Step 1: Understanding the Concept:
Canine systemic viral infections exhibit multisystemic clinical syndromes involving respiratory, gastrointestinal, and central nervous systems.
Step 2: Detailed Explanation:
Canine Distemper Virus (CDV, a Morbillivirus of the Paramyxoviridae family) causes a classic multisystemic disease in dogs featuring biphasic (diphagic) fever, catarrhal oculonasal discharge, severe respiratory distress, myoclonus/seizures, and hyperkeratosis of the footpads and nasal planum (termed 'hard pad disease').
Step 3: Final Answer:
Hence, the viral agent is Canine distemper virus.