Cattle Today

Cattle Today

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When it comes to animals persistently infected (PI) with Bovine Viral Diarrhea (BVD), the increased odds of premature death is the least of economic concerns. It's how these cattle bleed performance and return from herd mates and pen mates that can wreak havoc.

“The value of certified PI-free cattle is not exactly known, best estimates are between $5 and $15 per head based on the likely cost of PI cattle in stocker and feedlot operations, and the expected percentage of cattle that will test positive,” says Bob Larson, a veterinarian at Kansas State University. He shared the information with participants at the Elanco Professional Stocker Operators Symposiums held in June.

In other words, knowing up front there were no PI calves in the group would be worth that amount of money across the entire group. And, that may be a conservative number.

At the same symposiums, Bill Hessman, DVM, a consulting veterinarian with Haskell County Animal Hospital and Central States Testing, LLC at Sublette, KS shared highlights from a study he conducted for Cattle Empire, one of the nation's largest cattle feeding organizations. The cost per head exposed to PI in that operation's starter facility was $67.49/head, resulting in a total average cost per head across the entire population of $41.17/head. That's based on the largest PI trial I've heard of -- 21,743 head across 240 pens.

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 aren't implanted. Every animal was tested. PI animals were removed from some pens and left in others so Hessman and Cattle Empire owners, Paul and Roy Brown, could get a handle on how PI calves influenced pen health if they'd been in a pen then removed, left in a pen, or whether they existed in an adjoining pen to a non-PI set of calves, or ever had.

All said and done, the prevalence rate of 0.4 percent was just slightly higher than what's usually been seen in smaller trials. But, at least one PI calf was discovered in 71 of the 240 pens for a pen-infection rate of 31 percent. In other words, about one out of every three pens had been exposed to a PI calf.

These findings mirror those from a smaller trial (2,284 head in 24 pens) in which cattle were tested in Cattle Empire's finish facilities. Using close-out performance to compare between PI pens and non-PI pens, they found a prevalence rate of 0.31 percent and a pen infection rate of 21 percent. In sum, 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 the cattle exposed to PI animals, not to mortality and morbidity among the infected animals.

Based on feedlot research done in Canada and Texas, Larson explains, “Many pens or groups of cattle that contain one or more PI animal have higher numbers of non-PI cattle treated for disease than groups that do not contain any PIs; and gain performance in pens with PI cattle is usually lower than pens without PI cattle…When looking at economic loss due to PI cattle in stocker and feedlot operations, the high death rate of PI cattle is noticeable, but because PI cattle are relatively rare, the greatest economic loss is probably the reduced weight gain in the non-PI cattle that are constantly exposed to the BVD virus.”

Spun differently, Larson adds, “Because of this widespread economic loss, pens where PI cattle are removed or die soon after arrival probably have less economic loss than pens where PI cattle appear healthy and stay with the group—secreting BVD virus throughout the growing period.”

Hessman points out that while many PI calves die early on, some survive all the way to slaughter. Tracking those in Cattle Empire's starter-yard trial, only 25.6 percent of the calves died during the 60-day starter phase. Of those, 64 percent of the deaths were due to mucosal disease and 27 percent were due to respiratory disease. In the smaller trial in the finish yard, 71 percent of the PI calves survived to slaughter. Now consider this: “The greatest economic losses due to BVD occur on cow-calf farms, with stocker and feedlot operations experiencing less economic loss due to the disease,” says Larson. Here you're talking about impacts on conception rates, the percentage of calves weaned per cow exposed, production per acre, all those things that bind fixed-cost operators.

Busting the Bottom Line

According to Larson much of the economic damage stems from the virus' ability to wallop an animal's immune system so that other pathogens have the opportunity to take hold.

BVD-PI is also deceptive, for several reasons.

First, while a fair percentage of PI-infected calves die before weaning—if they weren't already aborted as fetuses—some that live show no clinical symptoms and can look the picture of health. Both Larson and Hessman showed pictures, demonstrating the fact. In other words, you can't see PI.

Next, Larson points out there are more than 200 BVD strains, so he says you're really talking about a cloud of viruses. Moreover, he explains the pathogen mutates easily meaning that virulence can change over time.

Finally, the prevalence rate of PI is so low that it can make some producers figure that it's not much of a problem. In round numbers, estimates are that about 10 percent of the nation's cow-herds have at least one PI animal. At weaning, Larson says the estimate is that 1 out of 200 calves or fewer are positive for PI.

But cattle that test positive for PI are the tip of the proverbial iceberg. PI cattle shed the virus their entire lives and they're loaded with it. As Larson pointed out, a common diagnostic test today involves taking a skin sample from the ear. For positive cases, these samples are overflowing with the virus. If the skin of an extremity is so saturated, imagine how much more so the innards are.

So, these PI-infected cattle shed the immuno-suppressive virus to any cattle they come into contact with. Whether or not these other cattle contract BVD, specifically, immune systems weakened by exposure increase the odds they will get sick with something else.

Incidentally, the only way to pass along PI-BVD is in utero during the last part of the first trimester of gestation or the first part of the second trimester.

Larson explains that's one reason PI discussion has increased within the industry over the past few years. Affordable and accurate diagnostic tests that have become available mean that PI cattle can be more easily identified. The fact that PI can only be transmitted from a PI-cow to its fetus means that the reservoir for infection is so narrow and rigidly defined that it can be managed and controlled. In fact, experts point out it could be eliminated from the industry if the industry aimed for it collectively.

Identify and Remove

Closer to home, for cow-calf operators, a growing number of veterinarians are saying that finding and eliminating PI animals is key to growing herd immunity. As mentioned earlier, there are a couple of tests available that run in the $3-$4 per head range. Veterinarians can help their clients develop cost-effective testing strategies as well as protocols for BVD vaccination and management.

According to Larson, variation in the cost of the disease itself comes at the hands of differences in virulence between strains, susceptibility of calves and animal density. The latter has to do with the fact that most cattle-to-cattle BVD transmission requires direct contact.

Ultimately, the only way to take care of the PI challenge is at the ranch; once they leave home, the industry is hung.

For perspective on how quickly and broadly PI can spread health problems, consider a Michigan State University study done a couple of years ago. Two groups of 92 head of feeder cattle were shipped from the Southeast. One of the loads included two PI calves. When the cattle arrived at the feedlot, half the calves from each load were vaccinated for BVD with a modified live virus. About 10 percent of the vaccinated calves that hadn't been exposed to the PI calves were pulled and treated for respiratory disease. About 13 percent of non-vaccinates that weren't exposed to the PI calves were treated for respiratory. Of the calves exposed to the PI calves, about 17 percent of those vaccinated on arrival were ultimately pulled for respiratory, compared to almost 30 percent of the non-vaccinates.

As time goes on, the benefits of controlling PI BVD at the ranch may impact the marketing side of the equation, too. As noted earlier, some buyers are already determining what BVD-PI is costing them and what it's worth to avoid.


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