Most cases of true pinkeye are caused by the bacterium Moraxella bovis, but in the past couple of years, we've been isolating more and more M. ovis from affected cattle, and I've been seeing reports from some areas of a 'new strain'(not really all that new, but relatively uncommon until recently) - Moraxella bovoculi - with high percentages of this new strain exhibiting resistance to tetracyclines.
So...those cases that haven't responded to LA-200/LA-300/Tetradure, etc., may, in fact, have M. bovoculi infection instead of, or in addition to infection with the other two more common species, Moraxella bovis and Moraxella ovis.