Bovine viral diarrhea virus (BVDV) was discovered over 60 years ago, yet researchers are still unlocking its secrets. One of the newest pieces of information about BVDV to come to light is the significance of prolonged testicular infections in bulls.

Previously, BVDV had been found in prolonged testicular infections following acute infection of immunocompetent bulls, says Dan Givens, DVM, PhD, Auburn University. “The primary purpose of a 2008 research study was to evaluate the production and maintenance of prolonged testicular infections after exposure to BVDV of seronegative bulls in varying circumstances.” The secondary objective of this study looking at prolonged BVDV infection was to assess the potential for transmission of BVDV via semen of bulls exhibiting a prolonged testicular infection. In total, 10 research trials were conducted as part of the 2008 study. 

BVDV trials

Givens explains that the first trial examined the duration of detectable virus in semen after intranasal inoculation of peripubertal bulls. The second to fifth trials examined the potential for prolonged testicular infections resulting from natural exposure of seronegative bulls to persistently-infected (PI) heifers.

In the last five trials, the potential for viral transmission from bulls exhibiting prolonged testicular infections to a small number of exposed animals (n=28) was evaluated. Givens’ research demonstrated that prolonged testicular infections can result in detection of viral RNA in semen for 2.75 years with infectious virus grown from testicular tissue 12.5 months after viral exposure. A type 1b strain of BVDV caused prolonged testicular infection after natural exposure of seronegative bulls to a PI heifer.

However, transmission of BVDV to susceptible animals was not detected in the final five trials of this research. “BVDV can persist in testicular tissue after acute infection for several years, but the potential for viral transmission from these prolonged testicular infections appears to be low,” says Givens.

It doesn’t appear that BVDV genotype matters in testicular infections, as it has been demonstrated that BVDV 1a, 1b, and 2 are capable of resulting in a prolonged testicular infection, explains Paul Walz, DVM, PhD, Dipl. ACVIM, Auburn University. “With respect to biotype, it only appears that non-cytopathic (NCP) strains cause this, but again NCP strains are most common in nature, so that probably doesn’t matter as much,” he says. ”It does appear that vaccination with cytopathic-containing modified-live vaccines does not cause prolonged testicular infection.”

What it means to producers

Givens believes that the most significant finding of this research is that the infections last so long. “Since they last so long, even if the transmission rate is low, it could later result in transmission and subsequent disease. The good news is that the rate of viral transmission appears to be low.” Givens adds that some bulls clear the virus while others maintain the virus in testicular tissue for at least 2.75 years without clearing it.

Aside from potential transmission to heifers or cows, Givens says testicular infection in bulls could affect fertility or the duration of fertility, but these effects have yet to be determined.

From a producer standpoint, the fact that transmission was so low or didn’t occur is encouraging, Walz says. “I believe that because transmission was low from bulls with prolonged testicular infections, producers still need to concentrate BVDV control efforts at identifying and eliminating the PI animals, enhancing immunity through vaccination and biosecurity.”

However, says Walz, even though Givens was unable to show transmission, the fact remains that he could identify viral RNA and culture the virus. “If I am trying to push for BVDV eradication, which is a step far beyond control, then this potential reservoir needs to be addressed before significant effort and money is put toward U.S. eradication,” Walz notes. He adds that we can look at our European colleagues when dealing with some of these potential reservoir issues. “The Scandinavian countries have been very successful in their eradication program without evaluating prolonged testicular infections and wildlife reservoirs. We can all agree that Norwegian cattle systems are not similar to U.S. systems, but they do provide an example of what can be done.”

In bulls with testicular infections, it is only present in the testicular tissue, rather than disseminated throughout the body as occurs with persistent infection. As a result, BVDV is only identified by tests on testicular tissue, and is not picked up by tests on other samples. “But until viral transmission is detected from this nidus of infection in the testicular tissue, I think that we should accept the current tests as an excellent indicator of the presence or absence of an epidemiologic significant infection,” Givens adds.

Three types of infection

Walz says there are three types of BVDV infections that have consequences to the testes. The first is persistent infection of the bulls. “These animals are PI because of immunotolerance, and they shed virus in semen as well as nasal secretions, feces, urine, etc.,” Walz explains. These animals are typically easy to identify as they are positive by skin biopsy immunohistochemistry (IHC), serum and white blood cell virus isolation.

The second type of BVDV infection in bulls is “persistent testicular infection”. This was described in a single bull that had infectious virus in his semen, but was negative for virus in blood. “To date, this is the only bull that has been identified,” Walz says. “This type of bull is a worst case scenario to a breeding herd, as this bull is capable of infecting heifers from his semen, yet he would be negative by conventional tests used to identify a PI animal such as blood tests or IHC.”

The third type of BVDV infection in bulls is “prolonged testicular infection”. These animals do not appear to be able to transmit BVDV to heifers, and they are even harder to identify since they would be negative by IHC (skin), blood tests, and even negative by standard virus isolation of semen. “The only real way to identify them is by polymerase-chain reaction (PCR) tests of semen, and that has to be done very carefully,” Walz cautions. It appears from Givens’ work that there is a “loss” of prolonged testicular infections over time. Even though Givens has demonstrated a bull positive for 2.75 years after intranasal inoculation, Walz adds, there is a gradual loss in bulls from day 90 after inoculation to 2.75 years. 

Vaccinating bulls

Givens says if bulls are exposed to a cytopathic strain initially (usually as a modified-live vaccine) they don’t develop a testicular infection and are protected as they get older. “Bulls should be vaccinated appropriately at least 30 days prior to breeding,” Givens states. “That gives a sense of security. The potential for that sense of security to be false appears to be very low.”

Walz adds that vaccination is very important in a herd, and all bulls entering a breeding herd should be test-negative for BVDV and vaccinated. “I would make sure the bulls were vaccinated with a killed product or were vaccinated with an MLV cytopathic-containing vaccine.” 


Testing semen for BVDV

Dan Givens, DVM, PhD, says for instances where concern about testicular infection is significant, semen can be tested for bovine viral diarrhea virus (BVDV) within 30 days prior to breeding by validated virus isolation techniques or by validated polymerase-chain reaction tests. While localized testicular infections are of some concern, persistent and acute infection of bulls are the most common causes of contaminated semen.

Due to limited epidemiologic significance, routine screening of semen from bulls that test negative on standard assays is not currently recommended, Givens says. If semen is not cryopreserved using Certified Semen Services guidelines, then a straw should be assayed using a validated PCR test to ensure the absence of BVDV prior to use on a farm with an existing biosecurity program.

“I would strongly discourage the use of positive semen even in vaccinated heifers or cows due to the potential for viral transmission to the fetus and production of persistently-infected (PI) animals,” Givens cautions.

Paul Walz, DVM, PhD, Dipl. ACVIM, agrees and says semen with a repeatable positive test for BVDV is suspicious. “I don’t think the risk of introducing BVDV into a naive population is worth the genetics of the individual bull,” he says. “You can certainly find semen that would be negative.”

Walz believes it comes down to an exercise in risk assessment and what level of risk someone is willing to deal with when it comes to BVDV in the testicles. “If I am in the business of selling semen, I would make sure the semen is tested with a validated PCR test before the sale,” he suggests. ”Because of wide dissemination of valuable semen and the potential for litigation, I would make sure my product is ‘tested’ free of BVDV. If I have a herd bull that is not PI who is only covering the 20 cows in my herd, then I probably wouldn’t test his semen because transmission is low or perhaps doesn’t even occur.”