What killed the herd, BVD or secondary infections?
All of this coronavirus scare has us remembering a bad BVD break at Nortex Feedyard in 2008. There were two different strains isolated in two pens, a 1b strain and a 2a strain. This resulted in a 76% morbidity rate and a 31% mortality rate within the two pens. On the day they arrived, they received vaccinations for MLV BHV-1, BRSV, BPIV-3 and killed BVDV 1a and 2a. Within 10 days, the morbidity rate was 49%. Within 15 days, the morbidity rate was 76%. At 15 days, the mortality rate was 13%. Postmortems were performed on all dead animals, what was found was 50% of the fatal cases showed severe BVD lesions, mostly in the larynx, trachea and esophagus.
These two pens were separated by an empty pen. Pen A had 1b, pen B had 2a. The only time the cattle came close enough to make direct contact was when they were moving pen A to be treated, as they had to pass by pen B via a cattle alley.
A colleague at the USDA became intrigued by these two strains of BVD as they seemed very virulent. We sent samples to her as she was going to see how BVD suppressed the immune system over the course of 14 days.
On her first study she infected an animal with both BVDV 2a and Bovine Coronavirus on day one. On day 7, nothing. Day 10, nothing, not even a fever.
On her second study she infected an animal with BVDV 2a, then on day 6 she infected the animal with Bovine Coronavirus, during the time of lowest white blood cell count. The animal died within 24 hours.
This goes to show that, if it wasn't for BVD suppressing the immune system, the cattle would have been fine. Maybe some sickness, but not a 76% morbidity rate and 31% mortality rate.