All it takes is one.

And if that one happens in your cow herd, it can wreak havoc any number of ways, says Gregg Hanzlicek, a DVM and director of field investigations at the Kansas State University (KSU) Veterinary Diagnostic Laboratory in Manhattan. “It’s going to be poor pregnancy rates, maybe some embryonic death, maybe some abortions,” he says.

And that’s just for starters. Bovine viral diarrhea (BVD) manifests itself in many ways, with scours or coccidiosis-like symptoms being only one, and not nearly the most devastating, result. Lighter weaning weights, increased respiratory disease and higher death loss are major problems affecting not just ranch results, but every place that calves go from there.

Usually, when BVD is a suspect, producers and veterinarians focus on the calves, reasoning that the calf crop is the most likely place for a persistently infected (PI) animal to hide. But that’s not always the case, says Dan Thomson, Jones professor of epidemiology and production medicine and director of the Beef Cattle Institute at KSU’s College of Veterinary Medicine.

“One of the more interesting cases we had was increased respiratory rate on calves still suckling the cows,” Thomson says. “We tested for multiple things but mainly BVD. The Trojan horse was the herd bull, a BVD PI. Every time they turned that PI bull in with the cows with the calves on the side, we’d get a respiratory outbreak in the calves.”

And that’s not accounting for the PI calves that were born the following year. Indeed, having a BVD PI animal in your herd can hurt you not just with this year’s calf crop. It’s the gift that keeps on taking, until it’s eradicated from your herd.

So does that mean you should test your bulls for BVD? As is often the answer, Hanzlicek says it depends. If everything is staying between the guardrails with your conception rate, calving percentage, weaning percentage and herd health, then you probably don’t have a problem, he says.

If that’s the case, he suggests working with your veterinarian to continue your pre-breeding vaccination protocols for both your cows and your bulls. In addition, avoid fenceline contact as best you can with neighboring cattle, and have a good biosecurity plan in place when you bring in cattle from unknown sources onto you ranch.

Bulls and more bulls

“If you’re buying bulls from unknown sources, or buy a young bull that hasn’t been tested for PI, then I’d for sure have him tested,” Hanzlicek says. That’s why he strongly encourages ranchers to know their sources.

“A lot of purebred bull breeders will test all their bulls and declare them PI-free before they sell them. That’s why we push buying from a reputable breeder, someone who is conscious of the disease,” he says.

The same thinking applies to heifers and mature cows purchased as herd replacements. “Any heifers I brought on the place that I didn’t raise, I would absolutely test those animals to make sure they’re not PIs,” he says. “If those heifers were pregnant when they came in, I’d not only test those heifers for PI, but every one of their calves to make sure they’re not PI. That’s really important.”

That’s because a PI animal is created during the first 30 to 150 days of gestation. If a cow or heifer is exposed to the virus during that time, she can pass it along to the calf. During this time of fetal development, the calf’s immune system can’t distinguish between good and bad bugs.

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If the fetus becomes infected with the BVD virus during this stage of development, the immune system assumes the virus is supposed to be there and doesn’t mount an immune response to it. What’s more, the calf may not show any clinical signs of having the disease, yet can continue to spew pathogenic virus over its entire lifetime.

And if the calves test PI-free, that can be an advantage at sale time. Data compiled in 2013 by Superior Livestock Auction showed an average premium of $2.97 per cwt for calves marketed as PI-free. While calf prices will vary week to week, if you assume a 600-pound feeder steer sells for $235 per cwt, that’s an additional $17.82 per head return. Even subtracting the cost of the test, which can range from $3.50 to $6 per head, it’s still a positive return.

The few, the proud

“Depending on what literature you look at, it’s estimated we have 1% of the calves born in the U.S. as PIs,” Thomson says. “Half of those die before weaning. So that gives us a prevalence of half-a-percent. And when you look at data coming into feedyards, that’s about where we’re at – 0.3% to 0.5%, depending on what route the cattle take getting to the feedlot.”

Thomson says research shows that about half the PIs, once they get to the feedlot, will either die or be railed after arrival. “So, 25% of the PIs born wind up going all the way to slaughter, and 75% die somewhere along the way, whether it’s at the ranch, the feedlot or the stocker operation.”

However, while the data show the number of PI calves in a feedlot isn’t high, they’re spread out. “In a study, we found 0.34% positive (as a BVD PI) but 25% of the pens. It’s not the number of animals, but the number of pens that are positive,” Thomson says.

That’s because many feedlot pens are comprised of calves from many different, usually unknown sources.

In fact, research shows that exposing a pen of feedyard cattle to a PI animal increases the risk of a respiratory disease outbreak. Research conducted by Thomson, along with Guy Loneragan at Texas Tech University and others, found animals in a pen with a PI are 43% more likely to be pulled and treated for respiratory problems. Other research found a PI calf in a feedyard pen can cause performance losses averaging $88.26 per head.

Translating that to your ranch, you’re at similar risk if you buy replacement cows, heifers and bulls from multiple, unknown sources.

If you see some of your performance metrics starting to slip, it may be time for some testing. Last fall, Thomson, says he got more calls and emails about increased abortion rates in fall-calving herds than he’s received in the past 10 years.

“And it’s not big increases. But the normal from the time we preg check to calving is estimated to be 2% to 3%. We were having people calling us with 7%, 8%, 10%. So something infectious is going on out there and we need to get on top of it,” he says.

If you have what you think is a high-risk herd, it might be time for some diagnostic thinking. “If we have poor reproductive performance, the things on my list are body condition score of the cows right at calving, and were the bulls fertility-tested before turnout? Then BVD and trich, in that order,” Hanzlicek says.

If your management measures up, but your cow herd isn’t where you want it to be, then testing is in order. “If you have a high-risk herd, we need to test all the calves, the cows without the calves, the replacement heifers and the bulls,” Thomson says. “But the big thing is, if you’re going to do this for biosecurity purposes, do this before the breeding season so we don’t propagate more PIs.”

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