At a time when it seems the cattle industry is faced with more disease issues than ever, there is one disease we have on the run. Attention to bovine viral diarrhea (BVD) virus over the last several years has led to several major efforts to rid BVD from the nation's cattle population.
In several states, special research and demonstration projects have been initiated to focus on eliminating BVD — Oregon, Michigan, Washington, Alabama, Georgia and Oklahoma, to name a few.
The Montana BVD-PI herd biosecurity project (see page 50) may be the largest and most comprehensive of the coordinated statewide BVD-control projects. It was initiated in 2006 to improve the overall health of Montana's cattle herd and add value to the state's calf crop.
The original focus of the project was to provide technical assistance and limited financial support to Montana ranchers who wanted to screen their herds for persistent infection (PI) with BVD. Since then, it's morphed into a demonstration of overall biosecurity practices that can prevent transmission of other diseases within and between cattle breeding herds.
First things first…
BVD is mainly a reproductive disease. Ranchers with infected herds see a decrease in the number of pregnant cows, as well as increased abortions, stillbirths and late-calving cows. There are very few cases of diarrhea, but the virus can lead to immunosupression in the newly infected animal, setting it and its herd mates up for attack from infectious diseases.
BVD begins and ends at the cow-calf level — the ranch or dairy. But BVD control in a beef or dairy herd doesn't have to be rocket science.
Management and control of BVD in most cattle herds is relatively simple if a producer remembers two ways the virus passes from one animal to another. The first is horizontal transmission — when a transiently (temporarily) infected animal releases the virus in its nasal and other secretions and the virus enters a susceptible animal through the mouth or respiratory tract. The second is vertical transmission of BVD virus via an infected dam's bloodstream to her fetus during pregnancy.
That fetal infection can result in fetal death, birth of a normal calf or birth of a PI calf — meaning that the infection lasts the entire life of the animal. It's important to note that PI females of breeding age not only are a source of horizontal transfer of BVD virus, but will always produce a PI calf themselves. The primary source of BVD virus is PI cattle; with transiently infected cattle considered a less important source.
Therein lies the problem
It's the PI calf from a non-PI cow, though, that's the smoking gun in nearly every BVD case. The PI calf exposes pregnant cows by producing and shedding huge amounts of very infective virus. Thus, the single most important step in controlling BVD is to identify and eliminate PI calves. Recent innovations in diagnostics allow us to affordably screen commercial cattle herds for the BVD virus.
The Montana Project relies on reverse transcriptase polymerase chain reaction (PCR) technology using pooled animal tissue samples. Results of peer-reviewed studies suggest that performing a reverse transcriptase-PCR assay on pooled fresh tissue samples is a “sensitive and specific method” of screening cattle for PI.
Diagnosis must be coupled with an animal identification system that allows efficient and accurate identification of BVD-PI animals so they can be removed from the herd.
The dam of a BVD-PI calf should be identified and tested for BVD-PI status, as there's a small chance the cow is also BVD-PI. A suspect-positive PI animal should be retested 3-4 weeks after initial testing to allow it the opportunity to clear itself of the virus in case a “hot” transient infection was detected.
While regular vaccination for BVD is critical and in most cases provides adequate protection, the tremendous amount of virus secreted by a PI animal can overwhelm an immunity level that would otherwise be protective under less severe exposure. Throw in breaks in nutrition, environmental stress, administration error and individual variation, and vaccination isn't the answer to eliminating BVD from a cowherd.
But good biosecurity — regular vaccination, strategic screening/testing and close attention to physical movement practices (physical biosecurity) — will go a long way toward keeping this disease on the run.
Frequently asked questions about controlling BVD at the ranch:
Can't I just test my cows for BVD?
Montana BVD-PI herd biosecurity project
Testing a pregnant cow will only tell you half the story — or less. A transiently infected cow could be PI-negative, but she could be carrying a PI calf and or be nursing a PI calf. And while she might be repeatedly transiently infected by her PI offspring, she can never become PI. Test calves first!
The caveat is purchased, bred replacement heifers. At minimal cost relative to the price of the heifer, it makes good biosecurity sense to test her before she's ever turned out at home. But realize you'll still need to test her calf.
Isn't a good vaccination program enough?
No. Vaccination alone (with either modified-live or killed vaccines) can't keep a cowherd free of BVD-PI cattle nor completely control BVD infection, according to the Academy of Veterinary Consultants and the American Association of Bovine Practitioners. Vaccines aren't 100% efficacious, administration is seldom perfect and individual animals react in various ways to vaccines.
Should I PI-test every year?
In most cases, testing doesn't have to be an annual event. Once you've established you don't have the virus circulating in the breeding herd — or you've eliminated it by finding and disposing of PI calves — you should be in good shape. Keep up a sound vaccination program and come back in about three years and conduct another whole-herd test.
Again a caveat! If you have a serious breach in biosecurity, consider whole-herd testing before the next breeding season. This advice doesn't apply to the seedstock producer. You have an obligation to sell every breeding animal — bull, cow and heifer — as “BVD PI-tested negative.”
What's a biosecurity breach?
Let's look at an example. One Montana rancher did a whole-herd test in spring 2006. A fire swept through his ranch that summer, burning down fences. He spent a month getting all his cattle home with no idea what they'd been exposed to. That's a breach in biosecurity. In 2007, he tested again, before breeding — fortunately with no PI cattle — but how could he have known unless he tested again?
Is a “closed herd” protected?
Unless you run cattle alone on an island far away, there's no such thing as a “closed herd.” Yes, good fences make good neighbors, but no fence can keep a virus out — or in.
Will a cow that had a PI calf this year have a PI calf next year?
Only if she's a PI herself. Otherwise, we suspect she's as likely to have another PI calf — or not — as any cow in the herd.
What do I do with PI calf?
Cut your losses and humanely euthanize it as soon as possible. Don't try to feed it, as 75% of the time it'll end up costing you more money and time than it will ever be worth. Plus, it's just another biosecurity risk that you don't need. You can eat it, but do not put the animal into commerce.
Are you telling me I can't spot a PI calf a mile away?
That's right. The only way to know the PI status of an animal is to test it for the virus.
What do you mean by a whole-herd test?
It involves all new-crop calves, open replacement heifers and breeding bulls in a herd. Calves can be tested anytime after they hit the ground. Don't forget the abortions, other oddball births and calves from late-calving cows. There's a reason a cow breeds late — it could be disease-related.
Is there a marketing incentive for cattle tested BVD-PI negative?
A rancher should test for BVD first for herd-health reasons. Any market rewards are icing on the cake. That said, one major video marketing firm in the U.S. is starting to see some price rewards for PI-tested negative calves. If you've tested, don't forget to tell the buyer — and do everything you can to squeeze another penny or two out of him. Good luck!
Clint Peck is a contributing editor and director of Beef Quality Assurance, Montana State University based in Billings, MT.
Montana BVD-PI herd biosecurity project
The Montana Project will continue through 2009 providing technical assistance and free BVD-PI biosecurity kits to volunteer participating Montana ranchers.
Calendar years 2006-2008
557 total herds screened (plus 82 “repeat” herds).
- 48 herds with at least one PI animal — 8.6% herds with PI.175,473 total head screened.
- 149 PI cattle found — 0.85 PIs/1,000 head screened.
- 135 new-crop calves.
- 6 weaned calves.
- 5 yearling heifers.
- 2 mature cows.
- 1 yearling purebred bull.