The prime reason for vaccinating breeding animals against bovine viral diarrhea (BVD) is to protect the fetus from becoming persistently infected (PI). But, don’t forget about the bulls, says Dr. Dan Givens with Auburn College of Veterinary Medicine in Alabama.

Givens recently wrapped up an in-depth study looking at the potential for BVD virus in semen to transmit disease, as well as the length of time BVD virus remains in the semen and testicular tissue. This and previous research shows that some bulls develop a condition known as prolonged BVD testicular infection following an acute BVD infection or vaccination with a noncytopathic modified-live BVD vaccine. PI bulls also shed the virus in their semen.

A prolonged testicular infection differs from a persistent infection in that the BVD virus is confined to the testicles. The virus replicates in the seminiferous tubules within the testicles and is shed in the semen.

Givens’ research project involved 10 separate trials. In the first trial, 23 seronegative bulls, nine to 15 months of age, received intranasal doses of BVD (Type 1a) virus grown from semen collected from a bull with a prolonged testicular infection. An animal is seronegative when there are no detectable antibodies to the virus in the bloodstream, as there would be if an animal had been vaccinated or previously exposed to the virus.

Though none of the bulls showed clinical signs of BVD infection, they all shed the virus in their semen for 10 to 14 days following the exposure. All bulls that tested negative for BVD on day 65 remained negative, however, BVD virus was still detectable in semen collected from five of the bulls 107 days after exposure. Subsequent testing showed that two bulls were still shedding the virus 848 days after initial exposure and one continued to do so through to 1,007 days.

Furthermore, BVD virus survives the process to prepare semen for artificial insemination. Previous trials have shown that susceptible heifers inseminated with semen containing Type 1a BVD virus produced PI offspring two to 15 per cent of the time.

Other trials in Givens’ project looked at the potential for BVD-laced semen from bulls with prolonged testicular infections to transmit disease in a natural breeding scenario on pasture, as well as the potential for Type 1b and Type 2 strains to cause prolonged testicular infections in bulls exposed to PI heifers. Findings confirmed that there is potential for Type 1b strains to cause prolonged testicular infections, but Type 2 appears to be limited in its potential to cause prolonged infections.

Reasons to vaccinate bulls

“Bulls are introduced to a new set of animals at the same time they are expected to achieve specific goals — to get the cows pregnant — within a short period of time,” Givens explains. A bull that hasn’t been vaccinated is susceptible to acquiring an acute infection, which can result in decreased fertility because the bull will be running a fever.

“Secondly, we want to make sure he doesn’t develop a prolonged testicular infection because there is still concern about long-term transmission of BVD virus from semen and the long-term impact on the bull’s fertility,” he says.

Givens’ recent research indicates the risk of BVD virus in semen transmitting the disease to other animals in a natural breeding/pasture scenario is low, however, due to the small number of animals in the trials, it was not clearly demonstrated that there is no risk.

A group of five seronegative heifers and six steers was exposed to a bull with a prolonged testicular infection (type 1a). All heifers were confirmed to be in calf within 21 days. One aborted on day 35, subsequently rebred and carried the calf to term. BVD virus was not found in blood samples taken from any of the calves, nor did any of the blood samples from the heifers and steers test positive. All remained seronegative for antibody titres, indicating that the virus had not been transmitted.

Hundreds of natural breedings would be necessary to accurately evaluate the potential for BVD virus to be spread via semen from bulls with prolonged infections. “There are questions that haven’t been answered, but we do know that we can prevent BVD by vaccinating the bulls,” he says.

While use of a noncytopathic product has been shown to induce prolonged testicular infections, Givens, in a 2007 trial, concluded that use of a cytopathic product will not result in prolonged infections.

Noncytopathic means that the virus doesn’t kill the cell it infects. All persistently-infected animals are infected with a noncytopathic strain of the virus that spreads throughout the body. A modified-live noncytopathic vaccine can have the same effect, he explains. Research clearly indicates that if you vaccinate a pregnant cow with a modified-live noncytopathic vaccine, there is a real chance her calf could be PI and if you vaccinate a bull with a modified-live noncytopathic vaccine, there is a real chance the bull will get a prolonged infection.

The latest information about BVD, including Givens’ presentation at the 2009 BVD Symposium, “Simple Targetted BVD Control” can be found at www.bvdinfo.org.This website was created by the National Cattlemen’s Beef Association and the Academy of Veterinary Consultants as a clearing house for BVD articles, presentations, research papers and news.