Most North American cattle are vaccinated for bovine viral diarrhea virus, yet the disease persists.
Virologist Frank van der Meer wants to find out if the vaccines are working and what kind of disease variability is found in persistently infected animals.
BVD is an infection in cattle that can lead to enteritis, abortion, fetal deformities and bovine respiratory disease.
“In the field, these viruses tend to change rapidly,” said van der Meer, a veterinary medicine professor at the University of Calgary.
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“Their outer core changes quite rapidly and they cannot be recognized by the antibodies that are induced by vaccines. That is my hypothesis.”
Most producers use a modified live vaccine made from a standard virus that was isolated a long time ago.
“There is still a lot of BVD around, even with the vaccination,” he said.
“There must be a reason for a mismatch because of induced immune responses and what can actually be found in the field.”
Van der Meer recently received a three-year $350,000 grant from the Alberta Livestock and Meat Agency to study the virus in beef and dairy cattle starting this fall.
He plans to collect samples of urine, feces and mucous from 10 dairy and 10 beef herds in Western Canada to learn which form of the virus is present.
No figures are available on disease prevalence but it is easy to find in beef and dairy herds.
“You don’t have to look very hard to find it,” he said. “People may have a hunch of what is going on, but the extent of the infection is, most of the time, not clear.”
There are three types of BVD in North America: 1A, 1B and Type II. More variability is found in the 1B viral form, and researchers know 1A and 1B cause the same type of symptoms. Type II is more pathogenic.
“The virus has the ability to change with every replication cycle,” van der Meer said.
“So every time a virus replicates, it will change. That is 100 percent sure and we find that in a lot of other viruses.”
He wants to monitor the changes and see how rapidly they can occur and how they can be blocked by vaccines.
“If that fit is still pretty good, we can be confident our vaccines are still doing their job,” he said.
“But if the fit is not as sufficient as we hoped, it would be we might need to update our vaccines and how we evaluate whether vaccines are still able to generate enough immune responses to block an infection.”
Cheryl Waldner of the University of Saskatchewan’s veterinary school maintains a wide database on this production limiting disease.
“Almost all cows are exposed to BVD at some point in their life. Many animals are exposed to IBR (infectious bovine rhinotracheitis), but it appears to be less common,” she wrote in an e-mail.
“As for clinical disease, that is harder to measure as many of these viruses present with similar clinical signs and sometimes the clinical signs are not serious enough to be noted by the producer.”
Van der Meer suspects producers underestimate the presence of the disease.
His study will also examine persistently infected animals that carry the virus but do not mount an immune response. They shed the virus in large amounts and are to blame for continuing problems with the virus.
Persistently infected animals are produced during a BVD infection in the first trimester of gestation. The virus establishes itself in the fetus, but the immune system doesn’t recognize it and it stays there for the rest of the calf’s life.
The animal then spreads the virus continuously in high amounts.
Waldner’s studies found that calf death risk was higher in herds where a persistently infected calf was present. These calves were more likely to be treated and typically weighed substantially less than herd mates at weaning.
The presence of persistently infected calves and serologic evidence of infection in weaned calves appeared to have the most substantial effect on productivity because of the higher risk of treatment and calf deaths and lower weaning weights.
A study from the fall of 2002 found that at least 18 of 203 herds had one or more persistently infected calves. They were more than 73 kilograms lighter than uninfected calves and six times more likely to be treated for infectious disease.
The number of treatments given to these calves was three times higher than normal.
Calves sampled in the fall at weaning with evidence of exposure to BVD infection were on average more than 13 kg lighter than calves that had not been exposed to the virus, after accounting for calf sex, breed, calf age, cow age, twin status, persistently infected status, treatment status, cow body condition score and calf vaccination status.
