In the world of cattle health, animals persistently infected (PI) with Bovine Viral Diarrhea (BVD) are a whole lot like taxes — it's the impacts you can't see that can rob profits, and it never stops.
“You'll miss more by not looking than not knowing,” explains Daniel Thomson, director of veterinary services for Cactus Feeders, Ltd., the largest cattle feeding organization in the world with a one-time capacity of 540,000 head.
Studies conducted by Cactus in conjunction with West Texas A&M University underscore how often PI animals and exposure to them are at the root of too many pulls, chronics and mortalities.
As an example, Cactus discovered — based on the ear-notch test for PI — that 2.6% of 1,930 head in their chronic pens were PI. Using the same diagnostic, 3-4% of all the mortalities in one of their yards were PI.
“A majority die within the first 30 days on feed, but they can string along for any number of days,” says Thomson. He recalls one example where an animal, ultimately discovered to be PI, keeled over the day before shipping.
In the Cactus studies, assuming 100,000 head of cattle placed, PI calves account for 42 mortalities and 36 railers. Multiply that by 12-weight fed cattle bringing $65/cwt. and that's $33,540 per 100,000 animals for mortality alone, not counting treatment cost along the way. Now apply that to the 24 million head or so of fed cattle each year and use current prices running past $95/cwt.
Moreover, the 0.3% PI prevalence rate documented in the Cactus study runs below the industry rule of thumb. It holds that 1% of all calves born each year are PI, and half of those will die before weaning. So, the industry average prevalence rate is estimated at 0.5% heading into the stocker pasture or feedlot.
That's part of PI's insidious nature. As Thomson explains, the heavier the cattle are coming into the stocker or cattle feeding operation, the lower the prevalence rate. The problem exits on a wider scale but plenty of PI cattle die before weaning, even though they have had ample opportunity to infect other cattle that do enter the market.
Plus, Thomson notes, you tend to see PI in clusters of calves coming from the same operations, which makes sense. He explains although the prevalence rate appears lower in put-together loads of cattle, it's because these clusters have been parceled out among larger groups of cattle, exposing more cattle along the way than if the problem had been contained within the clusters of infection.
Thomson points to research conducted by Michigan State and Auburn universities that evaluated the impact of PI exposure in transit from a sale location.
“There was almost a 100% increase in the pull rate for calves exposed to a PI animal, just on the truck,” he explains.
The study evaluated two groups of 92 head. One group went on a truck that contained no PI calves; the other load included two PI calves.
Half the calves on each load were vaccinated for BVD with a modified-live virus product. Among vaccinates exposed to the PI calves, morbidity ran 18%, compared to 29% for exposed non-vaccinates. Compare that to 10% for unexposed non-vaccinates. Morbidity for unexposed vaccinates was 13%.
Across the board, Thomson says PI calves represent roughly 5.8% of all cattle treated for bovine respiratory disease and 25% of mortality.
Plus, Thomson explains, not only do pens of cattle containing a single PI calf experience pull rates 33% higher than pens unexposed to PI, but all of the pens adjacent to the PI pen also run the same higher morbidity rate.
PI In Brief
“Basically, it's a reproductive disease. But, it's what we term a transition disease because it has ramifications in the stocker pasture and feedlot,” says Thomson.
For simplicity sake here, BVD infections are classified as either acute or PI. Acute cases result from exposure to — close contact with — PI animals.
PI is the result of fetal infection. Cows exposed to BVD prior to 125 days of gestation can infect the fetus in utero. Unaborted fetuses read the infection as part of their normal system and mount no response. Thus, the virus is allowed to multiply, and PI calves can shed the virus throughout their lives.
What's more, Thomson points out PI animals don't necessarily exhibit clinical symptoms. These would include profuse diarrhea, along with severe erosions and ulcers on mucosal surfaces (such as inside the mouth, between the toes). Another common occurrence is mucosal disease that results when PI animals harboring non-cytopathic BVD (a strain that doesn't kill cells) is exposed to a cytopathic (kills cells) variant.
Symptoms of acute infection can include fever, snotty noses, diarrhea and bovine respiratory disease. Acute or persistent, Thomson emphasizes BVD is an immunosuppressant disease and sets the stage for other infections to attack an already weakened immune system.
For the record, BVD is classified within two genotypes (Type I and Type II), both of which contain both cytopathic and non-cytopathic bio-types.
Biosecurity Is Key
“It's not something that's going away unless you get rid of PI herds. Vaccination appears to reduce the incidence, but doesn't eliminate it,” Thomson says. One reason for that is the seemingly infinite variations that exist between biotypes and genotypes, along with the virus's propensity for mutating.
That's why he believes biosecurity is essential in prevention and control.
“One of the first things people think when they hear the term biosecurity is protecting against terrorists. That's “security,” keeping out the threat of the intentional introduction of disease. Biosecurity is managing for decreased disease transmission between animals and between animals and humans.”
With that in mind, Thomson says basic sanitation is the foundation of biosecurity. It doesn't have to be elaborate, just clean. “How would you like to be the 20th person in line at the doctor's office to get stuck with the same needle?” he asks.
Hand in hand with sanitation, Thomson says isolating infected and exposed animals is central to controlling spread of the disease.
“We need to make sure our operations are set up to prevent re-exposure or further exposure of animals to diseases like BVD that are spread through close, direct, animal-to-animal contact,” says Thomson. “As an example, fence-line waterers are probably the biggest biosecurity breech we could imagine.”
Of course, all these precautions assume knowing what disease you're dealing with. According to Thomson, one reason outfits like Cactus are looking hard at PI is because the ear-notch test made available within the past couple of years finally provided a convenient, quick and affordable way to determine whether or not animals are PI.
Tie this diagnostic capability to the industry push for a standardized national identification system and Thomson believes the industry can reduce the PI challenge over time. In the mean time, he says organizations like the Academy of Veterinary Consultants are mounting coordinated strategies to reduce the problem. And, he says, more pre-conditioned calves are cropping up that have been tested for PI prior to sale.
Still, Thomson says the cattle industry has a long way to go, relative to the system-wide biosecurity already employed by competing species.
“How many pork producers do you know who would buy a weaner pig at a sale barn, take him home and throw him into the finishing barn? We do that in the cattle industry every day,” he says.
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