animals, typically resulting from muscle damage and tissue necrosis due to the bacteria's production of toxins. It is not a disease that can be contracted by dogs, which is why this answer is correct.

In contrast, Rabies, Distemper, and Hepatitis are all diseases that can affect dogs. Rabies is a viral disease that affects the central nervous system and can be transmitted through bites from infected animals. Canine Distemper is a contagious viral disease that impacts a dog's respiratory, gastrointestinal, and central nervous systems. Canine Hepatitis, caused by the Canine Adenovirus, primarily impacts the liver and can lead to severe illness in infected dogs. Therefore, the other options are diseases that do affect canine health, making Black Leg the only one that does not.