Bulls can spread the BVD virus through nose-to-nose contact as well as in their semen.
Editor’s note: Part IV in a BVDV series.
For most beef herds, bulls are a valuable investment. Often, care is taken to administer breeding soundness exams to ensure fertility, but are you focusing enough with your clients on diseases such as bovine viral diarrhea virus (BVDV) that can lead to abortions, persistently-infected (PI) calves and other clinical disease in the herd?
“The breeding soundness of bulls is a primary focus, and the potential for infectious disease introduction by bulls is often not given much emphasis,” says Kenny Brock, DVM, MS, PhD, Auburn University. “Bulls can contribute to the introduction and spread of BVDV in a herd through acute or persistent infections.” Research has also shown that acute BVDV infections in bulls can result in a temporary decrease in fertility. Brock notes that the virus does replicate in the testicle and may affect semen quality.
How bulls spread BVDV
Bulls can spread BVDV in many of the same ways cows can, such as through nose-to-nose contact, but they can also spread the virus through their semen. Persistently-infected bulls can be even more of a problem. “PI bulls shed a tremendous amount of virus in their semen and have been shown to cause the birth of PI calves,” says Victor Cortese, DVM, PhD, Pfizer Animal Health.
Brock adds that the offspring of a PI bull may or may not be persistently infected – one report showed only 25 percent of the calves born to a PI bull were persistently infected.
BVDV can also be spread easily to bulls. Cortese notes that the chance of unvaccinated bulls getting infected with BVDV while breeding PI or acutely infected cows is very high.
High-quality replacement bulls should be screened for PI BVDV.
Though it may not happen often, some PI bull calves can reach breeding age and shed virus. Cortese says if cows are well-vaccinated, they should be able to fend off an infection from a PI bull and not give birth to PI calves; however, he is not aware of any studies that have been done with PI bulls and vaccinated cows.
Bulls that are persistently infected are just as detrimental to a herd as any animal that is a PI, agrees Dan Goehl, DVM, Canton Veterinary Clinic, Canton, Mo., but he has never found a PI bull that has made it to breeding age. “Most of the time, the animals that are kept for bulls are the better-doing animals, so the chance that they are PI bulls is decreased. However, it’s not worth taking the chance and not testing those bulls.”
PI bulls are also able to pass on the virus and cause acute infections to cows, calves and other bulls in a herd. Cortese says if other bulls acquire an acute infection, they are usually able to fight it off, but while they are infected, they can also shed the virus.
“Sometimes producers will get discouraged because we are not finding positive BVDV animals, even though that is the point of testing,” Cortese notes. “But, I believe the test is still a very small investment to protect the larger investment of the bull or the greater economic loss that would occur if BVDV were allowed to be introduced to the herd.”
Stringent biosecurity needs to be continued even if a herd is negative for years, Cortese adds.
Testing bulls for BVDV
Most beef bulls at some point have a breeding soundness exam and get semen-tested, but often BVDV testing is forgotten. “Any new arrivals should be isolated and tested to determine the PI status,” says Cortese. He suggests that bulls be ear-notched and tested as PIs either as young calves destined for a bull battery, upon their first BSE exam or pre-sale.
Brock suggests to ear-notch day-old calves or collect blood for virus isolation at weaning. “Bulls should be screened early to identify and deal with any potential problems early.”
Victor Cortese, DVM, PhD, says the chance of unvaccinated bulls getting infected with BVDV while breeding PI or acutely infected cows is high.
Dan Goehl, DVM, believes BVDV-free status will become an important part of marketing seedstock animals.
To not test a new bull addition is irresponsible if there is any kind of BVDV biosecurity in the herd, says Goehl. “More seedstock producers are marketing their animals as BVDV-negative via the immunohistochemistry ear-notch test for PIs, but if not, I would have the bull tested before bringing it home.”
If a client is purchasing young bulls with the intention of raising them for breeding stock, Goehl recommends testing for negative PI status. If the bulls are born on the farm and are to be tested to meet the farm’s biosecurity plan, then it needs to be done before bull turnout while the young bull is still nursing.
Bulls and biosecurity
Goehl’s BVDV biosecurity program differs from herd to herd. He believes high-quality seedstock animals, such as replacement bulls and heifers, should all be screened for PI BVDV. “In a commercial herd, we often find it possible, depending on the risk assessment, to have an adequate biosecurity without testing every animal. Stringent vaccination is always a part of our biosecurity program.”
Herds that are deemed necessary to be BVDV-free have a very comprehensive BVDV biosecurity plan. Initially, all of the animals are tested and from then on, all new additions are tested. All of the calves are then tested prior to the breeding season.
“We know that PIs are formed from approximately 40-140 days gestation, so we want to be able to eliminate any possibilities before they can expose a pregnant cow,” says Goehl.
Preventing BVDV in bulls
Goehl uses a modified-live virus vaccine whenever the circumstances allow it. “Most bulls that we put through a seedstock program will have had two doses as a calf and another as a yearling. Then there should be an annual vaccine given to the bull at the same time it is given to the open cow herd.”
Basic strategies to keep bulls from contributing BVDV problems are simple, says Cortese. “Cull poor-growing or smaller bulls, maintain good vaccination programs in the home herd and quarantine new bulls until BVDV PI testing is back from the lab.”
ET programs and seedstock bulls
Due to the increased value of embryo transfer (ET) animals, Dan Goehl, DVM, believes it is critical for them to have BVDV-free status. “Donor animals are sometimes taken away from the farm to be flushed, and anytime they leave the farm, there is potential for exposure,” he says. “Also, ET calves often return to the seedstock herd, and it is important for these calves to remain clean to protect the entire herd. There is often significant financial cost involved in each pregnancy, so the importance of each pregnancy is amplified.”
BVDV not only causes the formation of PI animals but can cause embryonic resorption and abortion. “I believe BVDV-free status will become an important part of marketing seedstock animals,” adds Goehl. “We want our embryo transfer herds to be BVDV-free.”
Many of Goehl’s clients are in the seedstock business, and he says BVDV programs for the bulls are a bit more complex than the commercial herds. “One herd I work with has a high percentage of the calves raised as embryo-transfer calves in recipient herds owned by other producers. This means we have calves coming in from various herds and environments every year. It is hard enough to control the biosecurity problems on farms that you visit regularly, let alone ones that you have very little contact with. We want these calves ear-notched and IHC-tested before coming back to the main farm. We also want them to have received two doses of a modified-live vaccine.” These calves are then quarantined away from any bred animals for a period of time.
“I try to encourage the co-op herds to test prior to bull turnout, so if there is a problem, they can benefit from it the most,” Goehl says. “It does not help them break the cycle of BVDV if the calves are tested the week they are to return to the main farm.”
Testing bulls and certifying them as PI-negative also can increase their value and protect the seller from liability, adds Victor Cortese, DVM, PhD.
Kenny Brock, DVM, MS, PhD, says genetics companies have screening programs in place to screen semen donors and check semen lots for BVDV. For these reasons, he says it’s recommended to only use semen collected according to the recommendations of Certified Semen Services.