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The prevalence of cattle persistently infected with Bovine Viral Diarrhea (BVD) virus continues unabated in the feedlot and stocker pasture.
Why So Little Progress
“With the advent of newer vaccines and easier, less costly diagnostic tests, we haven’t seen the prevalence rate decrease,” Hessman says. He believes the reason is that the most prevalent strain of BVDV is strain 1b, a strain not included in current commercial modified-live vaccines.
Looking at the BVDV subtypes of PI cattle identified entering a feedyard in Kansas for 2004 to 2008 (1,263 head in all), 78.4% were subtype 1b, 11.9% were subtype 1a, and 9.7% were subtype 2a.
No one knows if the 1b strain is the most prevalent because it’s been inadvertently selected for through vaccination against the other strains, or if it’s always been the most prevalent and allowed to flourish in the absence of vaccination.
Consequently, when producers finally begin national herd expansion, Hessman expects PI-BVDV prevalence to increase, though not drastically.
“Heifers tend to be at the highest risk for producing PI calves because of their lack of maturity and lower level of basal protection than mature cows,” Hessman says.
Though he continues lobbying pharmaceutical companies to include strain 1b in BVDV vaccines, he says, “Even if we had a vaccine with 1b, it wouldn’t be the complete answer. With PI-BVDV, there is such a huge variation in the number of virus particles shed and other factors. Some PIs have the ability to shed extremely large quantities of BVDV virus, while other PI animals don’t shed at these high levels.”
Those factors include environmental impact and timing, commingling and other stressors, susceptibility of the recipient animal and, of course, the virulence of the particular virus.
All these variables mean that even when cattle are vaccinated effectively, the vaccination can be overwhelmed.
Though all PI animals are the result of in-utero infection, Hessman explains, “All PIs do not have the same effects due to the many variables, including the level of virus shed, virulence, recipient population susceptibility, etc.” He explains most BVDV in feedlots is mild or moderate. Then there are the extreme cases, like one he encountered in a Texas feedyard of high-risk auction market cattle that fell apart in 30 days. They’d been vaccinated multiple times, including for BVDV. The virulence of the virus was so extreme that it produced lesions that experts around the world had never seen before.
Currently, in the feedlot and stocker sectors, Hessman says, “most are using vaccination as the strategy to prevent it. Some join that with testing for and removal of PI animals, depending on the class of cattle … There is so much we still don’t know about the BVD virus itself,” Hessman says.
One thing that is known is that it continues to cost more than many realize.
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