“Disease prevention is more important today than it’s ever been because of the high capital requirements,” says Bill Hessman, DVM, Haskell County Animal Clinic at Sublette, KS.
Yet, when it comes to preventing Bovine Viral Diarrhea Virus (BVDV) and exposure to cattle persistently infected (PI) with the virus that can overwhelm immunity, there's been little progress made in a decade.
Hessman established Central States Testing (CST) in 2005. It was the first commercial laboratory to offer PI-BVDV testing. CST tests about 540,000 head annually – 90% of which are high-risk feedlot and stocker cattle. The prevalence rate based on that testing remains extraordinarily stagnant at 0.38-0.42% each year. Incidentally, Hessman estimates approximately 2 million head are tested industry-wide each year.
“The overall prevalence rate in the class of cattle I deal with has stayed the same, but I expect it to increase,” Hessman says (more later).
Total cost of BVDV was estimated at $1.9 billion annually in 2011. That was based on a cost of $35-$56/calf estimated by Julia Ridpath, a USDA Agricultural Research Service research microbiologist.
BVDV Increases BRD Risk
Though the prevalence rate of PI-BVDV is miniscule in feedlots, the impact of exposure to other animals is gargantuan.
Guy Loneragan, then at West Texas A&M University and now a Texas Tech University professor of epidemiology and animal health, led a study beginning in 2002 that estimated the prevalence of PI in 2,000 head of sale-barn calves arriving at a commercial feedlot.
“Relatively few PI cattle arrive at feedlots. However, those cattle are more likely to require treatment for respiratory tract disease and either become chronically ill or die than cattle that are not PI,” the study concluded. “In addition, they are associated with an increase in the incidence of respiratory tract disease of in-contact cattle.”
“BVD affects Bovine Respiratory Disease (BRD) and it does so through immunosuppression,” Hessman says. “BVDV appears to be a catalyst for the common pathogens we deal with because of the immunosuppressive effect of the virus.”
In Loneragan’s study, prevalence of PI on arrival was estimated at 0.3%. Cattle exposed to PI animals were 43% more likely to be treated for respiratory tract disease.
Loneragan emphasizes the study he led was observational in nature. “Just because we saw an association between exposure to PI animals and increased respiratory disease in cattle that could have been exposed to them doesn’t mean there is a causal relationship,” he says.
One outcome that Loneragan says did surprise him was that BVDV was 10 times more likely to be present in the study’s chronically ill and deads. Other studies have found similar implications. Still others, including a seminal one conducted by Hessman, found less statistical correlation.
Performance Is The Chief PI-BVDV Thief
“Our study was the first to look at performance, trying to understand the process of how BVDV works in the background robbing the metabolic process,” Hessman says.
He’s talking about 15,438 head of high-risk, sale-barn calves assembled in the Southeast and delivered to a starter yard in Kansas. They comprised 167 lots and were allocated among 172 pens for 66 days of backgrounding. For the record, 21,743 head arrived at the yard initially, but lots with fatalities prior to processing, or from which samples were not obtained, were excluded from the final analysis. Testing revealed 82 PI-BVDV animals in 70 lots for a prevalence rate of 0.50%.
The BVDV thievery revealed by the study is stunning.
“This study revealed that there is a detrimental impact from exposure to PI-BVDV cattle in a feedlot,” Hessman says. “Economically, this amounted to $93.52/animal in this study population at this feedlot. The largest segment of this loss was the result of performance losses of $88.26/ animal, and the balance of $5.26/animal resulted from an increase in the fatality percentage.”
Hessman explains that, though significant, the level of impact was reduced when disregarding outliers that had the most devastating effect. In the real world, unfortunately, removing outliers is not an option.
Hessman also points out the economic impact stems from the fact that BVDV infection is often subclinical – as much as 70-90% by some estimates. Animals persistently or acutely infected rob dollars through inferior performance, but you never know it.
In his study, Hessman says, “Feed efficiency or feed conversion may have provided the greatest impact to the economic outcomes because unexposed cattle had converted feed into body weight 55% more efficiently than did cattle with direct exposure.”
Hessman points out the cattle were not implanted and were limit-fed a moderate NEg ration.
“The effect on feed conversion likely represented the maximum difference, because this study represented only the first 66 days of the feeding period, and there would likely be some compensatory gains in the remainder of the feeding period,” Hessman explains. “On the other hand, there is no compensation for fatalities (deads-in calculation), and differences in fatality percentages among the exposure groups may have increased throughout the entire feeding period.”
Moreover, Hessman says, “A great number of animals exposed to PIs outperformed the mean of non-exposed cattle … The variability among PI animals can lend itself to misleading outcomes.”
For that matter, Hessman says some folks look at statistical significance and miss the point. He remembers another BVDV pioneer, Robert Fulton at Oklahoma State University, explaining the differences between statistical significance and biological significance.
In Hessman’s study for instance, the cost of gain for cattle exposed to PI animals (PI animals in lot on arrival and not removed; adjacent pens contained PI and non-PI cattle) was $6.31/kg, compared to $2.09/kg for cattle in pens where there were no PI animals in the lot on arrival, or in adjacent pens on arrival. Weight gain of the PI-exposed cattle was 34 kg compared to 50 kg compared to the unexposed ones (Table 1).
|Average treatment #||1.79||1.77||1.72||1.58||1.66|
Treatment Groups shown in Table 1:
PI – PI cattle in lot on arrival and allowed to remain in lot throughout the study; adjacent pens contained a mixture of PI cattle and non-PI cattle.
PIR– PI cattle were in the lot at arrival, but removed from the lot within 72 hour after arrival; adjacent pens contained a mixture of cattle from lots with no PI cattle at arrival or from which the PI cattle were removed within 72 hours after arrival.
NPIE– Non-PI exposed cattle in which there were no PI cattle in the lot at arrival but cattle were exposed because an adjacent pen or pens contained 1 or more PI animals.
NPIER– Non-PI exposed cattle in which there were no PI cattle in the lot at arrival but cattle were exposed because an adjacent pen or pens contained cattle from which the PI cattle were removed within 72 hours after arrival.
NPIU– Non-PI unexposed cattle in which there were no PI cattle in the lot at arrival and adjacent pens contained cattle from lots in which there were no PI cattle at arrival.
“This comparison shows the PI cost in the feedyard, to a yard that does not have any PIs on arrival – essentially the true cost of PI animals in the yard,” Hessman says.
Though it’s always a fool’s errand to infer results between studies or classes of cattle, Hessman says logic suggests that the prevalence of PI-BVDV is greater in the stocker sector than in feedlots simply because lighter, younger cattle are most likely to die from persistent BVDV infection.
“Prevalence is higher in younger animals because PIs have a greater mortality rate and tend to die out of the population over time; therefore, PI rate tends to decrease with age,” Hessman explains. “You’ll see the most PIs in a group at birth. Over time, as some PIs die, the prevalence will decrease.”
Spun another way, Hessman says, “There’s almost a linear reduction in PI prevalence as cattle weights increase.”
As an example, the prevalence among 2-weight calves was 0.64%; it was 0.38% for 5-weights, and .25% for 7-weights.
Plus, Hessman explains, younger, lighter cattle are more susceptible to infection and more likely to be commingled, another major factor associated with BVDV infection.
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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