Bovine viral diarrhea virus: its effects on estradiol, progesterone and prostaglandin secretion in the cow

Alterations in pregnancy-associated glycoprotein concentrations of pregnant sheep experimentally infected with bovine viral diarrhea virus

OBJECTIVE

To compare pregnancy-associated glycoprotein 1 (PAG1) concentrations in maternal (jugular vein) and fetal (uterine vein) circulations and amniotic fluid samples between pregnant ewes that were and were not experimentally infected with bovine viral diarrhea virus (BVDV).

ANIMALS

11 healthy pregnant yearling ewes.

PROCEDURES

Before study initiation, all ewes were naïve to BVDV and confirmed pregnant by transabdominal ultrasonography at approximately 60 days of gestation. At 65 days of gestation, ewes were intranasally inoculated with a noncytopathic BVDV type 1b strain (concentration, 10⁷ TCID₅₀/mL; 2 mL/nostril; n = 6) or an equal volume of BVDV-free viral culture medium (control; 5). A blood sample was collected for measurement of PAG1 concentration before inoculation. At 80 days of gestation, each ewe was anesthetized and underwent an ovariohysterectomy. While sheep were anesthetized, blood samples from the jugular and uterine veins and an amniotic fluid sample were collected for measurement of PAG1 concentration. Fetal tissues underwent real-time PCR analysis for BVDV RNA, and placental specimens underwent histologic evaluation and immunohistochemical staining for BVDV antigen.

RESULTS

At 80 days of gestation, BVDV RNA in fetal tissues and mild placentitis were detected in 5 of 6 BVDV-inoculated ewes. Mean PAG1 concentrations in the maternal and fetal circulations of BVDV-inoculated ewes were significantly less than those in control ewes. Mean amniotic fluid PAG1 concentration did not differ significantly between the 2 groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Concentration of PAG1 in the maternal circulation may be a useful biomarker for determining placental health in sheep after viral infection of the reproductive tract.

Mechanisms linking bovine viral diarrhea virus (BVDV) infection with infertility in cattle

Bovine viral diarrhea virus (BVDV) is an important infectious disease agent that causes significant reproductive and economic losses in the cattle industry worldwide. Although BVDV infection is known to cause poor fertility in cattle, a greater part of the underlying mechanisms particularly associated with early reproductive losses are not clearly understood. Previous studies reported viral compromise of reproductive function in infected bulls. In females, BVDV infection is thought to be capable of killing the oocyte, embryo or fetus directly, or to induce lesions that result in fetal abortion or malformation. BVDV infections may also induce immune dysfunction, and predispose cattle to other diseases that cause poor health and fertility. Other reports also suggested BVDV-induced disruption of the reproductive endocrine system, and a disruption of leukocyte and cytokine functions in the reproductive organs. More recent studies have provided evidence of viral-induced suppression of endometrial innate immunity that may predispose to uterine disease. Furthermore, there is new evidence that BVDV may potentially disrupt the maternal recognition of pregnancy or the immune protection of the conceptus. This review brings together the previous reports with the more recent findings, and attempts to explain some of the mechanisms linking this important virus to infertility in cattle.

Identification of a cell population that produces alpha/beta interferon in vitro and in vivo in response to noncytopathic bovine viral diarrhea virus

In vitro infection of bovine cells of many origins with the cytopathogenic bovine viral diarrhea virus (cpBVDV) results in the induction of alpha/beta interferon (IFN-alpha/beta), whereas noncytopathogenic BVDV (ncpBVDV) isolates have been shown not to induce IFN-alpha/beta in vitro. Similarly, cpBVDV induces IFN-alpha/beta in the early bovine fetus, but ncpBVDV does not. However, acute infection of naive cattle with ncpBVDV results in IFN-alpha/beta production. In this study, we identified and characterized a minor population of cells, present in lymph nodes that produce IFN-alpha in response to ncpBVDV. These cells expressed the myeloid markers CD14, CD11b, and CD172a but did not express CD4 and CD45RB. We also established that these cells produced IFN-alpha in the absence of detectable productive infection.

Do calcium-mediated cellular signalling pathways, prostaglandin E2 (PGE2), estrogen or progesterone receptor antagonists, or bacterial endotoxins affect bovine placental function in vitro?

The major objective of this experiment was to determine whether the bovine placenta could be stimulated to secrete progesterone, since the bovine placenta secretes little progesterone when the corpus luteum is functional. Secondly, we wanted to determine whether reported abortifacients or progesterone or estrogen receptor antagonists affected bovine placental prostaglandin secretion. The ovine placenta secretes half of the circulating progesterone at day 90 of pregnancy and PGE2 appears to regulate ovine placental progesterone secretion. Calcium has been reported to regulate placental progesterone secretion in cattle. Diced 186-245-day placental slice explants from six Brahman and six Angus cows were incubated in vitro at 39.5 degrees C under 95% air: 5% CO2 at pH 7.2 in 5 ml of M-199 for 1 h in the absence of treatments and for 4 and 8 h in the presence of treatments. Treatments were: vehicle; R24571; compound 48/80; IP3; PGE2; CaCl2; cyclosporin A; lipopolysaccharide (endotoxin) from Salmonella abortus equi., enteriditis, and typhimurium; monensin; ionomycin; arachidonic acid; mimosine; palmitic acid; progesterone, androstenedione; estradiol-17beta; A23187; RU-486; or MER-25. Jugular and uterine venous plasma and culture media were analyzed for progesterone, PGE2 and PGF2alpha by radioimmunoassay (RIA). Plasma hormone data were analyzed by a One-Way Analysis of Variance (ANOVA). Hormone data in culture media were analyzed for breed and treatment effects by a Factorial Design (2 breeds, 2-range of days, 21 treatments) for ANOVA (2 x 2 x 21). Since hormone data secreted by placental tissue in vitro did not differ (P > or = 0.05) by breed or range of days of pregnancy, data were pooled and analyzed by a One-Way ANOVA. Concentrations of PGE2 in uterine venous blood were two-fold greater (P < or = 0.05) in Angus than Brahman cows. PGE2 and PGF2alpha in vehicle controls increased from 4 to 8h (P < or = 0.05), but not progesterone (P > or = 0.05) Progesterone in culture media treated with RU-486 increased (P < or = 0.05) at 4 and 8 h compared to vehicle controls and was not affected by other treatments (P > or = 0.05). Concentrations of PGE2 in media at 4 and 8 h were lower (P < or = 0.05) when compared to controls except treatment with PGE2 at 4 and 8h and RU-486 at 8h (P > or = 0.05). PGF2alpha was increased (P < or = 0.05) by RU-486 at 8h and no other treatment affected PGF2alpha at 4 or 8 h (P < or = 0.05). In conclusion, modulators of cellular calcium signalling pathways given alone do not affect bovine placental progesterone secretion at the days studied and progesterone receptor-mediated events appear to suppress placental progesterone, PGF2alpha, and PGE2 secretion in cattle. In addition, PGE2 does not appear to regulate bovine placental progesterone secretion when the corpus luteum is functional and bacterial endotoxin does not appear to affect bovine placental secretion of PGF2alpha or PGE2.

Effect of bovine viral diarrhea virus infection on fertility of dairy heifers

A prospective field study in heifers from birth to first breeding was undertaken on two commercial dairies to assess the effect of bovine viral diarrhea virus (BVDV) congenital and post-natal infection (PNI) on fertility. A high BVDV Type 2 antibody titer (1:4096) at 10 months of age was associated with 32 more days to conceive, compared with a low titer (1:128). Conversely, infection with BVDV by 5-6 months of age and high BVDV Type 2 titers 1 month before conception or breeding was associated with improved fertility. Heifers with evidence of congenital BVDV infection had lower fertility than non-infected heifers (15-42 days longer time-to-first AI), which depended on BVDV Type 2 titers at 10 months of age. Neospora caninum infection was associated with additional services per conception (SPC) and Leptospira interrogans infection was associated with a delay in the time-to-first breeding. It appears that under field conditions, the effect of subclinical BVDV infection on subsequent heifer fertility may be due to a complex of interrelationships among multiple BVDV infections that depend on the type and timing of infection relative to reproductive development and events.

Reproductive consequences of infection with bovine viral diarrhea virus

Reproductive efficiency is imperative for the maintenance of profitability in both dairy and cow-calf enterprises. Bovine viral diarrhea virus is an important infectious disease agent of cattle that can potentially have a negative effect on all phases of reproduction. Reduced conception rates,early embryonic deaths, abortions, congenital defects, and weak calves have all been associated BVDV infection of susceptible females. In addition, the birth of calves PI with BVDV as a result of in utero fetal exposure is extremely important in the perpetuation of the virus in an infected herd or spread to other susceptible herds. Bulls acutely or PI with BVDV may bea source of viral spread through either natural service or semen used in artificial insemination. Management practices including elimination of PI cattle, biosecurity measures and strategic use of vaccination can be implemented to reduce the risk of BVDV related reproductive losses. Development of vaccines and vaccine strategies capable of providing better protection against fetal infection would be of benefit.

Differential effects of bovine viral diarrhoea virus on monocytes and dendritic cells

Various pathogens have been shown to infect antigen-presenting cells and affect their capacity to interact with and stimulate T-cell responses. We have used an antigenically identical pair of non-cytopathic (ncp) and cytopathic (cp) bovine viral diarrhoea virus (BVDV) isolates to determine how the two biotypes affect monocyte and dendritic cell (DC) function. We have shown that monocytes and DCs are both susceptible to infection with ncp BVDV and cp BVDV in vitro. In addition, monocytes infected with ncp BVDV were compromised in their ability to stimulate allogeneic and memory CD4(+) T cell responses, but DCs were not affected. This was not due to down-regulation of a number of recognized co-stimulatory molecules including CD80, CD86 and CD40. Striking differences in the response of the two cell types to infection with cytopathic virus were seen. Dendritic cells were not susceptible to the cytopathic effect caused by cp BVDV, whereas monocytes were killed. Analysis of interferon (IFN)-alpha/beta production showed similar levels in monocytes and DCs exposed to cp BVDV, but none was detected in cells exposed to ncp BVDV. We conclude that the prevention of cell death in DCs is not associated with enhanced production of IFN-alpha/beta, as proposed for influenza virus, but is by a distinct mechanism.

Studies of the pathogenesis of bovine pestivirus-induced ovarian dysfunction in superovulated dairy cattle

Two experiments (Experiment I, n=12 Holstein-Friesian heifers; Experiment II, n=8 Jersey cows) were conducted to investigate the pathogenesis of bovine pestivirus-induced ovarian dysfunction in cattle. In both experiments the cattle were superovulated with twice daily injections of a porcine pituitary extract preparation of follicle stimulating hormone (FSH-P), for 4 days commencing on Day 10+/-2 after a presynchronised oestrus. The heifers received a total dose of 30 mg and the cows 32 mg of FSH-P. Prostaglandin F(2alpha) (PGF(2alpha)) was administered 48 h after commencement of superovulation and all cattle were artificially inseminated (AI) between 48 and 66h after PGF(2alpha) treatment. In both experiments bovine pestivirus seronegative cattle (Experiment I, n=6; Experiment II, n=4) were inoculated intranasally with an Australian strain of non-cytopathogenic bovine pestivirus (bovine viral diarrhoea virus Type 1) 9 days prior to AI. Bovine pestivirus infection was confirmed by seroconversion and/or virus isolation in all of the inoculated cattle, consistent with a viremia occurring approximately between Day 5 prior to AI and the day of AI. Ovarian function was monitored in both experiments by daily transrectal ultrasonography and strategic blood sampling to determine progesterone, oestradiol-17beta, luteinising hormone (LH) and cortisol profiles. Non-surgical ova/embryo recovery was performed on Day 7 after AI. In Experiment II half the cattle were slaughtered on Day 2 and the remainder on Day 8 after AI, and the ovaries submitted for gross and histopathological examination including immunohistochemistry to demonstrate the presence of bovine pestivirus antigen. In both studies, comparisons were made between infected and confirmed uninfected (control) animals. Overall the bovine pestivirus infected cattle had significantly lower (P<0.05) ova/embryo recovery rates compared to the control cattle. There was evidence of either an absence (partial or complete) of a preovulatory LH surge or delay in timing of the LH peak in the majority (90%) of infected heifers and cows, and histologically, there was evidence of non-suppurative oophoritis with necrosis of granulosa cells and the oocyte in follicles from the infected cows. By contrast only 20% of the control heifers and cows had evidence of absence of a pre-ovulatory LH surge. These experiments collectively demonstrate that bovine pestivirus infection during the period of final growth of preovulatory follicles may result in varying degrees of necrosis of the granulosa cells with subsequent negative effects on oestradiol-17beta secretion which in turn negatively affects the magnitude and/or timing of the preovulatory LH surge.

Insemination of susceptible heifers with semen from a non-viraemic bull with persistent bovine virus diarrhoea virus infection localized in the testes

Bulls shedding bovine viral diarrhoea virus (BVDV) in semen and simultaneously having a high concentration of circulating antibodies may cause reproductive problems and spread the viral infection within cattle populations. To investigate this in detail, three heifers were inseminated with BVDV-infected semen from a non-viraemic, seropositive Holstein-Friesian bull, named 'Cumulus'. One control heifer was inseminated with semen from a healthy bull that was free of BVDV. All four heifers remained clinically healthy throughout the experiment. The conception succeeded in the control animal and in two of the three heifers inseminated with semen containing BVDV. The heifer with the failed conception was the only one that became systemically infected with BVDV. This animal was deemed non-pregnant by ultrasonic examination on day 34 after insemination and showed no signs of subsequent oestrus during the entire experimental period. At slaughter, 42 days after insemination, there were no histopathological changes in the ovaries and virus was not detected in ovarian tissue. The fact that seronegative dams served with semen from persistently infected bulls have occasionally produced persistently infected calves together with the present findings and the fact that non-viraemic, seropositive bulls can constantly shed BVDV, suggest that the use of semen from such bulls in BVDV-free herds could have far-reaching consequences, especially if it led to the birth of persistently infected (P1) calves.

Alpha/beta and gamma interferons are induced by infection with noncytopathic bovine viral diarrhea virus in vivo

In contrast to the results of previous in vitro studies, experimental infection of calves with noncytopathic bovine viral diarrhea virus (ncpBVDV) was found to induce strong alpha/beta and gamma interferon responses in gnotobiotic animals. These responses were associated with depressed levels of transforming growth factor beta (TGF-beta) in serum. The results of this study indicate that the immunosuppression caused by ncpBVDV is not associated with low interferon responses or elevated levels of TGF-beta.

Masking of two in vitro immunological assays for Mycobacterium bovis (BCG) in calves acutely infected with non-cytopathic bovine viral diarrhoea virus

Acute infection of calves, previously vaccinated with bacille Calmette-Guerin (BCG), with non-cytopathic viral diarrhoea virus (BVDV) resulted in the temporary suppression of two in vitro assays used to monitor Mycobacterium bovis infection. Lymphocyte proliferation and interferon-gamma production by whole blood cultures containing purified protein derivatives prepared from Mycobacterium avium (PPD-A) and M bovis (PPD-B) were markedly suppressed. The implication is that acute infections of cattle with non-cytopathic BVDV may temporarily compromise diagnostic tests for M. bovis infections and result in a failure to identify cattle with tuberculosis.

Bovine viral diarrhoea virus: its effects on ovarian function in the cow

Bovine viral diarrhoea virus (BVDV) is a major cattle pathogen responsible for a spectrum of symptoms, including reproductive failure. In this paper we investigate how BVDV interacts with the ovary. The viruses' tropism for the pre-ovulatory oocyte was studied by indirect immunohistochemistry. Two monoclonal antibodies, raised against the non-structural protein NS3 and the envelope glycoprotein E2 were used to probe cryo-sections cut from the ovaries of three persistently infected heifers. NS3 and E2 antigens were widely distributed within the ovarian stroma and follicular cells. NS3 was also localised within the proportion of oocytes. Overall 18.7% of the oocyte population had detectable levels of NS3. What is more, the proportion of antigen positive oocytes remained constant (P>0. 05) throughout the different stages of oocyte maturation. In a subsequent study seven cows were challenged with non-cytopathogenic BVDV (strain Pe515: 5x10(6) TCID(50)) to determine the oestradiol and progesterone responses to an acute infection. The sensitivity of the endogenous luteolytic mechanism was also established by analysing plasma prostaglandin F2alpha metabolite (PGFM) levels following an exogenous oxytocin (50 IU) challenge. The inoculation was given 2 days before a synchronised oestrus and was timed to ensure that viraemia occurred during the initial stage of corpora luteal development. Seven cows inoculated with non-infectious culture medium served as control animals and remained BVDV naive throughout the study. The BVDV challenge was followed by leucopenia, viraemia and sero-conversion. The virus also significantly (P<0.01) reduced plasma oestradiol levels between day 6 and day 11 post-inoculation (i.e. between day 4 and day 9 post-oestrus). However, the infection did not alter (P>0.05) progesterone secretion throughout the oestrous cycle or the plasma concentration of PGFM. These data indicate that bovine follicular cells and oocytes are permissive to BVDV at all stages of follicular development. They also show that a transient fall in oestradiol secretion may accompany an acute infection. In conclusion, this work has identified two potential routes through which BVDV can reduce fertility in the cow, namely impairment of oocyte quality and disruption of gonadal steroidogenesis.

The effects of bovine viral diarrhoea virus on cattle reproduction in relation to disease control

Bovine viral diarrhoea virus (BVDV) is a major reproductive pathogen in cattle. Infection of the bull can lead to a fall in semen quality and the isolation of infectious virus in the ejaculate, while infection in the cow leads to poor conception rates, abortions and congenital defects. BVDV also reduces the animal's resistance to other respiratory and enteric pathogens. The prevalence of BVDV is primarily due to the efficiency with which the virus crosses the placenta of susceptible females. Calves that survive infection during the first trimester of pregnancy are born with a persistent and lifelong infection. These persistently infected (PI) animals represent between 1.0% and 2.0% of the cattle population and continuously shed infectious virus. The availability of reliable diagnostic ELISA and PCR techniques, which can test milk or serum samples for virus or antibodies, has simplified BVDV surveillance and improved the prospects for control. Although PI animals are the principal vectors within and between herds, they can be readily identified and removed. By contrast, cows carrying a PI foetus are particularly problematic. These animals have been compared to 'Trojan Horses' because they are virus-negative and antibody-positive but they deliver PI calves. In general, acutely infected cattle are much less efficient vectors but infections at the onset of puberty have resulted in a localised and persistent infection within the testes. Under these circumstances, virus shedding into the semen may remain undetected. Transmission of BVDV can be controlled through vaccination or eradication. BVDV vaccine technology has been developing over the past 30 years, but currently available vaccines are still of the conventional inactivated or attenuated sort. In general, vaccination has not been applied with sufficient rigor to make a significant impact on the level of circulating virus, unlike the national and regional eradication programmes established in areas such as Scandinavia, Austria, the Netherlands and Scotland. Eradication confers the added advantage of improved herd health; however, it also creates a susceptible cattle population that needs to be protected by stringent biosecurity. In this article, we discuss how BVDV influences reproductive function, the potential for viral transmission during breeding and the measures that must be taken to avoid the spread of infection to susceptible cattle populations via semen, embryos, culture fluids and infected cows.