For every 100 dairy cattle, one is a carrier for BVD (bovine viral diarrhea). Such a “persistently infected” (PI) animal can shed several billion virus particles a day, serving as a reservoir of BVD in a herd. The frustrating thing is that a dairyman won’t be able to pick out the PI animal just by looking at her.
BVD can be “insidious” in a herd, responsible for “repeat breeding syndrome” and lost embryos, says Ron Schultz, a veterinary infectious disease expert within the vet school at UW- Madison. Schultz grew up on a Guernsey dairy farm in Pennsylvania. He warns that BVD is most definitely a dairy biosecurity concern.
BVD is a virus. Dairymen have likely already heard about Type 1 and Type 2 BVD. Those are different genotypes, and within each, there are many different strains of BVD virus - that differ significantly in their ability to cause disease. Type 2 BVD has been in the news as “hot” BVD, responsible for major “wrecks” in herds. Schultz notes that while some strains of type 2 are “hot,” there are Type 1 strains that “do the same thing.” BVD herd wrecks have occurred which producers used killed vaccine but did so improperly. They may have thought their cattle were safeguarded, when, in fact, they weren’t. These wrecks have resulted in dead calves and adult cows, major reproductive problems, and hemorrhagic disease in calves (bloody diarrhea).
BVD can cause one or more of the following problems: Abortion or early embryonic death, pneumonia, fever (a temp of 105 or greater), diarrhea, lameness and immunosupression, allowing other diseases to rear their heads.
Susceptible animals don’t necessarily get sick. Many show no signs of disease. Others can develop a fever with signs of pneumonia or without. Some will have mild to severe diarrhea. Occasionally some infected animals will have ulcers in their mouths. Pregnant animals may abort anywhere between 1 and 8 months after exposure.
BVD virus is shed in the manure, into the air and in body secretions. It can be brought into a herd by a replacement animal or her unborn calf. Fenceline contact with neighbor’s cattle can also be a route of transmission, if cattle are pastured.
Schultz says BVD mutates so much that it’s just a matter of time before a Type 3 comes along. However, he indicates it shouldn’t be something that keeps farmers awake at night if they practice tight biosecurity, get rid of any PIs in their herd, and properly vaccinate their cattle.
Schultz estimates that 90% of adult dairy cattle are antibody positive for BVD, meaning they’ve been exposed, it’s caused an immune response and is now gone. Of course, if they’ve been vaccinated, they’ll also be antibody positive.
How do PIs or BVD carriers come about? A dairy animal can only become persistently infected if exposed to a strain of BVD with certain characteristics, as a fetus, in the first 120 days of pregnancy. If exposed after 120 days, they can’t become PI. They can, however, range from normal to having birth defects, or they might be aborted. Fetuses exposed to some strains of BVD are usually killed, even in the first 120 days.
It’s the PI, exposed during that certain window inutero, that doesn’t recognize BVD as anything foreign in its body. A PI can potentially stick around for quite awhile in a herd, stealthily shedding virus and slowly creating more PIs in herdmates’ calves (exposed to the virus as fetuses in those first 120 days). What’s more, all of a PI’s offspring will also be PI. But, non-PI cows, as noted, can also have PI calves.
Schultz says it’s important to remember that a PI is the only permanent carrier of BVD. Vulnerable animals exposed to acute disease may become infected, shed the virus for a few days, recover and become immune. The PI remains infectious for her lifetime (or his lifetime, if it’s a bull), serving as a threat to other cattle. Schultz knows of one PI that in fact, placed high at World Dairy Expo.
The PI is a threat to biosecurity when it’s purchased and brought into a herd that hasn’t been properly vaccinated. It’s in such herds that major disasters are suffered. Don’t count on being able to pick out a PI in a group of cattle, Schultz warns. While some are poor doers, many appear completely normal. The only way to know for sure is to blood test.
It’s estimated that about 15% of the dairy herds in Wisconsin contain at least one PI animal. Some of the PI herds tested at the state lab have 5% or more PI animals. Schultz has found one herd with 17% of the animals being PIs. That herd had a bull that was PI, and he contributed to that exceptionally high percentage.
Farmers concerned about the possibility of PI animals being present in their herds should contact their own vets, who’ll know the proper samples to send into the lab. To look for PIs requires a blood test for the presence of the virus itself. It costs $5.00 per head (lab charge only). The false positives are “negligible,” and the false negative rate is only 5%. While that means the test will miss 5% of the PIs, Schultz says 95% test accuracy is “better than any other test available.” “That’s fantastic,” he says.
He advises any farmer having reproductive problems in his herd to test for BVD, a virus that has multiple effects because it can weaken the cow’s immune system.
Producers who want to expand will want to test incoming animals for BVD. And they’ll need to also make sure the home herd is properly vaccinated. It should also be noted that on occasion, it’s the home herd that has caused BVD problems in purchased animals, versus the other way around.
As for “purchased BVD,” the most common scenario, notes Schultz, goes something like this: “I brought in two heifers two years ago and we’ve never had the reproductive problems we have now.” For a $5.00 test of each of those heifers, that dairyman would have saved himself “a fortune,” stresses this UW BVD expert. Remember, 1 in 100 head of cattle is a PI. It may not sound like much, but if you were unlucky enough to bring in that one, “then you’re in trouble,” he warns.
While ideally, a farmer needs to test everything on his place, the bill can add up quickly. “If you need to start somewhere, start with the adult cow herd, which after all, “brings in the money,” says Schultz. “Calf problems are cheap compared to major problems in the milking string.”
An inexpensive way to “screen” a suspect herd for BVD - as an alternative to blood testing for the virus itself - is to focus on a group of young heifers. Don’t vaccinate them prior to six months of age, then take blood samples from 5 to 10 of them. Instead of trying to isolate the virus, the lab should look for BVD antibody If at least 20% (two or more out of the 10) come back with significant antibody titers, figure you have at least one PI in the herd - what Schultz calls a “virus factory.”
Once a farmer can rid his herd of the PIs - and their offspring - then he’ll have a “well-immunized” herd. That’s because a “PI animal can ‘vaccinate’ better than any farmer or vet with a syringe in his hand,” Schultz quips.
Schultz says even if a herd is well-vaccinated, that PI can still cause trouble. You can still have reproductive problems, because her herdmates’ fetuses are at risk. If they don’t become PI in the first 120 days, then they can be susceptible and be killed. Late-term abortions are often seen in PI herds. And over the years, there will be a buildup of PIs in the herd.
Once you’ve tested and culled all the PIs, Schultz reminds producers that they need to look at all the calves born in that herd for at least a year, as they might be PI. Once you get through that, then you’re “home free,” he says, as long as you don’t buy more PI animals and carefully vaccinate your herd for BVD.
He says perhaps the more common scenario in a herd that should warrant the dairyman putting an antenna up for PIs is seeing cows confirmed pregnant but then turning up open within the next cycle. If that’s happening to a fairly high percentage of the cows, beware.
Do not buy BVD-infected bulls. That PI bull produces the greatest number of PI calves in a herd. He’s the “ideal vector” to transmit BVD, says Schultz.
Blood test all incoming animals for virus and reject those that turn up positive. In addition, unborn calves carried by incoming bred heifers should be tested for persistent infection soon after birth. The virus test can be done at any age.
It’s always a good idea to isolate all newly purchased cattle for at least 30 days, preferably on another farm.
Schultz is happy to answer farmers’ questions about BVD (608-263-9888).
Reprinted with permission from AGRI-VIEW, March
12, 1998.
Written by Jane Fyksen-Crops/Livestock Editor