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.”