The pathogenesis of bovine virus diarrhoea virus infections

Distribution of viral antigen and development of lesions after experimental infection of calves with a BVDV 2 strain of low virulence

To examine the virus-host interaction in subclinical bovine viral diarrhea virus (BVDV) infections, the spread of a BVDV 2 strain of low virulence to different organs and the development of lesions were investigated. Eight colostrum-deprived, clinically healthy, 2-3-month-old calves were intranasally inoculated with 10(6) tissue culture infective dose of the naturally occurring BVDV 2 strain 28508-5 of low virulence, and 2 served as controls. Two calves each were euthanized at days 3, 6, 9, and 13 postinoculation (pi). Representative tissues were processed for histology and immunohistology. Signs of overt clinical disease were absent. However, a mild temperature elevation at days 7 or 8 pi and a moderate decrease of circulating lymphocytes occurred in all inoculated calves. The BVDV antigen was detected at day 3 pi in several lymphoid tissues. At day 6 pi, BVDV antigen was found widespread in lymphoid tissues and multifocally in intestinal epithelial cells but was associated with no or subtle lesions only. At day 9 pi, much less BVDV antigen was detectable, but there was severe depletion of lymphoid tissues. At day 13 pi, BVDV antigen had been cleared from most lymphoid tissues that were at variable phases of depletion and recovery. In conclusion, the BVDV strain of low virulence spread to lymphoid tissues and intestinal epithelial cells but was rapidly eliminated. Transient depletion of lymphoid tissues was followed by recovery.

Diagnostic dilemma encountered when detecting bovine viral diarrhea virus in IVF embryo production

Routine quality controls in production of bovine embryos by in vitro fertilization (IVF) should include screening all materials of animal origin for the presence of bovine viral diarrhea virus (BVDV). Using a reverse transcription nested polymerase chain reaction (RT-nPCR) assay, we detected BVDV in primary cultures of uterine tubal cells (UTC) that had been used during IVF procedures. The goal of our ensuing investigation was to determine its source and assess risks associated with the identified contaminant. Sequencing of the amplified 5' nontranslated region (NTR) of the viral genome confirmed a Genotype I BVDV contaminant. This viral contaminant was also identified by RT-nPCR in multiple samples of the same lot of fetal bovine serum (FBS) that was used in transport media by the laboratory that harvested the UTC. Both routine and enhanced roller bottle methods for virus isolation failed to detect BVDV in the FBS. Furthermore, virus neutralization assays did identify antibodies to Genotype I strains of BVDV in the FBS. After 7 days of co-incubation, neither cultured, washed UTC nor exposed, washed embryos were RT-nPCR positive for BVDV. Eight embryos produced in the contaminated system were nonsurgically transferred into eight seronegative cows. None of the embryo recipients seroconverted to BVDV. Thus, contamination of cell culture medium with BVDV did not result in transmission of the virus when IVF embryos were transferred. Failure to transmit disease was likely aided by serendipitous control from anti-BVDV antibodies in the FBS. However, a diagnostic dilemma was created when the RT-nPCR assays used to screen for BVDV were positive, yet attempts to isolate the virus were negative. This case study illustrates that if molecular assays are to be used to confirm the pathogen-free status of IVF embryo production systems, media components of animal origin (e.g. FBS) should be screened with molecular assays for BVDV as well as traditional virus isolation techniques.

Expression of the mouse Fc receptor B2 in bovine cells rescues the infectivity of conditionally neutralized bovine viral diarrhea virus

We examined the infectivity of bovine viral diarrhea virus (BVDV) particles opsonized with monoclonal antibodies on bovine cells expressing the murine Fcgamma receptor B2 (FcgammaRB2). Incubation of BVDV with each of five monoclonal antibodies (Mabs) to the envelope glycoprotein E2 led to efficient virus-neutralization, as evidenced by the failure to infect standard bovine testicle cells. In contrast, inoculation of four of these Mab-virus complexes onto transfectant bovine testicle cells expressing FcgammaRB2 resulted in a significant rescue of virus infectivity. Mab-virus complexes were 13.1, 7.37, 5.56 and 4.49 times more infectious for FcgammaR-expressing cells than for cells lacking FcgammaR. Because Mab-opsonized BVDV virion complexes uninfectious for standard cells may initiate productive infection in cells expressing the FcgammaR, the virion-Mab interaction should be described as a conditional neutralization. Interestingly, the infectivity of BVDV complexed with a specific virus neutralizing Mab (10f9) could not be rescued in FcgammaRB2-expressing cells. We postulate that attachment of antibody-virus complexes to FcR may only result in productive infection if the binding of antibody to virions does not interfere with post-attachment entry functions. Conditionally neutralized virions may play a role in the pathogenesis of any of the multiple diseases resulting from BVDV infections in cattle.

Phylogenetic analysis of the 5'-untranslated region of german BVDV type II isolates

On the basis of genetic differences, bovine viral diarrhoea viruses (BVDV) are subclassified into two distinct genotypes, BVDV type I and BVDV type II. We selected German BVDV type II isolates using the BVDV type I-specific monoclonal antibody WB160 and flow cytometric analysis for further characterization. For molecular characterization, a 288-bp fragment of the 5'-untranslated region (5'-UTR) of the selected isolates was investigated by reverse transcription-polymerase chain reaction (RT-PCR) and sequencing. Sequence comparisons of the partial 5'-UTR sequences and their phylogenetic analyses demonstrated that the 18 German BVDV type II isolates all belong to either subtype IIa (10 isolates) or subtype IIc (eight isolates). Nevertheless, the German BVDV type II isolates were genetically different (89.9-94.3% sequence identity) from the standard BVDV type II strain 890 from North America, which was recently classified as BVDV type IIa. Furthermore, a clear subdivision of the German BVDV type II isolates into two distinct subtypes (BVDV IIa Germany and BVDV IIc Germany) is shown. Viruses of both subgroups differed in the analysed 5'-UTR fragment from each other (91.6-95.2% sequence identity), but were highly conserved within the same German subtype (97.2-100% sequence identity). These findings are discussed in the context of BVDV type II origin, possible introduction into Germany, its epidemiology and impact for vaccine development.

Outbreak of foetal infection with bovine pestivirus in a central Queensland beef herd

OBJECTIVE

To describe a significant outbreak of foetal infection and subsequent losses due to bovine pestivirus on a 5200 ha beef breeding and fattening property in central Queensland. DESCRIPTION OF THE HERD: The affected herd consisted of 656 cows, including 269 recently purchased cows, and 221 heifers that were joined in December/January 1995/96. There were approximately 2500 cattle on the property.

INVESTIGATION

Following the purchase of 269 cows in October 1995, which were mingled with the existing cow herd, losses were experienced due to foetal infection with bovine pestivirus. These losses were recorded between 1996 and 1999 as: reduced pregnancy rates, losses between pregnancy testing (midpregnancy) and branding (calves averaged 3 months-of-age), losses due to pneumonia and ill-thrift between branding and approximately 12 months-of-age, and losses due to ill-thrift and the chronic wasting form of mucosal disease thereafter. All surviving calves were tested for bovine pestivirus in 1997 at an average of 10 months. Fifty-three calves were identified as persistently infected with bovine pestivirus. A further 110 calf losses could reasonably be attributed to bovine pestivirus infection. Persistently infected cattle were always unthrifty compared to their virus negative counterparts. Only one persistently infected calf was identified, on the basis of severe ill thrift, in the 1997 birth cohort and none in 1998.

CONCLUSIONS

This outbreak of foetal infection with bovine pestivirus resulted in significant production losses. These losses were recorded over the three years subsequent to the outbreak. Significant numbers of persistently infected calves were not evident among calves born in the two years after this outbreak.

Validation of a reverse transcription nested polymerase chain reaction (RT-nPCR) to detect bovine viral diarrhea virus (BVDV) associated with in vitro-derived bovine embryos and co-cultured cells

Sensitive RT-nPCR assays can be used for the rapid detection of viruses. The objective of this research was to validate an RT-nPCR assay for detection of BVDV associated with various samples collected from an IVF system. In 12 research replicates, we maintained matured COCs as negative controls or exposed them to 1 of 4 noncytopathic strains (SD-1, NY-1, CD-87, or PA-131) of BVDV for 1 h immediately before IVF. After 4 d of IVC, we harvested groups of 5 nonfertile ova or degenerated embryos (NFD) and some associated cumulus cells and transferred developing embryos and the remaining cumulus cells into secondary IVC drops. On the seventh d of IVC, cumulus cells, groups of 5 washed NFD and groups of 5 developed, washed embryos were harvested. We also collected single developed embryos after washing, washing with trypsin, washing and cryopreservation in ethylene glycol, or washing with trypsin and cryopreservation in ethylene glycol. All washes were performed according to International Embryo Transfer Society standards. Developed embryos and NFD were sonicated prior to assay. All samples were assayed for BVDV using virus isolation and RT-nPCR. The virus isolation and RT-nPCR assays determined that all negative control samples were BVDV-free. Virus was detected in association with all exposed cumulus cells and groups of developed embryos using both virus isolation and RT-nPCR. Results from viral assays of other exposed samples indicate enhanced sensitivity of the RT-nPCR assay. The RT-nPCR assay used in this research exhibited acceptable sensitivity, specificity, predictive value and repeatability for rapid detection of BVDV associated with the various samples obtained from an IVF system.

Virus neutralizing antibodies against a panel of 18 BVDV isolates in calves vaccinated with Rispoval RS-BVD

Seven of nine colostrum-deprived calves, free from infection with bovine virus diarrhoea virus (BVDV), were vaccinated with Rispoval RS-BVD on two occasions, 21 days apart, while the other two were kept as BVDV infection controls. The virus neutralizing (VN) serum antibodies induced by vaccination were tested for their ability to neutralize 18 European BVDV isolates, including laboratory reference strains and recent field isolates, both cytopathic and non-cytopathic biotypes as well as genotypes I and II. The strains were isolated in Belgium, France, Germany and the United Kingdom. While there were large variations in the vaccine-induced VN titres of the individual calves against all the strains, e.g. the titres against Osloss NCP, the European reference strain ranged from 1.7 to 6.7 (1:log2), serum from each animal was capable of neutralizing between nine and all 18 of the strains tested. Nevertheless, from the results of this study, it can be concluded that in colostrum-deprived BVDV seronegative calves, Rispoval RS-BVD can stimulate the production of VN antibodies capable of neutralizing a wide range of antigenically diverse European isolates of BVDV, including genotypes I and II.

Identificação do vírus da Diarréia Viral Bovina tipo 2 (BVDV-2) no sul do Brasil

Amostras do vírus da Diarréia Viral Bovina (BVDV), denominadas de BVDV tipo 2 (BVDV-2), foram inicialmente identificadas em surtos de BVD aguda e enfermidade hemorrágica e têm sido isoladas predominantemente na América do Norte. O presente artigo descreve dois casos de enfermidade gastroentérica/respiratória seguidos de isolamento e identificação de amostras de BVDV tipo 2 no sul do Brasil. Os vírus foram isolados de duas novilhas de diferentes rebanhos. Um dos animais apresentou enfermidade aguda, cursando com anorexia, atonia ruminal, diarréia escura ou muco-sanguinolenta, tenesmo e descarga nasal muco-purulenta. O outro animal desenvolveu enfermidade de longa duração (7 meses), caracterizada por crescimento retardado, anorexia, quadros recorrentes de diarréia, dermatite interdigital, hemorragias digestivas e genitais ocasionais, conjuntivite, artrite e pneumonia crônica. Congestão disseminada das mucosas, ulcerações extensivas e profundas na língua, palato e esôfago, áreas necróticas na mucosa do rúmen, áreas de congestão e ulcerações cobertas com fibrina no intestino delgado foram os achados mais proeminentes. Antígenos do BVDV foram demonstrados por imunohistoquí-mica no epitélio da língua, nos pulmões e em linfonodos mesentéricos. Amostras não-citopáticas do BVDV foram isoladas em cultivo celular a partir de leucócitos e do baço dos animais afetados e identificadas por imunofluorescência. Caracterização antigênica e análise filogenética desses isolados, e de outras duas amostras de BVDV isoladas de fetos coletados em matadouros, revelou tratar-se de BVDV tipo 2. A presença do BVDV tipo 2 na população bovina do Brasil possui um significado epidemiológico importante e pode ter conseqüências para o diagnóstico, estratégias de imunização e produção de vacinas.

The effect of different bovine viral diarrhea virus genotypes and biotypes on the metabolic activity and activation status of bovine peripheral blood mononuclear cells

The effects of cytopathic (cp) and non-cytopathic (ncp) bovine viral diarrhea virus (BVDV) on the cellular metabolic activity and activation status of bovine peripheral blood mononuclear cells (PBMC) were investigated. Cellular DNA and protein synthesis was determined by [3H]thymidine and [3H]valine incorporation, respectively, in phytohemagglutinin (PHA)-stimulated PBMC. All cp strains and most ncp BVDV strains significantly inhibited DNA synthesis in PHA-stimulated PBMC; however, only cp BVDV strains inhibited protein synthesis. A plaque assay and immunofluorescence test confirmed productive BVDV infection of PBMC. In addition, viral RNA synthesis was demonstrated in BVDV-infected PBMC by RT-PCR. The interleukin-2 receptor (IL-2R) was used as a marker for the activation status of BVDV-infected PBMC. The expression of IL-2R was preserved in virus-infected cells, even though DNA and protein synthesis was suppressed. These findings suggest a novel mechanism of virus-induced immune suppression in which BVDV inhibits basic metabolic activities of bovine PBMC. The activation signals, however, are maintained.

Prevalence of antibodies to IBR and BVD viruses in dairy cows with reproductive disorders

We determined the prevalence of antibodies to infectious bovine rhinotracheitis virus (IBRV) and bovine viral diarrhea virus (BVDV) in sera of dairy cows on 4 different farms in the Republic of Croatia. A high percentage (60.8%) of cows had various reproductive disorders. The results showed that seroprevalence of infectious bovine rhinotracheitis (IBR) was 85.8% and that of bovine viral diarrhea (BVD) was 79.2% in tested cows. Antibodies to both viruses were found in 80.8% of cows with reproductive disorders but in only 46.8% of cows without reproductive disorders. This difference was statistically significant (P<0.01), and indicated a connection between reproductive disorders and simultaneous infections with IBR and BVD viruses in dairy cows.

Level and duration of serum antibodies in cattle infected experimentally and naturally with bovine virus diarrhoea virus

Neutralising serum antibodies against bovine virus diarrhoea virus (BVDV) were monitored for three years in 35 cattle that were infected with the virus as calves; 24 of the calves were inoculated intramuscularly or intranasally, and 11 contracted the infection naturally. All the experimentally infected calves seroconverted within 14 to 28 days after inoculation, and all the animals still had high serum levels of antibodies to BVDV three years after infection. Determinations of antibody levels in milk and blood samples excluded the possibility that the calves had been reinfected with BVDV during the study.

Prevalence of bovine virus diarrhoea virus infection in Belgian white blue cattle in southern Belgium

A study was conducted in Southern Belgium to determine the prevalence of bovine viral diarrhoea virus (BVDV) infection in Belgian White Blue herds. Blood samples were taken from 9685 cattle, representing all the stock on 61 farms, by local veterinarians to screen for persistently infected animals and to determine their serological status against BVDV. Some of the herds (42.5%) were selected because of a prior positive diagnosis of BVDV or on the grounds of suspicion of BVD. A capture enzyme-linked immunosorbent assay (ELISA) was used to test for antigen. The prevalence of persistently infected animals was 0.75% overall and 1.46% in the 27 herds with at least one persistently infected animal. The prevalence of seropositive animals was determined with a competitive antibody ELISA and was found to be 65.5% for the animals as a whole but 53.8% for the herds without positively infected animals and 76.6% for the herds with at least one such animal. All the herds contained seropositive animals.

Distribution of bovine virus diarrhoea virus in tissues and white blood cells of cattle during acute infection

This study is performed to gain knowledge about the quantitative distribution of bovine virus diarrhoea virus (BVDV) in tissues and white blood cells (WBC) at different intervals after acute infection. Ten specific pathogen-free calves were intranasally inoculated with 10(5) 50% tissue culture infective dose of the non-cytopathic BVDV strain 4800. Twelve hours after inoculation tonsil biopsies were taken and WBC were collected daily for virus isolation and titration. Each day one calf was killed and virus isolations and titrations were performed from a range of tissues. The results indicate that BVDV first replicates in nasal mucosa and to high titers in the tonsil. The virus then appeared to spread to the regional lymph nodes and then disseminates throughout the body. The virus titers were highest in tonsil, thymus and ileum and were low in the WBC. Also after in vitro infection virus titers in WBC were very low, whereas, they were high in epithelial cells. Although the WBC might not be as important as other cells for replication of BVDV, they may play a role in the spread of the virus throughout the body.

Immunohistochemical evidence for the localization of bovine viral diarrhea virus, a single-stranded RNA virus, in ovarian oocytes in the cow

Bovine viral diarrhea virus (BVDV) is a single-stranded RNA virus responsible for enteric disease and reproductive failure in cattle. The virus can pass vertically from cow to fetus, causing abortion, birth of malformed calves, and calves born with persistent and life-long infections. In this study, we investigated the tropism of BVDV in ovarian tissue from persistently infected animals. Three heifers persistently infected with BVDV were euthanatized and their ovaries were recovered. A specimen of each ovary was taken (n = 6) for virus isolation, and the remaining ovarian tissue was stored at -70 C. Cryosections (6 microm) cut from each ovary were analyzed for the presence of BVDV antigens by indirect immunofluorescence. The immunofluorescent analysis employed two monoclonal antibodies, WB103 and WB162, previously raised against the nonstructural protein NS3 and the envelop glycoprotein E2, respectively. High titers (6.97 +/- 0.17 log10 tissue culture infective dose50/ml) of BVDV were recovered from 6/6 ovarian samples; NS3 and E2 were widely distributed within the ovarian stroma, the cumulus cell population, and the oocytes maturing in primordial, primary, and secondary follicles. Overall, 362/1,939 (18.7%) of the oocytes contained BVDV antigens, and there was no significant (P > 0.05) difference in the proportion of BVDV-infected oocytes recorded within the primordial (227/1,247, 18.2%), primary (122/630, 19.4%), and secondary (13/62, 21.0%) follicle populations. Although the developmental potential of the infected oocytes could not be established in the present study, we conclude that bovine oocyte and the cumulus cells are susceptible to BVDV infection.

Detection of bovine viral diarrhea virus in the ovaries of cattle acutely infected with bovine viral diarrhea virus

Bovine viral diarrhea virus (BVDV) is recognized worldwide as a major cause of economic loss in cattle. Infection with BVDV can result in several clinical outcomes. However, the reproductive consequences may be the most important. Infertility, early embryonic death, abortion, and congenital anomalies have all been reported following acute infection with BVDV. The cause of infertility following acute BVDV infection is not known. BVDV has been isolated from the bovine ovary and has been associated with chronic oophoritis. The purpose of this study was to identify the ovarian cell types infected with BVDV following acute infection. Twelve heifers were acutely infected with noncytopathic BVDV, and ovariectomies were performed between 4 and 60 days postinfection. BVDV was isolated on days 6 and 8 postinfection. Viral antigen was detected in macrophage-like cells and stromal cells in the ovarian cortex and oophoritis was evident from 6 to 60 days postinfection. These findings indicate that acute infection with BVDV may result in changes in ovarian function that could lead to reduced fertility.

Detection of cytopathic bovine viral diarrhea virus in the ovaries of cattle following immunization with a modified live bovine viral diarrhea virus vaccine

Economic loss from infection with bovine viral diarrhea virus (BVDV) is of worldwide concern. The unique pathogenesis and antigenic variability of BVDV have made this virus challenging to control. Vaccination programs are a major component of control and prevention strategies. Both killed and modified live vaccines are commercially available. Choice between killed and modified live vaccines is controversial. Of major concern is the safety of modified live vaccines. Little information is available on their tissue tropism and potential for causing pathology, especially with respect to the reproductive system. The objective of this study was to determine if BVDV could be detected in the ovary of cattle following immunization with a modified live BVDV vaccine. In 2 separate trials, 6 heifers and 4 mature cows were immunized with a modified live BVDV vaccine and ovaries were removed between 7 and 30 days postvaccination. Cytopathic BVDV was isolated from ovaries removed on days 8, 10, and 12. BVDV antigen was detected using immunohistochemistry on days 10-30. These findings are significant because replication of virus in the ovary could cause ovarian dysfunction, resulting in reduced fertility.

The immune response of cattle, persistently infected with noncytopathic BVDV, after superinfection with antigenically semi-homologous cytopathic BVDV

Cattle persistently infected (PI) with noncytopathic (ncp) bovine virus diarrhea virus (BVDV) are at risk for developing fatal mucosal disease (MD), which is considered to occur after superinfection with antigenically homologous cytopathic (cp) BVDV. In this study, we intranasally inoculated four PI-animals, that were PI with 2 ncp BVDV strains with 10(5) TCID50 antigenically closely related cp BVDV. Two PI-animals were inoculated with 10(5) TCID50 ncp BVDV and one PI-animal, with virus free cell culture medium. Two out of four PI-animals that were inoculated with cp BVDV, developed MD and were euthanized at day 17 and at day 24 after infection. Postmortem, both animals showed typical lesions of MD and cp BVDV was isolated. The other two PI-animals that were inoculated with cp BVDV did not develop MD and were euthanized at day 51. They showed ulcerations in the gastrointestinal tract, cp BVDV was isolated and neutralizing antibodies were detected. From the three PI-animals, that were inoculated with ncp BVDV or cell culture medium, cp BVDV was also isolated. Cross neutralization tests were performed and no antigenic differences could be detected between the cp strains isolated from the PI-animals. Lymphocyte subsets of these PI-animals were determined by flow cytometric analysis. Before superinfection, the percentages of gamma delta subsets were much higher in the PI-animals that did not develop MD than in nonviremic control animals and in the PI-animals that died of MD. From this study we conclude that the presence of antigenically closely related cp BVDV in PI-animals does not necessarily lead to the development of MD and that besides the antigenic relatedness between the persisting ncp BVDV and cp BVDV other factors, for instance the number of circulating gamma delta cells, might determine whether or not PI-animals develop MD.

The use of a mass-action model to validate the output from a stochastic simulation model of bovine viral diarrhoea virus spread in a closed dairy herd

The spread of bovine virus diarrhoea virus (BVDV) in a closed dairy herd maintained under typical management conditions is studied using two approaches. In the first instance a stochastic computer model is used to simulate the month-to-month changes in the infection status of each animal. These results are contrasted with the results of a mass-action model which uses three differential equations. A comparison of the two approaches indicates that the results are in broad agreement. The stochastic approach has the benefit of providing an estimate of the probability of the infection becoming extinct and the herd becoming BVDV-free for different herd sizes.

Bovine virus diarrhoea virus in semen from acutely infected bulls

The risk of spreading bovine virus diarrhoea virus (BVDV) from acutely infected animals to susceptible animals was investigated. Ten bulls from a herd with no previous history of BVDV were used. The bulls were demonstrated free from BVDV and such antibodies. Six of the bulls were inoculated intravenously with cytopathogenic virus, and 4 bulls were used as controls. Semen samples were collected during a period of 66 days after inoculation. The samples were examined for BVDV, and spermatological parameters were registered. Testes and epididymides were examined histologically post mortem. All inoculated bulls exhibited elevated temperatures between days 4 and 8 after inoculation, and BVDV antibodies were found in all of them on day 22. The control animals remained antibody negative. Non-cytopathogenic BVDV was isolated from seminal plasma from 2 bulls on day 7 after inoculation. Semen volume was significantly reduced from week 6 after inoculation. Percent abnormal sperm cells decreased in the same period. No significant differences were observed in sperm density or percentage of live spermatozoa. No pathological changes were found in the testes or epididymides.

Antigenically different pestivirus strains induce congenital infection in sheep: a model for bovine virus diarrhea virus vaccine efficacy studies

To study the efficacy and safety of bovine virus diarrhea virus (BVDV) vaccines there is need for a valid challenge model. We investigated whether sheep can be used in such a challenge model. We intranasally inoculated six groups (A-F) of seronegative sheep at day 49 of gestation with either of five antigenically different BVDV strains and one border disease virus strain. A seventh group (G) was housed for 10 days with a persistently infected calf and an eighth group (H) served as control. From each group half of the sheep were killed at 2 weeks, and half at 4 weeks after infection. For virus isolation five organs were collected from the sheep and seven from the fetuses. All sheep of groups A and H remained seronegative in the ELISA and in the serum neutralization test. At 2 and 4 weeks after infection virus was isolated from almost all fetal organs in six groups. In group A and in the control group no virus was isolated from the fetal organs. The virus distribution patterns in fetuses from sheep housed with the persistently infected calf or intranasally inoculated with the same strain were similar. We concluded that (i) antigenically different BVDV strains can induce congenital infection in sheep and that (ii) the consequences of a contact infection were similar to those after intranasal infection. In a second experiment we infected two groups of seronegative sheep with one of the strains used in the first experiment, before mating. A control group was left uninfected. The sheep were served and all sheep were challenged with antigenically homologous or heterologous BVDV at day 49 of pregnancy. Three weeks after challenge, sheep were killed and the procedure as in the first experiment was followed. None of the fetuses sheep were virus positive whereas all fetuses of the control sheep were virus positive. Hence, the immune response after BVDV infection protects fetuses against homologous and heterologous infection during pregnancy. Sheep may therefore be used in vaccination-challenge experiments to evaluate BVDV vaccine efficacy in preventing congenital infection.

Distribution of virus and viral antigen in specific pathogen-free calves following inoculation with noncytopathic bovine viral diarrhea virus

Gross and microscopic lesions and distribution of virus were studied in specific pathogen-free calves (SPF) 10 days post-inoculation (PI) with bovine viral diarrhea virus (BVDV). To investigate possible differences in tissue tropism between BCDV isolates, two clinically and antigenically different noncytopathic (ncp) isolates of BVDV were compared in the study. Four calves were exposed to noncytopathic (ncp) BVDV 7937, and four to ncp-BVDV 126. Two additional calves that were not exposed to virus served as controls. Both ncp-BVDV 7937 and ncp-BVDV 126 induced mild disease characterized by variable fever and anorexia. Lymphoid depletion was evident in Peyer's patch of four calves and the thymus of two calves exposed to BVDV. Differences between these isolates in the distribution of BVDV or BVDV antigen in tissues of inoculated calves were not found. High concentrations of BVDV and BVDV-specific antigen were detected in the thymus, Peyer's patch, and mesenteric lymph node of all exposed calves. BVDV was shown to infect cells of the bone marrow without causing microscopic lesions. High concentrations of BVDV were recovered from the bone marrow of all calves exposed to BVDV and BVDV-specific antigen was demonstrated at this location in six of these calves. Platelet counts of calves exposed to BVDV were significantly reduced during infection, which resulted in thrombocytopenia in one calf. Focal areas of necrosis were observed in squamous epithelial cells of the tonsil and ruminal mucosa. BVDV-specific antigen was found in and adjacent to these foci. Calves exposed to ncp-BVDV 7937 had slightly more severe clinical signs than those exposed to ncp-BVDV 126.

Foetal infections with bovine viral diarrhoea virus in Argentina

The frequency of isolation of bovine viral diarrhoea virus (BVDV) from primary tissue cultures and organs from bovine foetuses was studied between 1992 and 1994. Around 25% of primary tissue cultures were BVDV positive. Primary testis cultures were inoculated with homogenates of spleen, kidney, lung and liver from 52 foetuses. Cells were passaged twice and BVDV antigen investigated by indirect immunofluorescence. Non-cytopathic BVDV was detected in at least one organ in 11/52 foetuses (21.2%): 6/10 spleens, 4/7 kidneys, 7/9 lungs and 3/5 livers. Cytopathic BVDV was detected in lung and kidney from two foetuses. Since only gamma-irradiated sera are used in the laboratory and only inactivated BVDV vaccines are applied in Argentina, it was concluded that these isolations represented field infections. In addition to the 11 virus positive foetuses, two foetuses were positive for BVDV antibodies, which suggested a 25% prevalence of infection. These results stress the need for disease control on a herd basis and the requirement for biological reagents of bovine origin for the detection of BVDV.

Epidemiology of bovine viral diarrhea virus

Prevalence studies around the world show that BVDV is widespread in most cattle raising countries. There are significant differences, however, in prevalence between areas, probably the result of differences in cattle population structure and management practice. Direct contact with PI animals is probably the most important method of transmission of infection; however, field studies have shown that some limited spread of infection also occurs in the absence of PI animals. This may be due to contact with acutely infected animals or contact with other species infected with BVDV. Different ways of indirect transmission such as contaminated needles and equipment have been proven experimentally, and indirect transmission is considered to have some importance. If a PI animal is introduced directly into a dairy herd, most animals will be infected within a few months. On many occasions, however, a herd gets infected by other means than direct introduction of PI animals. In these cases, the infection is often spread to only a few animals after which the infections stops. The infection is then reinforced when PI animals are born. Slow and hence prolonged spread of infection in herds without PI animals also has been described, but the mechanism is not fully understood. Family lines of PI animals delivering PI calves are fairly common and can cause the infection to continue for several years. The clinical manifestations, acute BVDV, reproductive disorders, birth of malformed, weak and undersized calves, unthrifty PI animals, and mucosal disease often appear within certain periods. Large variation, however, can occur between herd outbreaks due to variation in virulence of the BVDV strain, housing of the cattle, and variation in transmission patterns. The extensive transmission of infection from PI animals makes different surveillance methods possible. Thus testing of a screening sample of a few young stock of antibodies and determination of antibody titer in bulk milk will often give good indication of presence of PI animals in herds not using BVDV vaccines. In herds using killed vaccine, determination of antibody titers among few young stock can show the presence of PI animals. The high incidence of infection combined with all the different damages that are seen after BVDV infection cause huge economical losses, which on a national level in the UK and Denmark (i.e., areas with widespread occurrence of infection) has been calculated as between 7 and 27 million pounds (between $11 and $42 million) per million calvings. Epidemiologic studies are important as a basis for selection of control strategy. Because of the variation in epidemiology between geographic areas, evaluation of a control strategy in an area preferentially should be based on epidemiologic studies in the same area.

The clinical manifestations of bovine viral diarrhea infection

BVDV infections may result in a wide range of clinical manifestations ranging from subclinical infections to a severe, highly fatal form referred to as MD. In recent years, a better understanding of pathogenesis has been achieved that has allowed for a better understanding of the different clinical forms of BVDV infection. Knowledge in this area continues to evolve. The clinical forms of BVDV infections are best understood by breaking them into categories that include infections in immunocompetent cattle, fetal infections, and infections in cattle that are immunotolerant to BVDV.

Development of a PCR amplification assay as a screening test using bulk milk samples for identifying dairy herds infected with bovine viral diarrhea virus

The approach of cDNA synthesis followed by polymerase chain reaction (PCR) amplification was used to develop a rapid screening test for the detection of bovine viral diarrhea virus (BVDV) in bulk tank milk samples. The initial development of this detection method was done using lactating Holstein cows; 1 acutely infected with BVDV following experimental inoculation and 2 persistently infected (PI) with BVDV. Viral RNA was extracted from somatic cells purified from whole milk using a guanidinium isothiocyanate and phenol/chloroform extraction method. Oligonucleotide primers were selected from the 5'untranslated region (5'UTR) and p80 region of BVDV genome. In the acutely infected cow, BVDV RNA was identified from days 6 to 10 postinoculation. Viral RNA extracted from somatic cells of milk from PI cows was detected by PCR using both 5'UTR and p80 primer sets. The sensitivity of PCR detection was determined by preparing dilutions of whole milk obtained from the BVDV persistently infected animals with milk from a BVDV-negative cow followed by purification of somatic cells and RNA extraction. BVDV was detected in milk serially diluted to 1:640 using PCR amplification. In addition, PCR amplification was 14.6 times more sensitive than virus isolation in detecting BVDV RNA in purified milk somatic cells. PCR detected BVDV RNA from a minimum of 580 somatic cells while the detection limit of virus isolation was 8500 cells. The sensitivity and specificity of BVDV amplification were confirmed by Southern hybridization analysis. BVDV RNA was detected using PCR in 33 out of 136 bulk milk samples collected from 124 individual herds using the 5'UTR primer set. These results indicate that PCR analysis of bulk tank milk samples may provide a rapid and sensitive method of screening herds for the presence of BVDV infections.

Estimation of herd incidence of infection with bovine virus diarrhoea virus (BVDV) in herds previously without animals persistently infected with BVDV

On a previous occasion, all animals in 9 herds had been bled for bovine virus diarrhoea virus (BVDV) and antibodies. No animals persistently infected (PI) with BVDV were detected. Three years later 10 animals in each herd were tested again. By this time 60 out of 90 previously seronegative animals had seroconverted. Seroconversions had occurred in 8 of the 9 herds corresponding to a incidence risk of herd infection of 0.52 per year. The incidence risk of seroconversions in individual animals was 0.31. Examination of young stock for antibodies and determination of antibody titer in bulk milk were good indicators for ongoing infections in the herds. The number of herd infections seemed to be higher than could be explained from purchase of PI animals.

Flow cytometric analyses of lymphocyte subsets in peripheral blood and lymphoid tissues of gnotobiotic calves during primary acute postnatal infections of bovine viral diarrhea virus

Eight 1-day-old gnotobiotic calves were exposed to one of 2 noncytopathic isolates of bovine viral diarrhea virus (ncp-BVDV 7937 and ncp-BVDV 126) and 2 similar calves were not infected and served as controls. Phenotypic analyses of peripheral blood mononuclear leukocytes at 3, 7, and 10 days postinfection (PI), and cell preparations of thymus, Peyer's patch, mesenteric lymph node, spleen, and bone marrow collected at necropsy, 10 days PI, were conducted using flow cytometric techniques on cells stained by an indirect fluorescent antibody assay using monoclonal antibodies specific for mononuclear leukocyte subsets. Significant fluctuations in specific subsets of peripheral blood mononuclear leukocytes were observed in calves exposed to ncp-BVDV 7937 on day 3 PI (cells expressing MHC class II), day 7 PI (B-cells), and day 10 PI (B cells and cells expressing MHCII). Significant phenotypic differences between groups were also detected in cell preparations from Peyer's patch and thymus. Calves exposed to ncp-BVDV 7937 had a significantly higher percentage of B cells than calves exposed to ncp-BVDV 126 and calves not exposed to BVDV. Calves exposed to ncp-BVDV 126 had a significantly higher percentage of CD2 (BoCD2) positive T cells than calves not exposed to BVDV. Fragility of thymic cell preparations was attributed to infection with virus. These results highlight the importance of the tropism of BVDV for cells of the bovine immune system and its role as a significant immunosuppressive agent capable of predisposing affected animals to infection with other agents.

Immune response to bovine viral diarrhea virus induced by anti-idiotypic antibodies

We have previously prepared rabbit anti-idiotypic antibodies (anti-Ids) against the neutralizing monoclonal antibodies (MAbs) specific for the gp53 of bovine viral diarrhea virus (BVDV). The anti-Ids, purified by sequential immunoaffinity chromatography, inhibited the immunizing MAb from binding to the original antigens and blocked BVDV infection of cell cultures. This study evaluated immune responses in mice to the purified anti-Id reagents. BVDV-specific neutralizing antibodies were induced by the anti-Ids. The antisera (Ab3) induced by the anti-Ids immunoprecipitated gp53 from BVDV-infected Madin-Darby bovine kidney (MDBK) cell lysates. However, lymphocyte-proliferative responses were specific only for the respective immunizing antigens. These results suggest that the anti-Ids may bear an internal image of the gp53 to stimulate production of antibody but not to stimulate a virus-specific cellular immune response in mice.

Viral antibodies in bovine fetuses in Argentina

In order to establish the prevalence of viral infections of the bovine fetus in Argentina, a serological survey for antibodies against viral agents currently affecting cattle in this country was conducted. Antibodies against foot-and-mouth disease virus (FMDV), bovine herpesvirus-1 (BHV-1), bovine leukaemia virus (BLV), bovine rotavirus (BRV), bovine coronavirus (BCV), bovine viral diarrhoea virus (BVDV) and parainfluenza-3 (PI-3) were investigated in a total of 315 fetal serum samples. Conventional techniques were used: indirect immunofluorescence (FMDV, BHV-1, BVDV and BCV), radial immunodiffusion (BLV), ELISA (BRV) and haemagglutination inhibition (PI-3). Antibodies against BHV-1, BVDV and PI-3 were detected in samples from fetuses in the second and third trimester of gestation, with a prevalence of 1.21 per cent (two of 165), 2.03 per cent (four of 197) and 5.08 per cent (nine of 177), respectively. Either antibodies or non-antibody factors able to bind to BRV and BCV antigens were detected with a prevalence of 2.44 per cent (five of 205) and 4.54 per cent (five of 110), respectively. In addition, 14.68 per cent of non-specific inhibitors of PI-3 mediated haemagglutination were found. No seropositives against FMDV and BLV were detected.

Immunogens of bovine viral diarrhea virus

Bovine viral diarrhea virus (BVDV) is a ubiquitous pathogen of cattle that induces economically important diseases affecting multiple organ systems. In the United States, over 150 biological products are licensed for control of BVDV. These products contain live or killed BVDV, and many products contain other viruses or bacteria. Potency tests for these vaccines are based on animal inoculation and serology. For live virus vaccines, titration of viral infectivity in cell culture is an accepted alternative to animal inoculation. The immunogens in a killed virus vaccine may be measured by enzyme linked immunoabsorbent assay. Immunogens of BVDV that stimulate a protective immune response have not been conclusively identified. Epitopes on a putative viral envelope glycoprotein, gp53, are involved in viral neutralization. Other viral glycoproteins, gp48 and gp25, are immunogenic but epitopes on these proteins do not stimulate production of antibodies that efficiently neutralize virus. Progress in developing meaningful in vitro assays for quantitation of BVDV immunogens awaits identification of viral proteins that stimulate a protective immunity.

The production and survival of lambs persistently infected with border disease virus

From 1985 to 1989 lambs persistently infected with border disease virus (BDV) were produced for comparative immunological studies by infecting 57 susceptible pregnant ewes between 50 and 60 days' gestation with Moredun or Oban strains of BDV. Ewes were infected either by injection with virus grown in cell culture or by housing with lambs excreting BDV. There was no significant difference in the outcomes of these different methods of infection. There was a significant difference in the number of viable lambs born to ewes receiving the two viruses. Of 41 ewes infected with Moredun virus 21 produced 32 live lambs of which 17 were reared to 1 month old (53% viability). Of 16 ewes receiving Oban virus 10 gave birth to 17 live lambs of which 15 were reared to 1 month old (88% viability). All the lambs born to ewes infected with Moredun BDV had varying signs of tremor and increased hairiness ("hairy-shakers") while those born to ewes infected with the Oban virus had no obvious clinical signs. Survival of the lambs was poor. Up until February 1991, 14 Moredun and 10 Oban sheep between the ages of 4 months and 5.5 yr had died from a variety of causes. The two commonest causes were a chronic wasting syndrome and a mucosal disease-like syndrome which was associated with the recovery of cytopathic BDV. Mating of unrelated persistently infected sheep was largely unproductive although 2 lambs were reared.

Immunity to bovine virus diarrhoea virus in calves: the role of different T-cell subpopulations analysed by specific depletion in vivo with monoclonal antibodies

Gnotobiotic calves were injected intravenously with murine monoclonal antibodies (mAb) directed against the BoCD4, BoCD8 or BoWC1 antigens that define the three major T-lymphocyte subpopulations in cattle. This produced a transient, specific depletion of each cell type in the circulation. Calves were then infected intranasally with a non-cytopathogenic biotype of bovine virus diarrhoea virus and the effect of the specific depletion with the mAb on viraemia and shedding of virus from the nasopharynx determined. Depletion of the cells expressing the BoCD4 antigen resulted in an extension of the duration of viraemia and an increase in the titre of virus in blood. No effect on nasopharyngeal shedding was noted. Depletion of either of the other two T-cell subsets that expressed the BoCD8 antigen or the BoWC1 antigen present on the gamma/delta T-cells had no demonstrable effect. These findings are interpreted as showing that the BoCD4+ cells play a pivotal role in controlling a primary infection with this virus but MHC class I restricted BoCD8+ T-cells are not a major effector mechanism. The BoCD4+ cells may be acting directly or be mediators of T-cell help.

Pestivirus gene expression: protein p80 of bovine viral diarrhea virus is a proteinase involved in polyprotein processing

Bovine viral diarrhea virus (BVDV), the prototypic pestivirus, possesses a positive-strand RNA genome with a single large open reading frame (ORF) encoding about 4000 amino acids. We have endeavored to elucidate the mechanisms involved in protein biogenesis by this pestivirus. Here, we present our studies on gene expression from the viral nonstructural protein coding region encompassing the carboxy-terminal 60% of the ORF. Previous sequence and modeling analyses predicted the amino-terminal region of the BVDV nonstructural protein p80 to be a trypsin-like serine proteinase. Using a mammalian cell transient expression system, we show that this region indeed possessed a proteolytic activity and, further, required the serine residue previously predicted to be the putative serine proteinase catalytic site. We found the p80-region proteinase activity was required for proteolytic processing of all viral nonstructural proteins. Cleavage by this activity at the amino and carboxy termini of the p80 protein itself likely occurred intramolecularly (in cis), since we were unable to demonstrate activity in trans at these sites. Cleavages at the three processing sites downstream of the carboxy terminus of p80 were shown to occur in trans. However, p80 proteinase activity alone was not sufficient for cleavage of the last of these sites. Another viral gene product, or specific condition, is implicated as a necessary cofactor for p80 proteinase activity at this site. Pestivirus polyprotein processing can now be compared to similar events by viruses of other groups. Finally, the potential role of p80 proteinase activity in the phenotype of cytopathic biotypes of BVDV is discussed.

Identification of cattle infected with bovine virus diarrhoea virus using a monoclonal antibody capture ELISA

A monoclonal antibody capture enzyme linked immunosorbent assay (ELISA) has been developed to detect pestivirus-specific antigen in the leucocytes of cattle infected with bovine virus diarrhoea virus (BVDV). A blind trial was conducted to compare the specificity of the ELISA with conventional tissue culture virus isolation on 215 blood samples submitted for BVDV diagnosis from cattle throughout Scotland. One hundred and sixty seven samples were negative by both ELISA and virus isolation and 47 samples were positive by both tests. One blood was negative by ELISA and positive by virus isolation.

The pathways for bovine virus diarrhoea virus biotypes in the pathogenesis of disease

BVDV infections of cattle ranges from the transient acute infections, which may be inapparent or mild, to mucosal disease which is inevitably fatal. On occasions the acute infections can lead to clinical episodes of diarrhoea an agalactia but as these syndromes cannot be reproduced experimentally, the pathogenesis remains unclear. The immunosuppressive effect of acute BVDV infections can enhance the clinical disease of other pathogens and this may be an important part of the calf respiratory disease complex. Although BVDV antigen has been demonstrated within the lymphoid tissues, for prolonged periods, the evidence for viral latency remains to be proven. Venereal infection is shown to be important in the transfer of virus to the foetus and congenital infections can cause abortions, malformations and the development of persistently viraemic calves. The two biotypes of BVDV, non-cytopathogenic and cytopathogenic, are described. Their sequential role in the pathogenesis of mucosal disease arises from the initial foetal infection with the non-cytopathogenic virus and the subsequent production of persistently viraemic calves. These calves may later develop mucosal disease as a result of superinfection with a "homologous" cytopathogenic virus and the possible origin of this biotype by mutation is discussed. Chronic disease is defined as a progressive wasting and usually diarrhoeic condition; it is suggested that this may develop following superinfection of persistently viraemic cattle with a "heterologous" cytopathogenic biotype.

Clinical and virological observations of a mucosal disease outbreak with persistently-infected seropositive survivors

A group of 14 four to nine month old calves, clinically healthy but persistently infected with bovine virus diarrhoea virus (BVDV), was obtained from a single farm, and reared as a group. Ten of them were male and were castrated soon after arrival. Signs of mucosal disease (MD) developed within a month and eight of the males had died or been killed on humane grounds by 2 months after purchase. The other two males and one of the females developed more chronic but progressive signs of MD and were killed during the next four months. The remaining three females showed only transient signs of MD followed by clinical recovery. They subsequently remained healthy up to slaughter at 2, 2.5 and 5 years respectively. These three survivors were persistently infected with BVDV, and shed virus in their mucous secretions, although two of them were also seropositive to the virus with fluctuating neutralizing antibody titres (at times as high as 1/960) to a range of BVDV strains including their own persisting virus.