I wonder if they don't test positive b/c maybe they'd been vaccinated at some time...?
I use the Col. Serum vaccine. Sometimes they get abscesses at the injection site....I treat them like "real" abscesses (isolate, treat, heal, release) and 90% of the time, they're 'abescess free' after that.
90% effectiveness beats no protection, when you have CL does in the herd, like I do.
I had a vaccine made at first. Spent $300.00 on it...
I dunno who messed up (my vet w/ the sample, the company who made it, the shipping company maybe let it get too warm at some point, or I used it "wrong"...) but CL spread like MAD during the time I used it. All of the does I used it on ended up w/ CL.
I switched to Col. Serum's Case Bac after that and had MUCH, much better luck.