|Briggs, Robert - Bob|
Submitted to: Canadian Journal of Veterinary Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/7/2005
Publication Date: 7/1/2005
Citation: Fulton, R.W., Briggs, R.E., Ridpath, J.F., Saliki, J.T., Confer, A.W., Payton, M.E., Duff, G.C., Step, D.L., Walker, D. 2005. Transmission of bovine viral diarrhea virus 1b to susceptible and vaccinated calves by exposure to persistently infected calves. Canadian Journal of Veterinary Research. 69:161-169. Interpretive Summary: Bovine viral diarrhea virus (BVDV) infections cause considerable loss to producers worldwide. While vaccines against BVDV have been available for over 40 years, control strategies based solely on vaccination have been ineffective. This study focused on the effectiveness of standard vaccination practices in a commercial feedlot situation. Animals infected with BVDV during gestation may be born with lifelong persistent BVDV infections. It is estimated that between 0.5% and 1.0% of cattle entering feedlots are persistently infected with BVDV. Further, it is common practice to vaccinate feedlot animals against BVDV as they enter the lot. This study demonstrated that PI animals are very efficient sources of infection and that animals vaccinated 3 days prior to exposure to a PI animal were not protected against BVDV infection. It was also suggested that differences between current field strains and vaccine strains contribute to vaccine failure. The importance of this study is that it demonstrates that current management techniques may contribute to vaccine failure. Vaccination at entry into feedlots does not give the animal sufficient time to mount a protective immune response and failure to remove PI animals severely compromises control measures. Finally this research indicates that the protection provided by current vaccines is not broad enough to prevent infection with diverse field strains.
Technical Abstract: Bovine viral diarrhea virus (BVDV) persistently infected (PI) calves represent significant sources of infection to susceptible cattle. The objectives of the study were to determine if PI calves transmitted infection to vaccinated and unvaccinated calves; to determine if BVDV vaccine strains could be differentiated from the PI field strains by subtyping molecular techniques, and if there were different rates of recovery from peripheral blood leukocytes (PBL) versus serums for acutely infected calves. BVDV PI calves infected with BVDV 1b were placed in pens with nonvaccinated and vaccinated calves for 35 days, with PBL, serums, and nasal swabs collected for viral isolation and serums for viral serology. In addition the transmission of bovine herpesvirus 1 (BHV-1), parainfluenza-3 virus (PI-3V), and bovine respiratory syncytial virus (BRSV) was monitored during the 35 day observation. BVDV subtype 1b was transmitted to both vaccinated and nonvaccinated calves, including BVDV1b seronegative and seropositive calves, after exposure to persistently infected (PI) calves. There was evidence of transmission both by: (1) viral isolation from peripheral blood leukocytes (PBL) and/or nasal swabs; and (2) seroconversions to BVDV1b. For the unvaccinated calves, 83.2%, seroconverted to BVDV1b. The high level of transmission by PI calves is illustrated by seroconversion rates by nonvaccinates in individual pens, 70% to 100% seroconversion to the BVDV1b. BVDV was isolated from 45 of 202 calves in the study. These included BVDV1b in ranch and order buyer (OB) calves plus BVDV strains identified as vaccinal strains that were in modified live virus (MLV) vaccines given to half the OB calves 3 days prior to the study. The BVDV1b isolates in exposed calves were detected between collection days 7 and 21 after exposure to PI calves. BVDV was recovered more frequently from PBL than serum in acutely infected calves. BVDV was also isolated from 2 of seven calves’ lungs dying with pulmonary lesions. Two of the calves dying with pneumonic lesions in the study had been BVDV1b viremic prior to death. The BVDV1b was isolated from both vaccinated and nonvaccinated calves. There were cytopathic (CP) strains isolated from MLV vaccinated calves during the same time frame as the BVDV1b isolations. These viruses were typed by polymerase chain reaction (PCR) and genetic sequencing, and most CP were confirmed as vaccinal origin. A BVDV2 NCP strain was found in only calf, an OB calf, on multiple collections, and the calf seroconverted to BVDV2. This virus was not identical to the BVDV2 CP 296 vaccine strain. The use of subtyping is required to differentiate vaccinal strains from the field strains. This study detected 2 different vaccine strains, the BVDV1b in PI calves and infected contact calves, and a heterologous BVDV2 subtype brought in as an acutely infected calf. The MLV vaccination with BVDV1a and BVDV2 components only 3 days prior to PI exposure did not protect against BVDV1b viremias or nasal shedding. There were other agents associated with the bovine respiratory disease (BRD) signs and lesions in this study including Mannheimia haemolytica, Mycoplasma spp, parainfluenza-3 virus (PI-3V), bovine respiratory syncytial virus (BRSV) and bovine herpesvirus 1 (BHV-1).