As others have stated, an antibody titer to BVD &/or IBR only mean that your animal's immune system has 'seen' those viruses at some point in the past - whether a field exposure or vaccination. And, as has been stated, a single serologic test doesn't tell you anything more than that. A 'convalescent' sample, collected 3-4 weeks later should demonstrate an increasing/decreasing or stable titer - which would allow us to determine if the animal was or was not responding to a recent infection. Since your bull was probably vaccinated prior to you purchasing him, and again last fall, I'd fully expect him to have a measurable antibody titer.
BVD is insidious. Antigen testing - that is, testing for the presence of the virus itself or viral nucleic acids, are the way to go to determine if an animal is actually infected, or more importantly, if they are a Persistently Infected(PI) animal. Ear notches may be tested by PCR or immunohistochemistry for BVD, or, as we do at the lab where I work, with an Antigen-Capture ELISA - which will work on ear notches, whole blood, or serum.
If I had, or was called out to see, an adult animal that had chronic scours, as you've described, I'd be wanting to do a serologic test and fecal culture for Johne's Disease.
We're seeing more and more cattle farms with populations of nematode parasites that are resistant to ivermectin. The Cydectin may still be effective; it's in the same class with ivermectin but is more potent - but sometime in the not-too-distant future, you're going to reach a point where Cydectin doesn't work either.
But, unless you do a fecal egg count before deworming and again 7-10 days later, you really have no idea whether your 'deworming' was effective. An effective dewormer(given at the appropriate dosage) will reduce fecal egg counts by 90% or more - if you don't get that 90% reduction in egg count, you've got resistant worm.
