Editor’s note: last in a beef BVDV series.
There’s not an abundance of research out there that can give a true picture of how much of a problem bovine viral diarrhea virus (BVDV) is in the feedlot. However, using data from several studies, a weighted estimate was calculated indicating a prevalence of persistently infected (PI) BVDV calves arriving at the feedlot of approximately 0.2 percent (Journal of the American Veterinary Medical Association, March 2005).
Guy Loneragan, BVSc, PhD, West Texas A&M University, and Dan Thomson, DVM, PhD, Kansas State University, conducted studies to estimate the prevalence of PI BVDV cattle. Three groups were tested for persistent infection upon arrival. A group of 2,000 600-pound. Missouri sale barn origin calves had a prevalence of 0.3 percent. A group of 1,995 400-pound southeastern sale barn origin calves had a prevalence of 0.15 percent, and no PI animals were found in a group of 1,400 northern calves. Another study published by Annette O’Connor, DVSc, found a prevalence of 0.23 percent in 4,705 calves. “All of these studies seem to agree that we can expect about two PI animals per 1,000 placements,” says Loneragan.
This scruffy red calf has the typical look of a steer persistently infected with BVDV.
What happens to feedlot PIs
Taylor et al looked at a pen of PI animals that arrived from one ranch. All of these animals were railed or died prior to reaching harvest weight. “Their study might indicate few survive to harvest,” says Loneragan. “However, all the PI animals were penned together and, presumably, provided a pretty good challenge for each other. We think that in these cases some will survive to harvest, and based on the percent that arrive, percentage that die and percentage that are railed, about 50 percent will survive to harvest."
Loneragan and Thomson found the same thing. “All the PIs we identified ended up being treated,” says Loneragan. “Other people certainly have found that PI animals can stay healthy and don’t necessarily get sick, and some do go to harvest. Not all end up in the dead pile or are realized.”
PI animals, however, factor significantly in the sick, chronic or dead pile. “Our work indicates that PI animals are more likely to get sick than non-PI animals,” notes Loneragan. “The PI animals are also more likely to die or be realized from the feedlot. “We found the prevalence in both of these groups to be roughly 3 percent. Logically, if it’s 0.2 percent at arrival and 3 percent of the deads and realizers are PI, obviously PI animals are over-represented in the dead and chronically ill animals. So even though they may make it to harvest, they are likely to end up ill or dead. That said, however, several studies have shown that PI animals can make it to harvest, and based on some of our estimates of prevalence at arrival, in the dead pile and in realizers, it seems possible that about half of them go to harvest.”
“Our research shows that animals with lung lesions will have lighter carcasses of 15 to 50 pounds per carcass at slaughter relative to animals without lung lesions,” adds Thomson. “All of the PIs in our studies were pulled and treated for BRD. The decrease in performance of the individual animal due to BRD is quite significant.”
Thomson believes that the more important question might be to understand the decrease in performance for the whole pen. “Performance might be better in the pens where the PIs actually die or are removed than the pens that still have the chronic shedding of virus from the PI cattle.”
Exposure to others
In a production setting, the BVD virus is a great example of biosecurity needs. “As an industry, we need to separate biosecurity and security,” says Thomson. “Security is prevention of intentional exposure of animals, such as terrorism. Biosecurity is prevention of disease from animal to animal.”
Fenceline water tanks are a great example of biosecurity breaches between pens. “We are recommending that producers use a receiving area for new cattle to prevent contamination between pens of new and already-started cattle,” says Thomson. “Also, we are changing the way we manage hospitals in feedyards. We are starting to experiment with biocontainment areas for animals that are treated more than once, and we are experimenting with disinfecting the treatment and processing areas between different groups of cattle.”
Looks can be deceiving. This normal-looking steer is also persistently infected with BVDV.
Photo credit:: Guy Loneragan, BVSc, PhD
PI animals, particularly those that do not die, have the chance to expose their penmates or other cattle through fenceline contact to BVDV. “PI animals that don’t die soon after arrival allow for a long-term exposure,” notes Loneragan. “Because feedlot operations are not all-in/all-out like swine operations, you can have a pen of long-fed cattle with one or more PI animals next to a pen of stressed, high-risk new arrivals. Even though the new-arrival group may not have a PI animal, the pen next to it could have one that is shedding.”
So what effect can PI animals have on penmates? “We’ve performed some limited but very interesting studies,” says Loneragan. “One study found the incidence of respiratory disease was approximately 40 percent greater in those that were exposed to a PI animal. We classified animals as exposed if they were either in the pen or they were in an adjacent pen and shared a fenceline. We were able to estimate that approximately 15 percent of disease events in the entire study population of calves were associated with the PI animal. Dr. Thomson has highlighted that disease costs much more than merely medicine and labor. It seems apparent that PI animals that shed a virulent form of BVDV result in substantial productivity losses for the feedlot industry.”
How long does it take an animal to become sick after exposure to a PI animal? That we don’t know. But because transmission to penmates occurs (as well as across pens), it makes the possibility of a feedlot buying certified PI-free animals and keeping them healthy challenging at this point. “With the low prevalence of BVDV in the industry, essentially all cattle coming in are low-risk for PI,” says Loneragan.
Thomson agrees with Loneragan to a point. “We don’t know the economic implications; however, removing the animals certainly decreases the long-term exposure to the BVD virus.”
Feedyards procure cattle primarily based on economic considerations. If those certified PI-free animals are not perceived to be more economical, then feedlots won’t buy them. If they are in a pen next to a PI animal, the in-contact nature of the feedyard may just negate everything that was paid for. “A lot of the ideas are very good right now, but unless you can procure an entire alley, section of the feedyard or even the whole feedyard as certified-free, there are no data to indicate if it is economical at this point in time,” adds Loneragan. “No one knows exactly what PI animals cost the feedyard. If we know how much it costs per animal placed, then we might have an idea of how much we have to spend to try and control it.”
This is why Thomson believes new biosecurity measurements need to be applied. “Now that we know that the PI cattle are there, we have some justification. There are probably other viral pathogens that are causing us grief that we don’t even know of yet. I also think that placing the calves in a receiving area allows you to manage the new cattle differently and decreases some production costs.”
Shared water tanks between pens can allow for nose-to-nose BVDV transmission from PI animals in one pen to cattle in an adjacent pen.
Photo credit: Guy Loneragan, BVSc, PhD
Screening PI animals
Is it worth screening animals for PI BVDV upon arrival to the feedlot? Maybe or maybe not. Loneragan says it sounds like a good idea because it does appear that PI animals do impact surrounding animals, but “we don’t know if it’s economical to do that on arrival,” he says. “There’s a lot of work to be done before I would recommend screening and removal of these persistently infected animals at arrival. While it sounds like and probably is a great idea, I don’t want to recommend something that costs more than it saves. We have to answer important questions such as, ‘Is the exposure on the trucks, etc., sufficient to mute the benefit of removing the PI animals at arrival?’”
Thomson believes screening might have merit. “I agree with Dr. Loneragan; we certainly don’t want to recommend something that costs more than it is worth. One problem with BVDV screening is that no one understands the total economic losses due to decreased performance, death loss, labor, etc. We started screening at some feedyards this year and have been pleased with the results. I do know that if there was ever a time in the market that it would be justified to screen for BVDV, it would be now with the high price of feeder cattle.”
Two issues need to be considered. First, if cattle have been exposed to PIs prior to arrival, does the adverse effect disappear when PI animals are removed or has the damage been done? Second, the potential for false positives always exists with any test. Because the prevalence at arrival is so low, it is possible that several false positives will be identified. The false positives may outweigh the cost of identifying and removing the true positives.
“If we rely on simple screening and culling, we could inadvertently cull too many non-PI animals,” says Loneragan. An option to get around this would be to apply a second test to all that are initially positive, which would reduce the likelihood of culling a non-PI animal and make it more economically feasible.
And, if PIs are discovered in a pen, is it logistically possible to remove them? “We have to individually identify all the animals, and no test is real-time and chuteside, and we’d have to find them in the pen a day or two after the test has been performed,” says Loneragan. “So there are a lot of other complications in doing it as a screening process in the feedlot.”
Keeping PIs out of the feedyard
To negate the effect PI cattle have on the economics of a feedyard requires those calves to not enter it in the first place, which means programs back at the ranch. “The site of greatest deleterious impact of BVDV is on the cow-calf ranch, and that is where control must begin,” says Loneragan. “Most ranches will be negative, and a few of them will be positive. When you have a positive animal on a ranch, you are more likely to find several positives. Some would estimate that the prevalence of positives on a ranch is as low as 5 percent and as high as 15 percent. A colleague of mine recently disclosed to me that approximately 30 percent of a calf crop from one ranch was PI based on IHC testing. Control at the feedyard, I think, is probably effective but not nearly as effective as control prior to arrival at the feedlot.”
Thomson agrees. “BVDV is a cow-calf disease. We need to move on with eradication of the disease at the ranch level. The ranch probably has more to gain economically than any other segment of the industry from eradication of BVD virus.”
The first step is try to estimate the magnitude of excess disease, morbidity, mortality, other animal loss, performance loss, etc., that is attributable to PI animals, says Loneragan. “If you can estimate the magnitude, you can estimate the cost. Then we can start to evaluate programs and identify how far back down the chain you have to go to make it economical.”
• Loneragan, G.H., Thomson, D.U., Montgomery, D.L., Mason, G.L., Larson, R.L. (2005). Prevalence, outcome and animal health consequences of feedlot cattle persistently infected with bovine viral diarrhea virus. Journal of the American Veterinary Medical Association 226:595-601.
BVDV exposure on the truck
In 2002, a research study by Grooms, Brock and Norby focused on exposure of non-PI animals to PI animals being hauled in the same truck.
Two groups of feeder cattle (92 head each) were identified in Alabama and transported in separate trucks to a feedlot at Michigan State University. Two persistently infected calves (Type I and II) were added to one truck prior to shipment. Upon arrival at the feedlot, one-half of the animals (46 head) in each truckload were vaccinated with a modified-live BVDV vaccine. Each truckload of cattle was kept in separate wings of the feedlot.
Samples for virus isolation and serology were collected on days 0, 7, 14, 28, 56, 84, 112, 140, 168. Calves were weighed every 28 days to determine average daily gain (ADG). Lung scores and carcass data were collected at slaughter. Virus was isolated in both vaccinated and non-vaccinated calves exposed to PIs. All calves exposed to PIs seroconverted to BVDV while only vaccinated calves in the non-PI-exposed group seroconverted to BVDV.
The percentage of animals treated was increased in the PI-exposed group: PI exposed and vaccinated (18 percent), PI exposed and non-vaccinated (29 percent), non-PI exposed and vaccinated (13 percent), non-PI exposed non-vaccinated (10 percent). Although ADGs were different at early weigh periods, there were no differences observed at the end of the trial. There were no differences in carcass characteristics or lung scores between the groups.
“Basically, we started seeing increased morbidity from BVDV exposure in the first week of commingling,” says Dan Grooms, DVM, PhD, Michigan State University. “This makes sense as the incubation period is usually two to five days, and the close exposure would have likely spread the virus through the group very quickly.”
Grooms adds that nearly all of the non-vaccinates had seroconverted by day 28 of exposure. “The bottom line is that by the time the non-PI calves were trucked to Michigan from Alabama with PI calves, they were exposed to BVDV.”