Vet Column 9-21-09 |

Vet Column 9-21-09

David L. Morris, DVM, Ph.D.
Fort Collins, Colo.

IBR, or infectious bovine rhinotracheitis, is a viral respiratory disease of beef and dairy cattle. Although it can be associated with other clinical disease syndromes of cattle, the respiratory associated form has been recognized since the virus was first isolated from clinically infected cattle in 1956. Vaccines, both modified-live and inactivated, have been widely used to control respiratory disease in cattle since then, particularly feedlot cattle.

In recent years, increases in the numbers of reports of IBR outbreaks in feedlot cattle vaccinated against the causative virus have occurred. These outbreaks take place late in the feedlot stage of production and are apparently not vaccine brand-specific. The question then surfaces regarding efficacy of the available vaccines relative to the field strains that might now be occurring. To evaluate this possibility, investigators from the University of Saskatchewan, the University of Nebraska, and Pfizer Animal Health recently published results concerning this possibility.

IBR field strains were isolated from clinical cases obtained in 2004 and 2003, and were from animals that were diagnosed with IBR, yet had been vaccinated against IBR. These isolates were then used to challenge experimental calves that had been vaccinated against IBR using a modified-live viral vaccine composed of a combination of strains based upon one of the original isolates used early on in the development of IBR vaccines. Sixty-three, 4- to 6-month-old crossbred beef calves, with no circulating antibodies for IBR, were used in this study.

In one replicate of the study, calves were vaccinated eight days after weaning and were subsequently challenged 30 days after vaccination. In the second replicate, calves were vaccinated 59 days after weaning and were challenged 97 days after vaccination. Challenge occurred following trailered transport of approximately 70 miles to more closely simulate natural conditions. On arrival, all calves were challenged by aerosolization of inoculum in a sealed trailer.

Results of this study indicate that proper use of vaccines based upon early IBR isolates conveyed acceptable cross protection and disease sparing effects to animals exposed to more recent, virulent IBR isolates in this experimental model. IBR vaccines are effective.

An important issue regarding bovine IBR and other herpes viruses is their genetic make-up relative to risk for change or mutation. IBR is a double stranded DNA virus and is much less likely to change genetically, or mutate, than a single stranded RNA virus such as bovine virus diarrhea (BVDV). Based upon their genetic make up alone, one would not expect as much antigenic differences, or genetic mutation, to occur with IBR virus as compared to BVD virus.

So what then might explain the late stage production differences observed regarding IBR outbreaks, particularly in feedlot cattle? Although some highly technical explanations can be theorized, one cannot forget that vaccinating and immunizing may be two different things. If one administers a vaccine under a high period of stress, or the animals become exposed to virulent challenge prior to developing a full immune response, or the animals are exposed to other disease agents that might suppress the immune response, disease protection may not be adequate. This also does not account for improper handling of vaccines at the time of administration such as ambient temperature, improper storage, outdated vaccine use, or time interval following reconstitution. Proper use of IBR vaccine along with management is important in immunizing cattle against this disease.

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