Bovine viral diarrhea virus: An updated American College of Veterinary Internal Medicine consensus statement with focus on virus biology, hosts, immunosuppression, and vaccination

Control of bovine viral diarrhea virus (BVDV) in cattle populations across most of the world has remained elusive in spite of advances in knowledge about this viral pathogen. A central feature of virus perseverance in cattle herds is the unique mechanism of persistent infection. Managing BVDV infection in herds involves controlling persistently infected carrier animals using a multidimensional approach of vaccination, biosecurity, and identification of BVDV reservoirs. A decade has passed since the original American College of Veterinary Internal Medicine consensus statement on BVDV. While much has remained the same with respect to clinical signs of disease, pathogenesis of infection including persistent infection, and diagnosis, scientific articles published since 2010 have led to a greater understanding of difficulties associated with control of BVDV. This consensus statement update on BVDV presents greater focus on topics currently relevant to the biology and control of this viral pathogen of cattle, including changes in virus subpopulations, infection in heterologous hosts, immunosuppression, and vaccination.

Attenuated lymphocyte activation leads to the development of immunotolerance in bovine fetuses persistently infected with bovine viral diarrhea virus†

Bovine viral diarrhea virus continues to cost the cattle industry millions of dollars each year despite control measures. The primary reservoirs for bovine viral diarrhea virus are persistently infected animals, which are infected in utero and shed the virus throughout their lifetime. The difficulty in controlling the virus stems from a limited understanding of transplacental transmission and fetal development of immunotolerance. In this study, pregnant bovine viral diarrhea virus naïve heifers were inoculated with bovine viral diarrhea virus on day 75 of gestation and fetal spleens were collected on gestational days 82, 97, 190, and 245. Microarray analysis on splenic RNA from days 82 and 97 revealed an increase in signaling for the innate immune system and antigen presentation to T cells in day 97 persistently infected fetuses compared to controls. Reverse transcription quantitative polymerase chain reaction on select targets validated the microarray revealing a downregulation of type I interferons and lymphocyte markers in day 190 persistently infected fetuses compared to controls. Protein was visualized using western blot and tissue sections were analyzed with hematoxylin and eosin staining and immunohistochemistry. Data collected indicate that fetal immunotolerance to bovine viral diarrhea virus developed between days 97 and 190, with mass attenuation of the immune system on day 190 of gestation. Furthermore, lymphocyte transcripts were initially unchanged then downregulated, suggesting that immunotolerance to the virus stems from a blockage in lymphocyte activation and hence an inability to clear the virus. The identification of lymphocyte derived immunotolerance will aid in the development of preventative and viral control measures to implement before or during pregnancy.

Comparison of reproductive protection against bovine viral diarrhea virus provided by multivalent viral vaccines containing inactivated fractions of bovine viral diarrhea virus 1 and 2

Bovine viral diarrhea virus (BVDV) is an important viral cause of reproductive disease, immune suppression and clinical disease in cattle. The objective of this study was to compare reproductive protection in cattle against the impacts of bovine viral diarrhea virus (BVDV) provided by three different multivalent vaccines containing inactivated BVDV. BVDV negative beef heifers and cows (n = 122) were randomly assigned to one of four groups. Groups A-C (n = 34/group) received two pre-breeding doses of one of three commercially available multivalent vaccines containing inactivated fractions of BVDV 1 and BVDV 2, and Group D (n = 20) served as negative control and received two doses of saline prior to breeding. Animals were bred, and following pregnancy diagnosis, 110 cattle [Group A (n = 31); Group B (n = 32); Group C (n = 31); Group D (n = 16)] were subjected to a 28-day exposure to cattle persistently infected (PI) with BVDV (1a, 1b and 2a). Of the 110 pregnancies, 6 pregnancies resulted in fetal resorption with no material for testing. From the resultant 104 pregnancies, BVDV transplacental infections were demonstrated in 73 pregnancies. The BVDV fetal infection rate (FI) was calculated at 13/30 (43%) for Group A cows, 27/29 (93%) for Group B cows, 18/30 (60%) for Group C cows, and 15/15 (100%) for Group D cows. Statistical differences were observed between groups with respect to post-vaccination antibody titers, presence and duration of viremia in pregnant cattle, and fetal infection rates in offspring from BVDV-exposed cows. Group A vaccination resulted in significant protection against BVDV infection as compared to all other groups based upon outcome measurements, while Group B vaccination did not differ in protection against BVDV infection from control Group D. Ability of inactivated BVDV vaccines to provide protection against BVDV fetal infection varies significantly among commercially available products; however, in this challenge model, the inactivated vaccines provided unacceptable levels of BVDV FI protection.

Virus detection by PCR following vaccination of naive calves with intranasal or injectable multivalent modified-live viral vaccines

We evaluated duration of PCR-positive results following administration of modified-live viral (MLV) vaccines to beef calves. Twenty beef calves were randomly assigned to either group 1 and vaccinated intranasally with a MLV vaccine containing bovine alphaherpesvirus 1 (BoHV-1), bovine respiratory syncytial virus (BRSV), and bovine parainfluenza virus 3 (BPIV-3), or to group 2 and vaccinated subcutaneously with a MLV vaccine containing bovine viral diarrhea virus 1 and 2 (BVDV-1, -2), BoHV-1, BRSV, and BPIV-3. Deep nasopharyngeal swabs (NPS) and transtracheal washes (TTW) were collected from all calves, and whole blood was collected from group 2 calves and tested by PCR. In group 1, the proportions of calves that tested PCR-positive to BVDV, BoHV-1, BRSV, and BPIV-3 on any sample at any time were 0%, 100%, 100%, and 10%, respectively. In group 1 calves, 100% of calves became PCR-positive for BoHV-1 by day 3 post-vaccination and 100% of calves became PCR-positive for BRSV by day 7 post-vaccination. In group 2, the proportions of calves that tested positive to BVDV, BoHV-1, BRSV, and BPIV-3 on any sample at any time were 50%, 40%, 10%, and 0%, respectively. All threshold cycle (Ct) values were >30 in group 2 calves, irrespective of virus; however, Ct values <25 were observed in group 1 calves from PCR-positive results for BoHV-1 and BRSV. All calves were PCR-negative for all viruses after day 28. Following intranasal MLV viral vaccination, PCR results and Ct values for BRSV and BoHV-1 suggest that attempts to differentiate vaccine virus from natural infection is unreliable.

Vaccination of cattle against bovine viral diarrhea virus

Bovine viral diarrhea virus (BVDV) is responsible for significant losses to the cattle industry. Currently, modified-live viral (MLV) and inactivated viral vaccines are available against BVDV, often in combination with other viral and bacterial antigens. Inactivated and MLV vaccines provide cattle producers and veterinarians safe and efficacious options for herd immunization to limit disease associated with BVDV infection. Vaccination of young cattle against BVDV is motivated by prevention of clinical disease and limiting viral spread to susceptible animals. For reproductive-age cattle, vaccination to prevent viremia and birth of persistently infected offspring is considered more important, while also more difficult to achieve than prevention of clinical disease. Recent advances have been made in the understanding of BVDV vaccine efficacy. In terms of preventing clinical disease, current BVDV vaccines have been demonstrated to have a rapid onset of immunity and MLV vaccines can be effectively utilized in calves possessing maternal immunity. For reproductive protection, more recent studies using multivalent MLV vaccines have demonstrated consistent fetal protection rates in the range of 85-100% in experimental studies. Proper timing and administration of BVDV vaccines can be utilized to maximize vaccine efficacy to provide an important contribution to reducing risks associated with BVDV infection. With improvements in vaccine formulations and increased understanding of the protective immune response following vaccination, control of BVDV through vaccination can be enhanced.

Evaluation of reproductive protection against bovine viral diarrhea virus and bovine herpesvirus-1 afforded by annual revaccination with modified-live viral or combination modified-live/killed viral vaccines after primary vaccination with modified-live viral vaccine

The objective of this study was to compare reproductive protection in cattle against bovine viral diarrhea virus (BVDV) and bovine herpesvirus 1 (BoHV-1) provided by annual revaccination with multivalent modified-live viral (MLV) vaccine or multivalent combination viral (CV) vaccine containing temperature-sensitive modified-live BoHV-1 and killed BVDV when MLV vaccines were given pre-breeding to nulliparous heifers. Seventy-five beef heifers were allocated into treatment groups A (n=30; two MLV doses pre-breeding, annual revaccination with MLV vaccine), B (n=30; two MLV doses pre-breeding, annual revaccination with CV vaccine) and C (n=15; saline in lieu of vaccine). Heifers were administered treatments on days 0 (weaning), 183 (pre-breeding), 366 (first gestation), and 738 (second gestation). After first calving, primiparous cows were bred, with pregnancy assessment on day 715. At that time, 24 group A heifers (23 pregnancies), 23 group B heifers (22 pregnancies), and 15 group C heifers (15 pregnancies) were commingled with six persistently infected (PI) cattle for 16days. Ninety-nine days after PI removal, cows were intravenously inoculated with BoHV-1. All fetuses and live offspring were assessed for BVDV and BoHV-1. Abortions occurred in 3/23 group A cows, 1/22 group B cows, and 11/15 group C cows. Fetal infection with BVDV or BoHV-1 occurred in 4/23 group A offspring, 0/22 group B offspring, and 15/15 group C offspring. This research demonstrates efficacy of administering two pre-breeding doses of MLV vaccine with annual revaccination using CV vaccine to prevent fetal loss due to exposure to BVDV and BoHV-1.

Prevention of abortion in cattle following vaccination against bovine herpesvirus 1: A meta-analysis

Bovine herpesvirus 1 is ubiquitous in cattle populations and is the cause of several clinical syndromes including respiratory disease, genital disease, and late-term abortions. Control of the virus in many parts of the world is achieved primarily through vaccination with either inactivated or modified-live viral vaccines. The purpose of this meta-analysis was to determine the cumulative efficacy of BoHV-1 vaccination to prevent abortion in pregnant cattle. Germane articles for inclusion in the analysis were identified through four online scientific databases and the examination of three review and ten primary study article reference lists. A total of 15 studies in 10 manuscripts involving over 7500 animals were included in the meta-analysis. Risk ratio effect sizes were used in random effects, weighted meta-analyses to assess the impact of vaccination. Subgroup analyses were performed based on type of vaccine, MLV or inactivated, and the type of disease challenge, experimentally induced compared to field studies. A 60% decrease in abortion risk in vaccinated cattle was demonstrated. The greatest decrease in abortion risk was seen in studies with intentional viral challenge although vaccination also decreased abortion risk in field studies. Both inactivated and modified-live viral vaccines decreased abortion risk. This meta-analysis provides quantitative support for the benefit of bovine herpesvirus 1 vaccination in the prevention of abortion.

Diagnosis and Control of Viral Diseases of Reproductive Importance: Infectious Bovine Rhinotracheitis and Bovine Viral Diarrhea

Both bovine viral diarrhea virus and bovine herpesvirus 1 can have significant negative reproductive impacts on cattle health. Vaccination is the primary control method for the viral pathogens in US cattle herds. Polyvalent, modified-live vaccines are recommended to provide optimal protection against various viral field strains. Of particular importance to bovine viral diarrhea control is the limitation of contact of pregnant cattle with potential viral reservoirs during the critical first 125 days of gestation.

BVDV vaccination in North America: risks versus benefits

The control and prevention of bovine viral diarrhea virus (BVDV) infections has provided substantial challenges. Viral genetic variation, persistent infections, and viral tropism for immune cells have complicated disease control strategies. Vaccination has, however, provided an effective tool to prevent acute systemic infections and increase reproductive efficiency through fetal protection. There has been substantial controversy about the safety and efficacy of BVDV vaccines, especially when comparing killed versus modified-live viral (MLV) vaccines. Furthermore, numerous vaccination protocols have been proposed to protect the fetus and ensure maternal antibody transfer to the calf. These issues have been further complicated by reports of immune suppression during natural infections and following vaccination. While killed BVDV vaccines provide the greatest safety, their limited immunogenicity makes multiple vaccinations necessary. In contrast, MLV BVDV vaccines induce a broader range of immune responses with a longer duration of immunity, but require strategic vaccination to minimize potential risks. Vaccination strategies for breeding females and young calves, in the face of maternal antibody, are discussed. With intranasal vaccination of young calves it is possible to avoid maternal antibody interference and induce immune memory that persists for 6-8 months. Thus, with an integrated vaccination protocol for both breeding cows and calves it is possible to maximize disease protection while minimizing vaccine risks.

Evaluation of BHV-1 antibody titer in a cattle herd against different BHV-1 strains

Although modified-live multivalent vaccines, such as PregGuard GOLD and Bovi-Shield Gold, have been used routinely in both beef and dairy cattle in the US, abortion and respiratory diseases still occasionally occur following vaccination. To determine whether the antibody induced by the multivalent vaccine can recognize BHV-1 isolates from aborted animals, BHV-1 antibody titer was evaluated with two isolates from abortion cases and two vaccine BHV-1 viruses. Cattle serum was collected from a dairy herd that was vaccinated annually with Bovi-Shield Gold 5 vaccine. Among the 28 cattle tested, no statistical significant difference in serum neutralization titer was observed when test virus was either vaccine virus or clinical isolates. It suggests that the BHV-1 antibody from the vaccinated cattle can recognize both the vaccine virus and clinical isolates. However, it is noticed that cows at 5 years old or older had a significantly lower BHV-1 antibody titer on average than the average of SN titer in 3 year-old cows. Similarly, cows at 5 years or older had a significantly lower BVDV antibody titer than cows at about 2 years of age. In addition, cattle vaccinated within 0-2 months had a significantly higher BHV-1 titer than those that received vaccination 6 months or greater prior to titer measurement. In contrast, cattle that received a vaccination 6 months prior had a significantly higher anti-BVDV antibody titer than those vaccinated within 1-2 months. The BVDV antibody titers remained relatively unchanged between 6 months and 1 year post-vaccination. Our study suggests little antigenic variation exists between BHV-1 disease isolates and BHV-1 of the multivalent vaccines. In addition, BHV-1 antibody titer is relatively lower at 6 months post vaccination in those tested animals. However, the BVDV antibody titer remained relatively high after 6 months from time of vaccination.

Impact of species and subgenotypes of bovine viral diarrhea virus on control by vaccination

Bovine viral diarrhea viruses (BVDV) are diverse genetically and antigenically. This diversity impacts both diagnostic testing and vaccination. In North America, there are two BVDV species, 1 and 2 with 3 subgentoypes, BVDV1a, BVDV1b and BVDV2a. Initially, US vaccines contained BVDV1a cytopathic strains. With the reporting of BVDV2 severe disease in Canada and the USA there was focus on protection by BVDV1a vaccines on BVDV2 disease. There was also emphasis of controlling persistently infected (PI) cattle resulted in studies for fetal protection afforded by BVDV1a vaccines. Initially, studies indicated that some BVDV1a vaccines gave less than 100% protection against BVDV2 challenge for fetal infection. Eventually vaccines in North America added BVDV2a to modified live virus (MLV) and killed BVDV1a vaccines. Ideally, vaccines should stimulate complete immunity providing 100% protection against disease, viremias, shedding, and 100% fetal protection in vaccinates when challenged with a range of diverse antigenic viruses (subgenotypes). There should be a long duration of immunity stimulated by vaccines, especially for fetal protection. MLV vaccines should be safe when given according to the label and free of other pathogens. While vaccines have now included BVDV1a and BVDV2a, with the discovery of the predominate subgenotype of BVDV in the USA to be BVDV1b, approximately 75% or greater in prevalence, protection in acute challenge and fetal protection studies became more apparent for BVDV1b. Thus many published studies examined protection by BVDV1a and BVDV2a vaccines against BVDV1b in acute challenge and fetal protection studies. There are no current BVDV1b vaccines in the USA. There are now more regulations on BVDV reproductive effects by the USDA Center for Veterinary Biologics (CVB) regarding label claims for protection against abortion, PI calves, and fetal infections, including expectations for studies regarding those claims. Also, the USDA CVB has a memorandum providing the guidance for exemption of the warning label statement against the use of the MLV BVDV in pregnant cows and calves nursing pregnant cows. In reviews of published studies in the USA, the results of acute challenge and fetal protection studies are described, including subgenotypes in vaccines and challenge strains and the results in vaccinates and the vaccinates' fetuses/newborns. In general, vaccines provide protection against heterologous strains, ranging from 100% to partial but statistically significant protection. In recent studies, the duration of immunity afforded by vaccines was investigated and reported. Issues of contamination remain, especially since fetal bovine serums may be contaminated with noncytopathic BVDV. In addition, the potential for immunosuppression by MLV vaccines exists, and new vaccines will be assessed in the future to prove those MLV components are not immunosuppressive by experimental studies. As new subgenotypes are found, the efficacy of the current vaccines should be evaluated for these new strains.

Cow attributes, herd management, and reproductive history events associated with abortion in cow-calf herds from Western Canada

The primary objective of this study was to identify herd management and cow characteristics that are associated with abortion in cow-calf herds in Western Canada. Reproductive events were closely monitored in 29,713 cows in 203 herds from the beginning of the breeding season in 2001 through the calving season in 2002. Herd management and cow-level risk factors such as age, body condition score, and previous reproductive history were measured through a series of herd visits by project personnel and detailed individual animal records maintained by the herd owner. Pregnancy status was assessed in fall of 2001 by the herd veterinarian. Cows most likely to abort were replacement heifers, cows that were more than 10 years of age, cows with a body condition score of less than or equal to or 5 of 9 at pregnancy testing, or with twin pregnancies. Cows vaccinated for bovine viral diarrhea virus and infectious bovine rhinotracheitis and bred on community pastures were less likely to abort than cows from community pastures that were not vaccinated. Cows bred on community pastures that were not vaccinated were also more likely to abort than cows that were not on community pastures regardless of vaccination status. Adverse calving-associated events such as severe dystocia, problems such as uterine prolapse or retained placentas, abortion or calf death within 1 hour of birth were also associated with an increased risk of abortion the subsequent calving season after accounting for all other factors.