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.