Going After BVD
The stage is set to put the bovine viral diarrhea (BVD) virus on the run. Armed with a fresh arsenal of scientific information, a developing picture of BVD’s economic impact and workable approaches to livestock biosecurity, the U.S. cattle industry is ringing in a new era in BVD control. In fact, the consensus of participants in a recent BVD control summit is that BVD’s days as the nation’s, if not the world’s, most costly viral livestock pathogen could be numbered.
The disease is considered the root of many evils. Bob Larson, a University of Missouri-Columbia veterinary beef cattle specialist, says this includes inhibiting conception and causing abortion in infected females.
“Even mild BVD virus infections of breeding animals can cause failure to conceive,” he says. “Reproductive efficiency can be decreased due to fatal birth defects following fetal infection.”
More and more, researchers and veterinary practitioners are realizing though, that a larger negative health effect of the BVD virus is suppression of the immune system.
“This means infected animals are more susceptible to other disease,” Larson says. Infection with the BVD virus has been associated with several respiratory disease complexes both at the ranch and in the feedlot.
Persistently infected animals
Horizontal infection from one animal to the pregnant dam can lead to vertical transmission of the BVD virus to her fetus during pregnancy. Fetal infection can lead to embryonic death, birth defects and stunting. Or, calves can be normal and healthy at birth.
An infected newborn calf, normal appearing or not, can be persistently infected (PI) with the virus — meaning the infection lasts the entire life of the animal. The BVD-PI animal is generated when a fetus becomes infected with virus during the first 125 days of gestation.
“We know the BVD virus is best transmitted and maintained within and between cattle populations by PI cattle,” says David Smith, a University of Nebraska-Lincoln Extension veterinarian. “PI cattle shed high amounts of the virus, serve as virus reservoirs and are the most important source of virus transmission.”
Researchers remind ranchers that: “once a PI calf, always a PI calf” — and fetal infection is the only known means of “creating” a PI animal.
The result of introducing a PI animal into a beef herd depends on the timing relative to the breeding season and the immune status of the herd during early gestation. Even without vaccination, the number of PI animals and the amount of BVD virus infection in a herd seems to be self-limiting, unless the herd has a lot of additions.
“Although a high percentage of PI animals die at or near birth, or at least by weaning, up to 50% may survive and enter a breeding pool — or reach the feedyard,” Larson explains. “PIs that live to be breeding females aren’t only a source of horizontal transfer of the virus, they’ll always produce a PI calf themselves.”
PI prevalence in the general U.S. beef cattle population ranges from 0.13-2.0%. In addition, about 4% of U.S. beef herds are expected to have at least one PI animal.
PI price discovery
The cost of having at least one PI animal in the beef herd ranges from $14.85 to $24.84/cow/year. Once the calf hits the feedyard, the costs can skyrocket.
Bill Hessman, DVM, Haskell County Animal Hospital, Sublette, KS, has been looking into PI’s economic impacts in his High Plains feedlots. His research at nearby Cattle Empire Feedyards (see BEEF Feeder, June 2005) indicates there’s a $47/head cost for every animal going into the feedyard because of PI exposure.
“We can enjoy a significant savings in overall costs if the feedyard is entirely PI-free,” Hessman says. “Before long, we’re going to be seeing a price-discovery process with regard to BVD PI-free herds because of better health and improved performance.”
Lucy Rechel, Yerington, NV, has been a pioneer in BVD-PI biosecurity. She’s in the third year of a BVD elimination program at Snyder Livestock, which she owns with her family. Her operation includes a 4,000-head breeding stock development.
The realized cost of PIs keeps rolling in as she delves deeper into BVD control. Rechel says in one set of commercial calves from one herd, the elimination of PIs resulted in a reduction of total medication costs of $4,213.13 one year to $137.47 in the next. That’s a cost she no longer has to pass on to the consigner.
Overall, she says her feedyard medication usage dropped nearly 90% in October and November, the two critical months following receiving time.
“If that’s not incentive to eliminate BVD PIs from a herd, I don’t know what is,” she says.
The benefits of BVD control can only be realized through a consistent program that rewards cattle producers for their efforts, says Jim Kennedy, director of the Colorado State University veterinary diagnostic lab at Rocky Ford.
“In the initial phases, the rewards may come in the form of a pricing advantage,” he says. “Later, when control has been more fully accepted by the industry, rewards are more likely to be increased production — and increased market access.”
No more excuses
Patsy Houghton, Heartland Cattle Co., McCook, NE, also sees the value of BVD biosecurity and PI elimination.
“The cost of even low PI prevalence in cow herds can be very, very high,” she says. “BVD PIs often exist in chronic feedyard pens at the rate of 21% and account for 3-4% of all deads.”
Houghton says ranchers should look at BVD PI control as an investment, not an expense. And, she’s a firm believer in addressing the point of BVD infection — the mother cow.
“If we can keep the BVD virus from reaching the fetus, we’ll prevent any more BVD PIs from being produced,” she says.
And, because PIs begin at the cow-calf level, Hessman and Houghton, like Rechel, are asking the rancher to initiate PI-control measures and nip the problem in the bud. Hessman says his feedyard clients are ecstatic over the biosecurity networks already being formed by their cow-calf and stocker cattle suppliers.
“I hear a lot of veterinarians say they don’t have BVD in their areas,” Hessman says. “But, I think we’ve under-diagnosed BVD.”
He says BVD is a disease that’s been around cow-calf operations for a long time but ranchers have been “comfortable” with it — even though it’s costing an estimated $2 billion/year.
“We’re seeing the evolution of practical and relatively inexpensive ways to address the disease, and there’s no excuse for it to remain a herd health problem,” he says. “Cattle Empire Feedyards is now PI-free, and we’ll do everything we feasibly can to stay that way.”
Commercial cow-calf producers have long been reluctant to screen herds for the presence of PI animals due to the relatively high cost of testing and diagnosis. Immunohistochemistry (IHC) and fluorescent antibody tests have been a useful tool to detect the BVD virus in skin samples (usually ear notches).
The latest innovation in PI diagnosis to quickly and accurately screen herds for removal of PI animals is the reverse transcriptase polymerase chain reaction (PCR) technology using pooled animal tissue samples, usually ear-notches.
“Most BVD control experts are recognizing pooled PCR testing as a method allowing cost-effective screening for cow-calf operators,” Larson says. “By pooling samples, the expense of screening herds with suspected low prevalence is minimized.”
A single PI animal can be detected in pools of 50-250 samples. If the initial pool is PCR-positive, it must be split and retested to find the one or more individuals in the pool infected with the BVD virus.
If tissue samples are collected for pooled PCR testing from all suckling calves before breeding, PIs can be identified and removed from the herd to prevent contact with pregnant females.
The first year a rancher adopts this strategy, all suckling calves, open cows, replacement heifers and bulls should be tested. If any calf tests positive, its dam should be tested. In subsequent years, if strict biosecurity is in place, only suckling calves and any purchased animals need to be screened for PI.
“Testing for PIs is a once-in-a-lifetime event,” Larson adds. “If an animal is PI-positive, it will always be positive, and the PI-negative animal will always be negative.”
Biosecurity and ID
In order to remove cattle that might be PI positive, the rancher must individually ID the tested animals. The ID system doesn’t need to be complicated — simple panel-type ear tags or other ID systems already in use. Each tissue sample needs to be associated with the animals ID number, however.
Phil Kesterson, DVM, Trail Animal Health Clinic, Bridgeport, NE, says one of the biggest testing challenges is getting the right sample into the right tube. While branding is often a good time to test in many operations, it can add to the chaos of the event.
“But, past the mechanics of testing and getting past the mentality of whether or not we should test if we suspect BVD,” he says, “we now have good science behind testing.”
In addition to screening, surveillance should include necropsies of as many aborted fetuses, stillborns and pre-weaning deaths as possible.
“If you end up losing calves, if you have any non-biological pairs, you need to test both the cow and calf,” Kesterson adds.
When thorough testing indicates the BVD virus isn’t present in the herd, the issue is then one of biosecurity, Smith says.
“The herd must be protected from direct exposure to cattle from other herds that may have the BVD virus — either transient or PI,” he explains. “Isolation and quarantine of new additions, before or after testing, is essential in maintaining a BVD PI-free herd.”
And, like other scientists and veterinary practitioners, he recommends a vaccination strategy based on a modified-live virus (MLV) vaccine.
There are several MLV and killed vaccines on the market. These vaccines may contain different strains and concentrations of the viruses, different adjuvants and may induce different levels of response. Cow herd BVD vaccination programs are primarily designed to prevent fetal infection.
“This is immunologically more difficult to achieve than protection from clinical disease,” Larson explains. “Vaccination provides some fetal protection when the dam is experimentally challenged, but that protection doesn’t extend to 100% of fetuses of exposed dams.”
Most scientists recommend an MLV but strongly advise producers to work with their local veterinarian to develop a vaccination strategy.
“Veterinarians should consider efficacy studies when evaluating the use of vaccines in the field,” Larson says. “Vaccination should be considered a component of an overall BVD control program, not the answer.”
Five simple steps to BVD control
- Test all your cattle for BVD persistent infection (PI).
- Vaccinate your cattle annually with a modified-live vaccine.
- Test all additions to the herd for PI.
- Test each year’s calf crop for PI.
- Establish a whole-herd BVD biosecurity plan.
BVD’s 10 commandments
Setting goals is the first step for a producer who wants to design a BVD control plan and reap the rewards of a national BVD elimination movement. With the help of a practicing veterinarian, a cost-effective plan can be developed to address any ranch, stocker or feedlot situation.
The first step, as advised by the National Cattlemen’s Beef Association BVD Working Group of the Cattle Health and Well Being Committee, is to determine if BVD is currently a problem. The Working Group led by Bob Smith, DVM, Stillwater, OK, has suggested a list of “commandments” a producer serious about eliminating BVD from his herd should consider.
I. Determine if your herd is at high or low risk for having persistently infected (PI) cattle.
II. Identify and eliminate any PIs from your herd.
III. Prevent other PIs from entering your herd.
IV. Vaccinate to reduce the risk of the disease.
V. Monitor your herd for the presence of the BVD virus.
Thou shalt not:
I. Sell PI animals but for direct slaughter.
II. Mix purchased pregnant cattle with your herd until they and their calves have been tested for existence of PIs.
III. Buy bred heifers unless they test PI-negative and are properly vaccinated.
IV. Use animals of unknown BVD status as embryo transfer recipients.
V. Rely on vaccination alone for BVD control.
PI Sorting Pays Big Money
With the advent of practical, affordable diagnostic tests, producers have been able to start unraveling the immuno-suppressive impact that cattle persistently infected (PI) with bovine viral diarrhea virus can have in their programs.
Now, the industry is getting an idea about how much PI costs, or how much avoiding it may be worth.
Consider a trial conducted by Bill Hessman, DVM, of Central States Testing and the Haskell County Animal Hospital at Sublette, KS. A feedlot client, Cattle Empire (CE), LLC, was wondering how prevalent the disease and pen-infection rate were in its operation. And to what extent were PI calves impacting their bottom line?
Ouch! According to Hessman, the cost of calves exposed to PI in that operation is $67.49/head, a total average cost across the entire population of $41.17/head.
That’s based on 21,743 head across 240 pens.
One out of three pens exposed
The trial began in July 2004 at one of the firm’s starter yards (10,000-head capacity) where cattle are limit-fed for 60 days and not implanted. Every animal was tested. PI animals were removed from some pens and left in others so CE owners, Paul and Roy Brown, could get a handle on how PI calves influenced pen health.
They found the prevalence rate of 0.4% was just slightly higher than that found in smaller trials. But at least one PI calf was discovered in 71 of the 240 pens, with a pen-infection rate of 31%. In other words, about one out of three pens had been exposed to a PI calf.
Despite longstanding industry conjecture, Hessman found the infected calves weren’t more likely to come from one particular state than another (10 states were represented). What did increase the likelihood, though, was the order buyer.
Of the 15 buyers who bought 300 or more head represented in the study, the PI prevalence rate, by buyer, ranged from 0% to 2%. It turned out buyer behavior contributed to the fact some were more likely to send PI calves. In particular, Hessman said calves bought as singles or doubles through the auction were more likely to be infected.
The resulting pen rate of infection was just as startling. Of buyers purchasing three or more pens, the rate ranged from 0-70%.
The findings mirror those of a smaller trial (2,284 head in 24 pens) in which cattle were tested in CE’s finish facilities. Using closeout performance to compare PI- and non-PI pens, CE found a prevalence rate of 0.31% and a pen infection rate of 21%. The economic damage in that trial was $47.43/head in the pens exposed to PI.
Keep in mind the bulk of the damage came from lost performance in cattle exposed to PI animals, not mortality and morbidity among infected animals. Hessman says, while many PI calves die early, some survive to slaughter.
Of those in CE’s starter-yard trial, only 25.6% of the calves died during the 60-day starter phase. Of those, 64% of the deaths were due to mucosal disease and 27% to respiratory disease. In the smaller trial in the finish yard, 71% of PI calves survived to harvest.
Marketing PI screening
Though I’m not aware of economic data as extensive as this in other industry segments. I suspect the damage would be at least as significant.
Consequently, Hessman says cow-calf producers and stocker operators may find added marketability for PI-screened calves. Buyers may be willing to pay more for calves already tested.
Using CE’s starter-yard trial, Hessman points out the $41.17 cost PI calves levied on the entire population is equivalent to about $8.23/cwt. on a five-weight calf. That’s not what cattle feeders would likely be willing to pay, but at least part of that would provide added negotiation power for producers with screened calves or PI-free herds.
That’s in addition to the economic advantages of identifying and removing PI animals from the herd to begin with.
It seems this kind of economic impact might make buyers more willing to share in the cost of screening calves they’re considering for purchase.
It’s already apparent that such major feedyards as CE are trying to wrap their arms around the problem. Judging by conversation at the recent Elanco Professional Stocker Operators Symposium, where Hessman presented this information, at least some of this nation’s largest and most cost-progressive stockers also are in the midst of trying to sort out PI calves as early in production as possible.
Management programs and control strategies designed to eliminate the bovine viral diarrhea (BVD) virus infection in cattle herds must consider two ways the virus spreads within a herd.
- From one animal to another (horizontal transmission) as infected animals secrete the virus and the virus enters a susceptible animal through the mouth or respiratory tract.
- From an infected dam’s bloodstream to her fetus during pregnancy (vertical transmission).
Horizontal transmission of BVD virus to calves or adult cattle results in a temporary (transient) infection that’s usually mild, but can occasionally result in severe disease.
The main negative health effect of BVD is it can inhibit conception and/or cause abortion in susceptible females. It also suppresses the immune system, making infected animals more susceptible to other diseases. In addition to contributing to disease in infected cattle, horizontal infection from one animal to a pregnant dam can lead to vertical transmission of BVD virus to her fetus during pregnancy.
Fetal infection and the PI
The BVD virus is able to cross from the dam’s bloodstream to the fetus with high efficiency. Fetal infection can lead to early embryonic death, abortion, birth defects and stunting.
The lesions associated with fetal infection of the BVD virus include brain malformations, spinal cord defects, cataracts and other eye abnormalities, sparse hair coats, a short lower jaw, growth retardation and lung immaturity.
The immune status of the dam, the stage of gestation, and the characteristics of the virus itself are important factors in determining the result of BVD virus infection of pregnant cows and heifers. If the fetus recognizes the virus as an infection, it can fight it off and result in the birth of a normal calf; or it can become a persistently infected (PI) calf, carrying and spreading the virus its entire life.
PI cattle are the result of fetal exposure to the non-cytopathic biotype of BVD virus prior to the development of a fairly mature immune system at about 125 days of gestation. Although a high percentage of PI calves die at or near birth, or at least by weaning, as many as 50% of PI calves may survive long enough to enter the breeding pool or a feedlot.
The spread of BVD
The primary source of BVD virus is PI cattle. These animals are a much more efficient transmitter of BVD virus than transiently (temporarily) infected animals because they secrete higher concentrations of virus for a longer period of time.
BVD-PI females of breeding age are a source of horizontal transfer of BVD virus. In addition, they will always produce a PI calf.
As a result, PI suckling calves are considered to be the primary source of BVD virus in breeding herds — depending on the timing of the introduction relative to the breeding season and the resulting immune status of the herd during early gestation.
Horizontal transmission of BVD virus to susceptible cattle has been shown to occur after only one hour of direct contact with a single PI animal. Horizontal transmission of the virus from either PI or transiently infected animals to susceptible cattle can occur directly via nose or mouth contact with virus-containing body fluids. In addition, air transmission over short distances seems likely.
Persistent infection has a clustered distribution, which means a few herds may contain several PI cattle. Clustering of multiple PI animals in a herd is primarily due to exposure of numerous susceptible dams to a PI or transiently infected source of BVD virus prior to day 125 of gestation.
Economic effects of PIs
Infection with BVD virus has been associated with respiratory disease outbreaks in feedlot situations. PI cattle also can spread the virus to susceptible cattle during marketing and trucking.
PIs have been shown to have an impact on health performance of susceptible penmates and cattle in adjacent pens.
The cost of one PI animal in a cow-calf herd reportedly ranges from $14.85 to $24.84/cow/year. Cow-calf producers should work with their veterinarian to determine if their herd is likely to be experiencing production and economic losses due to BVD.
The economic value of whole-herd screening for PI animals in cow-calf herds is influenced by:
- The likelihood of finding at least one PI animal in the herd,
- The negative production effects when PI animals are present,
- The cost of screening and identifying PI animals, and
- The value of animals sold.
Because of the potential production and economic losses associated with BVD, all cow-calf producers should work with their veterinarians to establish biosecurity plans to reduce the risk of buying or creating a PI animal. Biosecurity plans for BVD in cow-calf herds include a sound vaccination program and use of diagnostic testing for PI status of any herd replacements or additions.
Biosecurity also involves application of a vaccination protocol to reduce the risk of fetal infection in the event of cow herd exposure to an animal shedding BVD virus.
Modified-live vaccines (MLV) have inherent properties that may enable them to stimulate more complete protection against transplacental infection. For that reason, one recommendation is to vaccinate unstressed, healthy heifers with a MLV.
Vaccine administration should be timed so a protective immune response coincides with the first four months of gestation.
Bottom line: While vaccination does provide some protection from fetal infection, BVD virus control is generally achieved by a combination of:
- Diagnosis and removal of PI cattle,
- Vaccination with an MLV, and
- A biosecurity system that prevents the introduction of PI animals into the herd and minimizes contact with infected animals.
Bob Larson is a University of Missouri-Columbia DVM and director of Veterinary Medical Extension & Continuing Education.
For more information, go to the “BVD Information Links” posted on the Internet by the Academy of Veterinary consultants: www.avc-beef.org.