A new, highly contagious and sometimes deadly canine flu is spreading in kennels and at dog tracks around the country, veterinarians said yesterday. The virus, which scientists say mutated from an influenza strain that affects horses, has killed racing greyhounds in seven states and has been found in shelters and pet shops in many places, including the New York suburbs, though the extent of its spread is unknown. Dr. Cynda Crawford, an immunologist at the University of Florida's College of Veterinary Medicine who is studying the virus, said that it spread most easily where dogs were housed together but that...