Written By: Bradford P Smith, DVM, DACVIM, Professor Emeritus
When I was a fairly new veterinarian back in the early 70’s, I had a case that really caught my attention. A valuable 10 month Holstein bull had been recently shipped from the east coast to California, and it had developed fever, oral erosions on the dental pad, severe diarrhea, and weight loss. Frankly it looked as though it planned to die. The bull had leukopenia and was positive for BVDV. I informed the owner and then treated him symptomatically with fluids and antimicrobials, plus soft palatable feed. By some miracle he improved over the next 10 days and I kept the owner informed. Finally he looked so good that I called to tell them he was ready to go home. As I walked back into the barn, there was the bull lying dead in his stall! Calling the owner back to tell him the bull had died, only 20 minutes after I had told them he was better, was one of the most excruciating calls I have ever had to make. Our pathologists determined that he had a large cardiac infarct, which probably had caused a fatal arrhythmia. A new one on me!
Perhaps that is what makes BVDV such an intriguing pathogen…it has worldwide distribution, has very complex biology and it seems to have many ways to do harm. The history is long and convoluted. At first it was thought that there were two separate viruses, one that caused BVD and one that caused mucosal disease, a fulminant fatal condition. Later research demonstrated that in utero fetal infection with a biotype called noncytopathic or NCP (because it does not cause visible cytopathic effects in cell culture) could result in an immunotolerant carrier calf, which later would develop a super infection with the cytopathic (CP) biotype and show mucosal disease. Still later it was determined that the CP biotype can occur by molecular rearrangement of the original NCP biotype, as well as by exposure to an external CP biotype. The antigenic relationship of the CP and NCP biotypes determines whether mucosal disease, chronic BVD, and even chronic BVD followed by recovery occurs. The literature is full of papers about BVDV.
In addition to all this complexity, BVDV can cause early embryonic death, congenital defects and abortion. In feedlot animals, it is immunosuppressive and a contributor to bovine respiratory disease complex (shipping fever). Small wonder then that there are many vaccines available, and that new information is constantly emerging. As you learn about BVDV, be prepared to find out that something different has just been discovered. We certainly don’t have all the answers yet! Be sure to visit the BVDV PowerPage at VetPrep.com to review notes which review some of the most important details about this complicated disease.