The primary reason why cats are more likely to contract avian flu is because they like to catch birds, and will even interfere with dead bird carcasses. This means of course they are likely to be in far more contact with potential disease-carriers than dogs. However, the potential transfer of bird flu from birds to cats increases the risk of the virus strain mutating into one that passes more readily between other mammals – including dogs and humans. I would think the chances of a mammalian outbreak being limited to just one species is unlikely, however no one can say whether the disease will develop into one that can readily infect any mammal.