“What caught our eye was the number of late-day pulls that we couldn't explain,” says Scott Anderson, assistant manager for CRI Feeders of Guymon, OK. “There were clearly signs of bovine viral diarrhea virus (BVDV), though the cattle had been vaccinated for it numerous times. When our veterinarians took cultures, they were finding the BVDV 1B subtype.”
One of those veterinarians is Scott Crain, who owns Cattle Health Management (CHM), a feedlot veterinary consulting service at Meade, KS.
“We know we have BVDV 1B making cattle sick at all stages of the cattle-feeding period,” Crain says. “We know we have that happening with cattle vaccinated multiple times for the BVDV 1A and 2A available in commercial vaccines.”
In the yard of another CHM client, Crain says cattle owned by the feedlot from the stocker pasture forward began expressing clinical BVDV symptoms and one died, three weeks before closeout. It was 1B.
This particular strain isn't new. In 2002, BVDV expert Julia Ridpath of USDA's National Animal Disease Center told members of the Academy of Veterinary Consultants that BVDV 1B was an emerging subtype.
By last year, 1B was the predominant BVDV strain isolated in cattle persistently infected (PI) with BVDV entering two feedlots in Kansas (78.3%) and Oklahoma (64.8%). That's according to a multi-year study conducted by USDA's Agricultural Research Service.
But there are no commercially available vaccines today that contain BVDV 1B. Even if they did, odds are the pathogen's dynamic nature would allow it to outrun commercial vaccines that must undergo years of research, development and federal approval.
Different every day
Like human influenza, different strains and subtypes of BVDV combine to create new subtypes (antigenic shift) or mutate into entirely new strains (antigenic drift). Also like human influenza, BVDV is an RNA virus, with a propensity to mutate, collect genes from other viruses and recombine.
That's what got Crain to pondering how scientists decided which particular flu strains to include in human vaccines each year. The answer is antigenic cartography.
In oversimplified terms, antigenic cartography involves mapping the genes of the virus strains isolated from people infected with the flu and then comparing them to strains in the vaccines available. Specifically, scientists look at differences between antigens, which induce antibody production that can produce an immune response to the virus. That tells researchers whether a current vaccine protects against other strains and subtypes, or how much of the new pathogen can escape the immune response generated by viral antigens contained in the current vaccine.
This is the approach CHM and a coalition of other like-minded veterinarians from across the U.S. are using to tackle BVDV 1B through a practice they've developed — Professional Veterinary Associates (PVA) — which represents about 5.5 million head of fed cattle. Again, in the simplest terms, PVA developed a protocol that begins with participating veterinarians collecting and submitting BVDV samples to a national virus data bank.
“Learning from researchers who developed the process for human influenza, we need as many field strain isolates of the virus as we can get our hands on,” Crain says. “With human flu, they evaluate 10,000 isolates each year, taken from flu-infected people, in order to determine which isolates to add to the new vaccine.”
The veterinarians map the genes of submitted samples and compare them to strains of the BVDV in vaccines they're using. If they encounter a strain or subtype that's escaping immune response, such as 1B, they build an autogenous (custom-made) vaccine containing the specific strains or subtypes they're isolating.
Though commonplace in the swine industry, autogenous vaccines have a shadowy past in the cattle business where they've often been equated to bathtub mixes or outright snake oil.
“You must have the right antigens in the autogenous product to be effective,” says Nate McDonald, CHM managing director. “The key to getting the right antigens in an autogenous product is the antigenic cartography process, coupled with a surveillance program that continuously identifies the pathogens currently causing disease in the field.”
Crain emphasizes that the approach is intended to supplement commercial BVDV vaccines, not replace them. One reason 1B is currently the predominant subtype found in cattle infected with BVDV is because commercial vaccines contain the known BVDV genotypes 1A and 2A.
By law, autogenous vaccines can only contain killed strains of a virus, but modified-live vaccines (MLV) prompt a faster immune response, McDonald points out.
“MLVs maximize immune system stimulation, but only to the antigens contained in the vaccine or to antigens that show some level of cross protection,” McDonald explains. “When antigenic cartography shows us we have an isolate-causing disease that's not in the commercial product or offers limited cross-protection, then filling the gap in the coverage with an autogenous killed product enhances the total health program and gives us broader coverage.”
Next Page: Promising early results
Promising early results
When CHM approached CRI Feeders with the novel approach, Anderson says, “we were eager to give it a try. Especially on these bigger cattle, it gets expensive when one falls out.”
PVA veterinarians began using the autogenous vaccine in April. So far, comparing animals vaccinated for 1B to those not vaccinated for it, McDonald says pull rate and retreatment rates have been halved. But that's based on 200,000 head and during a time of year when disease challenge is the least.
Doug Hilbig of Hilbig Veterinary Services, Lakin, KS, says early indications are that the approach is curbing the trend of late-day pulls, especially on higher risk cattle. “I expect we'll also see fewer problems with high-stress cattle as we move into fall,” Hilbig explains. He has been using the protocol in some of his client yards, including Brookover Feed Yards at Garden City, KS.
“This is a difficult time for feedyards to think of adding cost,” Crain says, “But the idea is to spend a little more upfront for vaccine to save lots more later on. We know antibiotics don't prevent disease.” Cost of the autogenous vaccine is $1.50/head.
“Our initial impression is favorable,” Anderson says. “If we can do something relatively simple like this to keep cattle healthy and in the pen, we're going to do that.”