not necessarily, can send in swabs and tests from a flue hot herd and it isn't unknown for the results to come back negative.  have to test for specific strains, also labs can be expensive so they usually don't test for flue unless it's specifically asked for.  sulfa +baytril is a pretty potent combination, more than likely is viral, could also be a prrs flareup.  been some talk about prrs ramping up earlier this year.